Bidirectional onboard chargers (OBCs) are becoming mainstream commercial charging equipment for electric vehicles (EVs) because of their compactness, flexibility, and demand-response capabilities for power backup. This thesis focuses on the novel phase shift (PS) modulation techniques for efficiency improvement for bidirectional OBCs, including two-stage onboard chargers (TSOBCs) and single-stage onboard chargers (SSOBCs). A comprehensive overview and investigation of the state-of-the-art solutions of bidirectional OBCs are presented. It reviews the current industrial status, industrial applications, and future trends and challenges. A detailed overview of the promising topologies for bidirectional OBCs, including two-stage and single-stage structures, is also discussed in this thesis. Traditional PS modulation has been widely used in the back-end DC/DC converters of the TSOBCs because of its simple implementation. However, it is challenging to keep high efficiency at boundary operating points within wide specifications. Therefore, to improve efficiency at the boundary point for TSOBCs, the hybrid multiple phase shift (HMPS) modulation technique with minimal peak current optimization is presented to maximize the zero-voltage switching (ZVS) range. Compared to traditional single phase shift (SPS) modulation, the experimental results verify that the presented HMPS modulation strategy provides 1%-2% higher efficiency at the boundary points. On the other hand, an improved compact SSOBC topology and novel PS modulation techniques are proposed. Since the traditional PS modulation is challenging for AC/DC converters to keep a unity power factor (PF), novel PS modulation techniques are presented for the proposed SSOBC. Firstly, a sinusoidal single phase shift (SSPS) modulation introduces a sinusoidal phase shift to maintain a high PF and high efficiency within a wide operating point. However, due to the high current at the zero-crossing point of the grid voltage of the SSPS modulation, the novel adaptive sinusoidal single phase shift (ASSPS) modulation is presented to address this issue, which reduces conduction loss and increases efficiency. Secondly, based on the ASSPS modulation, the adaptive sinusoidal extended phase shift (ASEPS) modulation with minimal peak current optimization is presented to introduce one more degree of freedom to extend the ZVS flexibility, which reduces switching loss. Moreover, the minimal peak current optimization reduces transformer current, further decreasing conduction losses. Therefore, the power loss is minimized. Finally, this thesis presents the general design guideline of a 6 kW Silicon Carbide (SiC)-based bidirectional SSOBC, contributing to the further development of bidirectional SSOBC application. Experimental results verify the operating principle and high PF of the proposed SSPS, ASSPS, and ASEPS modulation. 1 kW experimental testing has validated that the peak efficiency is 95.3% with ASSPS modulation and 95.9% with ASEPS modulation. Compared to the existing pulse width modulation (PWM), the ASSPS modulation increased efficiency by 1.1%, and ASEPS modulation further increased by 1.7%. / Thesis / Doctor of Philosophy (PhD)

In this study, onboard chargers for EVs are investigated and a design of bi-directional onboard chargers is proposed and simulated. The goal of the charger is to be built in the future to be used in the test setup at Uppsala University. The charger consists of two stages: a power factor correction (PFC) converter, which converts AC voltages and currents from the grid side to DC while maintaining a unity power factor, and a bi-directional buck-boost converter, which regulates the charging and discharging current of the battery. The model was built using MATLAB/SIMULINK and the d-q synchronous reference frame was utilized to implement the current controller of the PFC converter, while the bi-directional buck-boost current controller was constructed using DC pulse width modulation. The Proportional and Integral gains were tuned using the MATLAB single input and single output tool (sisotool). The converter's topologies, structure, and corresponding mathematical model were investigated, and the charger was simulated and tested for charging and discharging modes. The battery voltage, current, and state of charge were monitored during all modes of operation to evaluate the performance of the buck-boost controller, and the functionality of the PFC controller and filter was tested by measuring the currents and voltages on the AC side. The charging and discharging efficiencies were mapped under various battery voltages and current sets to determine the performance of the charger under different operating conditions. The charger demonstrated excellent performance during charging and discharging modes and recommendations for future work to improve the efficiency and performance of bi-directional charging systems were provided.