Analysis of Voltage Sharing of Series-Connected SiC MOSFETs and Body-Diodes

In recent years, SiC MOSFETs have gained strong attention in medium-voltage power conversion applications. To increase the blocking voltage level, series-connection of SiC MOSFETs is an attractive solution but may suffer a severe voltage unbalance issue. To gain insights into the voltage unbalance issue, this article presents a detailed study of the impact of parasitic capacitors on the voltage sharing of series-connected SiC MOSFETs and body-diodes. The parasitic capacitors are categorized into two groups for analysis: 1) parasitic capacitors from gate terminal; 2) parasitic capacitors from drain/source terminals. The study reveals that gate parasitic capacitors affect gate miller-plateau voltage and ultimately the dv/dt during the turn-off. The voltage sharing will be worse under higher turn-off current or larger gate resistor. The drain/source parasitic capacitors will introduce additional capacitance across device drain-source terminals which results in an unbalanced voltage sharing. The position of switching unit and the heatsink connection schemes will affect the distribution of drain/source parasitic capacitors to cause different voltage sharing results. The drain/source parasitic capacitors will also cause voltage unbalance of series-connected body-diodes under different conditions. To verify the analysis, the voltage sharing between two series-connected 10 kV SiC MOSFETs is tested under different parasitic capacitors conditions.