Enhancing partial discharge inception voltage of DBCs by geometrical variations based on simulations of the electric field strength

As a consequence of higher voltages in power modules the partial discharge (PD) has to be studied more closely for the electrical insulation such as direct bonded copper (DBC) substrates. PD is a key degradation mechanism of insulating materials. This paper focusses on the partial discharge inception voltage (PDIV) of DBC substrates based on Al2O3 or AlN ceramics and the influence of the localization of the upper metallization relative to the lower metallization. Due to high electric field strengths in the vicinity of the edge of the metallization PD occurs in the voids of the ceramic material. Simulations of the electric field strength were carried out to analyze the effect of the layout of the metallization, the relative position of the upper to the lower metallization. The results of the simulation show a possible reduction of the electric field strength at the edge of the metallization due to geometrical variations. The experimental validation was done by phase resolved partial discharge measurements (PRPD) on different DBCs with varying ceramic materials and thicknesses. Each DBC was varied with respect to metallization layout and a dependency regarding the PDIV could be identified and thereby lead to a possible improvement by an adaption of the relative position of the upper to the lower metallization. This design optimization allows a PDIV enhancement of up to 20 % concerning the established design.