Fast Space Charge Measurement of Power Module Packaging Insulation Under Square Wave Voltage Based on High Repetition Frequency Pulsed Electroacoustic Method

High voltage power module packaging insulation such as silicone encapsulating elastomer endures unipolar square wave voltage with short transient time (~100 ns, high dV/dt) due to the recent booming of wide bandgap (WBG) semiconductor, which results in space charge accumulation and easier packaging failure compared with the traditional power equipment operating under 50/60 Hz sinusoidal stress. However, the charge dynamic behavior under the high-frequency square wave voltage with short transient time (rising/falling edge) is seldom reported due to the lack of fast space charge measurement. This article proposes a fast space charge measuring system based on a high repetition frequency pulsed electroacoustic (PEA) method. The measuring system mainly includes a nanosecond stimulating pulse generator with repetitive frequency up to 3 MHz based on the Blumlein line, a high voltage square wave source capable of generating the same electric stress as that in WBG power module packaging, and an electrode system with properly set resistor and capacitor. Then, space charge accumulation with time in silicone elastomer under square wave voltage is observed and analyzed by the proposed sub-megahertz repetition PEA measurement. In addition, the effect of the transient time of square wave voltage on charge transport is explored, showing that more charges reside in the tested silicone elastomer for the 100 ns transient time than the $10~\mu $ s transient time. The charge decay near the cathode, anode, and bulk part also presents different patterns which may be related to the energies of trapped charges, electrode-insulation contact, and internal electric field distribution.

I. Introduction

High voltage power modules are widely used in flexible dc transmission, high-voltage direct energy storage, solid-state transformers, electric transportation drives, and other electric power conversion applications. Compared with low-voltage power electronic devices, the use of high-voltage power modules can reduce the number of cascaded power units, significantly increase power density, provide key support for reducing carbon emissions, improve system flexibility, and solve energy shortages [1], [2], [3]. The use of wide bandgap (WBG) power semiconductors has greatly improved the performance of high-voltage power modules, and the research on matching packaging technology is crucial to maximizing the advantages of WBG semiconductors in high-voltage power modules. Due to the presence of body parasitic diodes or external parallel diodes, the package insulation (encapsulating insulation and substrate insulation) of high-voltage power modules such as SiC MOSFET and IGBT mainly withstands unipolar square wave voltages [4], [5]. The encapsulating material such as silicone elastomer encloses the substrate and power chips to form a typical “triple point” as shown in Fig. 1. Due to the extremely uneven geometric structure, the electric field and temperature are generally high at the triple point positions. The electric field and temperature at other positions decrease as they are away from the triple point, hence forming a “double gradient” distribution of the electric field and temperature in the power module package [6], [7]. Since the triple point positions are where the high temperature and electric field are located, space charge injection and accumulation should be investigated. Fig. 1.

Structure of a typical wired-bonded power module.