Power operating limits: how to choose the right power supply Page 30
Power supply designers take a hard look at soft magnetics Page 38
FEBRUARY 2019
Power Electronics Handbook
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Digitally Enhance Your Analog Controls
Combine the Speed of an Analog Controller With the Flexibility of a Digital Microcontroller No system can perform without reliable power supplies. Our Digitally Enhanced Power Analog (DEPA) family of products combines the performance of an analog Pulse-Width Modulation (PWM) controller with the configurability of an 8-bit PIC® microcontroller (MCU). The combination of these methods allows the addition of digital features to a reliable, easy-to-implement analog control loop, including fast transient responses, high efficiencies, reliable gain and phase margins. Adding the ability to measure and respond to changes with tailored algorithms improves the robustness of the system, while offering diagnostic and communication options. The single-chip solution can accept a high-voltage input and regulate a wide output current or voltage range, which allows you to maintain robust operation within an unstable environment. Discover how the flexibility of our DEPA products can enhance your next design.
Key Features •
Fast and efficient power conversion with analog current-mode control loop
•
Flexible control with an integrated MCU
•
Dynamically settable hardware protections enable robust operation
microchip.com/FlexiblePower The Microchip name and logo and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks are the property of their registered owners. © 2019 Microchip Technology Inc. All rights reserved. 1/19 DS20006124A
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POWER ELECTRONICS HANDBOOK
Now or later?
No time for time-of-use energy savings ONE
of the big justifications for smart home electronics and internet-of-things pipedreams is a concept called time-of-use electricity billing. Long used for industrial concerns, the plan is to bring TOU billing to households, so they’ll be charged more for electricity they use during periods of peak use, less for juice consumed otherwise. The idea is to give people incentives for, say, running the dishwasher late at night rather than right after dinner. Right now, TOU billing for consumers can only be found in a few pilot projects. But a lot of consumer electronics gizmos coming off drawing boards today list TOU billing as among their main justifications. Without TOU billing, it’s tough to see why you’d bother with second-by-second control of your water heater and home air conditioning. Indications are that appliance makers may be in for a big downer if they see a bonanza in TOU-inspired gadgetry. That seems to be one of the messages coming out of a study of a residential TOU trial conducted by researchers from The Ohio State University and the University of Southern Calif. Surveying about 19,000 TOU customers in the southwestern U.S., the researchers found that most of them had a limited understanding of their own energy use – good news, you’d think, for purveyors of gear that reads out energy use to the millisecond. The problem was that consumers thought they’d save money on TOU rates by strategically using their appliances. In actual experience, consumers typically saw little difference in their bills. Apparently, that realization was disappointing, and giving consumers a better grasp of their energy use didn’t help matters. The researchers explain that people think the appliances they frequently interact with use more energy than they really do. Writing in the journal Nature Energy, they said “People probably overestimate the amount of their energy use that can be shifted through highly visible actions, such as lighting, versus less visible actions such as heating and cooling….In addition, optimism bias may lead to perceiving greater ability to shift
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electricity use away from on-peak time (and more control over bills) than is truly possible.” Reading between the lines, it’s not hard to see what happened in this pilot program which took place during summer months: Consumers arrived home from work, typically found the internal temperature of their residence was somewhere near 90°F, and immediately hit the a/c, peak rates or no. Then to save money, they turned off a light bulb or two. When the utility bill arrived, sadness ensued. The results of the study don’t bode well for TOU promoters. Even more worrisome is that the people who took part in the study were probably more sympathetic to TOU than the general population. All of them voluntarily opted in for the billing scheme. Those in the study were also better educated, older, and included a higher percentage of homeowners than the U.S. average. Expect a lot more resistance to TOU billing rolled out to a broader base of much less understanding consumers, a few of whom will undoubted be convinced TOU is some kind of government conspiracy. When that happens, the entities expressing the most disappointment won’t be consumers but rather the companies whose economic success depends on selling TOU contrivances.
LEE TESCHLER, EXECUTIVE EDITOR
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HELPING YOU PROTECT WHAT
MATTERS
MPD CAN BE FOUND IN SECURITY DEVICES
AROUND THE WORLD
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CONTENTS FEBRUARY 2019
THE POWER ELECTRONICS HANDBOOK
02 07
NOW OR LATER? NO TIME FOR TIME-OF-USE ENERGY SAVINGS
KEY FACTORS IN POWER SUPPLY SELECTION
It pays to understand a few important electrical parameters that can dictate the kind of applications specific power supplies can handle.
11
SWITCHERS SAY BYE-BYE TO EMI
38
POWER SUPPLY DESIGNERS TAKE A HARD LOOK AT SOFT MAGNETICS
The promise of high efficiency and small size brought by super-fast switching power supplies could be delayed by a lack of magnetic materials that are up to the task.
44
LED BULBS THEN AND NOW: TEARDOWN OF THE ECOSMART 60-W EQUIVALENT LED BULB
Compared to the LED bulbs of only a few years ago, modern-day versions are simpler and assembled via more automated methods.
EMI can emanate from high di/dt loops found in some switch-mode supply topologies. New controllers overcome these difficulties by integrating key components inside the chip package.
16
LINEAR VS. SWITCHING POWER SUPPLIES: NOT ALWAYS AN EASY CHOICE
There are some uses that demand the low noise and fast response only available through traditional linear supplies.
20
POWERING GRAPHICS PROCESSORS FROM A 48-V BUS
New converter topologies and power transistors promise to reduce the size and boost the efficiency of supplies that will run nextgeneration AI platforms.
30
POWER OPERATING LIMITS: HOW TO CHOOSE THE RIGHT POWER SUPPLY
Not all power supplies react to over and under-voltages and currents in the same way. It pays to know the tradeoffs of different approaches to protection.
34
WHY WIDE BANDGAP HEMPTS EXCEL AT EFFICIENT POWER CONVERSION
High electron mobility transistors reduce power supply size thanks to a special make-up that eliminates sources of energy loss.
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POWER SUPPLY SELECTION
Key factors in power supply selection It pays to understand a few important electrical
FLORIAN HAAS | TRACO POWER GROUP
parameters that can dictate the kind of applications specific power supplies can handle.
A
few basic factors go into the selection of low-voltage dc/dc and ac/dc power supplies for OEM use. At the risk of stating the obvious, the key objective is to bring the input voltage to a new potential. With that in mind, there are six parameters that determine most of the qualities the supply should have.
1
Whether the input potential is divided
2
The input and output voltage ranges and the output current demand
3
The space available for the supply
4
The end product and the regulations that apply to its industry or area
5
The environmental conditions the supply will see
6
The degree of reliability the supply must provide
There are a few electrical parameters pertaining to supplies that need detailed specifications. They include how to deal with supply ripple and conducted noise, the handling of inrush current, thermal considerations, and electromagnetic compatibility (EMC) constraints. Designers typically quantify power supply requirements by measuring the power consumption of host equipment. In this Swiss manufacturer Traco Power has a diverse dc/dc converter product regard, every measurement changes the portfolio that offers in excess of 140 standard product series that includes state of the circuit; the impact of any more than 25 3-W dc/dc ranges. Shown here are the TVN 5 series measurement should be minimized. This with ultra-low ripple and noise; the TEQ 300 series which is certified to philosophy implies performing a fourEN 50155 for Railway applications and the TSR 2 series of step-down wire measurement even for simple tests. regulators with efficiency to 96% and a replacement to linear regulators. Measurement of current and voltage with independent test leads minimizes the impact of lead resistance on measured values. eeworldonline.com | designworldonline.com
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POWER ELECTRONICS HANDBOOK Four-wire measurement
Four-wire measurements supply current via a pair of leads called force connections. These generate a voltage drop across the impedance to be measured. A pair of voltage leads called sense connections sit immediately adjacent to the target impedance, so they do not include the voltage drop in the force leads or contacts. Because almost no current flows to the measuring instrument, the voltage drop in the sense leads is negligible. Customarily, the sense wires are the inside pair while the force wires are the outside pair. Exchanging the force and sense connections can degrade measurement accuracy because more of the lead resistance is included in the measurement. The force wires may have to carry a large current when measuring small resistances so they must be large enough to handle the anticipated level of current. The sense wires can be of a small gauge.
Circuit connections for a typical four-wire measurement with a source and load.
DEALING WITH INRUSH In a typical power supply, the ac current flows through a diode bridge rectifier and then flows into a filter capacitor. At power on, there is an inrush of current because the filter capacitor goes into its charging phase and acts like a momentary dead short. This inrush current can cause a number of problems. The usual remedy is to temporarily introduce a high resistance between the input power and rectifier in the form of an inrush current limiter. An inrush current limiter often takes the form of an NTC (negative temperature coefficient) thermistor in series with the diode bridge and sometimes in series with the filter capacitors. The thermistor gives the filter capacitor time to charge without the inrush current fully hitting the load. When energized, the NTC thermistor self-heats. The heating eventually lowers the thermistor resistance. As the resistance reaches a low value, current can pass through without degrading normal operation or power efficiency. The inrush current limiter remains at this steady-state condition, allowing the current to flow through unaffected. It should also be said that inrush current – or specifically, the heating that can arise from inrush current – can degrade the life of electrolytic capacitors often used as input filters. The reason is that high temperatures tend to shorten the life of electrolytic capacitors. Another factor that can cause such heating is excessive ripple current. It is common practice to measure the power supply ripple and noise on the output, though these parameters are generally spelled out in spec sheets. As a quick review, ripple voltage is
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the residual periodic variation of the power supply’s dc output voltage . It arises because of incomplete suppression of the alternating waveform after rectification. Ripple itself is a composite (non-sinusoidal) waveform consisting of harmonics of some fundamental frequency which can be tens of kilohertz to megahertz for switching supplies. Several parameters describe ripple; they include the peak (usually peak-to-peak) value of the ripple voltage; the root mean square (RMS) value of the voltage which is a component of power transmitted; the ripple factor γ, the ratio of RMS value to dc voltage output; the conversion ratio (also called the rectification ratio or efficiency) η, and form-factor, the ratio of the RMS value to the average value of the output voltage. Output ripple is undesirable for numerous reasons. It causes heating in dc circuit components because ripple current passes through parasitic elements such as the ESR of capacitors. Similarly, ripple forces designers to keep the value of parasitic elements low. Ripple voltage also forces components being powered to have higher peak voltage ratings than would otherwise be necessary. If the ripple frequency is in the audio range, it could become audible if the circuit being powered is for audio reproduction. Ditto for ripple frequencies near those by video displays. And ripple voltage that gets into digital circuits can reduce the logic threshold, as would any form of supply rail noise, making logic circuits more susceptible to incorrect outputs and corrupted data. Usually, noise and ripple can simply be reduced with two parallel-switched capacitors, for example, a 100 nF metal film capacitor and a 10 µF electrolytic capacitor, always bearing in mind that values presented on datasheets can be influenced by other factors during end usage. eeworldonline.com | designworldonline.com
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POWER SUPPLY SELECTION
Probing dc/dc converter pins For accurate measurements of dc/dc converter parameters, the scope probe head ground ring and measurement tip must be in direct contact with the converter pins. To compare results with manufacturer data, oscilloscope bandwidth is limited to 20 MHz, a common value for laboratory work.
Specific industries – such as industrial equipment and medical equipment, railway/transit equipment and communications equipment – generally each have their own standards for conducted and radiated EMI. Power supplies used in those industries must follow the applicable standards. For example, CISPR (in English, International Special Committee on Radio Interference) 11 is the international product standard for EMI originating from industrial, scientific and medical equipment. CISPR 11 applies to equipment that includes wireless power transfer chargers, Wi-Fi systems,
induction cooking hobs, and arc welders. In addition, other standards bodies such as the International Electrotechnical Commission (IEC) issue EMI standards. More specifically, IEC 61000-6- 3 applies to products in residential/ commercial/light-industrial applications, while IEC 61000-6-4 covers heavy-industrial uses. Certain industrial end equipment may have dedicated system-level standards that reference CISPR 11. For instance, IEC 61131-2 provides emission requirements for programmable controllers and their associated peripherals. Other system-
An example of inrush voltage in a LED lamp (yellow line). The voltage pattern in the lamp is in purple. The LED reaches its maximum current draw of about 10 A at the point where it switches on (point T, in orange). Within 10 msec current draw has returned to 300 mA.
level standards include IEC 61800-3 and IEC 61326-1, which dictate EMC requirements for adjustable-speed motor drive systems and laboratory equipment, respectively. Virtually all CISPR-based test standards specify limits for conducted emissions measured up to 30 MHz, except for CISPR 25, where the applicable upper frequency extends to 108 MHz. EN 55015, based on CISPR 15 for lighting equipment, has a measurement range extending down to 9 kHz for some apparatus. Use of a dc/dc converter with an internal filter doesn’t guarantee adherence to EMC values because EMC compatibility can often be affected by several components. In many cases, the output connections must include one to protective earth for safety reasons, and this can have a significant impact on EMC. Usually, the power supply manufacturer can offer advice regarding how to adhere to EMC values. Most power supply manufacturers provide help in the form of suggestions for suitable filters on their websites. For example, suggestions can be downloaded directly from the relevant device’s page at www.tracopower.com. If you can’t find the circuit diagrams for the product you have selected, don’t hesitate to contact the manufacturer directly on the phone or by email.
References Traco Power, www.tracopower.com
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Get Ahead of the Thermal Curve
Measure Power, Monitor Temperature and Turn on Your Fan Before it Gets Too Hot! How many parts does it take to accurately measure temperature and manage power? With our high-side current sensors, it could be as few as one. For example, the EMC1701/2/4 family has one current sensor and can monitor one, two or four temperature channels respectively. Power consumption has long been a leading indicator for thermal management. Measuring diverse power sources with multichannel chips closes the thermal information gap. For example, the PAC1933 can simultaneously measure a 1V Field Programmable Gate Array (FPGA), USB Type-C™ at 20V and a memory rail. Review our entire offering of high-side current sensors and DC-power monitors, including 36-hour on-chip accumulators and 16-bit precision multi-rail monitors.
microchip.com/DC-Power-Monitor The Microchip name and logo and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks are the property of their registered owners. © 2019 Microchip Technology Inc. All rights reserved. 1/19 DS20006057B
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BYE-BYE TO EMI
Switchers say bye-bye to EMI EMI can emanate from high di/dt loops found in some switch-mode supply topologies. New controllers
TONY ARMSTRONG | POWER BY LINEAR ANALOG DEVICES INC.
overcome these difficulties by integrating key components inside the chip package.
EMI generation in buck converters
IT
goes without saying that PCB layout sets functional, electromagnetic interference (EMI) and thermal behavior of every power supply design. Switching power supply layout is not black magic, but it is often overlooked until late in the design process. Many switchmode power supply designers are familiar with the design complexities and nuances of switch mode operation. But a lot of these old hands are literally retiring and leaving the industry! Consequently, more and more digital designers are being asked to take on switch-mode supply designs for no other reason than too few analog power supply designers to get the job done. Most digital designers know how to design with a simple linear regulator; it is less clear they are equipped to handle more complex designs such as step-up mode (boost) or even a buckboost topology (buck and boost modes combined). This leaves many electronic systems manufacturers wondering how their switch-mode supply circuits will get done. Companies that make ICs for switch-mode supplies are aware of this brain drain. So they are devising chips that help remove some of the complexity involved in the design of switch-mode circuitry. To understand these developments, consider the example of the basic buck regulator as diagramed in the nearby schematic. High di/ dt and parasitic inductance in the switcher “hot” loop causes electromagnetic noise and switch ringing. EMI emanates from the high di/dt loops. The supply wire as well as the load wire should not have high ac current content. Accordingly, the input capacitor C2 should source all the relevant ac currents to the output capacitor where any ac currents end.
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ADI — Power Electronics HB 02-19.indd 11
A simplified synchronous buck regulator and the high di/dt loops its topology creates.
During the on cycle with M1 closed and M2 open, the ac current follows the solid blue loop. During the off cycle, with M1 open and M2 closed, the ac current follows the green dotted loop. Most people have difficulty grasping that the loop producing the highest EMI is not the solid blue nor the dotted green. Only in the dotted red loop flows a fully switched ac current, switched from the zero to I peak and back to zero. The dotted red loop is commonly referred to as a hot loop because it has the highest ac and EMI energy. It is the high di/dt and parasitic inductance in the switcher hot loop that causes electromagnetic noise and switch ringing. To reduce EMI and improve performance, one must minimize the radiating effect of the dotted red loop. If we could reduce the PCB area of the dotted red loop to zero and buy an ideal capacitor with zero impedance, the problem would be solved. However, in the real world, it is the design engineer who must find an optimal compromise.
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POWER ELECTRONICS HANDBOOK
5-V 2.5-A step-down converter
The source of the high-frequency noise is the energy from switching transitions that is coupled though parasitic resistors, inductors and capacitors and creates high-frequency harmonics. So, knowing where the noise is generated gives clues about how to reduce it. The traditional way to reduce noise is to slow the MOSFET switching edges. Designers can mitigate these edges by slowing the internal switch driver or by adding snubbers externally. However, these measures will reduce the efficiency of the converter because they increase switching loss – especially if the switcher runs at a high switching frequency, say 2 MHz. There are several reasons for running at 2 MHz, which is relatively high for a switching supply: This switching rate enables the use of physically smaller external components such as capacitors and inductors. For example, every doubling of switching frequency leads to a halving of inductance value and output capacitance value. In automotive applications, switching at 2 MHz keeps noise out of the AM radio band. Filters and shielding can also be employed, but at the price of more external components and circuit board area. Spread-spectrum frequency modulation (SSFM) could also be implemented – this technique dithers the system clock within a specified range. SSFM reduces EMI in switching regulators. Although the switching frequency is most often chosen to be outside the AM band (530 kHz to 1.8 MHz), unmitigated switching harmonics can still violate stringent automotive EMI requirements within the AM band. Adding SSFM significantly reduces EMI both within the AM band as well as other regions. Or, one could simply use ADI Silent Switcher technology. It delivers high efficiency, low EMI, and sustains high switching frequencies with no tradeoffs.
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5-V 4-A step-down converter
The most obvious difference between the LT8610 synchronous step-down regulator and the LT8614 Silent Switcher is the addition of a second VIN and ground pin which permits the connection of special filtering capacitors.
SILENT SWITCHER TECHNOLOGY A Silent Switcher breaks the trade-off between EMI and efficiency without the need for slowing the switch edge rates. Consider the LT8610. It is a 42-V input-capable, monolithic (FETs inside) synchronous buck converter that can deliver up to 2.5 A of output current. It has a single input pin (VIN) at its top left corner. Compare this device with the LT8614, another 42-V input capable, monolithic synchronous buck converter that can deliver up to 4 A of output current. The LT8614 has two VIN pins and two ground pins on the opposite side of the package. This is significant, because it is part of what allows “silent” switching. Placing two input capacitators on opposite sides of the chip between the VIN and ground pins will cancel the magnetic fields. Additionally, there are opposing VIN, ground and input caps to enable magnetic field cancellation (right-hand rule applies) to lower EMI emissions. With the Silent Switcher capability of the LT8614, we can reduce the parasitic inductance by using copper-pillar flip-chip packaging. Reductions in package parasitic inductance come from eliminating the long bond wires of a wire-bonded assembly technique; the bond wires induce parasitic resistance and inductance. The opposing magnetic fields from the hot loops cancel each other out and the electric loop sees no net magnetic field. eeworldonline.com | designworldonline.com
2/19/19 4:09 PM
BYE-BYE TO EMI
LT8614 radiated EMI performance
The LT8614 radiated EMI performance lets it pass the most stringent CISPR25 Class 5 limits.
The LT8614 Silent Switcher technology has been tested against a current state-of-the-art switching regulator, the LT8610. Testing took place in a GTEM cell using the same load, input voltage, and the same inductor on the standard demo boards for both parts. We found that using the LT8614 Silent Switcher technology brings a 20-dB improvement compared to the already good EMI performance of the LT8610, especially at more-difficult-to-manage higher frequencies. Switching supplies based on the LT8614 need less filtering compared to other sensitive systems. Furthermore, the LT8614 exhibits benign time-domain behavior on its switch node edges. The LT8614 exhibits impressive performance, but it is not the end of the road. The LT8640 step-down regulator also features Silent Switcher architecture and delivers high efficiency at frequencies up to 3 MHz. Assembled in a 3×4mm QFN, the monolithic construction with integrated power switches and inclusion of all necessary circuitry yields a minimal PCB footprint. Transient response remains excellent and output voltage ripple is below 10 mVP-P at any load, from zero to full current. The LT8640 allows high-VIN-to-low-VOUT conversion at high frequency with a fast-minimum top switch on-time of 30 nsec.
Integration marches on
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The LT8640 and the more integrated LT8640S. The part number of the new higher integrated version carries an “S” suffix.
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PowerShuntAd_DesignWorld8_17.qxp_Layout 1 1/2/19 11:47 AM Page 1
Power Shunt
POWER ELECTRONICS HANDBOOK
Current Sense Resistors
Help you Make the Leap from
Concept to Reality
KOA Speer’s continually expanding line of Power Shunt Current Sense Resistors are the ideal solution to help you optimize your power system design. Our new Power Shunt Current Detectors are designed for DC to DC converters, inverters, batteries, motor controls, automotive modules, power supplies or any other power management application. KOA Speer Power Shunts Deliver:
To improve EMI/EMC, the LT8640 can operate in spreadspectrum mode. This feature varies the clock with a triangular frequency modulation of +20%. Here, a triangular frequency modulation varies the switching frequency between the value programmed by an external resistor (RT) to approximately 20% higher than that value. The modulation frequency is approximately 3 kHz. For example, when the LT8640 is programmed to switch at 2 MHz, the frequency will vary from 2 MHz to 2.4 MHz at a 3-kHz rate. When spread-spectrum operation is selected, Burst Mode operation is disabled, and the part will run in either pulse-skipping mode or forcedcontinuous mode. One possible difficulty with Silent Switcher control is that placing the filter capacitors too far from the LT8614 on the PCB can still lead to operational problems. Silent Switcher 2 eliminates this possibility by integrating the filter capacitors -VIN caps, IntVCC and Boost caps – inside a new LQFN package. This integration puts all the hot loops and ground planes within the packaging. The result is lower EMI and a smaller footprint thanks to fewer external components. And the PCB layout is much less sensitive to component location. Silent Switcher 2 also enables better thermal performance. The large multiple ground exposed pads on the LQFN Flip-Chip package facilitate the extraction of heat from the package and into the PCB. The elimination of high-resistance bond wires also boosts conversion efficiency. The EMI performance of the LT8640S easily passes the Radiated EMI Performance CISPR25 Class 5 peak limits with a wide margin. Silent Switcher technology can also be found in the LTM8053 and LTM8073 micromodule regulators where everything is virtually integrated with just a few external caps and resistors. Finally, it is worth pointing out that the reduction in PCB space made possible by Silent Switcher devices can also reduce the number of PCB layers needed.
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REFERENCES Analog Devices Inc., Power by Linear, www.analog.com/en/ products/landing-pages/001/ power-by-linear.html
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POWER ELECTRONICS HANDBOOK
Linear vs. switching power supplies: Not always an easy choice There are some uses that demand the low
ALEX K ARAPETIAN | ACOPIAN TECHNICAL CO.
noise and fast response only available through traditional linear supplies.
ASK
almost any engineer about linear power supplies and the likely instinctive reaction will be, “Sorry, I can’t use them — they’re too inefficient.” Any possibility of using a linear supply usually ends right there; it is as though you’re asking them to go back to vacuum-tube AM radios. Still, a good engineer knows that it’s wise to not make decisions based on assumptions and clichés, but rather on honest assessments of priorities and alternatives. This maxim also applies when deciding between a switching or linear power supply. Like many engineering decisions, this one depends on the specifics of the application, the features and functions it needs, priorities, and acceptable tradeoffs. First a quick review of the basics. A linear power supply first converts the high-voltage ac from the line into lower-voltage ac using a transformer. It then converts the low-voltage ac into an unregulated dc voltage via a rectifier and capacitor filters. An error amplifier with a voltage reference as one input and the output dc as the other controls a series-connected pass element. The error amplifier compares the reference to the output and regulates the output voltage by dropping excess voltage in the pass element, hence the designation “linear” supply. This closed-loop design ensures the supply output stays at the nominal voltage despite changes in supply line or load values. The downside of this arrangement is that the pass element is always in its active region. It dissipates power regardless of the power it is also delivering to the load; this is the major source of inefficiency in a linear supply.
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2 • 2019
Now consider a line-powered switching supply. Here, the line ac is usually first converted to unregulated dc (again, via a rectifier and filter). Then the supply regulates that dc voltage down to the desired voltage. There are many topology variations of the switching concept, but all have a similar underlying principle. Again, an error amplifier compares the regulator output value to an internal reference, but here the pass element is rapidly switched on and off with a pulse-width modulation or pulse-frequency modulation scheme. The output pulses are filtered to form low-ripple dc and the resulting waveform becomes the dc voltage output. Because the pass element is either completely on or off, it is always in a mode where its dissipation is minimal. The main losses arise from the series onresistance when the pass element conducts and switching losses as it transitions between on and off states.
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LINEAR VS. SWITCHING POWER SUPPLIES
The linear supply from Acopian Power Supplies (left) is a factor of ten larger and heavier than a comparable switching supply (above) that is also from Acopian, but the linear unit has beneficial attributes which the switcher supply cannot match.
LINEAR VS. SWITCHING SUPPLIES The typical linear supply has an efficiency of about 20% to 40%. This figure pales in comparison to the efficiency of a typical switching supply which comes in at 60% to 80% and can reach 90% in some cases. There are also major differences in size and weight. The switching supply is much smaller and lighter, largely because of the smaller transformer, discrete semiconductors, and passive components. For example, a 250-W linear power supply would require 600 in3 (a little under 9,000 cm3) of mounting space and would weigh 26 lb. (about 12 kg). A comparable ac/dc switching power supply could occupy one-tenth that volume and weigh just 2 lb. (0.9 kg).
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Acopian — Power Electronics HB 02-19.indd 17
The size of the switching supply is also a function of its switching frequency, ranging between a few hundred kilohertz for larger supplies to as high as a few megahertz for those supplying less power. Operation at high frequencies permits the use of smaller passives and reduces the overall footprint although overall efficiency will drop because of higher switching losses. Why would an engineer even consider a linear supply with its lower efficiency, larger size and higher weight? Here, noise on the dc output and transient response come into play. The linear supply is a continuousfunction unit with no discrete time clocking or switching action. The linear supply itself does not generate any EMI or RFI. As a result, its output is virtually free of any noise and ripple. Any noise at the load arises outside the supply itself from pickup in the power wiring between the supply and load. Chokes and other filter components as well as careful cable routing can attenuate this noise. In contrast, the switching supply is inherently a source of noise. Its output contains a fundamental at its clock frequency as well as numerous
2 • 2019
harmonics. Typical noise levels are on the order of hundreds of microvolts to tens of millivolts. These levels are unacceptable for many applications where the output voltage is at singledigit levels or the load is sensitive to supply rail noise. Switching-based noise can be filtered to some extent but is difficult to eliminate entirely. In addition to generating noise on the output cables, the supply also radiates noise which can induce unexpected and frustrating problems elsewhere in the system. While filtering can attenuate the output noise to acceptable levels, the problem of radiated noise is much more difficult to manage. Further, the frequency of the switching-induced noise may interfere with other clocked signals, resulting in beat frequencies and other interference. To head off such difficulties, the switching supply’s clock frequency may need to be synchronized with the system clock. In addition, there are increasingly stringent regulatory limits on the amount of noise a power supply can generate in different frequency bands, both as a function of power supply
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POWER ELECTRONICS HANDBOOK
Voltage regulation in linear supplies wattage and global location. Some switching supplies meet the regulatory mandates by using spread spectrum clocking to spread the noise energy across a wide band. This way, the noise does not exceed allowed limits at the clock frequency or its harmonics. While this technique works in the “legal” sense to meet mandatory standards, the supply noise can still affect internal system circuitry.
The linear power supply uses an all-analog closed-loop feedback approach to control its pass element such that the supply output voltage tracks the internal reference voltage. In the block diagram, the voltage divider at the right-hand side allows the user to set the output-voltage versus reference-voltage comparison ratio.
Typical SMPS configuration
TRANSIENT RESPONSE There is also the issue of transient response to sudden changes in load. The all-analog linear supply can be tuned to optimally respond to step-changes in load. The goal is to respond fast without overshoot or ringing, and a properly designed supply can provide this. In contrast, the closed-loop dynamics of the switching power supply are much more difficult to control. The switching supply may be slower or excessively overshoot and ring before it stabilizes, depending on design specifics and clock frequency. There is no doubt that the switching supply offers major benefits in efficiency, weight and size. But before assuming it is the right or only choice, engineers should consider the application as well as the impact of noise and ripple. There are installations where these attributes are not the priorities; instead, low ripple and output noise are the top priorities along with superior transient performance. Applications in this category include extremely low-noise amplifiers, advanced signal processing and data acquisition systems (including sensors, multiplexers, A/D converters and sample and hold circuits) and precision automatic test equipment (ATE) and laboratory test equipment.
REFERENCES Acopian Technical Co., www.acopian.com
In the SMPS supply, the output voltage is regulated by clocked on/off pulse-width modulation of the pass element, resulting in low losses and high efficiency. But the output contains clock noise that must be filtered to establish a clean dc output voltage.
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POWER ELECTRONICS HANDBOOK
Powering graphics processors from a 48-V bus ALEX LIDOW | EFFICIENT POWER CONVERSION
New converter topologies and power transistors promise to reduce the size and boost the efficiency of
Si-based 1/8-brick converter
supplies that will run next-generation AI platforms.
ARTIFICIAL
Intelligence (AI), gaming, cloud computing, and autonomous vehicles all employ the latest generation of graphics processors (GPUs) in lieu of CPUs. The reasoning is that GPUs offer higher computational density than traditional CPUs in terms of acquisition cost, size, and power requirements. The implications for power architecture seem clear; trends in vehicular power design are increasingly influenced by the power demands of electronics for autonomous systems. AI-based vehicles on the drawing boards generally use 48 V as the dominant voltage on the board with the GPU, and the final voltage must be somewhere around 1 V or less. Power levels are already around 1.5 kW and could soon hit 3 kW per GPU. What is less clear is the architecture necessary for getting from 48 V to 1 V at these power levels. There is a case for having isolation between the 48-V input and the point-of-load (POL) converters. It stems from two requirements. The first comes from the telecom industry where 48-V backup batteries supply a minus 48-V rail that must be isolated from the positive rails
VIN
A typical commercial eighthbrick converter based on silicon MOSFETs and its basic specs (top) and the EPC9115 eighthbrick converter using eGaN FETs for all power switches V along with its specs (bottom). IN
eGaN 1/8-brick converter
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31 A
P OUT
300 W
η MAX
96.1%
VIN
38-55 V
VOUT
12 V
I OUT
42 A
P OUT
500 W
η MAX
96.7%
38-55 V 9.6 V
I OUT
31 A
P OUT
300 W
η MAX
96.1%
38-55 V
VOUT
12 V
I OUT
42 A
P OUT
500 W
η MAX
96.7%
Bottom view
Top view
20
9.6 V
I OUT
VOUT
VIN
38-55 V
VOUT
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2/19/19 3:30 PM
POWERING GRAPHICS PROCESSORS
eGaN-based dc-dc converter
necessary to run the digital electronics. The second requirement stems from the range of voltages that might be seen at the nominally 48-V rail. With a range of 36 to 72 V, safety considerations dictate a need for isolation. Limiting the input range to less than 60 V eliminates the isolation requirements. Reducing the input voltage range to between 36 and 60 V (or even less in many cases) also reduces regulation requirements. The primary question is whether it is more economical or efficient to regulate at both the 48 VIN – X VOUT supply and the POL converter, or whether the burden of precise regulation can go only on the POL with the first stage delivering an output voltage proportional to the input voltage. There is no universal right answer to this question, but there are more and more applications that do not require the first stage regulation and operate as dc transformers (DCX). EXAMINING CANDIDATE ARCHITECTURES We will examine four different architectures as well as a spectrum of intermediate bus voltages:
2
A regulated, non-isolated 48-to-12-V and 48-to-5-V buck converter, followed by a 12 or 5-to-1-V buck converter.
3
An un-regulated 48-to-12-V or 48-to-6-V LLC (inductor-inductorcapacitor) topology, followed by a 12 or 6-V buck converter and,
4
A regulated single-stage hybrid converter that goes from 48-to-1 V without an intermediate stage.
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EPC — Power Electronics HB 02-19.indd 21
eGaN converter efficiency
Efficiency (%)
1
The more traditional 48-to-12-V isolated and regulated “brick” followed by a non-isolated 12-to-1-V buck converter.
A simplified schematic and plot of efficiency vs load current for several input voltages of the EPC9115 dc-dc converter.
I OUT (A)
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POWER ELECTRONICS HANDBOOK
eGaN FET power shortage A simplified schematic of the EPC9093 development and a photo of the power stage holding the EPC2053 eGaN FETs.
We start by examining the initial stage of conversion, then look at second-stage efficiencies at various voltages. These results can lead to conclusions about which architecture is most efficient, has the highest power density, and costs the least. Isolated brick-format dcdc converters are widely used in applications that need high reliability, efficiency, and robust performance in complex systems. The isolation takes place via a transformer. This approach has the twin benefits of providing a voltage transformation along with complete separation of input and load ground planes. The former helps maximize efficiency, and the latter is useful for both safety and to prevent or mitigate EMI and ground-loop problems. The state-of-the-art in a DOSA (Distributed-power Open Standards Alliance) standard eighth-brick converter is arguably a design using eGaN FETs. The basic design is a hardswitched, fully regulated PWM 48:12-V buck converter with a full-bridge input and a center-tapped synchronous rectifier output. Typical silicon-based eighth-brick converters show maximum
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output power levels near 300 W and peak efficiencies of approximately 96%. Such converters include custom heat sinks to function at these high-power levels. Such converters typically run at 150 kHz to 175 kHz to mitigate switching losses. The low switching loss of the GaN transistors permit a doubling of the switching frequency to 300 kHz, which allows a great reduction in inductor and transformer size and a large boost in output current and power. The resulting converter is capable of fully regulated operation at an output current of 42 A, or 500 W at a 12-V output. It has realized a peak efficiency of 96.7% at 30 A (the max load current of the siliconbased converter), with a full-load efficiency of 96.4%. The transistors account for 28% of the total loss (approximately 1% of the total output power). On the other hand, the magnetic components account for > 48% of the total loss. Thus the transistors are no longer the main limiting factor in the converter design.
2 • 2019
A view of the EPC2053 100-V eGaN FET with 3-mΩ typical on-resistance.
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POWERING GRAPHICS PROCESSORS
eGaN power stage efficiency
Efficiency (%)
Non-isolated dc-dc converters can be physically small, efficient, and cost effective when based around GaN FETs. In one case, a 25-A, 48 V to 5-12-V converter was constructed using the EPC2053 eGaN FET. The EPC9093 GaN development board, configured as a synchronous buck converter, yields a main power stage area of only 10x9 mm, which is half the size of its silicon equivalents, and can produce an output voltage ranging from 5 to 12 V. The EPC2053 is a Generation 5 eGaN FET rated at 100 V with a 3-mΩ typical on-resistance capable of carrying a 32-A continuous current and operating with up to a 150°C junction temperature. The EPC2053 has lower parasitic capacitances and on-resistance than its silicon counterparts, yielding faster switching and lower power losses even at higher switching frequencies. These qualities enable compact converters to produce high output power. When stepping down 48 V to 12 V at a 700 kHz switching frequency, the EPC9093 hits a peak efficiency of 97% when powering a 15-A load and maintains the efficiency above 96.5% with a 25-A load.
Output current (A)
A simplified schematic of the EPC9093 board and its power efficiency up to a 25-A output current for 5, 9, and 12-V output at a 700 kHz operating frequency using EPC2053 eGaN FETs.
LCC converter
Simplified schematic of an N:1 LLC converter configured with center tapped rectifier. Below, the 1-MHz, 900-W-capable, 48-to-12-V LLC converter along with its dimensions.
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POWER ELECTRONICS HANDBOOK
LCC converter efficiency
Efficiency (%)
Power efficiency as function of output power at 40, 48, and 60-V input voltages. The LLC converter efficiency performance was measured for loads ranging up to 900 W. The efficiency performance shows that the LLC converter easily exceeds 98% over a wide load range and input voltage range.
Output power (W)
The 48-V Step Down LLC dc Transformer employs the LLC topology to provide a high-power density and high efficiency in dc-dc power conversion. This converter can maintain a high efficiency over a wide operating range when operated as a dc transformer with a fixed conversion ratio, making it well suited for applications having relaxed requirements for output voltage regulation. The LLC can operate at frequencies high enough that parasitic elements can serve as circuit components, helping to minimize the physical component count. A nearby figure shows the schematic of a N:1 full-bridge LLC converter with a center-tapped synchronous rectifier. This circuit operates with ZVS for all the switching devices. The primary side devices are EPC2053 transistors, and the secondary rectifiers are the 1.15-mΩ-typical, 30-V-rated EPC2023. A 1-MHz, 900-W-capable, 48-to-6-V LLC converter has also been designed with an 8:1 ratio transformer using a 14-layer PCB and with a magnetizing inductance of 2.2 µH. The primary side devices are EPC2053 transistors and the secondary rectifiers are the 1.15-mΩtypical, 30-V-rated EPC2023, with each rectifier having two devices connected in parallel. A new 48-to-1-V Dual Inductor Hybrid Converter (DIHC), based on the Dickson switched-capacitor converter, has been proposed to address the drawbacks of conventional approaches. The DIHC employs two interleaved inductors at the output and eliminates two large synchronous switches in the hybrid Dickson converter. These modifications let
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POWER ELECTRONICS HANDBOOK
Photo, with dimensions, of the 1-MHz, 900-W-capable, 48-to-6-V LLC converter. The graph shows power efficiency as a function of output power at 40, 48, and 60-V input voltages.
Efficiency (%)
LCC converter efficiency
Output power (W)
Dual-inductor hybrid converter A schematic of the 6-to-1 dual inductor hybrid converter and the physical prototype.
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POWERING GRAPHICS PROCESSORS
Dual-inductor hybrid converter efficiency
Efficiency (%)
Measured efficiency at a 48-V input with different output voltages. Note that at 48 VIN-to-1 VOUT the overall efficiency is 92% at 10 A. The converter efficiency remains higher than 90% down to a 20% load, important for data center applications where light-load efficiency is an issue.
Load current (A)
Commercial buck converter comparison switch conduction and flying capacitors contribute nearly 2X less to the dc output impedance compared to the hybrid Dickson converter and thus 2X smaller conduction losses than the hybrid Dickson converter. In addition, the two interleaved inductors with naturally self-balanced currents give the DIHC the same benefits as in multi-phase converters for high-current applications without additional current balancing complexity. The DIHC also uses a split-phase operation to realize complete softcharging for all the capacitors. A key benefit of the DIHC is that all flying capacitors are soft-charged/ discharged by inductor currents without a hard-charging mode. Because flying capacitors see complete soft-charging, the DIHC can use significantly smaller capacitors without a higher switching frequency. In addition, the inductors can be favorably sized for high power density because of the reduced switch voltage. The smaller capacitors and inductors give the DIHC a high power density and looks promising for highpower and high current applications. eeworldonline.com | designworldonline.com
EPC — Power Electronics HB 02-19.indd 27
VIN
Efficiency
V
%
Manufacturer
Part Number
Frequency
4 5
94
TI
TPS543C20
500 kHz
5
93.5
TI
TPS543C20
500 kHz
8
91.2
Murata
MYMGK00504ERSR
250 kHz
10
89.155
TI
PMP20023 TPS544C25
386 kHz
12
88.5
TI
PMP20023 TPS544C25
386 kHz
12
89
CUI Inc.
NDM2Z-25
320 kHz
12
88.2
Murata
MYMGK00504ERSR
250 kHz
12
89.9
EPC
EPC9059
1000 kHz
Topology comparisons
Attribute
Brick + 12 V Buck (Case 1)
48 V Buck + 12 V or 5 V Buck (Case 2)
LLC + 6 V Buck (Case 3)
Non Iso Reg Hybrid (Case 4)
Efficiency
87%
88%
92%
92%
Density
Low
High
High
Med
Cost
High
Low
Low
Med
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POWER ELECTRONICS HANDBOOK
REFERENCES 1. State-of-the-art in DOSA-standard eighth-brick converters: Glaser, J., Strydom, J. and Reusch, D., “High Power Fully Regulated Eighth-brick DC-DC Converter with GaN FETs,” in PCIM Europe 2015; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management; Proceedings of, 2015, pp. 406–413. 2. Non-isolated dc-dc converters: A. Lidow, J. Strydom, M. de Rooij, D. Reusch, “GaN Transistors for Efficient Power Conversion,” Second Edition, Wiley, 2014. 3. “Boosting Power Density in 48 V to 5-12 V DC to DC Converter Using EPC2053, with up to 25 A Output,” How2AppNote 009, Available at: http://epc-co.com/epc/Portals/0/epc/documents/ application-notes/how2appnote009%20-boosting%20power%20 density%20in%2048%20v%20to%205-12%20v%20dc%20to%20 dc.pdf 4. 48-V step-down LLC dc transformer: “Exceeding 98% Efficiency in a Compact 48 V to 12 V, 900 W LLC Resonant Converter Using eGaN® FETs,” How2AppNote 011, Available at: http:// epc-co.com/epc/Portals/0/epc/documents/application-notes/ How2AppNote011%20Exceeding%2098%20percent%20 Efficiency%20in%20a%20Compact%2048%20V%20to%2012%20 V%20Resonant%20Converter.pdf 5. “How to Exceed 98% Efficiency in a Compact 48 V to 6 V, 900 W LLC Resonant Converter Using eGaN® FETs,” How2AppNote 014, Available at: http://epc-co.com/epc/Portals/0/epc/documents/ application-notes/How2AppNote014%20Exceed%2098%20 percent%20Efficiency%2048%20V%20to%206%20V%20 Resonant%20Converter.pdf 6. N:1 full bridge LLC converter: Huang, H., “Designing an LLC Resonant Half-Bridge Power Converter,” Reproduced from 2010 Texas Instruments Power Supply Design Seminar SEM1900, Topic 3, TI Literature No. SLUP263 7. Ahmed, M. H., Fei, C., Lee, F. C. and Li, Q., “48V voltage regulator module with PCB winding matrix transformer for future data centers,” IEEE Trans. Ind. Electron., vol. 64, no. 12, pp. 9302-9310, Dec. 2017. 8. 48V-to-1V Hybrid Converter: G.-S. Seo, R. Das, ad H.-P. Le, “A 95%-efficient 48 V-to-1 V/10 A VRM hybrid converter using interleaved dual inductors,” in Proc. IEEE Applied Power Electronics Conference and Exposition (ECCE), 2018. pp. 38253830 9. Y. Lei, R. May, and R. Pilawa-Podgurski, “Split-Phase Control: Achieving Complete Soft-Charging Operation of a Dickson Switched- Capacitor Converter,” IEEE Transactions on Power Electron., vol. 31, no. 1, pp. 770-782, 2016. 10. D. Baba, “Benefits of a multiphase buck converter,” Texas Instruments Inc., 2012.
OPTIMIZING THE INTERMEDIATE BUS VOLTAGE We can now turn to the efficiency of the second stage. Multiplying the peak reported efficiency of various commercial POL converters with the efficiency of the first stage converters gives a reasonable estimate of comparative peak efficiencies going from 48 to 1 V. The best efficiency comes from coupling either the DIHC or the LLC with a 6-VOUT second stage. The DIHC topology, however, is relatively new and has yet to be widely adopted. New AI and gaming applications are quickly adopting the 48-VIN-to-6-VOUT LLC, coupled with a 6-VIN-to-1-VOUT buck converter because of its high efficiency, high power density, and low cost. Traditional bricks are going to be around for many more years but are probably not going to see much growth in leading-edge high-density computing systems. In all the topologies with 48 VIN, the highest efficiency comes with using GaN devices. This is due to their lower capacitance and smaller size. With recent pricing declines in GaN power transistors, the cost comparison with silicon-based converters now strongly favors GaN in all the leading-edge solutions.
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The Ultimate Power Couple High Current MSD Series
Low Profile LPD Series
With their high K and small size, these 1:1 coupled inductors are the perfect match for your SEPIC and flyback applications Offered in thirteen body sizes and hundreds of inductance/current rating combinations, our MSD/LPD families are perfectly coupled to all your SEPIC and flyback designs. The MSD Family offers current ratings up to 16.36 Amps, low DCR, coupling coefficients as high as K ≥ 0.98, and up to 500 Vrms windingto-winding isolation.
With footprints as small as 3.0 mm square and profiles as low as 0.9 mm, the LPD Family offers current ratings up to 5.6 Amps, DCR as low as 0.042 Ohms and coupling coefficients as high as K ≥ 0.99. You can see all of our coupled inductors, including models with turns ratios up to 1:100, at www.coilcraft.com/coupled. ®
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POWER ELECTRONICS HANDBOOK
Power operating limits: How to choose the right power supply Not all power supplies react to over and under-voltages and currents in the same way. It pays to know the tradeoffs of different approaches to protection.
POWER
supplies can experience operating conditions outside normal specified limits, such as input under- or over-voltage, or variations in load and ambient temperature. These conditions can cause responses such as shutdowns, performance degradation, or component failures. To minimize such difficulties, product designers must know how their supply will perform outside its specified limits. At the power supply input, voltage fluctuations on the ac supply line can over-stress mandatory protection and filtering components such as X-capacitors, Y-capacitors and metal oxide varistors (MOV). These all have known failure modes when exposed to voltages above their rated maximum. X-capacitors, for example, are designed to fail short and will typically open the fuse, shutting down the power supply. Y-capacitors, on the other hand, are designed to fail open. This fault may go unnoticed for some time, although the capacitor will cease to filter common-mode noise effectively. The effects of over-voltage on the fuse can depend on the fuse voltage rating or its withstand voltage. If the voltage across the fuse exceeds this rating, arcing may prevent the fuse from protecting the circuit as intended.
Typical ac input network
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RON STULL | CUI INC.
This condition increases the risk of fire and can cause problems at the input or in downstream circuitry. Over-voltages may also interact with parasitic elements in the power supply circuitry, possibly boosting voltage-related stress on power semiconductors. In a flyback converter, the peak voltage across Input components the power switch vulnerable to voltage is determined by a stress typically include combination of the input those for protection voltage and output and filtering such as voltage as well as the X-capacitors (CX1 and transformer turns ratio CX2), Y-capacitors (CY1 and leakage inductance. and in the figure CY2), This peak voltage can and metal oxide varistors be difficult to calculate (MOV). All have known and typically must be failure modes when measured directly. exposed to voltages Conversely, above their rated under-voltage causes maximum. X-capacitors, higher currents in also known as “acrosscomponents such as the-line capacitors,” are the fuse, rectifier and used between the wires carrying the incoming ac current. A capacitor failure in this position will usually cause a fuse or circuit breaker to open. Y-capacitors, also known as “line-to-ground capacitors,” are used where capacitor failure could lead to the danger of electrical shock if the ground connection is lost.
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2/19/19 3:34 PM
Efficiency
Power dissipated (W)
Efficiency & power dissipation
THE RIGHT POWER SUPPLY
In this example, the efficiency of a 200-W power supply falls by 1% below peak when operated at 20% above rated power, causing power dissipation to rise by 30%. Thus a small change in efficiency increases power dissipation exponentially.
Output power (W) Efficiency
power switches. The result can be extra internal heating that can lead to rapid failure or poor reliability. High current can also introduce a loss of inductance or a saturation of magnetic components such as the PFC (power factor correction) choke. In some topologies, such conditions can cause potentially damaging peak currents in power switches, a rise in their operating frequency, a loss of energy efficiency, or the power supply may shut down. In other topologies, low input voltage can affect the operating frequency or duty cycle and cause a supply malfunction. In LLC resonant converters, a varying of the operating frequency regulates the output voltage. If the input voltage drops, the frequency slows to boost the input-to-output gain and keep the output voltage stable. However, there is a minimum frequency below which further reduction lowers the gain and so can cause malfunctioning or power-supply failure. Overvoltage can also affect PFC circuitry. A boost-PFC converter will cease to regulate if the input voltage rises above the output. Of course, there are several ways to protect the power supply against excessive input-voltage variations. High-wattage power supplies often feature brown-out protection to initiate shutdown if the input voltage falls below a specified threshold. Other protection mechanisms allow the power supply to continue operating, although the performance may suffer. An LLC converter, for example, may clamp the operating frequency at a minimum threshold to prevent malfunctioning. While this helps protect the power eeworldonline.com | designworldonline.com
CUI v2 — Power Electronics HB 02-19.indd 31
Power dissipation
supply from failure, it will cause a loss of regulation at the output. OUTPUT OVER-CURRENT To minimize cost or reduce bulk, designers may be tempted to size the power supply for typical load requirements without considering transient load currents above the chosen rating. Most power supplies contain over-current protection, but there are various types. Some have a well-defined current limit close to the maximum output rating. A too-close current limit can force the supply to shut down frequently. Other schemes are more flexible and allow short-term output currents to exceed the rated limit. But supplies with this type of protection (or those with no protection) can experience a temperature rise due to over-current that can degrade performance or cause a failure of MOSFETs, diodes, resistors or even copper traces. Note that power dissipation rises linearly with current in diodes, due to their fixed voltage, and exponentially in MOSFETs and resistive components. Chokes and transformers have a more complex response to overcurrents. Besides causing internal heating from coil resistance, over-currents can cause higher core losses and magnetic saturation, worsening power dissipation and heat rise. Saturation may also stop the power supply from operating or make component failures more likely. In a buck converter, where the ripple current is directly related to inductance, loss of inductance due to saturation causes higher currents in the MOSFET and diode.
2 • 2019
The effects of parasitic inductances such as transformer leakage should also be considered. These effects can produce voltage spikes when switches change state, becoming larger at higher loads. An excessive spike can destroy a MOSFET or may cause current or voltage sensors to send inaccurate information to the controller, potentially harming performance or causing failure. Variations in power supply efficiency, particularly near the maximum rated load, also affect performance and reliability. Efficiency typically peaks below full load. Beyond the peak, efficiency falls, raising power dissipation exponentially as the load rises. The rising dissipation can not only heat up components but also prevent compliance with mandatory efficiency regulations. Another consideration is load regulation – the maximum change in output voltage from no-load to full-load. Operation beyond the specified load range can drop the output voltage below the regulated voltage limit. Some smaller power supplies also specify a minimum load current. Operating the unit below this limit can compromise regulation in the same manner as exceeding the maximum load current. CONSIDERING THE ENVIRONMENT Power supply selection must also consider environmental conditions. Excessively high or low temperatures can profoundly degrade performance and reliability. Some components, such as electrolytic capacitors, may see a 50% cut in useful life with just a 10°C rise in ambient temperature. Alternatively, low temperatures can cause problems such as embrittlement of solder joints, connections or component leads, resulting in early failure. Both upper and lower operating temperature limits are defined to ensure components will operate as their manufacturers specify. Their performance cannot be DESIGN WORLD — EE NETWORK
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Output voltage regulation
In some supplies, operating beyond the specified load range can cause the output voltage to fall below the regulated voltage limit. Some smaller supplies also have a minimum load-current below which regulation falls out.
Output voltage (v)
POWER ELECTRONICS HANDBOOK
Load (W)
Capacitance change (%)
Aluminum electrolytic cap temperature dependence
Temperature (°C) Typical behavior of an aluminum electrolytic capacitor over temperature.
guaranteed outside of this range. Ignoring temperature limits may degrade the power supply efficiency, output ripple, regulation or noise-emission parameters. Key components of the power supply may have either a positive or negative temperature coefficient (PTC or NTC). MOSFETs are PTC devices whose onresistance rises with rising temperature. Bridge rectifier diodes, on the other hand, are NTC devices; as the forward voltage drops with increasing temperature, so, too, does internal power dissipation and heat generation. Depending on the individual power supply, either the NTC or PTC devices will dominate as the temperature changes, causing the overall supply efficiency to rise or fall. Resistors used to sense operating conditions and control the power supply typically carry little current, so they are generally not vulnerable to excessive heating or power dissipation. But temperature change affects their resistance value. The resistance change may lead to unwanted changes in power-supply parameters such as the regulated output voltage. Other effects can include early or late triggering of protection mechanisms that depend on sensed current.
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At low temperatures the capacitance of electrolytic capacitors drops, resulting in more ripple current or a failure of the power supply to start up. In addition, the resistance of NTC devices, such as inrush-current limiting thermistors, will rise as the ambient temperature falls, which can reduce efficiency or prevent start up. Some power supplies contain overtemperature protection and will shut down until the temperature drops within the specified limit. Others may only include protection for specific components or sub-circuits, a practice which can cause problems if some parts of the power supply shut down while others continue operating. PTC devices such as MOSFETs are usually designed-in with some safety margin to protect against over-temperature. However, the margin depends on operating conditions such as input voltage and may be narrower in some parts of the operating range than others. Finally, designers should investigate the effects of over- or under-voltage on electromagnetic emissions (EMI). Over- or under-voltage at the input, or output over-current, can alter the properties of EMI-filtering components or cause over-stress that can impair their performance. Although difficult to estimate, the effect can be significant and may result in a failure to comply with EMC regulations. In all, designers must understand how a power supply may respond to all changes in input, output or environmental conditions, whether they are within specified limits or not. This kind of information helps designers assess system performance, reliability, longevity and compliance with technical regulations. References ac-dc power supplies, https://www.cui.com/ catalog/power/ac-dc-power-supplies dc-dc converters, https://www.cui.com/catalog/ power/dc-dc-converters
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2/19/19 3:35 PM
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POWER ELECTRONICS HANDBOOK
Why wide bandgap HEMPTs excel at efficient power conversion High electron mobility transistors reduce power supply size thanks to a special make-up that
ANDREA BRICCONI | WEI DENG INFINEON TECHNOLOGIES AG
eliminates sources of energy loss.
IT
is an exciting time for power supply designers. We are learning how devices based on wide bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride-on-silicon (GaN-on-Si) open the door to new capabilities, often through new topologies. Broadly speaking, the advantages of both SiC and GaN-on-Si stem from lower on-resistance, the ability to operate at higher switching frequencies, and improvements in such important figures of merit as heat tolerance. The resulting advances in efficiency and lowering of losses bring higher power densities and better reliability in end products. That said, silicon will continue to be a mainstream power technology, providing a good combination of power transmission and switching speeds. SiC is more suitable for higher voltages in the region of 600 V to 1.7 kV, at switching frequencies approaching 1 MHz. GaN-onSi is excels at switching voltages between 100 and 600 V at frequencies far in excess of 1 MHz. One major difference between these two WBG materials is that transistor structures (i.e., MOSFETs, diodes, etc.) based on SiC resemble those for silicon . With GaN-on-Si, we can fabricate high electron mobility transistors (HEMT) that bring new advantages for power electronics. HEMT structures are significantly different than traditional MOSFETs, and it is worthwhile to explore the basis for this new structure. A material’s bandgap is fundamental to its ability to carry a charge. The bandgap refers to the difference in energy between the valence band and conduction band (hence bandgap), between which electrons must pass
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during current flow. It is effectively the property that determines whether a material is a conductor, insulator, or semiconductor. Bandgap is measured in units of electron volts (eV), where a smaller number indicates a smaller gap and a better conductor. Semiconductor materials including silicon have a bandgap of between 1 and 4 eV, while a bandgap above 4 eV would typically indicate the material is an insulator. While silicon is a narrow bandgap material with an eV of just over 1, WBG materials such as SiC and GaN have an eV of between 2 and 4. This gives them advantages over silicon semiconductors that can be exploited for power conversion applications. The mobility and abundance of charge carriers also determines the semiconductor properties of a material. (As a quick review, charge carrier mobility is the speed at which the charge carriers move in the material in a given direction, in the presence of an applied electric field.) If there is a great abundance of charge carriers, the material will conduct well even if its charge carriers have a low mobility. Conversely, if a material’s charge carriers exhibit high mobility, it can be a good conductor even if there are relatively few charge carriers present. In this respect, the wide bandgap GaN matched with a narrow-bandgap silicon substrate is an ideal platform for transistors with a high level of electron mobility, hence the term HEMT. DELIVERING HEMTS One of the main structural differences between MOSFETs and HEMTs involves the direction of current flow. In a MOSFET, the charge carriers predominantly flow
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2/19/19 3:36 PM
POWER CONVERSION EFFICIENCY
Application roadmap for WBG power
vertically. In HEMTs, such as the Infineon CoolGaN series, the charge carriers flow laterally. The direction of flow is due to the way the underlying silicon substrate interacts with the upper GaN layer. This interface is where much of the design innovation has happened in recent years, allowing GaN-on-Si to become a production-ready, high volume solution. The CoolGaN fabrication process involves epitaxial growth of the transition layers on the silicon substrate to provide an efficient interface between the dissimilar crystalline lattice structures of silicon and GaN. This special interface overcomes the issue of an otherwise mismatched thermal expansion coefficient between the two materials. Further layers are deposited over the transition layers, including
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Infineon — Power Electronics HB 02-19.indd 35
A graph of switching frequency vs. power output, and the applicable power technologies, illustrates how wide bandgap (WBG) materials both complement silicon and offer new capabilities for power electronic devices. Transistors based on a range of substrates will have a place in future power conversion and management applications.
p-doped GaN, to form the transistor structure. Metallization and passivation steps complete the structure. The addition of a p-doped gate results in a higher threshold voltage, effectively making the device a normally-off enhancement mode transistor. This feature can render the device sensitive to gate over-voltage, an issue overcome by developing a selfclamping p-type gate structure. The GaN-on-Si HEMT has a conduction path that joins the drain and source, formed by the heterojunction created between the GaN and AlGaN layers. This bidirectional current path, referred to as the two-dimensional electron gas, or 2DEG path, is controlled by the gate potential which, as explained earlier, is held high enough
2 • 2019
to make the device normally-off and operate in enhancement mode, the preferred format for power electronics. The HEMT structure of the CoolGaN series delivers several key benefits over the superjunction MOSFETs commonly used in power applications. First, they are inherently more immune to the effects of temperature. This temperature tolerance brings a lower temperature coefficient of the onresistance, RDS(ON). A lower temperature coefficient – around 2.0 for GaN-onSi vs. 2.4 for silicon – reduces the conduction loss, which also contributes to the ability to operate at higher voltages. As mentioned earlier, GaN-on-Si HEMTs can switch at higher frequencies than MOSFETs based on silicon or SiC.
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POWER ELECTRONICS HANDBOOK
GaN-on-Si HEMT cross section
A cross section of the GaN-on-Si HEMT structure developed by Infineon for its CoolGaN family. The GaN-on-Si HEMT has a conduction path that joins the drain and source, formed by the heterojunction created between the GaN and AlGaN layers. This bidirectional current path, referred to as the twodimensional electron gas, or 2DEG path, is controlled by the gate potential which is held high enough to make the device normally-off and operate in enhancement mode, the preferred format for power electronics.
The reason is GaN-on-Si HEMTs lack an intrinsic body diode between the drain and source. The absence of a body diode is fundamental to what is probably the most important feature of GaN-on-Si, which is their vastly better reverse-recovery performance. In other transistor structures, a body diode significantly limits switching performance. The effect of a body diode in other forms of transistors results in a reverse recovery charge which has with it a peak current. Reverse recovery charge is the charge that accumulates in the PN junction of a MOSFET’s forward biased body diode. In most applications, current flows through the body diode twice for each switching cycle, causing charge to build up. Peak current in topologies that feature repetitive reverse recovery becomes inhibiting. The absence of a body diode and its associated reverse recovery charge and peak current is essential to the highfrequency operation of GaN-on-Si HEMTs. The ability to operate at high frequencies, in turn, allows them to make new switching topologies practical as well as play a role in halfbridge converters.
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Furthermore, device output capacitance largely controls the rate-of-change of the voltage across the transistor, VDS. Superjunction MOSFETs based on silicon have an output capacitance that is highly non-linear. Initially, voltage changes slowly and then exhibits accelerated change before reverting to a slower rate of change as it approaches its working maximum. In a GaN-on-Si HEMT, the output capacitance is nearly linear with a variance that is around a tenth of that seen in superjunction silicon power transistors. This linearity also contributes to the higher switching performance of GaN-on-Si. A direct result of this faster switching in conversion topologies is a reduced dead-time; the time between the change in the conduction path in a half-bridge configuration. Dead-time is an essential safety feature in converter topologies to avoid short-circuits between the two power rails. No conduction takes place during this time, which lowers the converter’s efficiency. In other types of transistors that cannot switch as quickly, the dead-time must be carefully calculated to avoid short-circuits. Because GaN-on-Si HEMTs switch faster, the dead-time can be shorter, boosting overall performance. eeworldonline.com | designworldonline.com
2/19/19 3:37 PM
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POWER CONVERSION EFFICIENCY
Also important is the overall system solution. Conventionally, the size of the passive components in a switching topology scale down with higher switching frequency. GaN-on-Si HEMT operating at higher frequencies can use smaller passive components, increasing the power density. The resulting footprint is smaller than an equivalent silicon MOSFET design. Better power density is perhaps the single most consistent demand in power conversion. GaN-on-Si HEMTs will bring power conversion that make possible smaller end devices requiring less energy. Sustainability is also important and is driving the development of more efficient solutions based on SiC and GaN-on-Si. All in all, Infineon
CoolGaN technology, complemented by Superjunction MOSFET CoolMOS and CoolSiC solutions, make up a complete portfolio of power semiconductors for every application.
REFERENCES Infineon Technologies AG, www.infineon.com
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POWER ELECTRONICS HANDBOOK
Power supply designers
take a hard look at soft magnetics The promise of high efficiency and small size
LELAND TESCHLER | EXECUTIVE EDITOR
brought by super-fast switching power supplies could be delayed by a lack of magnetic materials that are up to the task.
EYEBALL
the schedule for APEC, considered the premier technical conference for power electronics, and you’ll notice a number of sessions devoted to magnetics. The interest in magnetics stems from gallium-nitride and silicon-carbide semiconductors. These wide-bandgap semiconductors have “on”resistances and related losses that are one-tenth that of devices based on silicon which reduces both their inherent capacitance and resistor-capacitor time constant. The result is lower losses per switching cycle, a consequence that lets GaN and SiC-based switch-mode power supplies switch at much higher frequencies – sometimes on the order of hundreds of megahertz. Therein lies the reason for the APEC sessions on magnetics. Operation at these higher frequencies can drastically reduce the size of the magnetic components involved in energy storage, but it is tough to find magnetic materials that can operate at such frequencies without experiencing a lot of energy loss. So the quest is on to find better ones.
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Cores on inductors like these become more difficult to devise as the operating frequency rises. Researchers are now examining exotic candidate materials such as nano composites in an effort to handle power supply switching frequencies extending into the hundreds of megahertz.
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2/20/19 9:27 AM
SOFT MAGNETICS
Magnetization curve for a generic soft magnet A typical magnetization curve for a soft magnet with key parameters labeled: Ms, or the saturation magnetization; Mr, the magnetization remaining after an external field is removed; Hc, the value of the magnetic field necessary to remove magnetization after the magnetic material has saturated; and Χi, the initial magnetic susceptibility.
To understand the problem, and to comprehend some of the finer points made at APEC presentations, it pays to be familiar with the fundamental parameters that describe magnetic materials and their energy storage capacity. Several materials now under investigation show promise for improving some of these parameters but generally at the expense of others. HARD VS. SOFT MAGNETICS An easy way to introduce magnetic parameters is to start with the difference between hard magnetics and soft magnetics. Hard magnetics are permanent magnets. Two parameters distinguish these materials: Their coercive field, Hc, which is the strength of the magnetic field necessary to remove their magnetization; and Mr, which is the magnetization in the material that remains after its removed from an external field. For permanent magnets, Hc is huge (greater than 1,000 A/M) and so is Mr. These two properties eeworldonline.com | designworldonline.com
Soft Magnetics — Power Electronics HB 02-19.indd 39
are what allow permanent magnets to magnetize other materials. In contrast, inductive applications in power electronics need materials that have a low Hc and little Mr. The reason is that inductors in power circuits must be made of materials having a magnetization that will switch rapidly under the influence of a magnetic field created by current-carrying coils of wire. Soft magnets have these qualities. Soft magnets have two other properties that are important for inductive applications. First, they have a high relative magnetic permeability, µr. Magnetic permeability is the ability of a material to support the formation of a magnetic field within itself. It is often expressed as the ratio of a material’s permeability to that of free space, µ0, so that µr = µ/ µ0. The high µr of a soft magnet used in an inductor concentrates the magnetic field lines inside the winding of an inductor. It does so by orders of magnitude more than that of an air core. 2 • 2019
This concentration of magnetic field lines allows more storage of energy in the form of magnetic flux density. Of course, it’s desirable to fabricate soft magnetic devices that can handle magnetic fields that are as high as possible. The maximum magnetic field a soft magnet can handle is a function of another parameter called Ms or saturation magnetization. This is the highest amount of magnetization the material can support. TRADEOFFS IN SOFT MAGNETS Though soft magnetic cores boost energy density, they also are a source of energy loss, particularly as frequencies rise. A lot of the work going on in soft magnetic materials aims at reducing two kinds of losses, hysteretic and eddy current. The first kind of energy loss to consider is that arising because of hysteresis. Every time a soft magnet is magnetized and demagnetized, it traces out a magnetization curve. The area inside the curve is a measure of the energy lost. The property that gives rise to the magnetic hysteresis curve is magnetic coercivity. It is defined as the degree to which a material withstands an external magnetic field without becoming demagnetized. In other words, it is how much a material resists DESIGN WORLD — EE NETWORK
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See Us At APEC ‘19 • BOOTH 252 Anaheim, CA March 17-21
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KEY SPECIFICATIONS • • • •
Min inductance: 2 to 67.5 mH RMS current: 0.4 to 7.5 A Max DC resistance: 0.5 to 2.02 ohm Min leakage: 1.1 to 720 uH
Triad’s CMT908 Series Common Mode Inductor features a sealed encapsulated design that is ideal for rugged environments and where reliability is critical. It provides superior common mode EMI noise suppression for power supplies and other power devices.
KEY SPECIFICATIONS • • • •
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KEY SPECIFICATIONS • • • •
Rated Current: 0.45 to 2.3 A Rated Inductance: 10 to 100 mH Resistance: 279 to 1970 mOhms Stray Inductance: 200 to 2100 mH
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Triad — PE 02-19.indd 40
Triad_Pow_Noise_Suppr_PE_Mar_19_F.indd 1
Triad’s CMT Series Common Mode Inductors provide common mode EMI suppression for power supplies. They feature our precision coil windings to eliminate noise and to minimize the AC line transmitted interference often created by high frequency switching power supplies. They also meet various regulatory requirements and are constructed with UL rated 130°C materials.
Inductors help prevent EMI/RFI in a wide range of applications. Features include low leakage flux, high self-resonant frequency, high impedance at applicable frequency and low stray capacitance in section winding. Precision coil winding ensures excellent performance and reliability
APPLICATIONS • • • •
Min. Inductance: 0.45 to 120 mH Max. Inductance Difference: 100 to 2500 uH Max DCR: 0.1 to 3.7 Ohm Current Rating: 0.5 to 4 A
KEY SPECIFICATIONS • Min inductance: 1 to 50 mH • Max current: 1.7 to 20 A • Max DCR: 0.006 to 0.45 ohm
2/20/19 8:54 AM
2/4/19 2:52 PM
SOFT MAGNETICS
USEFUL TERMS IN POWER MAGNETICS demagnetization. Coercivity can also be viewed as the intensity of the applied magnetic field necessary = magnetization or volume density to reduce the magnetization of that material to zero M of the magnetic moment after the material’s magnetization has been driven to = saturation magnetization = saturation. Ms material’s max magnetization A point to note about the hysteresis curve is that it is traced out for each cycle of ac through the = magnetization remaining after Mr external field is removed inductor. So there are hysteresis losses for every ac cycle. Thus even soft magnetics with a low coercivity = magnetic permeability of vacuum µ0 (and a small hysteresis) can become lossy as = magnetic polarization J ( = µ0 M) operating frequencies rise. = M/H = magnetic susceptibility, H is A second major mechanism for losses in soft χ = M/H the magnetic field magnetics arises because of eddy currents. These are closed paths of electrical current generated in µr and χ indicate how much magnetic µr = µ/ µ0 = 1 + χ field it takes to change the direction of a conductor because of a time-varying magnetic magnetization in a given material. field. These current loops create a magnetic field opposing the change in magnetic flux. = how much magnetic field it takes to Hc Power losses from eddy currents rise with the remove magnetization after saturation, square of operating frequency. Thus they can be the called coercive field. main source of energy loss at high switching rates. The losses arising due to the combination of hysteresis and eddy current losses are sometimes referred to as core losses, particularly in the context Another point to note is that soft ferrites are of electric motors. Core losses in electric motors don’t ferrimagnetic. Most other soft magnetic materials are depend on the amount of the motor load. As outlined above, ferromagnetic. The main difference between the two lies with they depend on properties of the magnetic material, the flux their magnetic domains. In ferromagnetics, the vector sum density, and the switching frequency (which determines the of magnetic moments from all the domains results in what’s rate of change of flux density). Estimates are that core losses called a spontaneous magnetization that is zero. account for about 25% of the total losses in conventional ac In ferrimagnetics, the magnitude of the domain spins are electric motors but are much higher for motors operating at such that there’s a non-zero spontaneous magnetization of higher rpm. the material. The difference between the two phenomenon In power electronics, the most widely used soft generally isn’t particularly important unless you are studying magnetic material is ferrite. The good thing about ferrites material properties rather than applying magnetic material is their high electrical conductivity (10 to 108 µΩ-m) which in real circuits. But the ferrimagnetic nature of ferrites gives lets them keep eddy currents low. They are also relatively them a relatively low Ms, typically 0.4 MA/m or less at room inexpensive to produce because they are made like temperature. In addition, Ms in ferrites is more temperature ceramics. sensitive than in ferromagnetic materials. A point to note is that there are both hard ferrites Their high conductivity and a constant µr that extends and soft ferrites. Hard ferrites have a high coercivity and well into the megahertz frequencies will probably let ferrites are used to make ferrite magnets. Soft ferrites are what continue to find use in applications involving both a low go into inductor cores. The most common soft ferrites energy density and low magnetic polarization. Research into are manganese-zinc ferrite and nickel-zinc ferrite. MnZn improving their performance includes engineering their grain ferrites have magnetic permeability and saturation levels boundaries and embedding them in special substrates. exceeding those of NiZn ferrites and generally find use at Another class of soft magnetic material getting attention switching frequencies below 5 MHz. NiZn ferrites have a is that of amorphous alloys. More specifically, soft magnetic higher resistivity which makes them good candidates for amorphous alloys generally consist of nanograins of applications above about 5 MHz. ferromagnetic material embedded in an amorphous matrix Materials makers tweak the properties of ferrites by (glass ceramic). This material usually comes in the form of adjusting material composition and processing, adjusting thin (generally 5 to 50 µm) ribbons. The thin cross section is a factors such as temperature stability and magnetic function of the fabrication process which involves cooling the permeability. material too fast for it to form large crystals. eeworldonline.com | designworldonline.com
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Amorphous soft magnetic material has a high conductivity. This and the thin layers that typically characterize its form factor keep eddy current losses low. There are a lot of applications for this material at frequencies up into the tens-of-kilohertz range. Some of the formulations of amorphous soft magnets use nickel and cobalt compounds to boost Ms and the Curie temperature of the amorphous and crystalline phases, thus allowing higher operating temperatures. Nickel is less expensive than cobalt, so it gets more attention among researchers. One difficulty associated with amorphous material is that it is quite hard, so hard that it would rapidly wear out conventional cutting tools. Water-jet cutting, EDM, and grinding operations have all been used to shape amorphous magnetic cores. But all in all, amorphous materials tend not be used in situations demanding magnetic material with complicated geometries. In inductor cores where the geometry isn’t complex, however, amorphous soft magnets can boost energy efficiency. The classic example is in cores for distribution transformers. Distribution transformers provide the final utility voltage step down, reducing the voltage in the distribution power lines to the level used by businesses and residences. Distribution transformers are energized 24/7 even when not powering a load, so it pays to minimize their core losses. Estimates are that about 10% of the distribution transformers in the U.S. now use amorphous magnetic cores. Another class of soft magnetics is comprised of composite powders. Soft magnetic composites (SMCs) use micrometer-scale iron and iron alloys such as Fe-P, Fe-Si, and FeCo. Research is also underway in nanometer-sized SMCs though none are yet commercially available.
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SMCs offer a potential energy savings because their composite structure can be manipulated to reduce losses during magnetic cycling. Embedding conductive magnetic particles in an insulating matrix boosts conductivity (10−3 to 10−1 µΩ-m) and reduces eddy current losses, although magnetic hysteresis losses rise. This increase in the coercive field arises because the particles retain some mechanical stresses. It’s thought that moving to nanometer-sized particles could make it easier to anneal away such stresses. Use of magnetic composites also makes it possible to tune the material’s µr and, in turn, its magnetic saturation qualities. Usually, circuit designers try to avoid saturating inductive cores by limiting the applied current, reducing the number of windings, making the core bigger, or giving the core an air gap to reduce µr. It turns out that spacings of SMC nonmagnetic material in the direction of magnetization effectively make up distributed air gaps. Distributed gaps avoid the magnetic flux leakage that arises in a core with
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a discrete air gap; only a negligible magnetic flux extends beyond the core. Additionally, this sort of distributed air gap also promotes soft saturation; µr drops slowly as saturation approaches, making saturation less disruptive. SMCs also have a relatively simple manufacturing process that involves mild forces and low temperatures. For example, an iron-powder SMC core typically goes through mild compaction and moderate heating of about 200°C for organic matrices and around 500°C for inorganic matrices. They can also be compacted into complex shapes without any extra machining. And their uniform physical make-up also lets composites suppress eddy currents equally in all directions. Right now, there’s a great deal of interest in formulating SMCs with nanoparticles. One reason is that reducing the particle size would cut eddy current losses to negligible levels. The problem is that the physics at these nano dimensions get complicated. For example, hysteresis losses have a complex relationship to particle size. The stresses and surface defects in
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SOFT MAGNETICS
micrometer-scale materials make hysteresis worse and worse still as particles approach the size of a single magnetic domain. The reason is single-domain particles do not reverse their magnetization via the usual means, by domain wall motion. Instead, it takes place through rotation of the magnetic moments within each particle. It takes a lot of energy to get this rotation in the largest single-domain particles. When the nanoparticles reach a critical size, generally in the tens of nanometers, there can be enough thermal energy to freely reorient the particle’s direction of magnetization. The temperature above which this is true is called the superparamagnetic blocking temperature. The particles no longer
exhibit magnetic hysteresis and are referred to as superparamagnetic. This situation sounds good from the standpoint of a core material; there’s no Hc and µr can be high. The weakness of the scheme is that there must be enough nonmagnetic matrix material to hold the individual nanoparticles apart. If magnetic nanoparticles come close together, they can magnetically couple and form ferromagnetic domains, leading to domain walls and magnetic hysteresis. Researchers figure nanocomposite SMCs probably can’t be much more than 50% magnetic material. Still, the nanocomposite idea looks like a promising way of making materials with low losses.
REFERENCES Pre-APEC Power Magnetics @ High Frequency all-day workshop, https://www.apec-conf.org/ news/artmid/437/articleid/35/ registration-open-pre-apecworkshop-on-power-magneticshigh-frequency-by-psma-andieee-pels-on-march-16-2019
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Soft Magnetics — Power Electronics HB 02-19.indd 43
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LED bulbs then and now: Teardown of the EcoSmart 60-W equivalent LED bulb Compared to the LED bulbs of only a few
LELAND TESCHLER | EXECUTIVE EDITOR
years ago, modern-day versions are simpler and assembled via more automated methods.
IF
you tore down an LED bulb manufactured a few years ago you’d likely find evidence of hand soldering and kludgy design practices. We found both in evidence when we examined LED bulbs back in 2015. We took bulbs from several manufacturers that all received the highest rankings from Consumer Reports. Several of them used epoxy potting material apparently to both add stability to the screw threads and to help manage thermal dissipation. A number of these mass-produced bulbs also displayed evidence of hand soldering. The most typical location was in making a connection between the LED plate and the circuit board holding the bulb electronics, but some bulbs contained other instances of solder globs that looked as though they had been done by hand. Back then, it was also common to see bulbs carrying sizable heat sinks. Many of the bulbs we looked at had metal heat-spreading components weighing in at a few ounces. And the circuitry driving the LEDs tended to be comprised of at least a dozen discrete components placed around the LED driver IC.
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LED — Power Electronics HB 02-19.indd 44
LED bulb assembly practices circa 2015 included epoxy potting material used as a structural element for the bulb screw threads and base contact, beefy metal heat sinks (top), and handsoldered wire connections to the LED plate.
Things have changed quite a bit in four years. We recently procured a new batch of 60-W equivalent LED bulbs to see the advances that have ensued since 2015. Like the last batch, these, too, were selected because they all got high ratings from Consumer Reports. Consider the EcoSmart A19 LED bulb, which is assembled in China but comes from the Lighting Science Group in Florida. This 9.5-W bulb illustrates how simple LED bulb electronics can be so long as the bulb doesn’t need to be dimmed. Cut away the translucent plastic cover and you’ll find a dozen LEDs sitting on the standard metal plate. The plate attaches to the bulb’s plastic housing via two Philips screws and to the PCB electronics via two connectors.
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LED BULBS THEN AND NOW
The EcoSmart LED bulb with and without its plastic translucent dome. Visible on the LED plate are the two simple connectors used to attach the LEDs to metal prongs on the PCB.
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LED — Power Electronics HB 02-19.indd 45
The connectors are worth a comment. They are simple and apparently inexpensive. They make a connection when metal posts on the PCB are pushed through them. They also seem to be designed for one-time use, fine for an LED bulb. Simple though they are, these connectors are in contrast to the typical LED plate connection scheme we found in Chinese-made bulbs four years ago -- the typical means of electrical connection was via soldered wires. Also interesting is the method used for connecting the electronics to the bulb base contact and the metal screw threads. A press-fit connector makes contact with a metal post extending up from the base contact. The metal screw thread connection is via a metal strap that puts a spring-load against it for contact. The metal screw threads and base post are one assembly that presses on to the plastic housing that supports the translucent bulb and the PCB. The simple configuration of the bulb base contrasts with what we typically found on LED bulbs in 2015. It wasn’t uncommon to find the metal screw threads supported mainly by epoxy potting material rather than by the plastic housing. In some bulbs the metal threads didn’t even touch the plastic housing. The epoxy did all the work. Similarly, the base contact on some bulbs was separate from the metal screw threads. And electrical connections to the base and to the metal screw threads tended to be via discrete wires. The electronics for LED bulbs that don’t allow dimming can be simple. In the case of the EcoSmart bulb, a single IC (BP9916D) from Bright Power Semiconductor in China handles LED
2 • 2019
With the base contact and metal threads removed, the connection scheme to the PCB becomes visible. It consists of a spring-metal contact that touches the metal threads and a friction-fit connection to the center metal post; no epoxy potting necessary.
driving chores. It is a buck constant-current device and contains a 500-V power MOSFET for handling LED current. The circuit we found on the bulb PCB is basically the same as the reference circuit on the BP9916D data sheet. Three capacitors, one inductor, a diode bridge, one resistor, one discrete diode, and a fuse for safety are the basic circuit components. There are an additional three resistors that seem to be there as part of a test circuit. The main resistor is a sense resistor used to convert the buck converter from a voltage-output to a current-output device. The discrete diode seems to be there to head off any ringing from the switched inductor/capacitor.
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A view of the PCB board out of the plastic housing shows the pins used to make contact with the LED plate, the spring metal contacts making electrical connections to the metal screw threads and base, one of the slide rails into which the PCB fits, the metalized internal wall of the plastic housing (probably done for EMI concerns), and the plastic posts and screw receptacles for the LED plate hardware.
Bright Power LED driver reference circuit
The Bright Power reference circuit for the BP9916D.
A point to note about the electronics is that the only heat sink is the metal LED plate itself. The PCB fits in the plastic housing via plastic slots. There is a metalized area on the inside of the housing, but it doesn’t touch the PCB – it’s probably there for EMI shielding. Bright Power doesn’t provide any details about the internal circuits of the LED driver chip. One explanation it does provide is that the chip operates in critical conduction mode, meaning that the current in the inductor goes to
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LED — Power Electronics HB 02-19.indd 46
Two ICs make up the bulk of the LED driver, a diode bridge and the BP9916D from Bright Power. Also visible are the anti-ringing diode, the current-sense resistor, and other resistors that seem to be for testing purposes.
zero before the next switching cycle initiates. Circuits that operate this way generally include a means of sensing when the inductor current hits zero, and this sensing usually takes place through a small sense winding used to show when voltage on the winding has dropped to near zero. However, a point to note about the Bright Power chip is that it doesn’t need a sense winding. The data sheet mentions something about a patent-pending MOSFET driving technique, which may have something
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to do with its ability to operate in critical conduction mode. All in all, the EcoSmart bulb is an interesting example of how LED bulb assembly techniques have advanced over the past few years.
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2/19/19 3:41 PM
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