Partial discharge (PD) occurring in packaging insulation under square pulse is one of the main causes of power module failure. Different rise times of the square pulse usually lead to different PD characteristics. This paper focuses on the effect of the square pulse rise time on PD characteristics of power module packaging insulation and a down-mixing PD detection method is applied to extract PD signals under square pulse with ultra-short rise time. A PD specimen based on the structure of power module packaging is fabricated and PD experiments under square pulse with three different rise times of 120, 500, and 1000 ns are performed. The experimental results show that the partial discharge inception voltage (PDIV) gradually decreases as the rise time increases. The statistics of PD magnitude indicate that a shorter rise time could result in a larger average PD magnitude. According to the experimental phenomena, the mechanism of the rise time effects on PDIV and PD magnitude are analyzed based on the electron appearance stochastic delay theory. A shorter rise time of square pulse may result in a higher PDIV of the packaging insulation, but more PDs with large magnitude will occur, which can lead to serious degradation of encapsulation and further influences insulation reliability.

partial discharge,square pulse,rise time,power module packaging insulation