Electrical Trees in Silicone Gel Influenced by Different Frequency and Pulse Width 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 DBC (Direct Bounded Copper) ceramic substrate. The voltage level of power module increases from 6.5 kV for Si-IGBT to 10~25 kV for SiC power module, which may lead to electrical tree initiation in silicone gel. Studies on electrical tree focuses on AC sine wave and unipolar voltage with low frequency, distinguishing from the unipolar square wave voltage with high frequency in power module. This paper studies the characteristics of electrical tree 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, 200Hz, … 10 kHz, and the pulse width is 10%, 30%, … 90%. The trees observed in this paper are branch-like or bush-like trees, where the newly generated branch exhibits as a string of bubbles or very thin filament channel. It is found that the voltage amplitude has a significant effect on the propagation of electrical tree. High voltage amplitude leads to denser branches and longer tree length. 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. However, the pulse width mainly influences the electrical tree morphology. This study may be useful for the improvement of insulation and design of power module with high voltage and high frequency.