PhD defence by He Du on Impact of Short-Circuit Events on the Remaining Useful Life of SiC MOSFETs and Mitigation Strategy

He Du, Department of Energy Technology, will defend the thesis "Impact of Short-Circuit Events on the Remaining Useful Life of SiC MOSFETs and Mitigation Strategy"

TITLE

Impact of Short-Circuit Events on the Remaining Useful Life of SiC MOSFETs and Mitigation Strategy

PHD DEFENDANT

He Du

SUPERVISOR

Professor Francesco Iannuzzo

CO-Supervisor

Associate Scientist Paula Diaz Reigosa

MODERATOR

Professor Xiongfei Wang

OPPONENTS

Associate Professor Pooya Davari, Dept. of Energy Technology, Aalborg University (Chairman)
Associate Professor Marina Antoniou, The University of Warwick, United Kingdom
Head of Research Stefan Mollov, Mitsubishi Electric R&D Centre Europe, France

ABSTRACT

Power semiconductor devices are critical components in power electronic systems. Thanks to the unprecedented performance compared with silicon-based devices, silicon carbide (SiC) devices are desirable for high-efficiency and high-power-density power converters. However, the proliferating demand for SiC MOSFETs raises concerns for long-term reliability and short circuit is one of the critical aspects as the devices withstand both high voltage and high current at the same time.
With the development of faster fault detection and short-circuit protection, the risk of catastrophic failure can be reduced, which allows the device to withstand many non destructive short-circuit events during its life. Typically, the remaining useful life (RUL) of the devices can be estimated based on the condition monitoring parameters, i.e. the ageing indicators and it only considers the wear-out failures under normal operating conditions. Therefore, this Ph.D. thesis aims at investigating the impact of short-circuit events on the RUL of SiC MOSFETs.
The thesis starts with an analysis of the degradation indicators under short-circuit conditions to identify the one which can best reflect the short-circuit degradation process and two test approaches were performed. Then, static and dynamic characteristics measurements were performed during repetitive short-circuit tests, respectively and the purpose is to investigate the impact of short-circuit degradation on the normal operating performance of SiC MOSFETs. As the case temperature is subjected to real application conditions, its influence is also considered by performing the repetitive short-circuit tests at different case temperatures.
Thereafter, power cycling tests were used to evaluate the wear-out ageing and lifetime of the devices. Mixed power-cycling/short-circuit tests were performed to assess the impact of short-circuit degradation on the power-cycling conditions, and further on, the RUL by applying different short-circuit stress. To deeply analyse the degradation mechanisms, failure analysis tools (lock-in thermography, focused ion beam, and scanning electron microscope) were used on both failed and functional device. Besides, a transient thermo-mechanical simulation based on finite-element-analysis software was achieved to verify the degradation mechanisms. Finally, a mitigation strategy comprising a top-side thermal mass is studied.
The main contribution of this thesis is the strong implication of short-circuit events on the RUL of SiC MOSFETs. With a larger number of events, the higher gate leakage current results in higher junction temperature swing and less number of cycles to failure. The root cause of gate leakage current is the mismatch of coefficient of thermal expansion between Al and SiO2, which forms the SiO2 interlayer crack, and the molten Al flowing through the crack, which builds a conductive path between gate- and source terminals. Moreover, sintered Cu foil on the top side could be an effective approach to mitigate this short-circuit degradation.

THE DEFENCE IN ENGLISH - all are welcome.

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