Finite element electrothermal modelling and characterization of single and parallel connected power devices

Power modules typically comprise of several power devices connected in parallel for the purpose of delivering high current capability. This is especially the case in SiC where small active area and low current MOSFETs are the only option due to defect density control and yield issues in the epitaxial growth of SiC wafers. Electrothermal variations between parallel connected devices can emerge from manufacturing variability, non-uniform degradation rates, variation in gate driving just to mention a few. The impact of electrothermal variation between parallel-connected devices as a function of device technology is thus important to consider especially since failure of the power module requires only failure in a single device. Furthermore, the impact of these electrothermal variations in parallel-connected devices on the total electrothermal ruggedness of the power module under anomalous switching conditions like unclamped inductive switching is important to consider for the different device technologies. In this thesis, the impact of initial junction temperature variation, switching rates and thermal boundary conditions between parallel-connected diodes have been evaluated for SiC Schottky and silicon PiN diodes under clamped and unclamped inductive switching. Finite element simulations have been used to support the experimental measurements. Similar studies have been performed in CoolMOS super-junction MOSFETs, silicon IGBTs and SiC power MOSFETs. New insights regarding the failure of parallel connected devices under unclamped inductive switching have been revealed from the models and measurements. Overall, the thesis makes a major contribution in the understanding of the electrothermal performance of parallel connected devices for different transistor and diode technologies.