Supervisor:
Stig Munk-Nielsen

Co-Supervisor:
Thore Stig Aunsborg

Assessment Committee:
Yajuan Guan (Chair)
Dr. Jin Wang, Professor, Center for High Performance Power Electronics Director, The Ohio State University
Associate Professor, Davide Fabiani, Department of Electrical Energy and Information Engineering, University of Bologna

Moderator:
Asger Bjørn Jørgensen

Abstract:

With the advancement of renewable energy technologies, power electronics are playing a crucial role in renewable energy systems, grids, and transportation. As traditional silicon (Si) power semiconductors approach their performance limits, there is an increasing focus on wide bandgap (WBG) materials such as silicon carbide (SiC). SiC-based power semiconductor devices offer excellent performance in high-temperature, high-frequency, high-voltage, and high-power density applications. SiC-based medium voltage (MV) power modules, capable of handling voltages of 10 kV and above, reduce the quantity of series-connected devices and simplify system architecture. However, the higher operating voltages and compact size of MV power modules lead to increased internal electric field strengths. Traditional packaging technologies struggle to handle these strong electric field, which increases the risk of partial discharge (PD) and deteriorates insulation performance. This PhD thesis investigates PD performance and proposes new strategies to improve the partial discharge inception voltage (PDIV) and insulation performance of MV power modules.

The thesis develops a framework for evaluating and optimizing the insulation performance of MV power modules using finite element method (FEM) simulations and PD experiments. As well as voltage endurance tests on power module substrates. An insulation lifetime model is established to explore the relationship between insulation lifetime, operating voltage, and PDIV, offering guideline for determining safe operating voltage ranges.

The thesis also examines the effect of modifying the bottom-layer copper structure of substrates on PDIV, revealing that removing the bottom-layer copper significantly improves PDIV. A new module design with complete removal of the bottom copper demonstrates a 67% increase in PDIV without a grounded liquid-cooled heat sink and a 41% increase with it.

Furthermore, the thesis proposes an innovative method for optimizing electric field distribution using guard rings and internal voltage dividers. This method increases PDIV by approximately 40%, offering a cost-effective solution to enhance PDIV without increasing thermal resistance or complicating the DBC manufacturing process. This method is adaptable and can be extended to multi-ring structures or combined with other optimization techniques to further reduce electric field and improve PDIV.