Unclamped Inductive Switching Robustness of SiC Devices With Parallel-Connected Varistor

Unclamped inductive switching (UIS) robustness of SiC devices with parallel-connected varistor was evaluated to design the cutoff current capability of solid-state circuit breakers (SSCBs). Because the operation of UIS tests is similar to that in the interruption of SSCBs, UIS tests of SiC devices without varistor and with a parallel-connected varistor were implemented. It was found that the cutoff current of SiC devices with the varistor was much larger than that without varistor. The effect of varistor on the increase of cutoff current depended on the device type and rating current. The cutoff current was 3–6 times higher for planar MOSFETs and 5–10 times higher for trench MOSFETs compared with no varistor condition. In contrast, the effect of varistor for JFET was small because the gate drive condition strongly affected the current switching time from SiC-JFET to varistor. The index of rating current for cutoff current capability was changed by parallel varistor connection because the destruction mechanism of SiC devices was changed because of the change in self-heating timing during the UIS.