Optimal Design and Control of High Power Density Bi-Directional CLLC Converter With Integrated Transformer for Wide-Range EV Battery Charging

This paper presents effective and practical control and design methodologies for electric vehicle (EV) on-board chargers (OBCs) utilizing bidirectional power factor correction (PFC) and CLLC resonant converters. The proposed system enables high-efficiency, high-power-density battery charging over a wide output voltage range from 320 V to 820 V. This is achieved by combining pulse frequency modulation (PFM) of the CLLC converter with dynamic DC-link voltage regulation through the PFC stage, ensuring optimal performance across varying operating conditions. A robust soft-start strategy is introduced that employs secondary-side precharging of the CLLC stage and gradual activation of phase-shift modulation (PSM). This approach effectively limits inrush current, minimizes switching stress, and reduces electromagnetic interference (EMI) during startup, thereby enhancing overall system reliability. To improve power density and thermal performance, a compact integrated transformer design is proposed. The structure leverages inherent leakage inductance to eliminate the need for discrete resonant inductors and incorporates a center multi-gap and center-hole configuration to facilitate efficient heat dissipation. A systematic transformer design methodology is also presented to realize the desired gain characteristics while meeting thermal and dimensional constraints. The effectiveness of the proposed control and design techniques is validated through an 11 kW hardware prototype, achieving a peak efficiency of 96.7% and a power density of 1.5 kW/L. These results demonstrate the proposed approach’s viability for wide-range, high-power battery charging applications in next-generation EV platforms.