Modelling SiC MOSFET module threshold voltage (VTH) and impact of parallel device ΔVTH on short circuit robustness

In SiC modules, where several power MOSFETs are connected in parallel for high current conduction capability, the issue of threshold voltage (VTH) variation between individual devices can become problematic. In instances where the standard deviation of device VTH is minimized by appropriate VTH pre-screening, non-uniform VTH shift under bias-temperature-instability can lead to increased VTH dispersion and potentially poor current sharing. Although non-destructive for normal operation, VTH dispersion in surge events like short circuits can cause premature module failure. In this paper, experimental investigations backed by theoretical models have been developed to explain the relationship between device VTH dispersion and module VTH shift. This is done for Si and SiC MOSFETs. Five Si/SiC MOSFETs are paralleled in a custom printed-circuit-board allowing individual VTH and module VTH measurements. It is shown that the module VTH is typically between the smallest VTH and the mean VTH, depending on the VTH standard deviation. Using empirical models for MOSFETs in weak inversion derived from the measured subthreshold gate transfer characteristics, the model can estimate module VTH from a dispersion of individual MOSFET VTH. It can also predict the impact of non-uniform VTH shift between constituent devices on the overall module VTH. Finally, the impact of VTH mismatch on current sharing during short circuits in parallel connected SiC MOSFETs is presented. The results show that VTH mismatch in parallel connected SiC MOSFETs reduces the short circuit withstand time from 5 μs to 4.5 μs compared to VTH matched SiC MOSFETs.