Characteristics of Electrical Tree Growing on Gel-Substrate Surface for Power Module at High Frequency Positive Square Wave Voltage

Electrical tree growing on the gel-substrate interface is the main form of insulation failure in power module packages. Studying electrical tree behavior can reveal the insulation degradation process, enabling targeted measures for inhibition. However, treeing behavior along the interface are currently unclear. For the wide frequency range of operating requirements of power modules, electrical tree characteristics growing on the gel-substrate interface under positive square wave voltage with a frequency of 1 kHz~20 kHz were investigated in this paper. First, electrical tree characteristics at different frequencies were analyzed in terms of initiation, morphology and propagation. Further, the mechanism of electrical tree characteristics and the effect of frequency were investigated. The results show that the electrical tree inception voltage (ETIV) decreases with frequency. ETIV decreases more from 1 kHz to 10 kHz than from 10 kHz to 20 kHz due to the weakening of the electric field by homopolar space charge. Higher frequency reduces charge injection depth and increases dielectric loss, resulting in a denser initial tree morphology. The electrical tree evolves from branch-like to bush-like when 1 kHz<
f
<10 kHz, while evolves from bush-like to pine-like when 10 kHz≤
f
≤20 kHz. Electrical tree morphology transformation is influenced by partial discharges, material properties, charge transport, and tree conductivity. Carbon depositions increase with frequency, affecting electrical tree morphology by increasing tree conductivity. The growth rate decreases with length for 1 kHz<
f
<10 kHz, but decreases first and then increases for 10 kHz≤
f
≤20 kHz. The field driven tree growth (FDTG) model is used to explain the characteristics of tree propagation when 10 kHz≤
f
≤20 kHz.