Analysis of Overvoltage Across Blanking Capacitor Due to Junction Capacitance of Sensing Diode in Desat Protection of SiC MOSFET

Wide-band-gap devices exhibit rapid switching capabilities, enhancing conversion efficiency and reducing converter size. However, their low short circuit withstand time (SCWT) capacity necessitates swift and precise fault detection and protection mechanisms. The reduction in fault detection time in desaturation (Desat) based protection requires minimising blanking capacitance. When the blanking capacitance is comparable to the sensing diode junction capacitance, the reverse recovery current of the sensing diode and oscillations across the device lead to overvoltage across the blanking capacitor during device turn-off and discontinuous current mode (DCM) operation, respectively, which leads to spurious fault detection. Based on variable junction capacitance of sensing diode, this paper presents an analytical method to estimate overvoltage across blanking capacitor during turn-off. This paper also derives a transfer function relating the device voltage oscillations with blanking capacitor voltage oscillations during DCM, which shows the impact of reduced blanking capacitance on overvoltage. Simulations and experiments validate the accuracy of the analytical methods proposed. Additionally, a recently proposed voltage clamping method is implemented to clamp such overvoltages, and experimental results on an 800-V(dc), 50-A(rms), and 50-kHz asymmetric H-bridge (AHB) converter are presented.