Cutoff Current Capability of SiC-MOSFETs with Parallel Connected Varistor under UIS Condition

Unclamped inductive switching (UIS) robustness of SiC MOSFETs with parallel connected varistor was evaluated to design the cutoff current capability of solid-state circuit breakers. Because the operation of UIS tests is similar to that in the interruption of solid-state circuit breakers, UIS tests of SiC MOSFETs without varistor and with a parallel-connected varistor were implemented. It was found that the cutoff current of SiC MOSFETs with the varistor was 3 to 6 times larger than that without varistor. The index of rating current for cutoff current capability was changed by parallel varistor connection, because the destruction mechanism of SiC MOSFETs was changed because of the change in self-heating timing during the UIS. From dependence of cutoff current on rating current, load inductance and varistor voltage, it is verified that the cutoff current of SiC MOSFET with varistor was limited by saturation current and current filament.