UIS Ruggedness of Parallel 4H-SiC MOSFETs

We have studied the UIS (Unclamped Inductive Switching) ruggedness of SiC MOSFETs in parallel. We show that UIS ruggedness of parallel MOSFETs is a function of the difference in their breakdown voltage (Δ-BVDSS). As expected, for large Δ-BVDSS UIS, ruggedness is dominated by the lower BVDSS transistor. Somewhat unexpectedly, for small enough Δ-BVDSS, UIS ruggedness is better than the sum of its two transistors. Specifically, the energy that parallel transistors of low Δ-BVDSS can sustain depends on the peak current and is 10%-20% higher than the sum of the energies of the individual transistors. We explain the physical mechanism of this effect and extend the concept to the case of more than 2 parallel transistors. These findings are important for the efficient design of power circuits with multiple die in parallel.