GaN and SiC: on track for speed and efficiency
Margery Conner -August 25, 2011
Silicon power MOSFETs are approaching their theoretical performance limits for both on-resistance and gate charge. Although silicon switches will continue to eke out gains, the increases will come more slowly and be smaller. Enter wide-bandgap materials: SiC (silicon carbide) and GaN (gallium nitride). Both technologies have for several years found use in RF amplifiers. Advances in process manufacturing are driving down the costs and increasing the power capability of power switches employing these materials, and proponents of both claim that they will supplant silicon MOSFETs in switched-mode-power-supply designs in which power density and efficiency are key attributes.
The size of a power supply’s components determines its power density; the largest components are the inductors and capacitors, and the MOSFET is a distant third. Capacitors and inductors shrink with higher switching frequency, which is topping out for silicon MOSFETs. Switching losses due to the gate charge would cancel any further increase in switching speed. As for the MOSFET itself, shrinking the die of a silicon MOSFET unfortunately increases the on-resistance. MOSFETs have traditionally measured their performance figure of merit as the on-resistance times the gate charge. For silicon devices, one parameter trades off against the other. Decreasing on-resistance usually comes from increasing the area of the device, but that increase in area comes with an increase in gate charge and, hence, capacitance, which decreases the device’s speed.
Wide-bandgap materials have lower on-resistance by an order of magnitude than silicon materials, with no correspondingly great increase in gate charge. The problems have been the difficulty and expense of working at high power with GaN and SiC materials. GaN and SiC transistors have for years found use in RF amplifiers, and SiC Schottky diodes commonly work as rectifiers in high-voltage power supplies because they have a fraction of the reverse-leakage current of silicon diodes. In addition, SiC has a high thermal conductivity, so an increase in temperature doesn’t degrade the device’s switching parameters. SiC devices target high-voltage applications that require a blocking voltage of 1200 to 1700V. At these voltages, silicon IGBTs (insulated-gate bipolar transistors) are more common switching devices than MOSFETs, but IGBTs are more expensive, and fewer engineers are familiar with IGBTs’ design constraints. SiC FETs share some of the same drive characteristics as silicon MOSFETs, such as being normally off, and can take advantage of MOSFETs’ relatively large base of design engineers.
Among available SiC power MOSFETs is Cree’s recently introduced SiC CMF20120D series. The $93.75 devices have a 1200V blocking voltage and a maximum on-resistance of 110 mΩ at a drain current of 20A and a gateto- source voltage of 20V (Figure 1). On-resistance increases by only 20% at the maximum operating temperature of 150°C. The devices have a gate charge of less than 100 nC throughout the input-voltage range, simplifying gate-drive requirements, and the devices have a forward-voltage drop of less than 2V at a 20A load current, reducing losses. Cree also offers the Z-Rec rectifier, a companion family of SiC power diodes, which has essentially no reverse recovery at 600, 650, and 1200V breakdown voltages. The devices have current ratings of 1 to 20A at 600V, 4 to 10A at 650V, and 5 to 20A at 1200V. The devices are also available in chip form with currents of 10 and 25A at 1700V.
Also in the market, Rohm Semiconductor recently announced its SCS1xxAGC series of SiC Schottky diodes, which maintains low forward voltage over a wide operating-temperature range. For example, the 10A-rated SCS110AGC part has forward voltage of 1.5V at 25°C and 1.6V at 125°C. The short typical reverse-recovery time of 15 nsec enables high-speed switching and minimizes switching loss. Prices range from $6.38 for the 6A version to $24.60 for the 20A version. Rohm will this year follow up its SiC diodes with SiC FETs; it also plans to release 600V transistors in Japan with 1200V SiC FETs in 2012. SemiSouth also makes SiC Schottky-diode rectifiers for solar-inverter and power-factor-correction applications and offers a SiC power JFET, the normally on 1200V SJEP120R085.
The nearly $100 price of SiC MOSFETs may condemn them to niche markets, but keep in mind that, when silicon MOSFETs debuted more than 30 years ago, their prices were also around $100 (in 1970s dollars), yet they now sell for a few dollars each. John Palmour, chief technology officer at Cree, expects the same downward trend in price for SiC devices, in large part because of the room for manufacturability and manufacturing efficiency (Reference 1). However, it’s also likely that SiC devices sell at their current price because that’s what the market will bear. Power efficiency has a price, and more efficient power supplies pay off in energy savings.
GaN devices have also had initial success as RF switching devices, generally at lower voltages than those of SiC. Manufacturers grow the GaN devices on sapphire substrates, involving high manufacturing costs. The breakthrough for GaN came with the ability to grow GaN structures on silicon. Initial GaN-on-silicon devices all operated at less than 100V, targeting use in the datacom-power-conversion market. GaN’s higher switching speed and efficiency allow dc/dc converters to operate in the megahertz region, saving space, reducing the need for heat sinks, and conserving power. International Rectifier was the first company to offer GaN-on-silicon power switching devices to the commercial market with the introduction of the IP2010 in early 2010. The device has a blocking voltage of 20 to 40V and targets point-of-load dc/dc converters. The company’s technology operates in depletion, normally on, mode but hides this characteristic from designers because the company offers the parts as complete driver stages rather than as discrete devices. For example, rather than quote a blocking voltage for the FETs, International Rectifier quotes the GaNpowIR stage by its input-voltage range of 7 to 13.2V, output voltage of 0.6 to 5.5V, output current of 30A, and operating frequency as high as 3 MHz. The drivers sell for $9 (2500).
EPC (Efficient Power Conversion) also offers GaN devices, for which it uses the trademarked name eGaN (enhanced-mode GaN). The company recently introduced the EPC2010 FET, which has a drain-to-source voltage of 200V, a maximum on-resistance of 25 mΩ with 5V applied to the gate, and a pulsed-current rating of 60A. Unlike International Rectifier’s GaN power-conversion-stage device, the EPC2010 is a discrete transistor. The drive is similar to a silicon power MOSFET, but some challenges in driving one exist. For example, because of the high switching frequencies, an eGaN circuit is sensitive to layout. The device also tolerates only a narrow range of gate voltages. To ensure that it’s on requires 4.5V, but it can tolerate only 6V. Considering the power transients you can expect in a power-converter environment, 1.5V is a narrow range of operation. Because the threshold is lower, you must drive it even harder when the gate gets close to ground to ensure that it stays below 1.4V rather than the 2.5V threshold you would encounter in a silicon MOSFET.
“There’s no true body diode in a GaN FET,” says Alex Lidow, founder and chief executive officer of the company. “There’s no reverse recovery loss, which is a performance gain. But when you do leave the FET on, it still has a forward drop of greater than 1.5V, so you have to be careful about dead time. None of these [drawbacks] are insurmountable, but you have to be careful.”
Recognizing an opportunity, National Semiconductor recently introduced a 100V half-bridge gate driver for use with eGaN FETs in high-voltage power converters (Figure 2). The LM5113 high- and low-side GaN-FET driver regulates the high-side floating bootstrap-capacitor voltage at approximately 5.25V to drive eGaN power FETs without exceeding the maximum gate-to-source voltage rating. The LM5113 also features independent sink and source outputs for flexible turn-on strength with respect to the turn-off strength. A 0.5Ω-impedance pulldown path provides a fast, reliable turn-off mechanism for the low-threshold-voltage eGaN power FETs. The LM5113 also integrates a high-side bootstrap diode, further minimizing PCB (printed-circuit-board) real estate, and provides independent logic inputs for the high- and the low-side drivers, enabling flexibility for use in a variety of both isolated- and nonisolated-power-supply topologies.
As for the future of GaN power devices, International Rectifier and EPC have both preannounced products for release by year-end with drain-to-source blocking voltages of 600V. International Rectifier’s new device (Figure 3) will depart from its original GaN-based power-converter stage and be a discrete switching device that pairs a low-voltage silicon FET in a cascode configuration in series with a GaN HEMT (high-electron-mobility transistor), meaning that “you get normally off operation in a three-terminal device whose drive requirements are the same as a typical silicon-based power [device],” says Tim McDonald, vice president of emerging technologies at the company. Further, you can drive it with standard gate drivers, with no special considerations about voltage limitations, overvoltage, or reliability, he adds.
GaN-power-device vendor Transphorm made its first public announcements at this year’s Applied Power Electronics Conference. It plans to introduce fully qualified 600V devices with on-resistances as low as 180 mΩ by the end of the year. The company uses both silicon and SiC substrates for its GaN devices, building early versions of its parts on SiC, which has a crystal structure that’s closer to GaN, and troubleshooting the process on SiC before it moves it to silicon. The device structure will be similar to International Rectifier’s approach, cascoding a low-voltage silicon FET in series with a high-voltage GaN HEMT (Reference 2).
“We looked at the problem of creating a normally off device in GaN and decided that the state of the art for GaN-gate construction limited the maximum positive voltage to 6V,” says Carl Blake, Transphorm’s vice president of marketing. “We viewed this [limitation] as a serious problem, which would limit the practical use of high-voltage GaN devices, so [we] decided to package two dice in a single package using the proven silicon technology as the current controller and the new GaN as an improved voltage-blocking device.” This approach enables power-design engineers to use available controllers and drivers and to focus on rapid performance improvement, he explains.
EPC has also preannounced 600V eGaN devices. According to EPC’s Lidow, no fundamental limitation exists for GaN devices’ blocking voltage, and GaN will rival and cost less than SiC for high-speed devices. It has come as a surprise to him that new applications have started using GaN simply as a silicon-MOSFET replacement. One such application is RF envelope tracking, he says. Lidow likens this application to a dc/dc power supply except that it follows the modulated signal up and down, creating an envelope that always biases the transistor a couple of volts higher than what it needs to generate an amplitude signal. This technique eliminates the heat that goes to waste in the transmitter. He says that the use of GaN enables power supplies to change their voltages when operating at 100 MHz, adding, “The most expensive part of an RF system is the transmitter, and now it has unloaded all of the waste energy that’s heating it up, allowing you to either pump out twice the power or eliminate some heat sinks.”
Lidow sees solar microinverters as other likely applications. The approximately 250W inverters find use in homes or businesses and compete in efficiency with 3-kW central inverters, which are currently more efficient than the lower-power, lower-voltage microinverters. The microinverters’ simultaneous requirements of high frequency, high voltage, and high power make the use of GaN HEMTs attractive.
Many industry participants believe that the second-source market for these new parts will mimic the early stages of the silicon MOSFET market and that many companies will jump in with their own takes on the technologies. For example, Microsemi plans to partner with EPC to develop a high-reliability version of EPC’s GaN HEMT.
You can reach Technical Editor Margery Conner at 1-805-461-8242 and firstname.lastname@example.org.
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