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Harnessing Power-over-Ethernet efficiently Page 44
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Why lithium batteries won’t fill up landfills CONSUMERS
with an inclination toward sustainability have been bummed out over recent revelations that most of the plastics they have carefully separated, washed, and placed in recycling bins typically end up in the local dump. So it may come as good news that dead lithium batteries are increasingly likely to avoid the same fate. The reason: A company called Li-Cycle says it has figured out how to fully recycle all the material in lithium cells. Until now, only the cobalt, nickel, and copper in these batteries typically got recycled. Everything else, including the lithium, went into the waste stream. Li-Cycle says there are several efforts happening worldwide aimed at recovering battery grade lithium. But they are typically on a bench or pilot scale. The company says it may be the only facility doing so that is up and running as a business. It also says it is the largest recycler of lithium-ion batteries in North America. The firm ‘s approach is to shred the batteries at numerous facilities spread out all over the country, then ship the now-compacted material to a central plant for extraction. Those familiar with lithium cells will realize that shredding them without starting a fire is a neat trick. Puncturing a lithium cell from a laptop, for example, generally makes the cell get super hot and eventually out-gas. A lithium-ion cell from a cell phone or toy that’s damaged frequently balloons up and may explode. Li-Cycle avoids such calamities during shredding by conducting the whole process, which has been patented, in an oxygen-free atmosphere. The resulting shredded material is inert. It consists of separated plastics, copper, aluminum, and cathode and anode material containing lithium, nickel, cobalt, graphite, and other metals. Li-Cycle’s tactic of shredding batteries at numerous distributed sites avoids the transportation costs which make it impractical to recycle several kinds of consumer materials. Many glass bottles tossed in recycling bins, for example, typically don’’t end up being used to make new glass. One reason is the overly long distance between most recycling facilities and used glass processors, called cullet suppliers, that lead to prohibitive shipping bills. Once the shredded material arrives at a Li-Cycle central hub, 95% of it gets recovered, the company claims. More important, the recovered materials are good enough to be reused in new lithium batteries. Li-Cycle also asserts the purity of these materials is on par with that of mined and refined versions and price competitive with virgin materials. But it turns out there are a tangle of thorny issues associated with tailoring reclaimed materials to the specs needed by battery manufacturers. Graphite compounds used in electrodes, for example, tend to have subtle differences in make up depending on the battery maker. So graphite reclaimed from lithium cells, at least in the near term, will probably end up in applications other than those involving batteries. That’s still better than what happens in ordinary battery recycling processes where graphite generally gets burned off as a waste product. All in all, it doesn’t look as though we’ll be seeing many lithium batteries in landfills.
LELAND TESCHLER • EXECUTIVE EDITOR
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5G TECHNOLOGY WORLD Delivers the Latest 5G Technology Trends
5G Technology World is EEWorldOnline’s newest site covering 5G technology, systems, infrastructure, and wireless design and development. Get caught up on critical 5G information, check out the following articles on 5GTechnologyWorld.com: Massive MIMO performance testing: Emulate the channel Performing MIMO testing using real-world conditions is critical for successful 5G deployments. www.5gtechnologyworld.com/massive-mimoperformance-testing-emulate-the-channel
5G is hot, keep your components and systems cool 5G’s antennas and the devices that drive them generate more heat than their LTE predecessors. That creates new cooling problems for wireless devices and systems. www.5gtechnologyworld.com/5g-is-hot-keep-yourcomponents-and-systems-cool
5G moves into production, causes test issues 5G Technology World talks with Teradyne’s Jeorge Hurtarte, who explains components and over-the-air production test of 5G components. www.5gtechnologyworld.com/5g-moves-intoproduction-causes-test-issues
IEEE 1588 adds timing performance while reducing cost and risk GPS and GNSS have been the standards for network timing, but they have security issues. A Master clock and IEEE 1588 reduces the risk and lowers installation costs. www.5gtechnologyworld.com/ieee-1588-adds-timingperformance-while-reducing-cost-and-risk
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CONTENTS POWER & ENERGY EFFICIENCY HANDBOOK | OCTOBER 2020
02 06
Why lithium batteries won’t fill up landfills Battery chemistries that excel in inaccessible applications Remotely located wireless devices require extra-long-life batteries to reduce the cost of ownership.
12
Navigating new safety standards for power supplies
16
Fusible resistors vs fuses
20
Driving modern haptics
23
How SiC boosts efficiency in power conversion
27
Liquid cooling for precise temperature control
30
Coming: New safety ratings for power adapters
33
Autonomous wireless charging keeps robots running
36
Boosting efficiency in fast-charge adapters
42
Why electrolytic capacitors blow up
46
Harnessing Power-over-Ethernet efficiently
More stringent requirements are scheduled to soon emerge from standards bodies that will affect a wide range of manufactured equipment.
Safety standards often dictate what component provides the best protection against overcurrents.
Piezo haptics now can provide a realistic sensation of a click or the resistance of a pushbutton.
The benefits silicon-carbide MOSFETs become evident from comparisons with their silicon counterparts.
Electronics that generate a lot of heat in a small volume often benefit from swapping a cooling fan for a system based on a liquid coolant.
Prepare yourself for the updated standards about to take hold in power equipment that includes external adapters.
Buck-boost regulators efficiently power wireless charging stations for mobile robots.
Gallium-nitride transistors help make charging circuits smaller and more economical.
Electrolytic capacitors have a reputation for failing spectacularly when mistreated.
A few simple techniques can boost the efficiency of modern PoE designs.
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POWER & ENERGY EFFICIENCY HANDBOOK
Battery chemistries that excel in inaccessible applications
Remotely located wireless devices require extra-long-life batteries to reduce the cost of ownership. Sol Jacobs • Tadiran Batteries
REMOTE
wireless devices serve as the cornerstone for all types of IIoTrelated applications, including asset tracking, system control and data automation (SCADA), environmental monitoring, AI, and machine learning, to name a few. Applications that are easily accessible and that operate in relatively moderate temperatures typically allow for use of numerous battery technologies, including inexpensive consumer alkaline and lithium-ion rechargeable batteries. However, the choice of power source becomes far more critical for wireless devices in scorching deserts, the frigid Arctic, or other harsh environments. Choice of battery technology is especially important where battery replacement is difficult or impossible, or if the application involves extreme temperature cycling. Specifying the right battery becomes
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even more important if the application requires 10+ year battery operating life to reduce the cost of ownership. It is useful to review the properties of ultra-long-life lithium battery chemistries to see how their benefits relate to the real world. There are two types of low-power wireless devices. The vast majority of them draw microamps of current and are powered by industrial-grade primary (non-rechargeable) lithium batteries. The second type of device draws current in the milliamp range. This level is enough to exhaust a primary battery relatively quickly. So a better approach may be an energy harvesting device in combination with an industrial grade rechargeable lthium-ion (Li-ion) cell to store the harvested energy. Low-power devices that require two-way connectivity often utilize a low-power communications protocol (i.e. WirelessHART, ZigBee, LoRa) along with a low-power chipset to maximize battery life. eeworldonline.com | designworldonline.com
LITHIUM BATTERY CHEMISTRIES Researchers from Cardiff University studying water channels beneath glaciers in Greenland and Antarctica developed the Cryoegg to beam data wirelessly. Designers chose a bobbin-type LiSOCl2 cell because it delivered high capacity and energy density, withstands temperatures as low as -30°C and can deliver periodic high current pulses twice daily for approximately two years. The Cryoeggs continually monitor changes in temperature, pressure, and electrical connectivity. Here, Mike Prior-Jones prepares a Cryoegg for deployment. Photos, courtesy Mauro Werder As the lightest non-gaseous metal, with a high intrinsic negative potential that exceeds all others, lithium offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all commercially available chemistries. Lithium cells operate within a normal operating current voltage (OCV) range of 2.7 to 3.6 V. They are also non-aqueous, making them less likely to freeze in extremely cold temperatures. Numerous factors must be considered when specifying a battery, including the amount of current consumed during active mode (including the size, duration, and frequency of pulses); energy consumed during ‘stand-by’ mode (the base current); storage time (as normal self-discharge during storage diminishes capacity); thermal environments (including storage and in-field operation); and equipment cut-off voltage, which drops as cell capacity is exhausted or during prolonged exposure to extreme temperatures. Perhaps most critical is the battery’s self-discharge rate, which can exceed the
amount of energy consumed to operate the device. Primary battery chemistries include iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), lithium thionyl chloride (LiSOCl2), alkaline, and lithium metal oxide chemistry. Lithium thionyl chloride (LiSOCl2) chemistry is overwhelmingly preferred for ultra-long-life applications. These cells can be constructed two ways: bobbin-type or spiral wound. Bobbin-type LiSOCl2 batteries feature higher capacity and energy density along with extremely low annual self-discharge, under 1% per year for certain cells that can operate for up to 40 years. Bobbin-type LiSOCl2 cells also feature the widest possible temperature range (-80 to 125°C) and a glass-to-metal hermetic seal to help prevent leakage. Battery self-discharge varies depending on the method of manufacturing and the purity of raw materials. For example, a superior quality bobbin-type LiSOCl2 cell can feature a self-discharge rate of 0.7% per year, retaining 70% of its original capacity after 40 years. By comparison, a lower quality bobbin-type LiSOCl2 cell can Comparing consumer and industrial Li-ion batteries have a self-discharge rate as high as 3% per year, losing 30% of its capacity every 10 years, making 40-year battery TLI-1550 (AA) Li-Ion Units life impossible. Industrial Grade 18650 Higher self-discharge rates may take years to detect, Diameter (max) [cm] 1.51 1.86 and theoretical test data is often misleading. So thorough due diligence is necessary when evaluating potential Length (max) [cm] 5.30 6.52 battery suppliers. Volume [cc] 9.49 17.71 Nominal Voltage
[V]
3.7
3.7
HIGH PULSES
Max Discharge Rate
[C]
15C
1.6C
Max Continuous Discharge Current
[A]
5
5
Capacity
[mAh]
Up to 1000
3000
Energy Density
[Wh/l]
129
627
Power [RT]
[W/liter]
1950
1045
Power [-20C]
[W/liter]
> 630
< 170
Operating Temp
deg. C
-40 to +90
-20 to +60
Charging Temp
deg. C
-40 to +85
0 to +45
[%/Year]
<5
<20
Cycle Life
[100% DOD]
~5000
~300
Cycle Life
[75% DOD]
~6250
~400
Cycle Life
[50% DOD]
~10000
~650
[Years]
>20
<5
Remote wireless applications increasingly require periodic high pulses to power two-way wireless communications. Standard bobbin-type LiSOCl2 cells are not designed to deliver high pulses. This can be easily overcome by adding a patented hybrid layer capacitor (HLC). The standard bobbin-type LiSOCl2 cell delivers low daily background current, while the HLC stores energy that can produce pulses of up to 15 A. The HLC also features a unique end-of-life voltage plateau that enables ‘low battery’ status alerts. Supercapacitors are generally not well suited for industrial applications due to various limitations, including: short-duration power; linear discharge qualities that do not permit full discharge of available energy; low capacity; low energy density; and a high self-discharge rate of up to 60% per year. In addition, supercapacitors linked in series require cell-balancing circuits that boost cost and take up space, use crimped seals that may leak, and draw more energy to shorten their operating life.
Self Discharge rate
Operating Life
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POWER & ENERGY EFFICIENCY HANDBOOK
Benefits of bobbin-type LiSOCl2 chemistry become clear from a review of remote wireless applications that employ these batteries. In one, researchers are studying water channels up to 2.5-km deep beneath glaciers in Greenland and Antarctica to better understand the effects of climate change on rising sea levels. To support this research, an innovative new instrument, dubbed the Cryoegg, was developed by Cardiff University. The Cryoegg transmits data wirelessly using radio transmissions, thus eliminating the need for bulky and expensive cables that can be damaged by glacial movement. The product designers were looking for a power source superior to lithium polymer batteries. Instead, they chose a bobbin-type LiSOCl2 cell that delivers high capacity and energy density and, is able to withstand temperatures as low as -30°C. It is capable of delivering periodic high current pulses to transmit data twice daily for approximately two years to continually monitor changes in temperature, pressure, and electrical connectivity. This application utilizes the 169 MHz Wireless M-Bus radio technology commonly found in AMR/AMI utility meter transmitter units (MTUs). Utility metering applications demand the use of bobbin-type LiSOCL2 batteries to reduce the risk of large-scale battery failure that can disrupt billing systems and disable remote start-up/shut-off capabilities. Another application involves the transport of scientific equipment across the Arctic environments. Oceantronics in Honolulu redesigned the battery pack for its GPS/ice buoy, replacing 380 alkaline D cells with a far smaller, lighter, and more economical battery pack using 32 bobbin-type LiSOCl2 cells and four HLCs. The new battery packs reduce size and weight by 90% (54 kg down to 3.2 kg). The compact size enables Oceantronics to make its device far easier to transport via helicopter to icebergs near the Sorth Pole. Switching from alkaline to LiSOCl2 chemistry also extended the operating life of the device many fold. In a third application, LiSOCl2 cells reduced the size and weight of line/connector sensors that monitor the status of electric power transmission lines located in desert environments. Southwire line/
Bobbin-type LiSOCl2 batteries feature higher capacity and energy density along with extremely low annual self-discharge, under 1% per year for certain cells that can operate for up to 40 years. Bobbin-type LiSOCl2 cells also exhibit a temperature range of -80 to 125°C and a glass-to-metal hermetic seal to help prevent leakage. connector sensors utilize bobbin-type LLiSOCl2 batteries to power the collection, aggregation, and transmission of data via a cellular network, monitoring temperature, catenary cables, and line current to warn the utility if transmission lines go down. Use of a bobbin-type LiSOCl2 battery enables these devices to be compact and lightweight (3.5 lb) for easier portability. These batteries can handle an extreme temperature range of -40 to 50°C and provide the high energy density necessary to deliver over 45 days of maintenance-free back-up power if no line current is detected.
ENERGY HARVESTING Energy harvesting is a candidate for applications that draw milliamps of current, enough to prematurely exhaust a primary battery. Photovoltaic (PV) panels are the most proven form of energy harvesting. Energy can also be harvested from equipment movement, vibration, temperature variances, and ambient RF/EM signals.
Oceantronics in Honolulu redesigned the battery pack for its GPS/ice buoy, replacing 380 alkaline D cells with a far smaller, lighter, and more economical battery pack using 32 bobbintype LiSOCl2 cells and four HLCs. The new battery packs reduce size and weight by 90% (54 kg down to 3.2 kg). The compact size enabled Oceantronics to make its device far easier to transport via helicopter to icebergs near the North Pole. Photo at left, courtesy of Sigrid Salo NOAA/PMEL. Photo on right courtesy Oceantronics.
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Powering robots that harvest, deliver and move
The most efficient power delivery for robots
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POWER & ENERGY EFFICIENCY HANDBOOK
For example, small solar panels are being combined with industrial grade Li-ion batteries to track the health and status of animal herds. Solar/ Li-ion hybrid systems also power parking meter fee collection systems, using AI-enabled sensors to identify open parking spots. Consumer-grade rechargeable Li-ion cells deliver a maximum operating life of five years and 500 recharge cycles and operate within a moderate temperature range (0 - 40°C), with no ability to deliver high current pulses. By contrast, industrial grade Li-ion batteries can operate for up to 20 years and 10,000 full recharge cycles, with an expanded temperature range (-40 to 85°C), and are able to deliver high pulses to power two-way wireless communications. In summary, long-life primary and rechargeable lithium batteries can reduce the cost of ownership for remote wireless applications. To ensure that the battery will last as long as the device, you must do your due diligence by requiring prospective battery suppliers provide well-documented long-term test results, in-field performance data under similar environmental conditions, along with numerous customer references.
LiSOCl2 cells reduced the size and weight of line/connector sensors that monitor the status of electric power transmission lines in deserts. Southwire line/connector sensors utilize bobbin-type LiSOCl2 batteries to power the collection, aggregation, and transmission of data via a cellular network, monitoring temperature, catenary cables, and line current to warn the utility if transmission lines go down. Use of a bobbin-type LiSOCl2 battery enables these devices to be compact and lightweight (3.5 lb). These batteries can handle a temperature range of -40 to 50°C and provide the energy density necessary to deliver over 45 days of maintenance-free back-up power if no line current is detected. Photos, courtsey Southwire.
References Tadiran, www.tadiranbat.com
Comparing primary lithium cell chemistries Primary Cell
Energy Density (Wh/1)
10
LiSOCL2
LiSOCL2
Li Metal Oxide
Li Metal Oxide
Bobbin-type with Hybrid Layer Capacitor
Bobbin-type
Modified for high capacity
Modified for high power
1,420
1,420
370
185
Alkaline
600
LiFeS2
LiMnO2
Lithium Iron Disulfate
CR123A
650
650
Power
Very High
Low
Very High
Very High
Low
High
Moderate
Voltage
3.6 to 3.9 V
3.6 V
4.1 V
4.1 V
1.5 V
1.5 V
3.0 V
Pulse Amplitude
Excellent
Small
High
Very High
Low
Moderate
Moderate
Passivation
None
High
Very Low
None
N/A
Fair
Moderate
Performance at Elevated Temp.
Excellent
Fair
Excellent
Excellent
Low
Moderate
Fair
Performance at Low temp.
Excellent
Fair
Moderate
Excellent
Low
Moderate
Poor
Operating life
Excellent
Excellent
Excellent
Excellent
Moderate
Moderate
Fair
Self-Discharge Rate
Very Low
Very Low
Very Low
Very Low
Very High
Moderate
High
Operating Temp.
-55°C to 85°C, can be extended to 105°C for a short time
-80°C to 125°C
-45°C to 85°C
-45°C to 85°C
-0°C to 60°C
-20°C to 60°C
0°C to 60°C
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Navigating new safety standards for power supplies More stringent requirements are scheduled to soon emerge from standards bodies that will affect a wide range of manufactured equipment. Sebastian Fischer, Andreas Flũhlern • Traco Electronic AG
STANDARDS
committees are working on a new, common standard for safety requirements governing audio/ video, information and communication technology–the IEC/ EN/UL 62368-1. The first version of IEC 62368-1 was published in 2010. The current EN 62368-1 in its 2nd edition has been in effect since 2016. The replacement of the previous EN 60950-1 standard will have a big impact on electrical equipment, and it will affect system developers who select and use power supplies. Electrical equipment put into circulation within the European Union market (EU, EFTA and other countries) must carry a CE label if the equipment is subject to an EU directive (such as the low-voltage directive). By means of this CE declaration, the manufacturer confirms the electrical equipment’s safety and/ or conformity. In this context, the term ‘electrical equipment’ pertains to a variety of devices such as industrial PCs, measuring equipment, and other instruments referred to as “terminal devices,” as well as components such as ac/dc switching power supplies which, in turn, may be part of a terminal device. As a rule, a harmonized standard is used for evaluating safety. An accredited certification body (e.g., TÜV, SIQ, UL, etc.) checks the terminal device's compliance with the standard's safety regulations. For many years, a widespread standard in this regard has been the EN 60950-1. This standard basically describes information technology equipment, but it is also used for evaluating electrical safety in many other areas. As a result, developers of terminal devices often rely on components, e.g., power supplies, that were also tested in accordance with the EN 60950-1 and for which there is proof of a safety check by means of a report and certificate. EN 60950-1 will no longer be valid in Europe as of 20 December 2020 (or, more precisely: whose presumption of conformity for the EU low-voltage directive will be withdrawn). EN 60950-1 will be replaced by EN 62368-1. The new EN 62368-1 standard follows a risk-based approach to assess whether or not a product is safe by defining various risk sources, protective measures and user groups. At dictates that a protective device be provided depending on the energy source's risk potential (e.g., electrical voltage) and the user's category
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Example of a protective device: Insulation / separation by means of increased clearance and creepage distances. (e.g., normal user). In an ac/dc switching power supply, a reinforced insulation between the 230-Vac input side and the dc side could serve as an example of such a protective measure (unless the user circle is restricted to specially trained people). In addition, depending on type and predominant area of use, other standards may also apply for power supplies. One is the EN 61010-1 standard which, in particular, will become the relevant standard for the power supply to control devices in industrial settings (control cabinets). Ultimately, the differences between the old EN 60950-1 and the new EN 62368-1 for the actual technical establishment of power supplies tend to be rather small and often do not require any design changes. If a manufacturer or importer previously certified their terminal device in accordance with the EN 60950-1, there is a high probability that they must now comply with the new EN 62368-1 standard after 12/20/20. Of course, it is also possible to apply another harmonized standard that is listed under the low-voltage standard (e.g., the EN 61010-1). Components such as dc/dc converters and/or ac/dc eeworldonline.com | designworldonline.com
SAFETY STANDARDS
A 450-W power supply unit with CE certification for IT applications (EN 62368-1) and medical technology (EN 60601-1). One is if the dc/dc converter is used in an environment with more stringent requirements regarding the converter's insulation for user safety (safe separation by means of reinforced or doubled insulation). Due to the normally higher input voltages (typically up to 160 Vdc), dc/dc converters for the rail sector are also subject to the low-voltage directive. For manufacturers of EN 62368-1- compliant terminal devices, it is often expected that even non-CE-required power supply components be certified in accordance with EN 62368-1 to reduce the testing effort. Like other manufacturers, the makers of power supplies are also affected by the certification bodies’ limited testing resources. Though (re)certification to the updated standard has been in the works for years, it's likely not all products will have been tested by 12/20/20.
switching power supplies for EN 62368-1 compliant terminal devices must also be certified in accordance with EN 62368-1. As a matter of principle, only power supplies subject to the EU low-voltage directive must be provided with a CE label. If such a power supply is certified according to the EN 62368-1, safety in compliance with the directive is assumed, and the power supply may be provided with a CE label and put into circulation. If such a power supply is “only” certified according to EN 60950-1, this presumption of safety will become void as of 12/20/20. After that date, the power supply may only be put into circulation if it has been certified according to EN 62368-1 (or another valid standard). No CE label may be issued for power supplies not subject to the directive. The PROVEN SHOCK, VIBRATION & limit below which power supplies are not NOISE REDUCING SOLUTIONS subject to the low-voltage directive is defined as 50 Vac or 74 Vdc. Thus ac/dc power supply units generally are subject to this directive and therefore must also have the EN 62368-1 certification as of 12/20/20 to be legally sold in Europe. Many industrial dc/dc converters have a nominal voltage ≤ 75 Vdc and are therefore not subject to the directive, which means terminal device manufacturers can put them into circulation without EN 623681 certification. Exceptions are possible.
RIA MATE
L
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POWER & ENERGY EFFICIENCY HANDBOOK
Power supplies tested for Amendment 2 of the EN 60950-1 will be accepted for use in EN 62368-1-mandatory terminal devices. The safety certification spells out whether the device was tested to Amendment 2. An example is this safety certificate for the Traco Power TIB product series with IEC/EN/UL 60950-1 conformity and the listed amendments. Traco Power’s portfolio includes over 300 product families with more than 5000 individual products. Over 98% of these are being recertified (if recertification is obligatory). However, there are also other manufacturers who use this deadline to streamline their product portfolios. This means that in most cases not all of the products are recertified, which may lead to difficulties for the users of these products when certifying the terminal device. However, many terminal device manufacturers have already certified their terminal devices to EN 62368-1 or are in the process of doing so. For them, the question is to what extent they can use power supplies that are currently (i.e., prior to 12/20/20) “only” certified according to EN 60950-1 but not yet according to the new EN 623681. In this regard, power supplies tested to Amendment 2 of the EN 60950-1 will be accepted by the certification bodies for use in EN 62368-1-mandatory terminal devices. Thus manufacturers that currently certify their terminal device according to EN 62368-1 will be able to use these power supplies without problems. Whether Amendment 2 was also tested can be seen in the safety certificate, if the manufacturer publishes one.
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However, as of 12/20/20 these power supplies must also have the EN 62368-1 certificate. In this context, the deadline of 12/20/20 refers to putting the product into circulation (EU import or availability on the market). For example, if the terminal device manufacturer received its EN 60950-1-certified power supplies prior to this deadline, the terminal device may also be used after 12/20/20. Thus, the terminal device manufacturer can sell terminal devices (with EN 62368-1 certification) that are equipped with an EN 60950-1 + Amendment 2 power supply even after 12/20/20, as long as this power supply was imported into the EU or sold within the EU prior to 12/20/20. In principle, this also makes it possible to accumulate a “transitional supply.” It should be noted that different situations arise outside of Europe. For example, in North America the UL 60950 remains valid, i.e., the UL 62368-1 will only be obligatory in case of new certifications. Devices certified to UL 60950 may also be put into circulation after 12/20/20. There are also countries (e.g., China) where the new standard is not accepted at all. However, exceptions apply regarding the 10 • 2020
use of components (such as power supplies); these may also be used with an EN 623681 certificate in terminal devices with EN 60950-1 certification. In a nutshell, power supplies used in terminal devices after 12/20/20 also require certification in accordance with the EN 62368-1. However, this is only mandatory if the power supply is also subject to the EU low-voltage directive, which is usually the case for ac/dc switching power supplies but not for many dc/dc converters. Terminal device manufacturers can already use power supplies tested in accordance with EN 60950-1, Amendment 2 in their EN 62368-1-certified terminal devices.
References Traco Electronic AG, http://www.tracopower.com
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POWER & ENERGY EFFICIENCY HANDBOOK
Fusible resistors vs fuses Safety standards often dictate what component provides the best protection against overcurrents. Todd Phillips, Saad Lambaz • Littelfuse, Inc.
MOST
people would probably agree that the time it takes to recharge their cell phone, gaming console, or tablet computer is both inconvenient and annoying. Faster charging requires a more powerful charger or power supply. The design of such supplies requires that designers emphasize device safety while also meeting cost, size, and efficiency constraints that are more stringent. As power levels rise, so too does the need for overcurrent protection. There are generally two different approaches to overcurrent protection: a conventional fuse or a fusible resistor. Fusible resistors have the benefit of combining overcurrent protection and inrush current protection in one component. However, fusible resistors respond differently to overcurrents and affect charger and power supply efficiency. A fusible resistor opens like a fuse when its current rating is exceeded. The component is generally a nichrome element with a melting temperature of around 1,400°C. Nichrome has a low thermal coefficient of resistance which allows the resistor to have a stable resistance over temperature. The 1,400°C melting temperature heats up surrounding components and the PCB during an overcurrent condition. Fuses are generally copper or silver elements with a melting temperature typically between 962°C and 1,083°C. Fuses also have a high thermal coefficient of resistance, at least a factor of 10 higher than a nichrome fusible resistor. Thus, the temperature of a fuse will rise faster during an overcurrent condition. The fuse resistance rises to bring the fuse to its melting point sooner. A fuse will prevent a heat build-up that happens when a fusible resistor experiences an overcurrent condition. The higher heat generated by the fusible resistor can damage other components and potentially lead to the ignition of nearby combustible components. The primary benefit of a fusible resistor is that its resistance limits inrush current. A fusible resistor that serves as the main overcurrent component in a power supply or charger can have
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Examples of fusible resistors and fuses. Top, the Vishay ACxx-CS Series fusible resistor. Bottom, the Littelfuse 443 Series fuse.
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OVERCURRENT PROTECTION How overcurrent protection methods compare Fuse
Fusible resistor
Fuse and NTC thermistor
~1000 °C
1400 °C
~1000 °C
0.05 Ω
10 Ω
1.6 Ω
Fuse time (.33 A -0.825 A through a 0.3A rating component)
120 s
16 min
120 s
For a 5 W power supply
0.0%
1.3%
0.2%
For a 10 W power supply
0.0%
2.9%
0.4%
For a 15 W power supply
0.0%
4.5%
0.7%
% efficiency budget required for a 25 W power supply
0.0%
7.5%
1.2%
% efficiency budget required for a 60 W power supply
0.1%
19.3%
3.0%
Melting point Typical resistance
% efficiency budget required for overcurrent protection
a resistance of 10 Ω. A fuse, in contrast, has a resistance ranging from milliohms to hundreds of milliohms. Designers can combine a fuse with an NTC thermistor to protect against overcurrents and limit inrush current. An NTC thermistor has a resistance that can be as high as 10 or 20 Ω initially;
however, it falls into the tens of milliohms range during steady-state operation. The fusible resistor seems to save space compared to a separate fuse and thermistor; however, the heat a fusible resistor generates could force components to be kept at least a half-inch away from
Performance comparison of a fuse, fusible resistor, and a fuse with an NTC thermistor in a nominally efficient, charger/power supply circuit.
fusible resistors rated up to 10 W and as much as an inch away from fusible resistors rated above 10 W. When chargers and power supplies must fit in cramped quarters, the component spacing associated with fusible resistors could be problematic.
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POWER & ENERGY EFFICIENCY HANDBOOK Where fuses and fusible resistors excel Fuse
Fusible resistor
Up to 8 times faster
-
2
1
Negligible
~ 14% at 25 W
Recommended for all, but specifically for 18 W and higher charger/power supplies
Suitable for 15 W and lower charger/power supplies
Response to overcurrent Number of components required for overcurrent and inrush current protection % charger/power supply efficiency budget required Recommendation for use
Fusible resistor protection
Fuse vs fusible resistor and when each component is recommended
EFFICIENCY STANDARDS Perhaps the most difficult challenge facing designers of power supplies or chargers is mandated efficiency standards. Efficiency standards such as Energy Star and Directive 2009/125/EC Ecodesign have become more demanding over the years. The higher the power output of the supply or charger, the higher the mandated efficiency. The efficiency requirements for three widely used levels of power output are:
A power supply/charger circuit that uses a fusible resistor for overcurrent protection and to limit inrush current.
Fuse + NTC thermistor
An example of a power supply/charger circuit using a fuse for overcurrent protection and an NTC thermistor to limit inrush current.
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• • •
60-W supplies and chargers must be 89% efficient 25-W supplies must be 86% efficient 5-W supplies can be less than 80% efficient.
Fusible resistors impact power efficiency because their resistance greatly exceeds that of a fuse. For example, consider a power supply rated at 25 W. Its required efficiency is 86%. The supply’s input power is 29 W (25 W/0.86). Thus, the supply can consume 4 W, its loss budget. A 10-Ω fusible resistor in the circuit can consume 0.59 W (I2R = (29 W/120 V)2 x 10 Ω) which is 14% of the loss budget. That leaves little room for losses in the active components such as the power transistors . A fuse with its milliohmlevel resistance would consume a negligible portion of the loss budget, giving the designer more freedom with the development of the complete circuit. Fuses and fusible resistors have different overcurrent and temperature-rise standards. Designers must be aware of the standards that apply to each device. Fuses for the North American markets are evaluated to the requirements of UL/CSA/ANCE Standards 248-1 and 248-14. For the European and international markets, fuses are evaluated to IEC Standard 60127-1 and either IEC Standards 60127-2, -3, -4 or -7. Different industry standards apply to fusible resistors. For North America, fusible resistors must comply with UL 1412 (in which the component is called a fusing resistor) and CSA Standard C22.2 No. 60065-03. For Europe and international markets, fusible resistors must comply with IEC 60127-8 (in which the component is described as a fuse resistor). eeworldonline.com | designworldonline.com
OVERCURRENT PROTECTION Minimum average efficiency (Standard Voltage, > 1W)
Government efficiency standards for power sources between 2007 and 2018 Standards for fuses are quite specific with regard to requirements for an allowable temperature rise. Fuses based on the North American standards UL/CSA/ANCE 248-1 and 248-14 can have a maximum temperature rise of 75°C during temperature testing. The test takes place at room temperature, typically 25°C. Hence the maximum fuse temperature is 100°C, and the fuse must remain intact during the test. For fusible resistors, UL 1412 does not define a maximum temperature rise. Instead, they are tested by gradually increasing the current through the component until it opens. Cheesecloth either surrounds the fusible resistor or sits at a specific distance from the component. The passing criteria is that the cheesecloth does not ignite. Cheesecloth, constructed from cotton, has an ignition temperature of about 400°C. Thus, the standards for a fusible resistor allow the component to operate at much higher temperatures than those for a fuse.
RECOMMENDED USAGE Fuses must comply with more rigorous standards and, as a result, perform much better than fusible resistors as overcurrent protection components. Because fusible resistors can be a source of ignition without sufficient spacing, they best serve in lower wattage power supplies and chargers likely to experience lower overcurrents and temperature rises. Fusible resistors generally provide suitable overcurrent protection for power supplies and chargers sized up to about 15 W. Higher wattage devices are usually best served by a fuse or a fuse and an NTC thermistor.
References Littelfuse Inc., www.littelfuse.com
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Examples of widely used fuses for overcurrent protection include the 8.5x4-mm 369 Series Radial Lead Fuses (top) with a 300-V rating, a time delay characteristic, and open time of 120 sec maximum at a 210% overload; (middle) the 10x3-mm 443 Series Surface Mount Fuses with a 250-V rating and a slow-blow characteristic as well as an open time of 120 sec maximum at a 250% overload; and (bottom) the 20x5-mm 219XA Series Glass Body Cartridge Fuses with a 250-V rating and a slow-blow characteristic as well as an open time of 120 sec maximum at a 210% overload. 10 • 2020
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POWER & ENERGY EFFICIENCY HANDBOOK
Driving modern haptics Piezo haptics now can provide a realistic sensation of a click or the resistance of a pushbutton. Sonja Taylor Brown • TDK
IN
the movie Ready Player One, teenager Wade Watts regularly enters a virtual reality simulator and virtual gaming environment. For citizens in this future dystopian civilization, the currency of the simulator is more stable than that of the real world, and, as a result, what happens there is more important than what happens on the outside. Wade eventually purchases a haptic suit so he can experience the physical sensation of touch from other users in the simulation. To many engineers, good haptic technology seems as far-off as what they see in the movies. However, automobiles, gaming controllers and cell phones all use haptic technology today. And there are some misconceptions about topics that include the differences between haptic actuators and piezo benders, costs of using piezo haptic technology, customization that allows realistic feedback, and how haptics technology stands up in harsh environments. Many users are familiar with the old-style haptic feedback on cell phone screens. Advantages of today’s haptic devices become evident from a quick review of older haptic methods. Cell phones generally have employed electromechanical haptic devices, either eccentric motors that spin an unbalanced
mass to create vibrations or linear resonant actuators (LRAs) containing a springmounted mass that vibrates back and forth linearly. For many haptic applications like gaming, these electromechanical devices are battery hogs. They are also slow. Start-up time for an eccentric motor, defined as the time to reach 90% of the rated acceleration, is usually about 50 to 100 msec, and stopping takes a similar amount of time. LRAs tend to have slightly quicker start-up times, but applications with repeated haptic events can still have problems with LRA latency. In contrast, today’s haptic devices are based on piezo technology and can provide a realistic sensation of a click or the resistance of a pushbutton. Devices and surfaces can be customized and miniaturized for different applications. Additionally, piezo haptics aren’t just actuators. They can also sense pressure. Thus the devices can be set up to sense varying levels of pressure to, for example, add an element of safety in industrial settings. Here, the humanmachine interface could require workers apply significant pressure to turn a system on or off so accidental “touches” don’t cause problems. On the other hand, piezo haptic actuators can be set up to register the most minute changes in movement for quick actuation of machinery where
Piezoelectric unimorph structure
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precision is imperative, as with a joystick in a forklift. Though haptics technologies vary, it is not unusual to find them all using the same driver ICs. One example of a modern haptic driver chip is the BOS1901 from Boréas Technologies. This device provides drive voltages up to 190 Vpp working from a 3-V supply, allowing it to drive both piezo haptic actuators as well as the electromechanical haptics that require higher peak-to-peak signals. To accommodate piezo actuators that can double as sensors, the chip incorporates a high-speed SPI communication bus to allow an MCU to monitor data such as actuator voltage for sensing applications. Despite commonalities in their drive electronics, there are differences in piezo technologies employed in haptics applications. Piezo haptic actuator technology is not the same as piezo bender technology. This is perhaps the biggest misconception about haptic actuators. As a quick review,
An example of a modern piezo haptic device is the TDK PowerHap 0904H014V060 actuator which has a unimorph structure in which ceramic piezoelectric elements having electrodes on both sides are bonded to one side of a metal plate. An ac voltage applied to the electrode causes the piezoelectric element to expand and contract, which causes warpage on the bonded metal plate. This unimorph structure enables the whole metal plate to vibrate with high efficiency. eeworldonline.com | designworldonline.com
DRIVING HAPTICS Eccentric rotating mass haptic a piezo bimorph, or bender, operates like a bimetallic strip in a thermostat. Layers of the piezo ceramic join to either a metal substrate or to other piezo ceramic strip layers. Energizing the piezo ceramic causes a deflection that is proportional to the applied voltage. There are drawbacks to bender technology, and many suppliers of modern piezo devices feel that benders have created a negative perception of haptic technology that has been difficult to overcome. It will take time to change this perception. One problem with piezo benders is that they don’t stand up to vibration, shock, and other environmental conditions as well as piezo haptic actuators, which can withstand large compressive loads and deliver much larger forces and accelerations. With piezo benders the force of the load generally acts directly on both ends of the ceramic. As a result, these actuators cannot withstand large loads and can easily crack, break and fail. They also can’t provide the force and acceleration necessary for a smooth user interface, resulting in a poor user experience. This makes them impractical for many applications in industrial, mobile and automotive settings. Now consider a piezo haptic actuator. It typically has a unimorph structure in which ceramic piezoelectric elements, having electrodes on both sides, bond to one side of a metal plate. Applying an ac voltage to the electrode expands and contracts the piezoelectric element, which warps the bonded metal plate. This unimorph structure enables the whole metal plate to vibrate with high efficiency. The resulting form factor is thinner than old style eccentric motors or LRAs. These piezo actuators also operate with drive signals of around 24 V, whereas piezo benders typically need driver voltages of 50 to 150 VPP. Piezo actuators additionally deliver tactile sensation to a wide area with a fast response, typically less than 15 msec. There are two types of piezoelectric haptics, single-plate and multilayer. The multilayer type can generate a larger displacement than that of singleeeworldonline.com | designworldonline.com
Piezoelectric haptic
Graphs compare the driving patterns and behavior of an eccentric rotating mass and the PiezoHapt, a modern piezo haptic actuator. While the eccentric rotating mass takes 0.1 sec or more to start moving, the PiezoHapt Actuator begins vibrating in a small fraction of that time. Because piezo haptic devices energize more quickly than eccentric rotating masses, lower power consumption is also one of the advantages. The output amplitude can also be changed via the drive voltage, making it is possible to express various, delicately controlled vibration patterns. Eccentric rotating masses can’t be controlled this way because their amplitude is determined by design.
plate element of the same thickness. Such actuators can transmit a vibration sensation to the skin. Moreover, because there is no solder joint associated with connecting a wire to the element , there is no extra load from it applied to the multilayer element, thus improving the amplitude efficiency. Because piezo haptics can be energized more quickly compared with eccentric rotating masses, the lower power consumption is also one of the advantages. As the input frequency or voltage rises, so, too, does power consumption. But it’s possible to minimize power use by judiciously managing the vibrations. Additionally, the drive voltage can serve as a means of manipulating vibration amplitude to express various,
delicately controlled vibration patterns. Of course, this sort of manipulation is impossible to realize with eccentric rotating masses or LRAs where amplitude is determined by design. Additionally, piezo haptic actuators are highly stable, reliable and rarely fail. As the primary human machine interface for many devices, they provide a good user experience – even in harsh environments. Piezo haptic actuators are robust and can give the sensation that the user has actually depressed a button. This behavior lets them serve in, for example, industrial applications such as control panels where users wear gloves. When the user touches the “button” on a control panel, the touch force can be used to trigger an actuator based on
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POWER & ENERGY EFFICIENCY HANDBOOK Simplified schematic, BOS1901 haptics driver
An example of a modern-day haptics driver chip: The BOS1901 from Boréas Technologies is an actuator driver with energy recovery. It can drive actuators with up to 190-V peak-to-peak waveforms while operating from a 3 to 5.5-V supply. The input digital stream is written in an internal FIFO over the digital interface to generate the desired output waveform.
the amount force applied. Users feel this tactile feedback based on voltage, vibrations, sound and motion that gives the sensation that a button has been pushed.
IT ’S NOT FRAGILE The second misconception is that piezo haptic technology is expensive. Piezo haptics may cost more than LRAs or eccentric rotating mass actuators. But unlike these older technologies, they can replace switches and buttons. Because there is no need for additional holes cut into the frame, the technology becomes inherently waterproof. And a single programmable piezo haptic switch can replace several parts including sensors, buttons, ESD protection and moisture protection devices. Thanks to their high force output and compact dimensions they can take the place of several solenoid actuators while providing the same experience in AR/VR glasses. Haptic technology can be used in a variety of applications besides AR/VR. Piezo haptic technology incorporated into automobile panels, for example, can allow the driver to “feel” the various buttons and options for selection on a menu. This sort of feedback boosts safety because it allows the driver to quickly discern one virtual button from another without having to look away from the road. A point to note is that piezo haptic actuators can work at virtually any frequency perceptible to humans. The voltage, amplitude, frequency, duration and waveform can be tailored to produce hundreds of different kinds of feedback from a button-like click to a heavy thump. This contrasts with LRAs which must be driven only at their resonant frequency.
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A third misconception is that haptic technology is fragile. This may be true of piezo benders which can break easily because the ceramic attaches directly to the movable object and a back plate. When the full external force is applied, the ceramic bends, stresses, and may eventually crack. Piezo haptic actuators don’t operate this way. Instead of bending, they contract in the X and Y axes when excited by the driving voltage and expand in the Z direction. There is essentially no wear. In fact, piezo haptic actuators have undergone billion-cycle tests and exhibited no degradation. Just the thing for Wade Watts’ haptic suit.
References TDK PiezoHapt actuators, https://product.tdk.com/info/ en/products/sw_piezo/haptic/piezohapt/technote/tpo/ index.html Boréas Technologies BOS1901 Technical Documents, https://www.boreas.ca/pages/bos1901-technicaldocuments
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SiC MOSFETs
How SiC boosts efficiency in power conversion The benefits silicon-carbide MOSFETs become evident from comparisons with their silicon counterparts. Rene Mente • Infineon Technologies Sizing up applications
NEW
650-V SiC MOSFETs make it practical to hit power conversion efficiency levels exceeding 97% in switched-mode power supply (SMPS) designs. Simply replacing silicon MOSFETs with SiC versions in typical resonant designs will improve efficiency, but the most significant gains come from rethinking designs completely. There are several factors to consider when determining when to opt for a new design approach. SiC configurations can bring higher power density (and thus smaller footprints, reduced thermal management requirements) that silicon devices can’t provide. The result ultimately can be a lower total cost of ownership (TCO). For an example, consider server/telecom power supplies. Here, higher switching frequencies with SiC MOSFETs bring greater efficiency, a lighter and smaller power supply, higher reliability because of simpler overall topology, and a more economical BOM. Consider a full-bridge totem pole PFC that utilizes two SiC and two low-ohmic superjunction (SJ) MOSFETs in four-phase operation over each ac cycle. In both the positive and negative phases of the cycle, one of the SJ MOSFETs continuously conducts while the corresponding SiC MOSFETs alternate between conducting (on) and off states. At each of the on/off states there is a short period in which one SiC MOSFET body diode conducts and the second MOSFET is actively turned on. Because of this hard commutation on a conducting body diode, the low reverserecovery charge and output charge (Qrr and Qoss) characteristics of the SiC devices are a major factor in system efficiency. eeworldonline.com
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Here’s how applications for SiC MOSFETs compare with those for silicon MOSFETs in terms of output power and switching frequency.
SiC MOSFETs in action
Full bridge Totem Pole PFC
LLC stage Synchronous rectifier
An example illustrating the benefits of 650-V SiC MOSFETs: A continuous conduction mode (CCM) totem pole power-factor control (PFC) stage and a resonant LLC converter utilizing 650 V SiC MOSFETs. A final synchronous rectification stage, operating at a lower voltage, employs ordinary silicon MOSFETs.
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POWER & ENERGY EFFICIENCY HANDBOOK 650 V CoolsiC in 3.3 kW CCM Totem Pole PFC 600 V CoolMOS in Dual Boost PFC Vin = 230 VAC; Tamb = 25°C
Efficiency comparison between a SiC CCM totem pole PFC and silicon MOSFET Dual Boost PFC. Here the IMZAxxxxxxxxx devices are all SiC MOSFETs. The IPZA60R037P7 is the silicon MOSFET.
Comparing output capacitance
While it’s possible to implement a highefficiency triangular current-mode (TCM) totem pole using silicon MOSFETS, doing so would require a more complex controller and driving scheme that would raise part count and costs. The most similar topology based on 600-V Si MOSFETs would be a Dual Boost PFC. We can use this configuration for efficiency comparisons with a design employing three variants of a 650-V SiC MOSFET (48 mΩ, 72 mΩ and 107 mΩ). The Dual Boost topology efficiency tops out at 98.85%. All three SiC MOSFETs exceed this mark, and the 48-mΩ device reaches at least 99% efficiency across an output range of 8001,800 W. It takes a 99% efficient PFC stage to reach a target of 98% system efficiency. Efficiency comparisons for the LLC resonant converter (the LLC moniker denotes the resonant components), expressed as diode losses, were calculated for a design that substitutes SiC MOSFETs for silicon devices with subsequent optimization of the converter dead-time settings. Efficiency improved by up to 0.5% at light loads. This improvement declined to 0.1% at 10-A output with RDS(on) being the main source of the losses.
Typical Coss output capacitance comparison between SiC and Si MOSFETs. Here the IMZAxxx devices are all SiC, IPWxxx is the silicon MOSFET.
CONTRIBUTING FACTORS SiC MOSFETs exhibit body diode behavior and linear output capacitance that are well-suited to topologies which have hard commutation on the conducting body diode in every switching cycle. For example, the reverse recovery charge (Qrr) of the Infineon CoolSiC 650-V MOSFETs
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SiC MOSFETs Transfer characteristics @ 25°C How the VGS vs ID transfer characteristics compare at 25 and 150°C for SiC (IMZAxxx) and silicon (IPWxxx) MOSFETs.
Transfer characteristics @ 150°C
is approximately 10 times lower than peak-performing fast diode silicon MOSFETs. This parameter is key in reaching the 99% peak efficiency in the PFC scheme discussed here. It is important to note that the forward voltage of the SiC body diode is a little less than four times higher than that in silicon MOSFETs. Thus, while it is possible to drive CoolSiC MOSFETs as though they were silicon devices, body diode conduction losses can boost efficiency without a change in driving strategy. The output capacitance (Coss) behavior of SiC varies across output voltage much more linearly than that of silicon MOSFETs. Importantly, the lower capacitance of silicon at higher voltages can be problematic in protecting the switch from voltage overshoot. Designers typically aim for 80% de-rating on the drain-source voltage. Experiments have shown that super-junction MOSFETs require an external gate resistor of 47 Ω during start-up in a LLC converter. The risk of exceeding de-rating guidelines is minimal when using a SiC MOSFET, which needs no external resistor to slow the switching frequency during turn-off in this experiment. Other important factors contributing to SiC MOSFET performance in this SMPS revolve around the stability of electrical characteristics across operating temperature ranges. For example, RDS(on) remains remarkably stable, with a multiplication factor of 1.13 from 25 to 100°C for a CoolSiC device compared to 1.67 for a CoolMOS silicon device. This behavior allows use of devices having an RDS(on) at 25°C exceeding that of silicon MOSFETs while not sacrificing conduction losses. The behavior of breakdown voltage across operating
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POWER & ENERGY EFFICIENCY HANDBOOK RDS(on) @ 11 A over temperature
temperatures also is favorable; SiC has a shallower and more consistent slope than silicon, with a relatively minor variation from the specified 650 V at 25°C. Current transfer characteristics also are stable across varying temperatures. Finally, the gate charge is on average 50% lower for this class of SiC MOSFETs compared to silicon devices with the same RDS(on).
How drive voltage for SiC MOSFETs affect RDS(on) over temperature.
DESIGNING WITH SIC MOSFETS The decision to use SiC MOSFETs yields the greatest benefit by adopting new strategies for gate driving. Infineon recommends a driving voltage from 0– 18 V. Compared to silicon-specific drivers (typically up to 15 V), gate-driving strategies can reduce the RDS(on) by approximately 18%. A second factor in driver design is the importance of minimizing negative gatesource voltage, which can have two root causes. One is inductance-driven oscillation of gate-source voltage during turn-off. The second is capacitance-driven negative gatesource voltage. In either case, voltages below -2 V can degrade RDS(on) over the device lifetime. A diode clamp is the most straightforward way to address negative voltage, and it has been shown to prevent any swing below -2 V, thus eliminating drift over time. Parasitic gate
inductance may also impact the negative voltage effect, and this problem can be mitigated with a Kelvin source connection. With the exception of the gate-driving strategy, key design considerations and constraints faced with SiC MOSFETs are similar to those for silicon MOSFETs. The additional robustness of SiC makes the newly available devices inherently easier to use. Thus, for high power applications where the design goal is greater than 97% system efficiency, 650-V SiC MOSFETs are on track to simplify things for power supply suppliers.
References Infineon Technologies, https://www.infineon.com/
Diode clamp examples
Driver
Driver
Diode clamp
Driver GND Power GND Gate-source voltages below -2 V can degrade RDS(on) over the device lifetime. A diode clamp is the most straightforward way to address negative voltage, and it has been shown to prevent any swing below -2 V and thus eliminate drift over time. Parasitic gate inductance may also impact the negative voltage effect, and this can be mitigated with a Kelvin source connection.
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THERMAL MANAGEMENT
Liquid cooling for precise temperature control Electronics that generate a lot of heat in a small volume often benefit from swapping a cooling fan for a system based on a liquid coolant. Greg Ducharme • Laird Thermal Systems
THE
specific heat of water is higher than the specific heat of air—about 4,200 J/kg°C vs. 1,005 J/kg°C. This is another way of saying that air performs better as a thermal insulator than as a thermal conductor. That, in a nutshell, is why liquid cooling can be more effective than air cooling. Liquid cooling systems can dissipate a large amount of heat in densely packed electronic enclosures to facilitate more complex system designs. Liquid cooling systems combine a high capacity for transferring waste heat with a high coefficient of performance (COP, the ratio of useful cooling provided to work required). There are two types of liquid cooling setups. First are liquid heat exchanger systems. They use either a liquid-to-liquid heat exchanger or a liquid-to-air heat exchanger to cool the coolant in a liquid circuit. In the first case, the coolant is often cooled below ambient temperature using a facility (often called primary) coolant loop. Alternatively, the aircooled system will cool the coolant to near ambient temperature. The remainder of both systems consist of a pump to circulate the coolant, often a tank to give the pump a constant supply of coolant, and a liquid circuit to transfer coolant from the heat source to the liquid cooling system. Second are liquid chillers (recirculating chillers). They use a compressor system instead of a liquid heat exchanger assembly. A chiller uses a vapor compression mechanical refrigeration system that connects to the coolant system through a device called an evaporator. Refrigerant circulates through an evaporator, compressor, condenser and expansion device. The evaporator functions as a heat exchanger such that heat captured by the process coolant flow transfers eeworldonline.com
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to the refrigerant. As the heat-transfer takes place, the coolant evaporates, changing from a low-pressure liquid into vapor. The coolant then flows to a compressor, which ensures the pressure in the evaporator remains low enough to absorb heat at the correct rate and raises the pressure in outgoing coolant vapor to ensure its temperature remains high enough to release heat when it reaches the condenser. The coolant returns to a liquid state at the condenser. The latent heat given up as the refrigerant changes from vapor to liquid is carried away from the environment by a cooling medium (air or water). Liquid cooling systems have several advantages over conventional air-cooled systems: High heat-pumping capacity: Liquid heat exchangers can reduce the thermal resistance of conventional heat sink fan dissipation
Liquid cooling systems can dissipate a large amount of heat. For example, in a medical X-ray system, a liquid heat exchanger system cools the coolant in a liquid circuit by the use of a liquid-to-air heat exchanger. The system contains a pump that circulates coolant and a liquid circuit to transfer coolant from the heat source to the liquid cooling system. An expansion device enables the cooling circuit to be completely sealed from the outside environment and compensates for the thermal expansion of the fluid over its wide operating temperature range.
Liquid-to-liquid cooling
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POWER & ENERGY EFFICIENCY HANDBOOK
mechanisms by a factor of 10 or more. This is due to the poor thermal properties of air versus coolants such as water. The value of the air heat transfer coefficient depends on parameters that include air velocity, air temperature, geometry of the surface in contact with the air, and others. For natural/ free convection, the value of heat transfer through air can be between 5 - 25 W/(m2 K) and for forced convection from 25 to 250 W/(m2 K). Contrast this with the heat transfer coefficient of water, 500 to 10,000 W/(m2 K). High heat-flux density: Heat flux density is a flow of energy per unit of area per unit of time. Liquid cooling systems can remove up to five times more heat per square area than conventional air cooling systems. This factor becomes advantageous in densely packed electronics without a lot of space for air flow. High COP: In a compressor system, dividing heat extracted from the evaporator by the work done by the compressor gives the system coefficient of performance. For example, if an application requires cooling a heat load of 3 kW, a standard compressor-based refrigeration system typically requires around 1 kW of energy to provide the proper cooling, which is more efficient compared to other technologies. (Note that COP is typically not used as an efficiency yardstick for air cooling systems.) Heat routing: Liquid cooling allows a small heat exchanger to sit at the heat source, which then routes heat away through a liquid circuit. This can be advantageous in densely packed electronics compared to conventional air-cooling systems. Air fans, for example, push air through the system, but the air they circulate can be well above ambient temperature if there are other hot electronics in close proximity. Rapid cooling: Cooling time is a function of cooling capacity. Because liquid cooling systems have larger cooling capacities than conventional heat-sink fans, they can dissipate heat more quickly. Lower noise: Systems having one kilowatt or more of heat to dissipate require large fans to get the job done. The resulting air cooling system tends to be noisier and more prone to vibration than the equivalent liquid chiller. Liquid cooling systems can use a variety of fluids such as water, deionized water, glycol/
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Liquid-to-air cooling
water solutions, dielectric fluids (fluorocarbons), transformer oil, hydrofluorocarbon (HFCs) refrigerants and natural refrigerants. The coolant that is right for a specific application depends on its qualities and thermophysical properties. Perhaps the most obvious properties to consider are the fluid’s phase transitions (boiling and freezing) and the chemical breakdown of the fluid’s chemistry. For example, ordinary water as a coolant is susceptible to biological fouling. Algae, bacteria or fungi can form depending on the system’s exposure to light and heat and the availability of nutrients in the wetted components. The resulting slime or biofilm can reduce the heat transferred between the fluid and wetted surfaces In the same vein, some water-based coolants may include antifreeze agents such as propylene glycol or ethylene glycol. Propylene glycol is much less toxic than ethylene glycol, and also has a higher specific heat. But it has lower thermal conductivity and higher viscosity. Usually, system designers try to keep the concentration of glycol low because of the superior performance of water over either glycol type. An additional consideration is the compatibility of the coolant with the materials at hand. Although stainless steel is usually excellent when there is a chance of corrosion, it also has a rather low thermal conductivity
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Probably the most well-known example of liquid-to-air cooling is the radiator-based system used to dissipate engine heat in vehicles. compared to other metals such as aluminum or copper. Aluminum and its alloys have good thermal conductivities ranging from 160-210 W/(m2 K). But aluminum tends to be susceptible to corrosion or pitting from impurities in unpurified water. Even mixed distilled water, glycols form acidic compounds under oxidation that can corrode wetted surfaces and create organic acid byproducts. There are other considerations when high power levels are involved. Cooling fluids cannot be permitted to facilitate arcing from high voltages to ground or to other surfaces. Similarly, coolants must have low electrical conductivity when working with voltages in the tens of kilovolts as for applications such as X-ray tube cooling. Dielectric fluids such as XG Galden or Fluorinert are employed for these purposes because they exhibit dielectric strengths in the tens of kilovolts per 0.10 in. A fluid with low electrical conductivity can build up static charge as a result of flow electrification. Fluids having eeworldonline.com
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THERMAL MANAGEMENT
a resistivity of 2×1011 Ω-cm or more (equivalently, 50 pS/m or less) are considered to be susceptible to static buildup. Deionized water, for comparison, has a lower resistivity. Ideally, the coolant fluid is inexpensive and nontoxic with exceptional thermophysical properties and a long operating life. The ultimate goal of maximizing coolant properties is to improve heat transfer between the fluid and the heat exchange surfaces it touches. Assessing the heat transfer coefficient directly in these cases requires use of correlations developed to calculate the coefficient for various specific geometrical conditions. In these correlations, two dimensionless parameters have dependence on the fluid properties. The Rayleigh number is associated with buoyancy-driven flow, also known as free convection or natural convection. The Prandtl number is the ratio of momentum diffusivity to thermal diffusivity. The process of assessing a particular fluid in an application can be cumbersome, particularly when there are several kinds and orientations of heat transfer convection surfaces in question. Short of a full thermal analysis, designers may use a less rigorous approach involving a figure of merit, such as the Mouromtseff number, to give a simpler basis for comparing fluids by taking into account some or all of the physical properties. Because liquid cooling systems can be quite complex, the optimal liquid cooling solution is often a variation of a standard solution or a custom configuration. There are many unique attributes that must be taken into consideration depending on the application. Many applications require precise temperature control of multiple liquid circuits, or multiple pressure settings to accommodate both low and high-pressure drop conditions. All in all, liquid cooling systems such as those devised by Laird Thermal Systems are designed to maximize temperature stabilization at above, below, or equal to ambient temperature. Systems are compatible with water, water- glycol, transformer oil, or various corrosion inhibitors. Liquid cooling systems from Laird Thermal Systems can be found in the analytical, medical, industrial and semiconductor markets.
References Coolant application note, https://www.lairdthermal. com/sites/default/files/ckfinder/files/resources/ Application-Notes/Common-Coolant-Types-andtheir-uses-in-Liquid-Cooling-Systems/CommonCoolant-Types-and-their-Uses-in-Liquid-CoolingSystems-Appnote.pdf Thermal wizard online spec tool, https://www. lairdthermal.com/index.php/thermal-wizardpeltier-home
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Compressor-based refrigeration
A conventional compressor-based refrigeration system contains four fundamental parts: the evaporator, compressor, condenser, and expansion device. The evaporator (cold section) is a heat exchanger where the refrigerant interfaces with the process fluid (air or liquid coolant in the case of a chiller) and evaporates (boils) from a liquid to a gas. During this change-of-state from liquid to gas, latent energy (heat) is absorbed at a constant refrigerant temperature. The compressor acts as the refrigerant pump and recompresses the gas. The condenser expels both the heat absorbed at the evaporator and the heat produced during compression into the ambient environment while the refrigerant returns to a liquid. The expansion device meters the amount of refrigerant flow while creating a reduction in refrigerant pressure to the point where it is ready to evaporate again. Compressor systems offer a more desirable setpoint range than air-based heat exchangers and can use 30% to 35% less power than a thermoelectric system. Modern compressorbased liquid cooling systems also offer much quieter operation in smaller and lighter packages compared to that of previous generations.
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POWER & ENERGY EFFICIENCY HANDBOOK
Coming: New safety ratings for power adapters Prepare yourself for the updated standards about to take hold in power equipment that includes external adapters.
A
power adapter is a barrier between lethal ac mains voltage and the dc power line for end equipment. Different environments, however, demand different adapter construction to meet safety levels.
Ron Stull • CUI Inc.
product’s application. To complicate matters, not all countries have adopted “international” safety standards; some have written their own, and others base their standards on the general framework of the international standard. One facet of safety, as it applies to a power adapter and the equipment it powers, is to ensure a user cannot make contact with lethal ac line voltages. Also, safety goes beyond the potential for electrical shock, to require that the adapter body uses low flammability materials that can enclose any fire that might arise. For the majority of commercial and domestic equipment, standards IEC 60950-1 and IEC 60065 apply, although these are both intended to be withdrawn in Dec. 2020. Thereafter, IEC 623681 applies. IEC 60950-1 covers the broad category of IT and office equipment, and IEC 60065 covers an equally broad range of audio and video equipment from video projectors and media players. At this time, IEC 62368-1 coexists with the other two, and prudent power adapter manufacturers are certifying their products against IEC 62368-1 and IEC 60950-1. When selecting an external power adapter, check the manufacturer’s datasheet to find out with which safety standard the adapter complies.
For example, domestic, IT, industrial, and medical end-uses all have their own safety requirements. The primary concerns are the amount of separation through air and across surfaces between hazardous voltages and outputs or casings. However, such criteria are application-dependent and also vary with the over-voltage category of the ac supply, degree of environmental pollution, the type of user who has access, and even operating altitude. Other parameters also can be critical such as the fusing arrangement and earth leakage current, particularly in medical environments. Safety rating requirements also depend on the point of geographical use with different countries adopting local variants of international standards or their own national versions. Of course, the selection of an ac-to-dc power adapter involves evaluating your product’s overall voltage and maximum-current requirements. Besides looking at the obvious parameters, you should also confirm the adapter conforms to the applicable safety Energy source Effect on the body standards. These can vary depending on the geographic Not painful, but may be detectable Class 1 regions of the world where the Painful, but not an injury product is distributed. Class 2 There are several internationally recognized Injury Class 3 safety standards that apply to power adapters. Some of these are specific to the
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Effect on combustible materials Ignition not likely Ignition possible, but limited growth and spread of fire Ignition likely, rapid growth and spread of fire
Classification of energy sources.
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SAFETY STANDARDS Insulation types and protection classes of power supplies Protection class I
Protection class II
Basic insulation
Basic insulation
and
Grounded chassis
and
Double insulation
or
Reinforced insulation
Supplementary insulation
Functional or operational insulation
Types of insulation and the two protection classes of power supplies.
A NEW APPROACH TO SAFETY STANDARDS The IEC 62368-1 standard covers all aspects of audio, video, IT, office, and communications equipment. The USA, Canada, and Europe have all adopted it. IEC 62368-1 not only unifies the two previous standards, it also marks a significant change in the approach to electrical safety. In the past, IEC 60950-1 was quite prescriptive about how safety features should be implemented within a power adapter and the associated end-equipment. The new standard is more flexible. It introduces
the concept of hazard-based safety engineering (HBSE). The underlying idea for HBSE is that the manufacturer’s engineering team is responsible for thoroughly reviewing and identifying any potentially hazardous energy sources and has implemented appropriate measures to prevent user harm. Harm
Input to output isolation test voltage
Input voltage
could include not only electrical shock but burns and injury from a resulting fire. In this regard, designers must consider all potential use cases and also all possible fault conditions. The standard does not cover medical equipment; the relevant standard IEC 60601-1 is still in force. The IEC 62368-1 standard provides categories for both the energy source and likely effects on the body and on the product--in this context, the power adapter-- materials and enclosure. Within an ac-to-dc power adapter, the ac line power is a Class 3 energy source. It’s energy is lethal if it passes directly through the adapter circuitry to the dc output and Output voltage
Power supply under test Input to ground isolation test voltage
Output to ground isolation test voltage
The basic format for testing isolation voltages for a power adapter.
Earth safety ground
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POWER & ENERGY EFFICIENCY HANDBOOK Third edition requirements by classification into the end-product. The energy source classifications Classification Isolation Creepage Insulation help the adapter engineering team decide on safety measures to be incorporated into the adapter. Different One MOOP 1500 Vac 2.5 mm Basic types of insulation are used to isolate lethal Class 3 Two MOOP 3000 Vac 5 mm Double energy sources from the end-product user. Insulation regimes in use are divided between Class One MOOP 1500 Vac 4 mm Basic I and Class II types of power supplies and adapters. In Two MOOP 4000 Vac 8 mm Double a Class I adapter, a protective ground(earth) connection is used to transfer any hazardous voltages that have IEC 60601-1 third edition means of protection. Here One managed to bypass any basic insulation methods. A Class II product MOOP and Two MOOP are different qualification levels. does not require a ground connection, relying instead on the use of double or reinforced insulation techniques to protect users. monitoring equipment, it is highly likely the patient will have Functional insulation, also termed operational insulation, purely monitoring electrodes or sensors attached to their body. The serves the purpose of preventing faults and allowing the equipment consequences of excessive leakage current or an insulation failure to operate normally. An example of functional insulation is the may prove fatal. The medical safety standard IEC 60601-1 applies to separation built-in between high-voltage PCB tracks. Basic insulation all medical equipment. It has undergone several iterations since its techniques include the use of insulation-covered wires within the inception in 1977 and is currently in its fourth edition. adapter. An additional outer layer of insulating material is called IEC 60601-1 approaches safety in the manner resembling that supplementary insulation. An example of basic insulation is the of IEC 60950. It further emphasizes insulation, leakage currents, and plastic cover between the metal ac line connectors inside the adapter air-gap clearances. The earlier second edition of the standard put enclosure and the adapter’s PCB. the potential use cases for medical equipment in three categories: Double insulation refers to an adapter that incorporates both in the vicinity of the patient but without touching the patient’s body, basic and supplementary insulation. Reinforced insulation provides equipment physically connected to the patient’s body (a blood the same level of protection as a double-insulated adapter but with pressure monitor, for example), or equipment such as an implantable only a single layer of insulation. defibrillator that has physical contact with the patient’s heart. Any form of insulating material used to isolate one part of a circuit Each of these types has specific isolation voltages, creepage, from another must be able to continue to insulate against high voltages and insulation limits stipulated. Edition three, ratified in 2005, took present within the circuit. Note that an air gap between conductors the initial concepts further and introduced the idea of a means of is considered a type of insulation. Within IEC 60950-1, the minimum protection (MOP). Recognizing that hazards for operators differ from clearance is specified as 4 mm for reinforced or double insulated those for patients, the third edition also introduced a means of patient methods, or 2 mm for basic and supplementary insulation. protection (MOPP), and a means of operator protection (MOOP). For a An air gap - creepage - specification is typically quoted at a patient attached to cardiac equipment, the power adapter must meet given working voltage, usually 264 Vac. The use of an air gap also MOPP criteria. For example, the CUI SDM65-U is a 65-W ac/dc external introduces the need to consider the environmental conditions, desktop power adapter that is designed for 2 x MOPP applications. because pollution and humidity significantly alter the ability of air to The fourth edition of IEC 60601-1 introduces the aspect of be an insulator. Typically, most insulators will withstand high voltages electromagnetic interference as a potential safety risk. With the but may breakdown above a given voltage. After a breakdown growing use of smartphones, Bluetooth headsets, and other wireless condition, there is no longer a safety barrier in place, and lethal peripherals, there is a potential risk that their emissions could disturb voltages may pass through. medical equipment. The introduction of electromagnetic immunity The point at which an insulator breaks down will depend on the levels (EMI) for medical equipment, and the possibility that the machine magnitude of the voltage, the material qualities, and the duration of itself could disrupt the operation of other equipment in the vicinity, has the applied voltage. During safety testing, a power supply adapter led to the definition of three intended-use environments: professional is tested for three different breakdown voltage conditions: input to healthcare facilities, home healthcare, and special environments. The output, input to ground, and output to ground. latter category includes places where high levels of electromagnetic The datasheet will list power adapter isolation characteristics. interference may arise, as from radiotherapy equipment. It should indicate the test voltage used and the duration for which it All in all, external power adapters maintain a safety barrier was supplied. For example, the CUI series SDI200G is rated with an between lethal ac line voltage and the dc output. are of paramount isolation voltage of 3,000 Vac for one minute. importance. The manufacturer’s datasheet lists all relevant safety Another important datasheet parameter is the leakage current. certifications and should also take into account both the geographical Leakage involves any current flowing from the adapter to the ground regions in which the product may be used as well as the application connection. A Class I power adapter uses the protective ground to the adapter will power. shield users from harm. On a Class II adapter, where no protective ground connection is in place, the user may form a path to ground for stray currents. IEC 60950-1 stipulates that leakage from a Class II References adapter must not exceed 0.25 mA. CUI Inc., www.cui.com/ Where the power adapter is powering an item of healthcare
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WIRELESS CHARGING
Autonomous wireless charging keeps robots running Buck-boost regulators efficiently power wireless charging stations for mobile robots.
LOGISTICS,
delivery and inspection industries increasingly rely on mobile robotic fleets. These fleets have become large enough that their users are trying to find ways of recharging them that don’t rely on human operators. One means of automating the robotic charging process is through use of equipment from WiBotic. The approach is to build wireless charging hardware into the robotic platform. The charging hardware sends the bot to a charging station when it senses the battery needs a charge. The bot positions itself near the charging station in a way that aligns transmitting and receiving coils for power transfer. The various possible configurations of ambulatory and flying robotic platforms complicate the design of the charging apparatus. Today, there are several ways of wirelessly charging batteries. The most common is inductive charging as is typical in cell phones. But inductive systems are only efficient when the antennas are extremely close. It’s tough to design robots and drones to position themselves accurately enough for reliable charging. Magnetic resonance charging offers more flexible positioning but has a relatively small sweet spot for maximum transfer efficiency. WiBotic technology incorporates both inductive and resonant systems via what’s called an Adaptive Matching system. It constantly monitors relative antenna position and dynamically adjusts both hardware and firmware parameters to maintain maximum eeworldonline.com | designworldonline.com
energy transfer efficiency across several centimeters of vertical, horizontal and angular offset. An embedded identification and communication system lets any robot charge from any station, even if the robots have different battery chemistries, voltages, and charging rates. APIs from WiBotic let computers on the same network as the transmitter monitor charging and set charging parameters. For example, operators might schedule robots for charging at their highest power level when they’re busy, but more slowly overnight to maximize battery lifespan. WiBotic chargers must carefully manage charging modes to optimize up-time while not degrading batteries through repetitive quick charges. This 10 • 2020
An aerial drone landing on a WiBotic charging pad. The WiBotic charging system automatically manages both inductive charging when the receiver sits close to the power transmitter and magnetic resonance charging for situations like this one where the receiver sits some distance from the power transmitter.
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POWER & ENERGY EFFICIENCY HANDBOOK
Visible on the TR-110 wireless charging station PCB are the cooling fans that dissipate heat from the onboard RF amplifier. The Vicor surface-mounted PRM (not visible) sits outside this airflow. variability in charging cycles and power levels – from 300 W for fast charging to 100 mW trickle-charging – also requires a power delivery architecture that matches a wide range of impedances. To efficiently accommodate the wide range of loads, WiBotic used a Vicor zerovoltage switching (ZVS) buck-boost PRM regulator. Its topology enables a high switching frequency (about 1MHz). High switching frequency reduces the size of reactive components, enabling a power density of up to 1,383 W/in3. The Vicor regulator is integrated into the RF transmitter onboard the WiBotic TR110 wireless charging station. The 48-V input (the regulator accepts a 36–75-Vdc range) is from an ac-dc power supply. The output voltage is dynamically controlled and trimmed from approximately 20 to 55 V as needed. The Vicor PRM handles full-charge and trickle-charge modes with no significant drop-off at lower power levels–a critical performance benchmark that defeated competing power components—as well as a ‘topping-off’ mode requiring a constant voltage for a 100% charge. High-
efficiency conversion yielded a maximum operating temperature of 45°C. The TR-110 wireless charging station employs active cooling to dissipate heat from the onboard RF amplifier, but the Vicor surface-mounted PRM sits outside this airflow. The zero-cross switching of the Vicor PRM module also minimized EMI/noise challenges and conducted emission/EMC requirements with no need for additional filters.
A fully autonomous mobile robot from Waypoint Robotics here moves toward a WiBotic wireless charging transmitter. The bot, which usually transports material in warehouses, has enough smarts to properly position itself next to the WiBotic charger. The WiBotic power transmitter-receiver adjusts power transmission mode to best suit the conditions at hand and the battery technology the Waypoint bot happens to carry.
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WIRELESS CHARGING
The WiBotic wireless charging system components: At left, the transmitter unit and transmitter antenna coil. At right is the receive antenna coil and the onboard charger unit, connected to a battery pack being charged. The transmitter unit generates a highfrequency wireless power signal that travels through a coaxial SMA cable to the antenna coil. The transmitter recognizes any incoming robot equipped with an onboard charger and delivers the right amount of energy. Supported battery chemistries include lithium ion, lithium polymer, lithium iron phosphate, lead acid, and nickel metal hydride. Robots with completely different battery voltages can share the same transmitter unit which automatically recognizes each robot and adjusts charge parameters accordingly.
The PRM is in a surface-mount package measuring 1.28x0.87x0.249 in [32.5x22.0x6.31 mm] which is compatible with standard pick-and-place and surface-mount assembly processes. It also provides a planar thermal interface area to enhance thermal conductivity. The compact size of the PRM helps keep the onboard charger small and light weight, thus promoting long operating cycles particularly for airborne fleets. For the stationary wireless charging stations, a dense power delivery architecture ultimately helps conserve valuable real estate in the deployment environment.
Optically Clear Silicone Adhesive Addition Cured MasterSil 157
Outstanding optical clarity Refractive index, 75°F: 1.43 Wide temperature range -175°F to +500°F Superior flexibility Elongation, 75°F: 110-140%
References Vicor, www.vicorpower.com WiBotic, www.wibotic.com
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POWER & ENERGY EFFICIENCY HANDBOOK
Boosting efficiency in fast-charge adapters Gallium-nitride transistors help make charging circuits smaller and more economical.
WHEN
it comes to fastcharging power adapters for consumer devices, high power density and high efficiency are now the two major technical requirements. With fast switching speed and low on-state resistance (Rdson), GaN transistors enable adapters that charge fast with high efficiency. Well-known mobile phone brands such as Huawei, Xiaomi, and OPPO, and more than 20 aftermarket brands, have launched GaN-based fast-charging adapter products.
For chargers below 75 W (30-65 W), quasi resonant (QR) flyback and active clamp flyback (ACF) are the main topologies, exhibiting efficiency close to 94% and power density at 20 W/in3. Chargers above 75 W (100-300 W) typically implement a two-stage topology which uses a power factor correction (PFC) circuit in the front stage and an LLC resonance circuit or other isolated dc/dc converter in the second stage. The target maximum efficiency is around 95%, and the power density should exceed 22 W/in3. Compared with traditional silicon MOSFETs, GaN transistors perform better. The advantages of wide bandgap (WBG) semiconductors let GaN circuits operate at higher switching speed with lower switching losses. To compare GaN transistors with silicon MOSFETs of the same die size, GaN has lower on-state resistance Rdson and lower operating temperatures. GaN transistors can support lower driving charge (Qg), gate drain charge (Qgd), and output energy (Eoss) with lower Rdson. A QR flyback circuit topology is widely used in adapter circuits because it is economical and reliable. Boosting the switching frequency to reduce the size of passive components such as transformers is an effective way to improve the power density of the adapter. However, this approach will inevitably cause additional switching losses and a temperature rise. There
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Jimmy Liu • GaN Systems Inc.
Power class: 30W-65W
Power class: 100W-300W
The main topology, power density, and performance requirements of adapters for consumer products.
QR flyback circuit
Losses of a typical QR flyback circuit.
are two types of QR flyback losses which are relative to the switching frequency. The higher the frequency, the higher the losses. During the turn-off period, the drain current peaks and the transistor is turned off with hard switching. The result is current-to-voltage IV crossover losses. This switching crossover loss can be evaluated 10 • 2020
by the parameters for total gate charge (Qg) and drain to gate charger (Qgd). During the turn-on period, the current drops to around zero, so there is no IV crossover loss. However, at high ac input such as 230 Vac,the QR flyback circuit doesn’t execute a zero-voltage turn-on, and there’s a discharge loss due to the eeworldonline.com
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FAST CHARGING GaN Total driving charge
Overlapping switching loss
Comparing the total charges and switching overlapping losses of GaN and silicon devices. The Qgd of GaN transistors is only 6% that of silicon MOSFETs.
GaN and silicon device Eoss
The Eoss of GaN transistors is only about 60% that of silicon MOSFETs, so the discharge loss generated by the switching capacitor in the GaN circuit is much lower.
EZDrive level switching circuit and drive waveform
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POWER & ENERGY EFFICIENCY HANDBOOK Level-switching circuit
Drain-source dv/dt
Drain-source dv/dt of rise and fall voltage is controlled by the EZDrive level switching circuit to minimize EMI.
Fit
MTTF (yr)
GaN Systems
0.896
1.27E+05
Other GaN supplier
89.639
1.27E+03
Flyback circuit
Fit
MTTF (yr)
GaN Systems
0.730
1.56E+06
Other GaN supplier
61.689
1.85E+03
GaN systems daughter card
Reference design of GaN-based 5X6-mm PDFN device + EZDrive level switching circuit daughter card. 38
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FAST CHARGING GaN Market solutions for GaN Systems' GaN-based charger transistor's parasitic capacitance. This capacitance discharging loss can be quantified by the output energy (Eoss). Figure of Merit (FOM) is an important index to evaluate the on-state and switching properties of power transistors. The smaller the value, the higher the performance. Input FOM represents the voltage and current overlapping losses generated during switching under the same Rdson. It is the most important index for evaluating transistors in a hard-switching circuit. With a similar Rdson (50-60 mΩ) the Qgd of GaN transistors is only 6% of that for silicon MOSFETs, leading to one-fifth the voltage and current overlapping losses. The FOM of the QR flyback gives the discharge loss caused by the parasitic capacitance at 200 V under the same Rdson. With a similar Rdson, the Eoss of GaN transistors is only about 60% of that for silicon MOSFETs, so the discharge loss generated by the transistor's parasitic capacitiance is much lower.
RELIABILITY Design engineers focus on three factors when developing chargers: product reliability for a long service life and low failure rate; total cost, including BOM and production costs along with transistor costs; and time-to-market, with the goal of significantly shortening the design cycle. Fast chargers ranging from 30 to 300 W can use 650-V 5x6-mm PDFN encapsulated GaN Systems transistors with Rdson from 150 (GS-065-011-1-L) to 450 mΩ (GS-065-004-1-L). GaN Systems starts with the Jedec standard for its qualification process and goes beyond it with multiples of extended test times. Additional reliability tests such as a switch dynamic life test at high temperature have been designed based on GaN transistor properties. Drivers for GaN Systems devices have a turn-on voltage of around 6 V and a turn-off voltage from 0 to 10 V. The output voltage for conventional drive charger control ICs is generally 12 V. So the control IC output voltage must undergo a level shift. GaN
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Systems uses a low-cost EZDrive level shift circuit where four simple separation devices (RUD, CUD, ZDUD1 and ZDUD2) convert the drive voltage with no overshoot or oscillations in the VGS of the GaN Systems transistor. The dv/dt drain-source drive voltage slope is controlled by the gate resistor Rg, thus improving the EMI design. Compared to other single-chip GaN schemes, the GaN Systems transistor plus EZDrive level shift circuit is more flexible and makes full use of the internally integrated driver of the control IC, keeping down cost. EMI can be optimized by controlling the dv/dt slope switching. A reference design integrates a 650-V 5x6-mm PDFN-encapsulated GaN Systems transistor with a daughter card containing the EZDrive level shift circuit. This approach allows designers to quickly replace the silicon MOSFET devices such as those in TO220 packages to evaluate the GaN Systems transistors. One example of a reference design for the fast-charging market is a 65-W high-power density (18.5 W/in3) PD charger equipped with a 650-V 150 mΩ GaN Systems device (GS-065-011-1-L). The QR circuit is inexpensive with the peak efficiency of nearly 94%. It meets the performance and standby power loss of CoC V5 Tier2, and the max temperature with case is below 65C°. The design has also passed safety standards and the EN55032 Class B EMI test for both conduction and
10 • 2020
GaN Systems solutions for the fast charger market with power ranging from 30 to 300 W. These solutions contain common power and circuit topology (including QR flyback, ACF, LLC Resonance, PFC, etc.), for extremely high efficiency and power density.
Reference design of 65-W PD fast charger
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POWER & ENERGY EFFICIENCY HANDBOOK Reference design of 300W charger
radiation and can also support a variety of USB-C protocol outputs. In another design, a 300-W high-power density ac/dc charger applies the GS66504B GaN Systems transistors in a synchronous PFC boost and LLC resonant topology with the peak efficiency up to 95% and power density at 34 W/in3. It meets the EN55032 Class B test on EMI conduction. The frequency of LLC resonant soft switching circuit is up to 500 kHz for high power density.
References GaN Systems, gansystems.com/
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A reference design for a 300W high-power-density ac/ dc charger uses GS66504B GaN Systems transistors in a synchronous PFC boost and LLC resonant topology with the peak efficiency up to 95% and power density at 34 W/in3.
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POWER & ENERGY EFFICIENCY HANDBOOK
Why electrolytic capacitors blow up Electrolytic capacitors have a reputation for failing spectacularly when mistreated.
POP
open a common LED bulb and you’ll often find an electrolytic capacitor occupying a place in the input from the ac line. Though illumination-grade LEDs generally have lifetimes exceeding 10,000 hours, the electrolytic caps in their base may not last nearly that long. There may be a variety of reasons for such bad outcomes. Perhaps the main reason for difficulties with electrolytic caps is their poor performance when exposed to reverse voltages. Electrolytics are polarized devices that only work well when applied signals on the cap’s positive terminal exceed that on the negative terminal. The sensitivity to polarity arises because of the cap’s construction. The most common electrolytic cap is the aluminum electrolytic. Its anode electrode (+) is a pure aluminum foil with an etched surface. A thin insulating layer of aluminum oxide acts as the dielectric of the capacitor. A nonsolid electrolyte covers the rough surface of the oxide layer, serving in principle as the cathode (-). A second aluminum foil called “cathode foil” touches the electrolyte and serves as the electrical connection to the cathode. The entire assembly is rolled up to form the distinctive cylindrical shape defining electrolytics. A point to note is that applying a positive voltage to the anode material in the electrolytic bath forms an insulating aluminum oxide layer. Its
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Leland Teschler • Executive Editor
thickness corresponds to the applied voltage. This oxide layer acts as the dielectric. After forming a dielectric oxide on the rough anode structures, a counter-electrode must match the rough insulating oxide surface. The electrolyte serves this purpose. The thickness of the dielectric is thin, typically measured in nanometers. The voltage strength of the oxide layer is quite high in the right direction. But exceeding the maximum voltage spec can make the capacitor look like an electrical short. The result can make for a noteworthy video as evidenced by the number of exploding capacitors available for viewing on YouTube. Here’s where polarity comes in: Applying a signal with the wrong polarity prevents the oxide layer from forming. The result, again, can be catastrophic failure. Of course, application circuits that are functioning properly will supply
Setting series capacitance balance resistance
Nichicon Inc. gives recommendations for balancing electrolytic capacitors wired in series so their individual leakage currents don’t cause problems.
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ELECTROLYTIC CAPS
Failure modes and their causes
signals of the right polarity to the electrolytic caps they use. The most common culprit for shortening electrolytic cap life is heat. A capacitor rated for 10,000 h at 25°C will be derated for use in higher temperatures--it may only be rated for 1,000 h at 85°C, even less at 105°C. The heat generally acts to evaporate the electrolyte and reduce the capacitance. Additionally, an electrolytic cap can heat up when it is in a circuit that repeatedly and rapidly charges and discharges it. And performance changes caused by high temperatures eeworldonline.com
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are temporary, Spec-sheet performance will reappear once the capacitor returns to a normal temperature (assuming it hasn’t been damaged by over-temperature). There are capacitors rated for long life at higher temperatures for when temperature is a problem. Another notorious factor for shortening electrolytic cap life is the ripple current the cap sees. Ripple current is common in power regulator circuits that frequently employ electrolytic caps. For complex electrochemical reasons, the higher 10 • 2020
Illinois Capacitor summarizes the failure modes of electrolytic capacitors this way.
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POWER & ENERGY EFFICIENCY HANDBOOK the ripple current, the greater and faster the cap degrades. Sensitivity to ripple current is a function of the cap construction and materials. Vendors specify operating life with different ripple-current values. Additionally, there are electrolytic caps designed specifically to handle high-ripple current. Unfortunately, supply chain issues can also impact cap performance. Substandard or outright counterfeit parts are increasingly common in procurement channels. It is relatively easy to make an adequate capacitor that will function acceptably in the short term. However, operating life can easily be substandard. For example, the capacitance of even high-quality electrolytic capacitors can drift from the nominal value over time. It is not unusual to large tolerances specified, typically 20%. Thus an aluminum electrolytic capacitor with a nominal capacitance of 47µF can be expected to measure anywhere between 37.6 µF and 56.4 µF. Tantalum electrolytic capacitors can have tighter tolerances, but typically have lower operating voltages. And
Example electrolytic capacitor case styles (Nichicon)
Examples of packaging used with for electrolytic capacitors. These packages, from Nichicon, vent gases through the rubber layer in the event of overheating.
It’s not a web page, it’s an industry information site So much happens between issues of R&D World that even another issue would not be enough to keep up. That’s why it makes sense to visit rdworldonline.com and stay on Twitter, Facebook and Linkedin. It’s updated regularly with relevant technical information and other significant news to the design engineering community.
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ELECTROLYTIC CAPS Electrolytic cap
Electrolytic capacitors visible on these circuit boards taken from 60-W equivalent LED bulbs made by Philips and Feit Electric. all bets are off with regard to the tolerance over time of substandard or counterfeit caps. It also pays to know the conditions under which cap spec sheet ratings apply. Rated capacitance is generally indicated as the value at 20°C and 120 Hz. The capacitance will be reduced at temperatures higher and lower than 20°C. Note, too, the spec for loss tangent. The loss tangent is defined as the tangent of the difference of the phase angle between capacitor voltage and capacitor current with respect to the theoretical 90° value. The difference is caused by the dielectric losses within the capacitor. The tangent of the loss angle (tan δ) is indicated as the value at 20°C and 120 Hz. This value will drop at higher temperatures and rise at lower temperatures. Additionally, both capacitance and tangent of the loss angle differ with frequency. Capacitance is lower at high frequencies, and tangent of the loss angle is higher at high frequencies. Capacitor impedance is generally expressed as the value at 20°C and 100 kHz. The impedance will be higher at lower frequencies. Storage conditions can also affect electrolytic cap performance. The leakage current in an aluminum electrolytic capacitor will rise if the capacitor is stored for extended periods. As with other capacitor parameters, the effect is more pronounced at higher storage temperatures. However, applying a voltage can reduce the leakage current . This is the principle behind reconditioning an electrolytic capacitor by applying a voltage. The the same reason, circuit designers should consider the initial increases in cap current when designing the equipment. The usual technique is to put a guard circuit in parallel with the cap. The packaging of the cap itself can cause problems. Note there is no isolation between the capacitor case and the cathode terminal. eeworldonline.com
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Manufacturers typically don’t specify the amount of resistance between the electrolytic capacitor case and the cathode terminal. The cap outer sleeves are also susceptible to damage. The sleeve may crack if exposed to high temperatures. Generally the outer sleeves are made from PVC, but the PVC is there for labeling, not to provide electrical insulation. Electrolytic caps typically also incorporate pressure vents that take the form of a thin area on the outer case, put there to handle pressure build-up if the cap is mistreated. It’s good to figure out where the vent is and allow a space above it. Finally, slight differences in capacitors wired in series or parallel can lead to issues. For example, electrical current may not balance evenly among capacitors in parallel. In power supplies, one outcome can be excessive ripple current one or more of the caps. Similarly, when two or more capacitors connect in series, the balance of the applied voltages must be taken into account so the voltages applied to each of the individual capacitors stay below the rated voltages. The usual approach is to install voltage divider resistors in parallel with each of the capacitors.
References Illinois Capacitor, https://www.illinoiscapacitor.com/ Nichicon Inc., https://www.nichicon.co.jp/ english/products/pdf/aluminum.pdf
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POWER & ENERGY EFFICIENCY HANDBOOK
Harnessing Power-overEthernet efficiently A few simple techniques can boost the efficiency of modern PoE designs.
THE
latest Power over Ethernet (PoE) technology delivers enough power to meet the needs of today’s WiFi6 routers, IoT gateways, and 5G small cells. PoE now delivers up to 71 W of power over standard Ethernet cables. At this power level, efficiency becomes critical to a PoE Powered Device (PD) system, which receives power via the Ethernet cable. There are several ways to improve the efficiency of the PD system and to boost the efficiency of the flyback converter commonly used to step the PoE voltage down to a usable level. PoE transmits power over the Ethernet cable by injecting ~50 Vdc onto the twisted pairs. The dc voltage is applied through the center taps of the Ethernet transformers, and either two or all four of the four twisted pairs in the cable deliver power. The latest IEEE 802.3bt PoE standard uses the four twisted pairs. Many papers and articles have been written describing in detail how the PoE standard works. For a PoE PD system, it is key to understand that the IEEE standard does not require that power from the Ethernet switch be injected in any specific polarity. Therefore, the PD must include rectifier bridges on the input to set the dc
Charlie Ice • Silicon Labs
polarity for the system. Furthermore, most PoE PD systems do not operate from ~50 Vdc but rather operate from a lower voltage, often 5 Vdc. This practice necessitates use of a dc-dc converter to step the PoE voltage down to something usable by the system. The PoE input and the dc-dc converter combine to determine the overall efficiency of the PoE PD system. The PoE PD interface includes input bridges to set the polarity of the PoE voltage, a power-enable FET, and the PoE interface IC which is sometimes combined with the dc-dc controller into a single device. There are three fairly simple ways to improve the efficiency of a PoE PD system that do not involve the dc-dc converter, and all three ways improve the efficiency of the PoE PD interface. The biggest improvement in efficiency comes from using FET bridges instead of diode bridges. However, it is also the
Ethernet cabling contains four twisted pairs of wires. IEEE 802.3af/at specifies power for two of the four twisted pairs, and IEEE 802.3bt allows for delivering power via all four twisted pairs.
Ethernet cabling
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POWER-OVER-ETHERNET Typical PoE-powered node
most expensive improvement to implement. Most PoE PD systems use diode bridges to define the positive and negative rails. The diodes are inexpensive and provide the option of using Schottky diodes to reduce the resulting voltage drop across the diode bridges. However, the power loss adds up quickly in a 71-W system, and the extra heat complicates compact designs. Use of actively controlled FETs for the bridges reduces the power loss and thermals considerably. Higher efficiency also results when the PoE interface IC gets power from an aux winding on the Ethernet transformer. PoE interface devices must be able to operate from the PoE input voltage. However, many of them also include the option to switch to an auxiliary supply once the dc-dc converter is running. In a flyback or forward topology, an extra winding can be added to the transformer to generate a low-voltage rail to power the PoE interface device. For example, a device that draws 5 mA of current will dissipate 250 mW (50 Vx5 mA = 250 mW), running from the PoE input. However, if running from a 15-V auxiliary supply, it dissipates 75 mW (15 Vx5 mA = 75 mW), a significant savings. While the efficiency gain is not as dramatic as that available through FET bridges, the auxiliary
The power architecture of a PoE powered access point with an intermediate voltage rail (red lines) distributed throughout the system. input is still a simple way to reduce power consumption. The third way to boost efficiency is via a power-enable FET with low on resistance. As the name implies, the power-enable FET enables power to the dc-dc converter once PoE negotiations have completed and power is applied to the PoE PD. All the power for the PoE PD system must flow through this FET, so reducing the FET “on” resistance is another easy way to improve efficiency. Many PoE
Inside a PoE PD supply
The internal design of a PoE PD interface power supply including the input bridges, PoE interface, and dc-dc converter.
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POWER & ENERGY EFFICIENCY HANDBOOK Simple flyback converter
The simplified topology of a flyback converter with an optocoupler providing isolated feedback to the controller. interface devices integrate this FET onboard. If the device can drive an external FET instead, selection of an external FET with lower “on” resistance can reduce power dissipation.
PoE POWER SUPPLY TOPOLOGY Flyback converters have served in PoE systems for years because they are efficient, inexpensive, and provide isolation. A flyback is typically used for systems needing up to 30 W of power. Beyond this level most designs employ a forward converter to stepdown the PoE voltage. A flyback converter uses a specialized transformer to both transfer power and to step down (or even up) the input voltage. Though ordinary transformers typically do not store energy, the situation in a flyback converter is different. That’s because the transformer is constructed as two coupled inductors with an air gap. Most PoE PD device manufacturers provide evaluation boards that include a dc-dc converter along with a custom transformer design. Designers should be careful when selecting a transformer differing from that used on the evaluation board: The transformer characteristics are essential to the operation of the flyback converter. Even a slight difference can
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cause instability or dramatically reduce the efficiency of the converter. Finlly, the use of a transformer allows a flyback converter to be isolated, with the a high impedance between the grounds on the primary and secondary sides. This quality allows the PoE PD device to safely provide user-accessible ports, grounded
data connections (such as USB), and minimizes the chance of ground loops. As with anything safety related, always consult with the relevant safety team or agency. It is useful to review the role of the transformer in the flyback. When the FET is “on” (closed) current flows through the inductor on the primary side. The inductor on the secondary side does not have any current flow, as the diode connecting the secondary to the V+ output is reverse biased. Notice the polarity of the transformer creates this condition, as the opposite polarities cause the secondary side voltage to be the reverse of the primary side. Once the current through the primary side inductor reaches a peak value, the controller opens the switch. The transformer will try to maintain its magnetic flux, and the only way to do so is to induce a current through the secondary side inductor to forward-bias the diode. The inductor current flow then charges the output capacitor and powers the load. Output voltage is primarily determined by the transformer turns ratio between the primary and secondary sides. When the switch closes, the cycle repeats with the output capacitor powering the load when the switch is closed. There are many details to a flyback
Current flow, switch closed
Flyback current flow when the switch is closed. Dashed lines represent no current flow. 10 • 2020
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POWER-OVER-ETHERNET converter that impact stability and efficiency. But a simple way to improve efficiency is use a dc-dc controller with a secondary side FET driver. In this configuration, a FET replaces the secondary side diode. The controller energizes the FET when the switch is open and de-energizes it when the switch closes. This technique improves efficiency by eliminating the voltage drop across the diode and replacing it with the voltage drop due to the FET on resistance. An additional point to note is that in an isolated flyback converter, a pulse transformer is required to transmit the gate-drive signal across the isolation barrier to the secondary side FET. Depending on the system output voltage and power level, replacing the output diode with a FET can boost the efficiency by up 5%. All in all, PoE PD efficiency can be pushed to greater than 90% from PoEinput to power-output by using a few, straightforward techniques. With up to 71 W of delivered power, PoE stands ready to power today and tomorrow’s WiFi, 5G, and IoT devices.
Current flow, switch open
Flyback current flow when the switch is open. Dashed lines represent no current flow.
References Silicon Labs, www.silabs.com
Flyback with secondary side FET network
Flyback converter with secondary side FET and pulse transformer highlighted in green.
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