Date of Graduation

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Zhao, Yue

Committee Member

Song, Xiaoqing

Second Committee Member

McCann, Roy A.

Third Committee Member

Zhang, Shengfan

Keywords

Power converter; Powertrain electrification; SiC

Abstract

With the increasing concern towards environment protection, electric vehicles (EVs) and hybrid electric vehicles (HEVs) have become popular in recent years. They not only help reduce carbon emissions, but also come with other benefits such higher efficiency and better user experience. Many countries and vehicle manufactures have made their plans to keep up with this trend in vehicle electrification. Powertrain is an assembly of components in a vehicle that pushes it forward. As for EVs and HEVs, their powertrains are composed of one or more power electronics converters, which are significant to the system. Silicon carbide MOSFETs have shown superior characteristics and thus are being widely used in different applications including EVs and HEVs. This dissertation focuses on key technologies to improve the performance of high-power SiC converters in EV and HEVs’ powertrain systems. The architecture of this dissertation can be divided into two major parts according to the targets investigated. The first part of the dissertation centers around the power converter topology in powertrain system, including DC-DC boost converter and inverter. The second part investigates two different current balancing methods to help realize balanced current sharing between parallel connected SiC MOSFETs, which is often used to increase power rating of converters. A DC-DC converter is applied in some EV/HEV powertrains to boost the battery voltage for the DC bus of inverter. Light load scenario happens frequently in EV/HEVs’ driving profile, but the efficiency of conventional boost converter is low in that range. Therefore, a composite DC-DC converter topology is proposed in chapter 2 to enhance the light load efficiency. A multi-variable optimization method during the converter operation is also studied for the same purpose. A 30-kW composite converter prototype was built and the light load efficiency improvement brought by the proposed optimization method has been validated in experiments. Inverter is another key power electronics converter in the electrified powertrain system. Soft switching technology may bring multiple benefits such as higher efficiency and reduced electromagnetic interference issue. The auxiliary resonant commutated pole (ARCP) soft switching inverter topology has been selected for high power motor drive applications. Simulations study and an optimization design method are presented in chapter 3. A 10-kW ARCP soft switching inverter was assembled. Double pulse test and R-L load continuous have been performed to validate the soft switching functionality. The ARCP inverter has been tested to have higher efficiency while lower voltage slew rate compared to the conventional hard switching inverter. A hybrid closed-loop current balancing method is proposed in chapter 4. Causes of steady-state and transient imbalance current issues are studied via simulation. The steady steady-state imbalance current is first suppressed by inserting inductors between AC output of paralleled half-bridge modules and the load. As for heavy-duty electrified vehicles which have large profiles, the inserted inductors can be implemented by the self-inductance of power cables. The remaining transient imbalanced current can be thereby sensed easily during the steady state with conventional low-cost sensors. Based on this, a closed-loop control by regulating modulation reference signals of different paralleled phase legs is introduced to further balance the remaining current differences. The proposed method has been applied to high power inverter with 2 and 4 SiC modules paralleled in each phase. A good current sharing has been observed in both cases. Active gate drivers have been widely investigated to regulate switching performance of SiC MOSFETs, which could also be adopted for current balancing purposes. An active gate drive control method is proposed in chapter 5 for steady-state current balancing, which is realized by controlling gate voltages in PWM scheme to regulate the equivalent on-state drain-to-source resistance of paralleled MOSFETs. A time delay is applied to different paralleled MOSFETs suppress the transient unbalanced current together with the proposed method. An active gate driver board was assembled and tested. The proposed method shows good robustness when tested with different number of SiC modules paralleled and at different temperatures. The currents and losses are balanced among all the parallel connected SiC modules on all the testing cases. The total losses in tests using proposed active gate driver method are not increased when compared to losses in tests using conventional gate driver method.

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