Understanding the Turn-off Behavior of SiC MOSFET Body Diodes in Fast Switching Applications

Abstract:
The turn-off behavior of SiC MOSFET body diodes differs from that of Si pn-diodes in IGBT circuits due to their unique device characteristics. For SiC MOSFETs in the 1200 V class, the output capacitance has a larger, and the bipolar charge of the pn-diode, a smaller impact. However, at elevated temperatures and high current densities, the bipolar charge is of equal importance as the capacitive charge, even for 1200 V devices. In fast switching applications, the commutation loop stray inductance has a significant impact on body diode turn-off, which translates into pronounced surge voltages and oscillations. For Si pn-diodes, turn-off losses are specified as reverse recovery energy. However, the conventional definition to determine Erec and Qrr may give misleading and inconsistent results for SiC MOSFETs that do not represent the actual device properties. This paper discusses the unique characteristics and various influences on body diode turn-off behavior, and clarifies the concept of reverse recovery losses for SiC MOSFETs in fast switching applications.