Impact of Leakage Currents on Voltage Sharing in Series Connected SiC Power MOSFETs and Silicon IGBT Devices

Low leakage currents and reduced temperature sensitivity in SiC power devices compared to silicon devices results in the voltage sharing under series connection being more stable even under temperature variation between the series devices. Leakage currents result from impact ionization in the voltage blocking drift region, hence, the device with the higher leakage current under a series arrangement will have lower output capacitance therefore will block less voltage in the series pair. Series connected power devices are required for voltage sharing in high voltage applications like grid connected converters and DC circuit breakers. In applications where series power devices may be at different junction temperatures as a result of the physical architecture of the converter cooling system or differential degradation of the packaging, the leakage current and switching synchronization is critical in the OFF-state. Snubbers are traditionally used for static and dynamic voltage balancing in series connected power devices, however, there is increasing interest in active gate control. Therefore, the impact of the power device technology on voltage sharing in series devices is important to enable snubberless operation via active gate control. This paper investigates voltage sharing in series connected devices by comparing SiC trench MOSFETs and Si IGBTs. Series devices have been tested under different temperatures and switching rates to investigate the impact of electrothermal variation on voltage imbalance. Measurements and finite element models have been used to support the analysis.