Temperature Dependence of High Dielectric Strength Potting Materials for Medium Voltage Power Modules

Voltage insulation inside power modules is paramount for functional and reliable operation. Dielectric potting materials are stressed as the overall size of these modules is reduced due to size, weight, and cost considerations while the operating voltage of these modules continue to increase. In particular, voltage ratings of silicon carbide (SiC) device technologies will continue to increase above 6.5 kV into the tens of kilovolts in the future. SiC devices are also often operated at higher junction temperatures 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 tends to decrease, which is a serious concern for a compact power module operating at many kilovolts. A plurality of high-temperature-rated, high dielectric strength potting materials was 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 >20 kV/mm, 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 simultaneously 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 volt per millimeter isolation rate often decreases. The performance degradation of these materials over temperature is plotted, and insulation thicknesses are suggested for use with medium voltages at operating temperatures above 175°C.