High power density on-board chargers (OBCs) for electric vehicles (EVs)

Nasr Esfahani, Fatemeh and Badawy, Ahmed and Ma, Xiandong (2025) High power density on-board chargers (OBCs) for electric vehicles (EVs). PhD thesis, Lancaster University.

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Abstract

The thesis presents a novel modular integrated on-board charger (MIOBC) specifically designed for electric vehicles (EVs). The proposed modular charger improves safety, fault-ride-through (FRT) capability, and battery monitoring and control by modularising the high voltage (HV) battery and power converters. The modular charger utilises single-stage and two-stage Cuk-based and SEPIC-based converter topologies as submodules (SMs), chosen for their high efficiency and continuous input and output currents. The single-stage design avoids the need for separate power factor correction (PFC) stages and bulky DC-link capacitors, while the two-stage design simplifies control and minimises voltage ripple. High-frequency (HF) transformers, operating in the range from 20 kHz to 50 kHz, are incorporated within the SMs to provide galvanic isolation, enhancing safety and enabling voltage boosting. The traction inverter (motor drive) is integrated into the on-board charger (OBC). This integration can improve power density by up to 20-30% by minimising the overall system size, weight, and cooling requirements. The MIOBC can operate in three modes: charging from the AC grid, normal driving (acceleration), and regenerative braking (deceleration). The thesis includes dynamic models of Permanent Magnet Synchronous Motors (PMSM) and EVs. Detailed mathematical modelling and power loss analysis for semiconductor devices are provided, along with switching principles for various operating modes, namely DC-AC rectification, AC-DC inversion, and DCDC conversion. A subinterval is added in the DC-AC mode to decouple the input and output sides of the SM, trapping the second-order harmonic component from the single-phase AC grid within the SMs’ passive components rather than transferring it to the HV batteries. State-space representations are used to derive line-to-output and control-to-output transfer functions under the small-signal AC assumption. These transfer functions are employed to develop required control systems at the system level and SM level for different operating modes. To address the phase lag (delay) caused by right-half-plane (RHP) zeros in the transfer function of the Cuk-based SMs, classical controllers, such as proportional-resonant (PR), proportional-integral (PI), and lead-lag compensators, are employed in the system-level control loops (outer loop). The loop-shaping method is utilised to design the controllers’ gains, according to the desired phase margin (PM), gain margin (GM), and bandwidth (BW). Given the trade-offs between system stability and responsiveness inherent in classical controllers, a model predictive controller (MPC) is used in the SM-level (inner loop) to explicitly account for RHP zeros. This approach improves both stability and transient response by incorporating RHP zeros into the control model. The inner loop controller receives the reference signals from the outer loop controller and regulates input and output currents. Therefore, it operates at a significantly higher speed compared to the outer loop controller. Moreover, control systems have been developed to balance battery packs by addressing mismatches in the initial state-of-charge (SoC) and temperature, thereby preventing over-charging or over-discharging. Furthermore, a modular PV-powered grid-tied EV charger is introduced, and control loops at the SM-level and system-level are developed. To validate the performance of the proposed modular EV chargers and the robustness of the control systems under different operating conditions, simulations (using MATLAB/Simulink and LTSpice) and experiments (using a Formula Student (FS) racing car) are carried out. The test bench features a configuration that includes battery segments with a total capacity of 5.7 kWh and a 68 kW PMSM driven by a TMS320F28335 eZdsp microcontroller. Experimental results validate the MIOBC’s effectiveness across its operational modes. In driving mode, the MIOBC accelerates the vehicle from 0 to 108 km/h in approximately 5 seconds while maintaining stable performance. In regenerative braking mode, the system efficiently converts mechanical energy back into electrical energy, showcasing its ability to recover energy during deceleration. During charging mode, the MIOBC handles up to 3 kW of power, demonstrating a 12% improvement in efficiency. In addition, the MIOBC demonstrates robust fault tolerance by effectively managing disturbances and maintaining stable operation, even when one or two battery segments are disconnected. In such scenarios, the system compensates by increasing the current through the remaining battery segments to keep the output power constant. The experiments also highlight the need for trade-offs in the controller’s BW and PM when using classical controllers in the outer loop controls. For instance, a PR controller with a larger PM of approximately 60 degrees and a lower BW of around 300 Hz provides enhanced stability but results in slower recovery times. Further testing with a 10 kW prototype demonstrates the effectiveness of MPC in addressing the challenges posed by RHP zeros. MPC offers significant improvements without compromising stability or responsiveness. The experimental results also indicate significant advancements in battery pack balancing, with a 66% improvement in SoC uniformity and a 62.5% improvement in temperature uniformity. Temperature differences are maintained within 10°C and reduced to within 5% of the average temperature, highlighting the effectiveness of the developed control strategies. Additionally, the modulation strategy for trapping second-order harmonic components is validated, evidenced by the presence of a second-order harmonic component at twice the grid frequency (100 Hz) in the inductor current. The thesis concludes with recommendations for future research.