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.
Since gate drivers are used to drive the power device, it is a critical piece of the power puzzle. One way to ensure an optimized design using SiC includes first carefully considering your choice of the gate driver. Simultaneously, it requires a close look at the key requirements of your design – efficiency, density, and, of course, cost – because there are always trade-offs, depending on the application requirements.
SiC MOSFET Versus Silicon IGBT
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. This allows the use of smaller filters and passives and it reduces the cooling needs.
From 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.
SiC vs. Si and IGBTs (Source: Microchip Technology)
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.
With silver 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.
This component reduction in size and weight, which translates into lower cost, means that SiC goes way beyond getting better efficiency.
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.
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. 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.
This is where the selection of the gate driver plays a big role. SiC has unique requirements for characteristics such as supply voltage, fast short-circuit protection, and high dv/dt immunity.
Selecting The Sic Gate Driver
When it comes to selecting the right gate driver for SiC switches, it takes a new mindset in thinking about the power solution compared to silicon-based devices. The key areas to look at include topology, voltage, bias, and monitoring and protection features.
The selection of the SiC driver is vital, and historically it was okay to use a sequential approach to selecting the gate driver. Prior to SiC you’d pick the IGBT first then the gate driver second then the busbars and the capacitor, etc. It’s totally changed.
You have to look at the whole holistic solution that you’re building and those tradeoffs at every step instead of this sequential approach that you have with IGBTs. It’s been an education for a lot of customers.
Topology, power level, protection and functional safety requirements, and the generation of SiC devices in use will dictate the kind of SiC driver needed for the application. For example, a non-isolated driver, which could require a lot of extra circuitry, is good for simpler applications, where not everything has to be integrated into the driver.
There are also isolated drivers that can handle the negative bias and isolation issues but will still need some kind of monitoring in the system, up to devices that offer further integration such as monitoring and protection circuitry and functional safety for automotive applications.
The checklist for deploying SiC the right way is to look at the topology and what kind of devices you have to drive then pick the gate driver, optimize the bias, figure out what kind of protection is needed, and then optimize the layout.
From a driver point of view, it’s having the right bias, so the right voltage capability, whether you need isolated or non-isolated gate drivers, how much protection is needed, which ties into the integration level [for protection and safety] or how much extra circuitry is required, he added.
One of the things that has hindered SiC a little bit is the realization that because of the higher switching speed it needs to be put into a package where the source inductance is eliminated. Source inductance can be nasty and cause a lot of ringing and additional power losses because it slows down the switching action.
This is where the layout engineer becomes your best friend because you really have to look at the layout to mitigate the ringing and optimize it for high speed switching. This includes minimizing the trace inductances and separating the gate loop from the power loop and proper bypassing [of the switched current path and broad frequency band] by selecting the right components.
What’s really critical is connecting the driver to the switch. You have to connect the ground of the driver directly to the source of the power switch because of stray inductances that can increase switching losses.
Key Points to Ponder
When selecting a SiC MOSFET the first critical question to ask is ‘do I need unipolar or bipolar driving’ for that component.
There are fast and robust drivers right in the market that can drive both Si as well SiC but what people need to be cautious about when moving from Si to SiC is how you drive it because silicon is driven with a typical voltage of 12 volts.
You are using 12-V to turn-on and using 0 V to turn-off, so the normal voltage range for the driver driving silicon components or superjunction MOSFETS is 0 to 12 volts and that’s across the board from any silicon component supplier.
On the other hand, SiC devices from different vendors will have different turn-on/turn-on voltages. There are SiC MOSFETs in the market, for example, that require +15 V to turn it on and -4 V to turn it off, or +20 V for turn-on and -2 or -5 V for turn-off, Ivankovic said. This requires a driver that enables the use of positive and negative voltages.
Thus, be careful about whether you need a unipolar gate driver or you need both a positive and negative to drive the component properly.