Supervisor:
Michael Møller Bech

Co-Supervisor:
Stig Munk-Nielsen

Assessment Committee:
Ariya Sangwongwanich (Chair)
Professor Dr.-Ing. Regine Mallwitz, Technische Universität Braunschweig
Professor dr. hab. inz, Jacek Rabkowski, Dyrektor, nstytut Sterowania i Elektroniki Przemyslowej

Moderator:
Asger Bjørn Jørgensen

Abstract:

With the growing demand for green energy transition, medium-voltage (MV) silicon carbide (SiC) power semiconductor technology has emerged as a promising solution to enhance efficiency, increase power density, reduce design complexity, and boost generation capacity in future renewable energy systems. This thesis addresses key challenges in high-power renewable energy systems based on MV SiC MOSFETs. In particular, parasitic capacitances lead to high switching currents Cdv/dt and extra switching loss 1/2CV2, while voltage spikes caused by parasitic inductances Ldi/dt are relatively less critical. The thesis identifies the sustained oscillations that can occur between paralleled MOSFETs, posing a reliability risk to power converters.

To understand and mitigate sustained oscillations, a small signal model is developed. Using a simplified symmetrical circuit model that includes the relevant parasitic elements, the analysis reveals impacts of parasitic elements contribute to the sustained oscillation between paralleled MV SiC devices. From these insights, practical design guidelines are derived and confirmed by experimental measurements. Based on the small signal model of power module with parasitic parameter and SiC MOSFET intrinsic characteristics, a novel mitigation method called differential damper is proposed for sustained oscillation among paralleled SiC MOSFET chips. By adding a small resistor between the gates of the parallel MOSFET chips, the sustained oscillation is effectively suppressed. The optimal value of this damping resistor is determined through both theoretical analysis and experiments, and tests show that it eliminates the sustained oscillations effectively. In addition, a power-converter platform combining low-voltage and MV SiC stages is designed and built for a wind-turbine application. This system-level platform allows investigation of related issues such as common-mode currents flowing into the control and protection circuits, and parasitic capacitances at the motor drive output. These factors can cause electromagnetic interference or malfunction.

Overall, this research provides a comprehensive study of sustained oscillations in MV SiC devices and its practical solutions. By presenting the modeling method, and experimental validation, the thesis helps to provide design guidelines for engineers and researchers in robust SiC-based power modules and converters.