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Temperature dependence of high dielectric strength potting materials for medium voltage power modules
Keywords: dielectric, materials, medium voltage
Voltage isolation inside power modules is paramount for functional and reliable operation of these devices. 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 transformer oils, greases, silicones, and parylene. The materials were cured according to their data sheet recommendations. Materials were selected with a dielectric strength greater than 20 kV/mm, an operating temperature range of 200 °C or higher, and low hardness and modulus of elasticity. The selected materials were intended to be used as a dielectric potting materials around all internal metal substrate, device, and wire bond components. Thus, it is important that the potting material be very conformal and not induce significant stresses onto the wire bonds or plastic sidewalls during thermal cycling. 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 gaps over which to test the dielectric materials. Although voltage isolation may increase with increased dielectric thickness the kV/mm isolation rate decreases. The performance degradation of these materials over temperature is plotted and derating guidelines are proposed for use with medium voltages at operating temperatures above 175 °C.
Chad B. O’Neal, Senior Research Engineer
Arkansas Power Electronics International Inc.
Fayetteville, AR

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