Degradation mechanism of epoxy-based composite sheet subjected to repetitive voltage pulses under high temperature

This paper presents an experimental study on insulation degradation of epoxy-based composite sheets with boron nitride (BN) for a circuit board in a pulse width modulation (PWM) control unit for inverter-fed motors. Polarity-reversed voltage pulses with damped oscillations are repeatedly applied to the 100μm-thick composite sheets at 200°C with a repetition rate of 1000 pps until complete breakdowns occur. Two kinds of electrode configurations, sphere-plane electrodes with defective mold including air and plane-plane electrodes by molding with great care, are used for experiments. Evaluation of pressure wave signals indicates that the area of partial discharge extension increases with increasing the applied voltage for the sphere-plane system. Results of lifetime measurements show that breakdown time lags for the sphere-plane system are normally distributed because the sheet is gradually degraded by the partial discharges in the air layer between the sheet and the sphere electrode. On the other hand, Weibull plots for the plane-plane system indicate that the breakdown mechanism is mainly based on a stochastic phenomenon caused by an electronic process.