High power density on-board chargers (OBCs) for electric vehicles (EVs)
Creators: name: Nasr Esfahani, Fatemeh
Publisher: name: Lancaster University
Year: 2025
Subjects: Electric vehicles
On-board battery chargers (OBCs)
Model Predictive Control (MPC)
Loop-shaping
Permanent Magnet Synchronous Machine (PMSM)
Control Design
Regenerative Braking (RB)

description: 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.