Dielectric Properties And Influencing Factors Of Silicone Gel For High Voltage IGBT Device Encapsulation

High-voltage IGBT(Insulated Gate Bipolar Transisitor)devices,as the core components of various kinds of flexible HVDC transmission equipment,are gradually developing in the direction of high voltage,high frequency and large current.Silicone gel is used as insulation material for IGBT device packaging.It bears unipolar repetitive pulse voltage during device operation and the voltage has a wide operating spectrum range.In addition,due to the increase of internal temperature caused by the on-off loss during device operation,the insulation properties of silicone gel face high temperature challenges.Under such a complex working condition,the variation of the dielectric properties of silicone gel will change the electric field distribution inside the device,thus affecting the insulation performance of the device.In order to accurately analyze the electric field characteristics inside the device,the broadband dielectric characteristics of silicone gel were studied from three aspects in this paper.Firstly,a broadband dielectric test platform for silicone gel was developed.In this paper,the principle of measuring broadband dielectric properties of silicone gel is introduced,and a method of measuring broadband dielectric properties of silicone gel using interdigital electrode is put forward in view of the problem that the silicone gel is viscous and easy to deform,and it will produce large error when silicone gel tested under parallel plate electrode.The equivalent capacitance parameters of interdigital electrode were calculated theoretically and experimentally,and the equivalent capacitance parameter was 4.65pF.Thus,the broadband dielectric characteristics of the silicone gel used for the packaging of high voltage IGBT devices were accurately obtained.Secondly,the influence of frequency on the dielectric properties of silicone gel was studied,and the dielectric relaxation model of silicone gel was obtained.The dielectric properties of silicone gel were tested in the frequency range of 0.01 Hz-1 MHz.The effect of frequency on the real and imaginary parts of the relative complex dielectric constant of silicone gel was obtained,and the dispersion phenomenon of the real and imaginary parts in the low frequency domain was analyzed.By analyzing the complex plane graph of silicone gel in the form of complex modulus,the Cole-Cole relaxation model which can accurately describe the broadband dielectric properties of silicone gel is obtained,and the frequency domain dielectric characteristic parameters of the Cole-Cole relaxation model are extracted.Finally,the effects of temperature and Long-term thermal stress on the broadband dielectric properties of silicone gel were analyzed.In the range of 10℃-200℃,it is found that the relative complex permittivity of silicone gel increased obviously with the increase of temperature in the low frequency range.Furthermore,the frequency domain dielectric characteristic parameters at different temperatures were extracted and fitted by the Arrhenius equation and the double-well model,and the influence of temperature on the frequency domain dielectric characteristic parameters was obtained.The effects of thermal stress time at 175℃ on the broadband dielectric properties of silicone gel were studied,and the relationship between relaxation time τ1、τ2 and thermal stress time was established.