Alex Lidow CEO of EPC
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CONTENTS
PULSE
Alex Lidow
EFFICIENT POWER CONVERSION (EPC) The CEO and Founder of EPC on why gallium nitride will become the new silicon.
Featured Products This week’s latest products from EEWeb.
How to GaN
An Introduction to Gallium Nitride (GaN) Transistor Technology
Smartbond™ SoC
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Reactive Power—Pt. 3
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A look into specific voltage and var solutions for power systems serving large urban areas.
RTZ
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PULSE
A
lex Lidow was born into power semiconductors. His father and grandfather founded International Rectifier back in 1947, so he was raised around resistors, diodes, transistors and capacitors. Initially attending Caltech for aeronautical engineering, Lidow had a professor who introduced him to semiconductors and he has been in love ever since. Lidow then went on to Stanford University to pursue his PhD. While at Stanford, he began working with Gallium Arsenide because he believed at the time that it would be the successor to silicon. When he realized he was wrong, however, he quickly got back into silicon by joining International Rectifier. His 30-year journey at International rectifier was focused around replacing the aging bipolar transistor through the development of power MOSFETs. In 2007—after being CEO of International Rectifier for 12 years—Lidow went off to start Efficient Power Conversion (EPC), to work on creating the gallium nitride transistors he believes will ultimately replace power MOSFETs. We spoke to Lidow about why he believes GaN is the true successor to silicon, what products EPC is developing, and the roadmap towards wide adoption of GaN devices.
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INTERVIEW
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PULSE “ GaN is very fast—much faster than silicon could ever hope to be. Our limitation when we use a silicon IC to drive our GaN devices is the fact that we’re using a silicon IC.”
How did your experience in semiconductors lead you to believe that GaN would replace silicon?
a market that is more and more willing to accept a very advanced technology that yields a much more efficient system.
While I was at International Rectifier working on this power MOSFET, I realized that there were several things that were needed for the power MOSFET to replace the old bipolar transistor. In order to cross this chasm of a new technology, I needed to find applications that couldn’t be done by bipolar transistors. In order to generate enough interest in the device, it had to be easy to use and reliable enough. Finally, in order to deal a knockout blow to the bipolar transistor, I had to be able to make it cheaper than the bipolar transistor. I bring up these lessons, because these are the same lessons that I am now applying to gallium nitride in our quest to displace the old power MOSFET.
What types of customers does EPC currently serve?
I was CEO of International Rectifier for 12 years until 2007. I left the company and started Efficient Power Conversion (EPC, based on the concept that we will make gallium nitride transistors that will replace power MOSFETs. That really is the beginning of it. We had two early requirements—one was to make an enhancement mode, which we accomplished in a year or two. The second thing we knew we had to do was to make it cheaper than a MOSFET, and we’re still working on that, but our roadmap says that in about three more years, we will achieve that. EPC started delivering a product to beta customers in 2009 and proudly released them in 2010 to all customers, and here we are in 2013 and we have about 500 customers. We are seeing
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I could put our customers into a few categories. I’d call one a business category—the early adopters. I will come back and re-split that into technical categories. But the early adopters are those guys that I mentioned that helped you cross the chasm who are the ones that see your device as enabling them to do something they couldn’t do otherwise. We really have four groups of customers that fit into that category. The first of them are working on wireless power transmission, particularly with the highly resonant, loosely coupled wireless power that you hear about. That wireless power transmission, I believe, within about 10 years will start seeing general applications that replace the wall socket. Instead of using a plug into the wall, you’ll just wirelessly transmit power to your television or your lamp. Between now and then, you’ll start to see wireless power transmission for cellphones, tablets, and even electric vehicles. A lot of these applications are standardized on using 6.78 MHz lower ISM band, which is unlicensed and therefor anyone could use it. That is a frequency that is very good for gallium nitride and very difficult for power MOSFETs. That group of customers is a willing and enthusiastic adopter of gallium nitride. Probably in late 2013 or 2014, you’ll start seeing cellphone chargers come out with this loosely coupled resonant wireless power system.
INTERVIEW A second application that is also an early adopter is LiDAR. When you see the Google mappers with the system on top, that is an example of a LiDAR system. The latest generations of that system uses our GaN devices. The reason is that you want to map data elements as fast as you can. The faster you can do it, the faster you can drive the car. It essentially develops a 3D observation of your surroundings as quickly as you can flash your lasers. If you want to go fast enough to actually drive that car all by itself, you need to do it pretty quickly. That’s one of the reasons why they are an early adopter of our GaN transistors. A third example of an early adopter is envelope tracking. Envelope tracking is an idea that has been around for about 80 years. If you want to transmit in radio frequencies, you use a carrier frequencies, say for example 2.4 GHz, which is very common today. You then amplitude modulate that carrier in order to send information. The bigger the bandwidth that you want to have in your information transmission, the more you have high peaks in your transmission and very low valleys. This creates something called a peak to average power ratio. If you could imagine wanting to transmit on a 4G LTE network, you will have about 10 times the peak power compared with the average power. Why is that important? It’s important because your design has to be able to operate at peak power, and yet you’re running more often at average power. If you use a fixed DC power supply on the RF amplifier, everything between peak and average is lost in the form of heat inside of the amplifier. If you could design a DC power supply that could track that amplitude envelope precisely, then you would actually never burn that energy. In the case of 4G LTE networks, if you had envelope tracking on your power supply, you would use less than half the energy to transmit.
Where would that be on the cellphone? It would actually be on the transmitter of a base station, a microcell, picocell, or on the power amplifier of a cellphone. It is very hard to make a power amplifier that precisely tracks an information envelope that may have a 60 MHz bandwidth. Just think of a DC to DC converter with a 60 MHz bandwidth for adjusting. That’s one of the reasons why this hasn’t really been a reality for 80 years,
because the transmission data rate has always been ahead of the bandwidth of the power transistors. But now, with GaN, we’re finally there. We can finally do envelope tracking, which involves making a very high speed, class-D amplifier that tracks these envelopes very precisely in conjunction with a linear amplifier—it’s kind of a hybrid amplifier. That is a class of customers that is growing for us and will be a large application of gallium nitride in the next 3 to 5 years.
Does this include handsets? Handsets are the final adopters in the journey—the first one being base stations. That’s a multi-billion dollar transistor market today and will be even bigger as you get into higher value things like envelope tracking. I believe that wireless power and RF envelope tracking are two applications that will be over a billion dollar markets for GaN transistors and GaN ICs.
What devices do you currently offer and how do you see them evolving over the next few years? We have a family of products that range from 200 volts all the way down to 40 volts. They go from just a couple of amps up to 33 amps of DC current capability. It’s really a matrix of die sizes, so we have die sizes that are large, medium, and small at different voltages. It’s meant to address about 70% of the power MOSFET market. The devices aren’t necessary optimal for each of those applications, but they are better than any MOSFET. It gives us the broadest target to aim at for our products. We are taking our roadmap in two basic directions from here. One is towards higher voltages. We’re going to, hopefully this year, launch 600 volt products. The second direction we’re going in is towards integrated circuits where we’re combining signal level capability on the same chip with our power devices. I think that this is an exciting possibility because gallium nitride is very amenable to monolithic integration of signal level and analog devices along with power devices.
Are there particular functions or existing ICs that you are targeting? Yes. We want to include in our power devices the driver function. The reason is that GaN is very fast—
Top View of an eGaN FET
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PULSE “ Gallium nitride is self-isolating, meaning it protects itself from the environment because the active GaN element on top of a piece of silicon is actually encapsulated in a thick insulating glass. With that, we made our lives much easier.”
much faster than silicon could ever hope to be. Our limitation when we use a silicon IC to drive our GaN devices is the fact that we’re using a silicon IC. So if we could integrate it right into our GaN device, we would have a higher performing product and it would save the user the problem of buying additional components, and having to make a very efficient layout in order to accommodate the high speed integration of the silicon driver with a GaN transistor.
all good and well, but it’s very difficult to get a high-volume CMOS foundry to allow you to bring in your gallium nitride wafers. We could intellectually show them that it wouldn’t add any cross-contamination, as you could imagine. Our solution was that we found a partner in Taiwan who agreed not only to invest in our company, but also to open up his CMOS foundry to our production. We’ve been producing in his foundry, side-by-side with his silicon wafers for his other customers, without any cross-contamination.
How did you find a manufacturing facility that would produce your gallium nitride wafers?
I’ll answer that question in two parts. When we got started five years ago, not only were we small and poor, but we had very little influence. When we went to traditional packaging sub-contractors in Asia, they didn’t want to have anything to do with us because we were way too small with way too long of a ramp for them. This is frustrating. We thought about it for awhile, and I remembered from my experience selling power MOSFETs for a long time that our customers continually complained about our packages. They complained that the packages added too much resistance, added too much inductance, were too big, and that they cost too much. Those elements rattled around in my head and I asked why in the world would we want to do
EPC is a completely fabless company. It really has to do with the fact that when we started this company, we didn’t have a lot of money. The thing that we could not afford to do is to build a wafer fab, so we did something that was quite unique; we designed a product to be manufactured in a silicon foundry. That’s
eGaN® FET Structure
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Could GaN be a drop-and-replace for a power MOSFET, or is there additional design work that needs to be done?
INTERVIEW a package. Gallium nitride is self-isolating, meaning it protects itself from the environment because the active GaN element on top of a piece of silicon is actually encapsulated in a thick insulating glass. With that, we made our lives much easier. When I was at International Rectifier, we produced a billion MOSFETs every quarter and the packaging cost was same as the wafer fab costs. You could say that packaging was half of the costs, which is pretty amazing. With one stroke of the pen, EPC eliminated that cost. We’ve been delivering our product with solder bars that are screen printed on in wafer form and forms a land grid array format in a flip chip. It is extraordinarily high performance because the package doesn’t add any inductance or resistance and doesn’t add any size, so you can cozy up all of the components right next to our chip. This increases the power density, but also reduces the signal path. That leads to the second part of your question about needing a replacement for the power MOSFET. The answer to that is, conceptually, yes. It looks like a MOSFET on a circuit diagram, it’s a three terminal device, it’s enhancement mode, and it turns current on and off. What’s different is that it’s extraordinarily high-speed. As a result, it is far more sensitive to parasitic elements like inductance that you might have in your design or PC board. In order to really get the performance out of your GaN transistor that you are paying for, you need to pay much more attention to the design of the device than you would with a power MOSFET. For that, we have a lot of literature that we’ve done and we also work with Texas Instruments to make an integrated circuit that makes it very small and can be efficiently laid out on a PC board, but also has protections inside of it so you don’t get trapped with over-voltaging your device because of the ringing that might occur when you drive it at high speeds.
As a company, does EPC provide customer support to someone who is new to GaN devices? You bet. I learned from my years in this industry that the customers that come back to you year after year are the ones that you serviced with high-level technical support. With that lesson, EPC has two layers of technical
support. We have field applications engineers located in the U.S. and in Asia. Those are the primary points of contact—they are very experienced designers that can go in there with any customer and help them get going very quickly. We also have a layer of support of PhD-level, top of the field applications engineers who work out of fixed labs. They are the ones that are generating the state-of-the-art in gallium nitride systems. If you look at the book that we wrote, GaN Transistors for Efficient Power Conversion, two of my co-authors are two of our applications engineers. They are in the lab making these reference designs that can then be used by our customers to very quickly get up and running in GaN.
What steps need to be taken to help with wide adoption of GaN devices? Gallium nitride transistors, today, cost more than power MOSFETs. As I said before, in order to replace old technology, it can’t just be higher performance, it’s got to be cheaper as well. If you have a high performance product, but it’s more expensive, it will be a niche product. If you have a low performance product that is cheaper, it will be a niche product. In order to have a displacement technology, you need to have a higher performance product that has to be cheaper. That’s why, when we founded EPC, we made it a goal to make an enhancement mode device and make it cheaper than a MOSFET. The elements to do that are pretty straightforward. I already told you how we cut the cost in half relative to a MOSFET by eliminating the packaging. We also use a foundry and we have half the number of processing steps that a power MOSFET wafer has. Therefore, our wafer fab costs are much lower than a power MOSFET. What’s more expensive is growing the heterostructure crystal on top of a silicon wafer. That is more expensive because the equipment we are using, which are called MOCVD reactors, were designed to make LEDs on small wafers and are inherently inefficient for our own particular growth needs for gallium nitride transistors. We’ve been working with several manufacturers of MOCVD equipment and recently received our second-generation machine that is establishing a whole new capability in terms of cost as well as performance of the device. That, we believe, will leverage into a third generation machine, which will bring the cost growing the epitaxial heterostructure to be about the same as growing epitaxial silicon. That’s when we’ll cross over and become cheaper than a MOSFET. That’s about three years away. ■
“I learned from my years in this industry that the customers that come back to you year after year are the ones that you serviced with high-level technical support.”
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PULSE
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TECH ARTICLE
Alex Lidow
CEO of Efficient Power Conversion (EPC)
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PULSE “How to GaN” is a monthly column written by Alex Lidow, CEO of EPC, the leader in enhancement-mode gallium nitride transistors. Written for power systems design engineers, the column will examine gallium nitride technology applied to power conversion. Initial articles will discuss the technology, followed by articles on basic design fundamentals, and supplemented by a series of articles on applications examples. The GaN Journey Begins HEMT (High Electron Mobility Transistor) gallium nitride (GaN) transistors first started appearing in about 2004 with depletion-mode RF transistors made by Eudyna Corporation in Japan. Using GaN on silicon carbide (SiC) substrates, Eudyna successfully brought into production transistors designed for the RF market. The HEMT structure was based on the unusually high electron mobility, described as a two-dimensional electron gas (2DEG), near the interface between an AlGaN and GaN heterostructure interface. Adapting this phenomenon to gallium nitride grown on silicon carbide, Eudyna was able to produce benchmark power gain in the multi-gigahertz frequency range. In 2005, Nitronex Corporation introduced the first depletion mode RF HEMT transistor made with GaN grown on silicon wafers using their SIGANTIC® technology. GaN RF transistors have continued to make inroads in RF applications as several other companies have entered in the market. Acceptance outside this market, however, has been limited by device cost as well as the inconvenience of depletion mode operation. In June 2009 Efficient Power Conversion Corporation (EPC) introduced the first enhancement-mode GaN on silicon (eGaN®) FETs designed specifically as power MOSFET replacements. These products are designed to be produced in high-volume at low cost using standard silicon manufacturing technology and facilities. Properties
Description
GaN
Si
The basic requirements for power semiconductors are efficiency, reliability, controllability, and cost. Without these attributes, a new device structure has no chance of economic viability. Let’s now look at the comparison between silicon, silicon carbide, and gallium nitride as the three most likely candidates for the dominant platform for next-generation power transistors.
Why Gallium Nitride? Silicon has been a dominant material for power management since the late 1950’s. The advantages silicon had over earlier semiconductors such as germanium or selenium could be expressed in four key categories: 1. Silicon enabled new applications not possible in earlier materials 2. Silicon proved more reliable 3. Silicon was in many ways easier to use 4. Silicon devices were lower cost In order for another semiconductor material to displace silicon in the next generation power transistors it must demonstrate its SiC superiority over silicon in these four areas.
Eg (eV)
bandgap energy
3.4
1.12
3.2
EBR (MV/ cm)
critical electric field for breakdown in the crystal
3.3
0.3
3.5
µ (cm2/Vs)
mobility of electrons
990–2000
1500
650
Table 1: Material properties of GaN, SiC, and silicon
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All of the advantages of silicon stemmed from its basic physical properties combined with a huge investment in manufacturing infrastructure and engineering. First, let’s look at some of those basic material properties and compare them with potential successor material candidates. Table 1 shows three key electrical
TECH ARTICLE properties of three semiconductor materials contending for the power management market.
Ron (Ωmm2)
One way of translating these basic crystal parameters into a comparison of device performance in a power transistor is to calculate the best theoretical performance that could be achieved in each of the three candidates. For power devices there are many characteristics that impact performance in a variety of power conversion systems available today. Five of the most important are conduction efficiency, breakdown voltage, switching efficiency, size, and cost. These device characteristics determine the achievable system frequency and power density.
101
Because GaN devices can be much smaller than silicon devices, and the electrons are more mobile than either silicon or silicon carbide, GaN HEMT transistors can switch much, much faster. Figure 2 shows a comparison of the transition time between a GaN transistor and two silicon power MOSFETs in a 12 VIN and 1.2 VOUT buck converter. The GaN transistor switches in about onefifth the time of the comparable 40 V silicon device and one-fourth the time of a 25 V silicon device. Score another one for GaN! The eGaN device’s lateral structure also lends itself to flip-chip packaging, which is a very high performance packaging solution due to the minimal resistance and terminal
SiC limit
10-1
GaN limit
10-2 10-3 10-4
102
101
103
104
Breakdown voltage (V) Figure 1: Material properties of GaN, SiC, and silicon
Using the data from table 1 (and adjusting for the enhanced mobility of the GaN 2DEG), we can calculate the theoretical minimum device on-resistance (the inverse of conductivity) as a function of breakdown voltage and as a function of material. As shown in Figure 1, SiC and GaN both have a superior relationship between onresistance and breakdown voltage due to their higher critical electric field strength. This allows devices to be smaller and the electrical terminals to be placed closer together for a given breakdown voltage requirement. GaN has an extra advantage compared with SiC as a result of the enhanced mobility of electrons in the 2DEG. This translates into a GaN device with a smaller size for a given on-resistance and breakdown voltage. Score one for GaN!
Si limit
100
25 V Si MOSFET 5.7 ns
˜
40 V Si MOSFET 7.5 ns
˜
40 V eGan FET 1.5 ns
˜
3 V/Div 5 ns/Div
Figure 2: Comparison of hard swtiching turn-on speed of eGaN FETs vs. silicon MOSFETs in a 12V - 1.2V, 20A buck converter. All three devices have similar RDS(ON) but different breakdown voltages shown.
The basic requirements for power semiconductors are efficiency, reliability, controllability, and cost. Without these attributes, a new device structure has no chance of economic viability.
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PULSE Device
LGA eGaN FET Package
Smallest Equivalent MOSFET Packages
inductance. Add to this a distinct die area advantage over silicon and the resultant solution is a superior power device in a high performance package that is significantly smaller than anything available today. Table 2 is a comparison of the size of the eGaN® FETs compared with equivalent onresistance MOSFETs. The double advantage of the efficient chip-scale LGA package and the smaller die size translate into a significant reduction in overall size occupied by the eGaN® FET on a PCB. GaN transistors stem from a relatively new technology and, as such, remain somewhat more expensive to produce than their silicon counterparts. This, however, is a temporary situation. As discussed in detail in Chapter 14 of the book, “GaN Transistors for Efficient Power Conversion”, there are no insurmountable barriers to achieving an even lower cost for an equivalent performance eGaN FET compared with a power MOSFET or IGBT.
Basic GaN FET Structure
Table 2: Comparison between power MOSFETs in various packages and eGaN FETs in LGA packages
Figure 3: Typical depletion mode AIGaN/GaN HFET structure with three metal-semiconductor contacts for the source, gate, and drain.
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The basic GaN transistor structure is shown in Figure 3. As with any power FET, there are gate, source, and drain electrodes. The source and drain electrodes pierce through the top AlGaN layer to form an ohmic contact with the underlying 2DEG. This creates a shortcircuit between the source and the drain until the 2DEG “pool” of electrons is depleted and the semi-insulating GaN crystal can block the flow of current. In order to deplete the 2DEG, a gate electrode is placed on top of the AlGaN layer. In many of the early GaN transistors, this gate electrode was formed as a Schottky contact to the top surface. By applying negative voltage to this contact, the Schottky barrier becomes reverse biased and the electrons underneath are depleted. Therefore, in order to turn this device OFF, a negative voltage relative to both drain and source electrodes is needed. This type of transistor is called a “depletion mode”, or d-mode, HFET. In power conversion applications, d-mode devices are inconvenient because at the startup of a power converter a negative bias must be applied to the power devices first or a short circuit will result. An enhancement mode (e-mode) device, on the other hand, does not suffer this limitation. With zero bias on the gate, an e-mode device is OFF and
TECH ARTICLE will not conduct current. When Efficient Power Conversion Corporation introduced the first commercial e-mode (eGaN®) FETs, it significantly reduced the level of difficulty designing power conversion systems with GaN transistors.
eGaN® FET Structure - Enhancement Mode EPC’s enhancement mode process begins with silicon wafers. A thin layer of Aluminum Nitride (AlN) is grown on the silicon to provide a seed layer for the subsequent growth of a gallium nitride heterostructure. A heterostructure of aluminum gallium nitride (AlGaN) and then GaN is grown on the AlN. This layer provides a foundation on which to build the eGaN® FET. A very thin AlGaN layer is then grown on top of the highly resistive GaN. It is this thin layer that creates a strained interface between the GaN and AlGaN crystals layers. This interface, combined with the intrinsic piezoelectric nature of GaN, creates a twodimensional electron gas (2DEG) which is filled with highly mobile and abundant electrons. Further processing of a gate electrode forms a depletion region under the gate. To enhance the FET, a positive voltage is applied to the gate in the same manner as turning on an n-channel, enhancement mode power MOSFET. A cross section of this structure is depicted in Figure 4. Additional layers of metal are added to route the electrons to gate, drain, and source terminals (see Figure 5 cross section). This structure is repeated many times to form a power device as shown in Figure 6.
Figure 4: eGaN® FET Structure
Figure 5: SEM micrograph of an eGaN FET
Summary In this first of a series of articles we introduced the concept that GaN-on-silicon power devices could be a superior replacement for the aging power MOSFET. We also described the structure of the two types of GaN transistors – depletion mode and enhancement mode. GaN FETs are smaller, faster, easy to use, commercially available, and will soon be less expensive than their silicon ancestors. In the next article we will discuss the basic tools a power system designer will need to capture the superior performance GaN transistors can deliver. ■
Figure 6: Top view of a completed eGaN FET. This device is rated at 40 V, 4 mΩ, and 33 Amperes
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POWER, SMALL SIZE
D
ialog Semiconductor is releasing a new SmartBond™ System-on-Chip wireless connectivity solution for Bluetooth. The DA14580 solution presents several advantages, including its small size and low power consumption, and will be able to be used for connecting wireless keyboards, wearable computers, proximity devices, and other short-range wireless devices. SmartBond™ is the lowest power solution in its market, at 3.8mA during transmission and reception. That low power consumption is “quite an achievement,” says Mark de Clercq, Product Marketing Group Manager at Dialog. The low power consumption of SmartBond™ will allow the battery life of a product to be doubled, or the battery size to be reduced by a factor of two. Though there have been other Bluetooth Smart products on the market for several years, such as health monitors like Fitbit Flex and Jawbone UP, SmartBond™ draws less than 50% of the power of any other solution. That means that a battery that would’ve lasted 2 years with previous generation technology could now last 4 years. For developers, SmartBond™ could be used for both embedded and hosted solutions. “We’ve included the Cortex M0 on the chip,” said de Clercq, which “enables you to basically develop the complete application on the chip itself, without the need for a microcontroller.” Some applications, however, require a much higher processing performance, and so SmartBond™ is still enabled to connect to an external controller.
There are three different packages being released for the SmartBond™, and different development kits to go along with all three packages. “Basically we are introducing this device in three flavors to offer developers all the design flexibility they need,” said de Cercq. “We have one that will be WL-CSP34, and it will be 2.5x2.5mm and have 12 GPIOs in terms of connectivity. The second one is a QFN40 package, and that will have a size of 5x5mm and offer 24 GPIOs. And then finally you have QFN48, which offers 32 GPIOs, and will have the size of 6x6mm.” With these different packages, Dialog hopes to offer customers a range so they can choose the best package for their application. The Bluetooth market is about to grow rapidly, Dialog believes. Possible competitors include ZigBee and Wi-Fi. However, because of the penetration of Bluetooth Smart Ready in consumer devices, Dialog believes Bluetooth Smart will do well in point-to-point applications such as wireless mice and TV Remote Controls. ■
(Left) Block diagram of the DA14580
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TECH ARTICLE
eactive Power Part 3: age and var solutions
e voltage/var problems associated with serving urban areas that import large shares of power during peak load hours have slowly been gaining recognition by the international electric power community. P. Jeffrey Palermo Executive Consultant DNV KEMA Energy & Sustainability
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PULSE Affected urban areas have often been geographically distant; hence, they may not have been aware of similar problems faced by other areas and thought their situation was unique. There is a clear need for broader awareness of this voltage/var issue and possible solutions. DNV KEMA Energy & Sustainability has observed common practices among urban areas in developed countries. To find a quantifiable common measure, we developed an indicator—referred to as the compensation ratio—based on total required transmission reactive compensation levels to support imports and total import amounts.
In a 2006 KEMA study, we compared important metropolitan areas around the world that had high levels of compensation: Mexico City, San Diego, New York City, South Florida, El Paso, Cape Town, Auckland, and Albuquerque. We selected these areas because they had these three characteristics in common:
• They are important population and commercial centers in their region. • They are at the end of the electrical network with a peninsular type of transmission connection to the rest of the larger grid. • Power imports are an important factor in serving the local load.
The most direct way to lower the compensation ratio—without reducing imports—is to build a new transmission line or upgrade the existing network. These are often capital-intensive projects, which may require new rights-ofway and involve timeconsuming licensing processes.
While every situation is unique, as a rule of thumb, the transmission reactive compensation ratio is a matter of concern once it exceeds 25%. A level above 50% is generally considered to be unacceptably high. Ratios for these eight metropolitan areas fell within the 15%-30% range as shown in the chart below.
0 Figure 1: Compensation ratios of selected major urban power importing areas
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Albuguerque
Cape Town
El Paso, TX
S Florida
New York City
10
San Diego
20
Mexico City
30
Calgary
40
Auckland
Compensation ratio (%)
50
TECH ARTICLE The most direct way to lower the compensation ratio—without reducing imports—is to build a new transmission line or upgrade the existing network. These are often capital-intensive projects, which may require new rights-ofway and involve time-consuming licensing processes. Increasing reactive compensation is often an economically attractive option to transmission expansion. Reactive devices, such as switched capacitors, SVCS, and STATCOMS, do not require extensive land, and they are not especially visible when compared with major new transmission lines. These characteristics make them much more acceptable to the public and, thus, to transmission system operators responsible for providing reliable electric delivery service. In most cases, reactive devices are much less expensive to build than new transmission lines. In moderation, such devices are useful additions to the options system planners and operators can use to relieve voltage/var problems and to provide improved operating flexibility. However, excessive dependence on these devices can increase the complexity of system design and operation and lead to increased risk of uncontrolled system collapse as discussed in part 2 of this series. There are two general types of solutions: increase the local real power (MW) supply (including demand management) or increase the import capability of the transmission system.
INCREASING THE LOCAL REAL POWER (MW) SUPPLY
adopt new demand management programs may not be trivial. As a result, system operators must be able to use the program’s features effectively and reliably.
Local-area energy storage
Installing energy storage devices in the load area can also be an effective solution. Modern storage provides both real and reactive power to both reduce imports and provide reactive support. They are especially effective in supporting the system following a contingency— where the stored energy is only needed for a limited time.
INCREASING THE POWER IMPORT CAPABILITY
Adding transmission at exiting voltage levels
The most obvious transmission solution is to increase the number of transmission lines that serve the area. Adding a new line decreases the impedance and reduces reactive losses. Adding a line to an area supplied through five major transmission lines will reduce the impedance by about 17%. To have a significant impact on reactive losses, however, multiple new lines are required. The cost of new transmission lines is directly related to the length and number of lines. A new line parallel to the most heavily loaded lines is usually more effective than others. Local opposition could require a portion of the new line to be placed underground, which would significantly increase costs.
Modifying existing transmission
An alternate solution is to modify the configuration of existing lines. Recent research has shown that changes in tower configurations and conductor spacing can reduce line impedances by up to 20%. To have a significant impact on reactive losses, multiple lines have to be modified. Also, the transmission has to be removed from service to make the changes. For heavily loaded import areas, it can be difficult to schedule the necessary outages.
New local-area generation
Perhaps the most obvious solution is to add generating capacity to the importing area. Local generation will reduce imports into the area and add to the local var supply. It will also decrease the possibility of voltage collapse and improve dynamic stability of the area.
Increased local demand management
Considering the opposition to new power plants and transmission lines in most urban areas, increased demand management programs can effectively reduce imports in areas nearing peak supply limits. It is important to recognize that this option only defers capital investment for future grid or generation expansion. Furthermore, the costs of incentives that encourage customers to
The cost of new transmission lines is directly related to the length and number of lines. A new line parallel to the most heavily loaded lines is usually more effective than others.
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PULSE The best solution for any situation depends on the specific characteristics of the power system involved.
The impedance of these transmission lines can also be reduced by adding series compensation. This involves inserting transmission capacitors in series with the lines. Series compensation is widely used in the Western United States where line lengths are rather long. To have a significant impact on reactive losses, multiple lines have to be modified. Care must be used in applying series compensation, since too much can cause other types of problems, e.g., resonant circuits that introduce new oscillation modes in the power system causes problems for system operation and can potentially damage equipment.
Adding transmission at higher voltage
Often, the most effective solution is to add a new higher voltage level of transmission— usually 500/765 kV. These lines have about half of the effective impedance of 230/345 kV lines. Such a line—with half the impedance— will have only one quarter of the reactive losses of a 230/345 kV line. The higher voltage lines require wider rights-of-way and taller towers, and they often encounter significant opposition because of routing and environmental concerns.
Adding HVDC transmission
A more expensive option is to add highvoltage direct current (HVDC) transmission to deliver real power (MW) into an area. HVDC transmission can reduce imports through the parallel AC transmission system and, thus, reduce the overall reactive losses associated with imports.
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An advantage of the HVDC line is that it requires slightly lower and narrower towers than comparable AC lines. While the cost of the tower and conductors are lower for HVDC, they have very high terminal costs, which require expensive electronics to convert between AC and DC. HVDC lines are usually used only when distances are long and power levels are high. In special situations, such as long submarine cables and connecting asynchronous systems, HVDC is also better than AC. The best solution for any situation depends on the specific characteristics of the power system involved. By understanding the needs, simulating possible solutions, and incorporating local guidelines and constraints, one can develop an effective solution for every situation.
About the Author
Mr. Palermo has more than 35 years of experience in the power sector, with specific expertise in system planning and sector restructuring. At DNV KEMA, Mr. Palermo is responsible for system planning and operating studies of generation and transmission systems within a range of multi-utility coordination schemes. He advises and assists utilities in developing and evaluating transmission plans -- including a wide range of system analyses using a variety of steady-state and dynamic system analysis tools and techniques. Mr. Palermo has appeared as an expert witness before FERC, Congressional committees, state legislatures, state and federal courts, arbitration panels, and state and provincial utility commissions. He has provided numerous lectures on restructuring, system planning and system operation. ■
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Get the Datasheet and Order Samples http://www.intersil.com
Power Factor Correction Controllers ISL6730A, ISL6730B, ISL6730C, ISL6730D The ISL6730A, ISL6730B, ISL6730C, ISL6730D are active Features power factor correction (PFC) controller ICs that use a boost topology. (ISL6730B, ISL6730C, ISL6730D are Coming Soon.) The controllers are suitable for AC/DC power systems, up to 2kW and over the universal line input.
The ISL6730A, ISL6730B, ISL6730C, ISL6730D are operated in continuous current mode. Accurate input current shaping is achieved with a current error amplifier. A patent pending breakthrough negative capacitance technology minimizes zero crossing distortion and reduces the magnetic components size. The small external components result in a low cost design without sacrificing performance. The internally clamped 12.5V gate driver delivers 1.5A peak current to the external power MOSFET. The ISL6730A, ISL6730B, ISL6730C, ISL6730D provide a highly reliable system that is fully protected. Protection features include cycle-by-cycle overcurrent, over power limit, over-temperature, input brownout, output overvoltage and undervoltage protection.
• Reduce component size requirements - Enables smaller, thinner AC/DC adapters - Choke and cap size can be reduced by 66% - Lower cost of materials • Excellent power factor over line and load regulation - Internal current compensation - CCM Mode with Patent pending IP for smaller EMI filter • Better light load efficiency - Automatic pulse skipping - Programmable or automatic shutdown • High reliable design - Cycle-by-cycle current limit - Input average power limit - OVP and OTP protection - Input brownout protection
The ISL6730A, ISL6730B provide excellent power efficiency and transitions into a power saving skip mode during light load conditions, thus improving efficiency automatically. The ISL6730A, ISL6730B, ISL6730C, ISL6730D can be shut down by pulling the FB pin below 0.5V or grounding the BO pin. The ISL6730C, ISL6730D have no skip mode.
• Small 10 Ld MSOP package
Two switching frequency options are provided. The ISL6730B, ISL6730D switch at 62kHz, and the ISL6730A, ISL6730C switch at 124kHz.
• TV AC/DC power supply
• Desktop computer AC/DC adaptor • Laptop computer AC/DC adaptor • AC/DC brick converters
100
VI
VLINE
Applications
+
VOUT
95
EFFICIENCY (%)
90
VCC ISEN
GATE
ICOMP
GND
ISL6730
VIN
FB
ISL6730A, SKIP
80 ISL6730C
75 70
COMP BO
85
65
VREG
60
0
20
FIGURE 1. TYPICAL APPLICATION
40 60 OUTPUT POWER (W)
80
100
FIGURE 2. PFC EFFICIENCY
TABLE 1. KEY DIFFERENCES IN FAMILY OF ISL6730
February 26, 2013 FN8258.0
VERSION
ISL6730A
ISL6730B
ISL6730C
ISL6730D
Switching Frequency
124kHz
62kHz
124kHz
62kHz
Skip Mode
Yes-Fixed
Yes-Fixed
No
No
Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2013 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.
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