A Technology Analysis of Voltage Sharing in Series Connected Power Devices

Series connected power devices are required for voltage sharing in high voltage applications like grid connected converters. With SiC considered as a strong contender for grid applications, the performance and reliability issues associated with voltage sharing compared to contemporary silicon bipolar devices is important to consider. 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 zero-temperature coefficient of the power devices determines the voltage sharing and loss distribution in the ON-state while the leakage current and switching synchronization is critical in the OFF-state. In the ON-state, the lower zero-temperature-coefficient (ZTC) point in SiC devices contributes to increasing voltage divergence with the higher thermal resistance device increasingly dissipating more power. In this case, the higher ZTC point in silicon bipolar devices is an advantage although it is a disadvantage for paralleling. Due to the absence of tail currents in SiC devices, they exhibit less voltage divergence during OFF-state transient. The different operating conditions between the series connected devices also determines the voltage sharing during the transient switching of the devices. Using finite-element and experimental measurements, this paper analyzes the technology dependence of voltage sharing in series devices during on-state, off-state and switching.