Date of Graduation

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Zhao, Yue

Committee Member

Hu, Han

Second Committee Member

Song, Xiaoqing

Third Committee Member

Chen, Zhong

Keywords

Magnetics; Medium Voltage; Power Electronics; WBG devices

Abstract

To develop high-performance high- power-density MV power converters, the emerging silicon carbide (SiC) devices are more attractive than their silicon (Si) counterparts, since the fast switch frequency brought by the SiC can effectively reduce the volume and weight of the filter components and thus increase the converter power density. From the converter topology perspective, with the MV dc distribution, the single stage isolated dc/dc converter are suitable for next-generation electric aircraft system due to soft switching and high power density. In this work, comprehensive static and dynamic characterizations were conducted for the latest 6.5 kV silicon carbide (SiC) MOSFETs from room temperature to 175 °C. A custom clamped inductive load test setup is developed to evaluate the switching performance and losses for the medium voltage (MV) SiC devices. Grounding paths due to the parasitic capacitances in the test setup are analyzed and reduced to mitigate impacts to drain current sensing during the switching period. It presents the impact of negative gate bias on the reverse recovery of the medium voltage (MV) silicon carbide (SiC) planar MOSFETs for high-temperature applications (from 25 °C to 175 °C). The difference between low voltage and MV SiC device of the 3rd-quad characteristic at different gate bias is introduced. The impact of negative gate bias on conductivity modulation and reverse recovery is discussed. A custom clamped inductive load (CIL) test setup is built for the MV SiC devices to evaluate the body diode performance. It is found that the reverse recovery is much worse under negative gate bias at high temperature. As a result, the negative gate bias leads to significant increase of reverse recovery charge and energy. In this work, an all silicon carbide (SiC) series-resonant converter (SRC) design is proposed and demonstrated to achieve a single stage dc-to-dc conversion from 3kV to 540V (±270V) for future electric aircraft applications. The proposed SRC consists of a neutral-point-clamped (NPC) converter using 3.3kV SiC MOSFETs on the primary side, a H-bridge converter using 900V SiC MOSFET on the secondary side and a high frequency (HF) transformer. The detailed design methods for the SRC power stage and the HF transformer are presented. Especially, a tradeoff between the complexity for the cooling system and the need for high power density and voltage insulation is addressed in the transformer design, leading to a novel multi-layer winding structure design to enhance the insulation capability and also the mechanical robustness. The proposed bobbin design is realized using additive manufacturing. The detailed analysis and modeling of the parasitic capacitance between sections introduced by fringe electrical field is presented. To validate the effectiveness of the proposed SRC design, a 25kW converter prototype using 3.3kV SiC discrete devices has been developed with a peak efficiency of 99.08% achieved in experimental studies. In addition, using these latest 6.5 kV SiC MOSFETs, a 25 kW all SiC series-resonant converter (SRC) is proposed to enable a single stage dc-to-dc conversion from 43 kV to 270 V aiming at applications on future electric aircraft with onboard MVDC distribution. The proposed SRC consists of a 2-level half-bridge stage using 6.5 kV discrete SiC MOSFETs on the primary side, a full-bridge converter using 900 V SiC MOSFET modules on the secondary side and a high frequency transformer. Compared to a 3-level neutral-point-clamped converter using 3.3 kV discrete SiC MOSFETs, the efficiency and power density of the 2-level 6.5 kV SiC devices based converter are increased. The MV transformer with separated winding and core structure is designed for lower parasitic capacitance, improved thermal performance, and better insulation between primary and secondary windings. To validate the effectiveness of the proposed SRC design, a converter prototype is developed and comprehensive experimental studies are conducted.

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