Magnetic Components such as EMI Inductor, Common Mode Choke, DC-DC Converter for IGBT Gate Driver Applications.
Silicon-carbide (SiC) MOSFETs have made big inroads in the power semiconductor industry thanks to a range of benefits over silicon-based switches. These include faster switching, higher efficiency, higher voltage operation, and higher temperatures that result in smaller and lighter designs. This has helped them find homes in a range of automotive and industrial applications. But wideband gap (WBG) devices like SiC also introduce design challenges, including electromagnetic interference (EMI), overheating, and overvoltage conditions, which can be solved by choosing the right gate driver.
Despite the inherent benefits of SiC, pricing is still a roadblock to adoption. If you look at SiC versus silicon on a part comparison basis, it’s going to be more expensive and difficult to justify unless designers look at the total solution cost, according to power IC manufacturers.
So let’s first address the applications, benefits, and trade-offs of SiC versus silicon MOSFETs or IGBTs. SiC FETs offer lower on resistance (thanks to a higher breakdown voltage), high saturation velocity for faster switching, and a 3× higher bandgap energy, which results in a higher junction temperature for improved cooling, and 3× higher thermal conductivity, which translates into higher power density.
There is industry agreement that low-voltage Si MOSFETs and GaN play in the < 700-V range and above that is where SiC comes into play with a little bit of overlap in the lower power range.
SiC is mostly replacing silicon IGBT type applications over 600 V and above 3.3 kW, and even more so at about 11 kW, which is really more of a sweet spot for SiC, which means high-voltage operation, low switching losses, and a higher switching frequency power stage, said Rob Weber, product line director, digital gate driver (AgileSwitch), Discrete and Power Management, Microchip Technology.
This allows the use of smaller filters and passives and it reduces the cooling needs, he said. “We’re talking system level benefits versus IGBTs, which is ultimately a reduction in size, cost, and weight.
“From a a loss perspective you can reduce the losses up to 70 percent, for example, at a 30-kHz switching frequency, and that is a result of some of the different characteristics of silicon carbide in terms of the breakdown field, electron saturation velocity, bandgap energy, and thermal conductivity,” said Weber.
The benchmarks that engineers look at is efficiency, which results in levels of improvement, but the other thing that is happening more and more in SiC is the system level benefits over IGBTs, said Weber.
“With silicon carbide you can operate at a higher switching frequency that enables you to have smaller external components that surround the immediate power stage like filters, for example, which are big, heavy magnetic devices; operate at higher temperatures or operate cooler due to the lower switching losses; replace a liquid-cooled system with an air-cooled system, and shrink the size of the heat sink,” he explained.
This component reduction in size and weight, which translates into lower cost, means that SiC goes way beyond getting better efficiency, he said.
However, in a part-to-part price comparison, SiC is still more expensive than traditional silicon-based IGBTs. “The SiC module will cost more from every manufacturer, but when you look at the total system, the SiC system costs are lower,” said Weber.
In an example shared by Weber, one customer was able to achieve a six percent reduction in system costs when using a SiC MOSFET.
Once the designer has made the decision to switch to SiC, they also need to look at the trade-offs. Power semiconductor manufacturers agree there are “secondary effects” like noise, EMI, and overvoltage that have to be dealt with.
“When you’re switching these devices faster, you potentially create more noise which will translate into EMI,” said Weber. “In addition, while SiC is great at higher voltage it is also much less robust than IGBTs for short-circuit conditions and you’re getting variability in your voltage, so you get overvoltage conditions, which is causing some designers to use higher voltage rated SiC devices, so they can control the overvoltage better and overheating.”