Temperature dependence of high dielectric strength potting materials for medium voltage power modules

Abstract

Voltage isolation inside power modules is paramount for functional and reliable operation. The dielectric potting materials are further stressed as the overall size of these modules is reduced due to size, weight, and cost considerations while the operating voltage of the modules continue to increase. Voltage ratings of silicon carbide device technologies will continue to increase above 6.5 kV into the tens of kilovolts in the future. Silicon carbide devices are also often operated at higher junction temperatures in order to take advantage of the high temperature capabilities of the material. As the module temperature increases, the dielectric strength of insulating materials in the module tend to decrease, which is a serious consideration for a compact power module operating at many kilovolts. A plurality of high temperature rated, high dielectric strength potting materials were tested for voltage breakdown and leakage current up to 30 kV and 250 °C. A range of different materials, both conventional and novel, were tested including silicones and parylene. Materials were selected with a dielectric strength greater than 500 V/mil, an operating temperature range of 200 °C or higher, and low hardness and modulus of elasticity with the intent of demonstrating the capability of blocking 20 kV or more in a reasonable thickness. A custom test setup was constructed to apply the voltage to test samples while measuring the breakdown voltage and simultaneous recording the leakage current. Test coupons were designed to provide a range of dielectric thicknesses over which to test the dielectric strength. Although voltage isolation may increase with increased dielectric thickness, the V/mil isolation rate often decreases. The performance degradation of these materials over temperature is plotted and deratings are suggested for use with medium voltages at operating temperatures above 175 °C.