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Growing the infrastructure for wide-bandgap designs

By Steve Ohr, Consultant, Semiconductor Industry Analyst

There is a quiet revolution going on in the field of power electronics. In a massive effort to improve the efficiency of power transfer devices, and to reduce the electrical energy they consume, engineers and scientists are turning to entirely new materials, new device configurations, new power supply topologies. Most dramatic are the power transistors manufactured with wide bandgap materials: Gallium Nitride (GaN) and Silicon Carbide (SiC). These materials build transistors and diodes with faster switching speeds (shorter turn-on/turn-off times), higher power densities with lower heat dissipation, but with high-temperature operation. They enable power conversion devices (silver boxes and modules) with smaller form factors and greater energy-transfer efficiency.

High Voltage Applications Proliferate
In a drastic shift from the rigors of deep-submicron CMOS (where you negotiate nano-amps-per-MHz), Power Electronics asks engineers to think about voltage sources on an entirely different scale: 600 volts, if you’re driving motors for home appliances; 1200 or 1800 volts, if you’re adding DC-AC inverters to the local smart grid. High voltage applications include large data centers (in which, a series of steps) the prevailing voltage is reduced from the 480VAC volts entering a large building to 12- or 5-DC at the compute server racks, supporting current flows on the order of hundreds of amperes. We’re also looking at factory floor applications — and home appliances —using 240-Volt AC power lines and 600 volt semiconductor devices to drive fractional horsepower motors. At 400 or 800 volts, we’re looking at electric cars and vehicles, and chargers, if not vehicle drive train components.
IGBTs vs. GaN and SiC

These applications are currently served by conventional silicon power transistors. Insulated Gate Bipolar Transistors (IGBTs) have among the largest footprints in high-voltage applications. They offer the on/off switching characteristics of metal oxide field-effect transistors (MOSFETs), but with a long tail (a “leisurely” turn off profile. Silicon MOSFETs offer a more definitive turn-on/turn-off, which makes “low voltage” transistors (eg, 30 volt) popular as +5-or 12-volt CPU drivers (and popular as 200- or 400-volt) automotive bus drivers.

The infrastructure — just now being built — will be rich with literature and reference designs for high-voltage circuits. But while legacy components (silicon transistors and diodes, ceramic and electrolytic capacitors are inexpensive and widely available (often for10 cents apiece), engineers must live the inhabitions on switching speed. It involves a search for faster, more robust switching devices. In operation, the higher the voltage and current, the more sluggish the switching operation becomes: Your 1000-volt IGBT has a 20-kHz commutation frequency.

There is a tradeoff, in power supply design, between switching speeds and voltage handling capabilities. MOSFETs serve PWMs in computers, but will not likely be used for 1200-V energy inverters. Wide bandgap devices look to replace IGBTs and silicon MOSFETs with higher switching speeds.

GaN devices are promising to impact the markets now held firmly by MOSFETs, including 600-V motor drives where the drivers are connected to 240-V AC mains. SiC devices with 1200 and 1800-volt capabilities will serve industrial power generation, or alternate energy sources.

Support for a Design Infrastructure
A high-speed/high-voltage design infrastructure is unfolding thanks to the efforts of a number of companies — despite an apparent partition between the developers of GaN and SiC devices. While companies like Infineon, through its International Rectifier subsidiary, is bringing both GaN and SiC devices to market, most semiconductor suppliers are developing transistors for one technology or the other. Infineon in fact is partnering with Texas Instruments and Wolfspeed (a Cree company) on JEDEC standards for wide bandgap power devices… The recently-formed (October 2017) JC-70 committee will initially have two subcommittees: Gallium Nitride (GaN) and Silicon Carbide (SiC). Ideally the specification will focus on Reliability Testing and Qualification Procedures. The deliverables will include datasheet and parameters; the datasheets increasingly point to reference designs.

Reference Designs support for Gan/SiC infrastructure
In addition to its support for a standardization of how GaN and SiC data is reported, Cree offers reference circuits for its C3M0075120K SiC MOSFET. A similarly helpful reference design is available for Infineon’s 1200-V CoolSiC transistor, an enhancement-mode device, using a positive gate drive (+15). Engineers’ natural tendency would be to turn the device “on” with a separate gate charge — and turn “off” the device by letting the gate drive fall to zero. At these elevated switching frequencies, however, you don’t get a complete return to zero (to ground). The residual appears as noise. The best performance is obtained, Infineon advises, by forcing a return to zero. This is accomplished with a negative drive (-5 V). Thus, a SiC power supply requires its own separate power supply for the gate drive.

Taking advantage of Silicon Carbide’s switching speed requires specialized gate drivers. A positive voltage (+15V) will turn the transistor on, but an additional bias (-5V) is required cull noise on the lower part of the duty cycle.

Despite the recommendation of separate gate drivers for the SiC, the circuit will make copious use of resistors and capacitors. Infineon’s reference design uses a fist full of passive components. The design note recommends segregating high-side add low-side gate drivers with resistors; it improves enables power supply stability and controllability. Recommended is 1.0µF to 2.2µF at the gate. Other capacitors include a 10nF to 0.1µF as bypass capacitors. Ceramic capacitors remain popular miniature power supplies.

Other Design Considerations
While SiC has three primary advantages over silicon transistors, according to Cree, taking advantage of these can be worrisome for circuit designers. Higher critical breakdown field supports the use of thinner oxides This means: an ability to process higher voltages than silicon transistors and less on-state resistance RDson. Thinner epitaxial layers will support the high blocking voltage in power devices. A SiC can construct a 4500 V power device with only a 40-50 µm “drift layer,” while a silicon device needs a 500 µm drift layer to handle a 4500-V potential.

The SiC MOSFETs now starting to appear have a different pin configuration that the familiar TO-247 packaged power device. Two source pins are intended to avoid impedance couplings to ground.

Designers used to three-terminal transistors — a base, collector, emitter for a BJT; a gate, drain, source for a MOSFET — might be thrown by the C3M0075120K. The SiC FET, in contrast, has two “source” terminals: a “driver source” and a “power source”. The driver source is essentially a reference terminal for the circuitry driving the SiC device’s gate. If the gate driver is referenced to the same ground that receives the load current, inductance in the load-current path can be troublesome. If the driver-source terminal is used as the reference for the driver circuitry, the negative effect of the inductance is reduced. The on-going challenge in the use of SiC includes the need for a 20-V gate drive, and a negative –2 V to –5 V bias. Using the device’s body diode, the forward voltage drop is as high as 4 V!

New capacitor types for high speed
Whether legacy passive components evolve to take advantage of GaN’s and SiC’s faster switching speeds is a matter of opinion. Engineers now accept that that new gate drivers capable of higher switching frequencies — specialized active components — will be required. Research on metal oxide-semiconductor capacitors on silicon carbide is showing promise (or, at least some interest). But many engineers believe that conventional passive components, albeit with some tweaking, can do the job. Film capacitors, for example, will exhibit increased impedance when operated above 100 kHz. That will change the driver requirement. Aluminum capacitors have the same problem above 30kHz in 200V applications. Ceramic capacitors remain the most versatile.

Interactive simulation tools for capacitors and inductors, tools which help component choices by simulating the behavior of components in a drive circuit chain

The list of capacitor makers offering engineers capacitors for high speed design includes EPCOS (film and electrolytics), Murata with high-voltage ceramics (also DC-DC converters), Panasonic high-voltage (635V) film capacitors. DK’s CeraLink” capacitors are designed for DC-DC energy transfer applications in SiC and GaN circuits. The capacitors are specifically designed for DC energy transfer and can support snubber capacitor applications where SiC and GaN circuits can “overshoot” on turn-off phases. which are used to reduce voltage overshoot at switch off phases. The CeraLink capacitors are effectively multilayer ceramics with very high-power densities. Their small package size enables embedding in motor drive inverters.

 

Components & Devices Technology

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