Electrical Trees in Silicone Gel Influenced by Different Frequency and Pulsewidth Under Unipolar High-Frequency Square Wave Voltage

As the encapsulation material, silicone gel is in contact with the high voltage electrodes of the semiconductor device and direct bonded copper (DBC) ceramic substrate. The voltage level of the power module increases from 6.5 kV for Si-IGBT to 10–25 kV for the SiC power module, which may lead to electrical tree initiation in silicone gel. Studies on electrical trees focus on ac sine wave and unipolar voltage with low frequency, distinguishing from the unipolar square wave voltage with high frequency in power modules. This article studies the characteristics of electrical trees in silicone gel under unipolar high-frequency square voltage, the real working environment of silicone gel. The imposed voltage is 10–18 kV, the frequency is 50 Hz, 200 Hz,…, 10 kHz, and the pulsewidth is 10%, 30%,…, 90%. The trees observed in this article are branch-like or bush-like trees, where the newly generated branch exhibits a string of bubbles or very thin filament channels. It is found that the voltage amplitude has a significant effect on the propagation of the electrical tree. High-voltage amplitude leads to denser branches and longer tree lengths. Similarly, the frequency can promote the growth of electrical trees. Higher frequency leads to longer length and higher fractal dimension, as well as higher growth rate. The pulsewidth, however, mainly influences the electrical tree morphology. This study may be useful for the improvement of insulation and design of power modules with high voltage and high frequency.