Ideal Switches Pose Their Own Challenges

This blog post was first published by United Silicon Carbide (UnitedSiC) which joined the Qorvo family in November 2021. UnitedSiC is a leading manufacturer of silicon carbide (SiC) power semiconductors and expands Qorvo's reach into the fast-growing markets for electric vehicles (EVs), industrial power, circuit protection, renewables and data center power.

Ask a power converter designer what they want from a semiconductor switch and the answer might be something like: "Low on-resistance, high off-resistance, and the fastest possible transition between the two states." The idea, of course, is that simplistically, this gives low power dissipation. SiC FETs come close to this ideal, with on-resistance now less than 6 milliohms in the 750V class and edge rates that are in the nanosecond region, promising efficiency figures of 99.5% plus for converters and inverters in the multi-kW range.

We now realize that unless we work towards zero connection inductance or wildly over-specify the switch voltage rating and allow for heavy EMI filtering, edge rates need to be controlled and ringing damped. The traditional way to limit the voltage spike has been to add series gate resistance RG(OFF), but this has its own problems, adding a delay to the waveforms that limits duty cycle and high-frequency operation – one of the headline advantages of a wide band-gap switch. A gate resistor also substantially increases switch losses and has little effect on the ringing.

A better solution is a simple RC snubber. You might balk at the idea of the large, hot resistor-capacitor network typically needed with IGBTs, but for SiC FETs, it's different. It is used mainly to damp the resonance between connection inductance and device capacitance, which is extremely low with SiC FETs. This means that a capacitance of around 200pF (2X or 3X of Coss(er)) is all that is typically needed with a few ohms of series resistance to provide the damping. Some power is dissipated in the snubber resistor, but the network acts to reduce overlap between switch-off voltage and current in both hard and soft-switched applications, so losses are actually less on this transition.

A snubber produces some dissipation on turn-on, so for a complete picture, total losses E(ON) + E(OFF) should be considered. The figure shows E(TOTAL) for a 40-milliohm. The blue line is with no snubber and 5 ohms for both RG(ON) and RG(OFF). The yellow line is with 5 ohms for RG(ON), zero ohms for RG(OFF) and with a 200pF/10 ohms snubber. There is an obvious net benefit, using the snubber at 40A of about 10.9W reduced dissipation when operating at 40kHz. At light loads, the situation is reversed, but dissipation is low at these levels.

A snubber is a great solution but is it an inconvenient overhead? If energy absorbed by a snubber resistor is evaluated in a typical application, it could be around 120µJ per cycle, equating to over 5W at 40kHz. However, tests show that most of this is dissipated in the SiC FET channel as it transitions through its linear region at turn-on, rather than in the snubber resistor. The result is that a 1W resistor is typically adequate in the snubber, which could easily be a surface mount type at this power level. The capacitor is small.

The designer can now happily tick off another item in their wish list for a perfect switch – a device that can be easily and cheaply tamed to reduce overshoots and ringing without compromising other benefits.