Origins of Soft-Switching Coss Losses in SiC Power MOSFETs and Diodes for Resonant Converter Applications

The recent development and commercialization of wide bandgap (WBG) power semiconductors, specifically gallium nitride (GaN) and silicon carbide (SiC), have driven the increase in switching frequency for soft-switching power converters, such as the Class E, Class Φ
₂
, and Class DE resonant inverters and rectifiers. However, prior literature has characterized numerous commercial GaN and SiC devices using the Sawyer-Tower circuit and discovered significant large-signal C
_oss
charge-voltage hysteresis. This C
_oss
hysteresis, equivalent to OFF-state energy loss, is highly dependent on the frequency and voltage across the device, hindering the efficiency and performance of MHz-range soft-switched converters. This article is the first to explain the origin of the C
_oss
loss in SiC power devices as charging and discharging conduction losses at the termination of the device. The loss characteristics relative to the operating voltage, frequency, dV/dt, and temperature are dictated by incomplete ionization. Incomplete ionization also highlights a significant inconsistency between the large-signal C
_oss
behavior and small-signal behaviors, which is often the model used in manufacturers' datasheets and SPICE simulations. The large-signal charge-voltage behavior is transient, where the charge in C
_oss
depends on the rate of the voltage swing across the device. We validate these hypotheses through mixed-mode simulations using the Sentaurus technology computer-aided design (TCAD) and experimentally using commercial and custom SiC devices.