Space Charge Characteristics of Silicone Gel for Power Module Encapsulation at High Temperatures

Abstract

With the advancement of power modules toward high voltage and high power, the encapsulation reliability faced by power modules has attracted great attention under increasing electrical stress and thermal stress. Silicone gel is prevailing for encapsulation with excellent electrical properties. Subjected to direct current (dc) voltages, space charge accumulation will cause local electric field distortion and partial discharge. Thus, it is the intention of this article to study space charge accumulation and breakdown characteristics of silicone gel at high temperatures. Based on the pulsed electroacoustic (PEA) method, experiments are carried out to obtain space charge distribution at high temperatures and homocharge is found to accumulate in silicone gel. The electrical performance is also characterized and analyzed by systematic dc tests. Temperature plays an important role in affecting insulation properties of silicone gel, causing a decrease in breakdown field and an enhancement in dc conductivity. Analyses show that the decrease in dc breakdown strength of silicone gel attributes to two reasons. For one thing, the intensification of charge injection leads to an overall increase in distortion rate of partial electric field. For another, the shallow traps are dominant in silicone gel and the molecular chains show high activity on the freedom volume as temperature rises. The measurement and analysis will provide references for further application of silicone gel in power module encapsulation.