A Three-Level T-Type <i>LLC</i> Converter With Adaptive Frequency PWM Control for On-Board Charger

The circulating current negatively impacts the performance of LLC converters by increasing losses and current ratings, reducing efficiency, and narrowing the zero-voltage switching (ZVS) range. This article proposes an adaptive frequency pulsewidth modulation control (AFPC) method to enhance the steady-state performance of a three-level T-type LLC (3LTLLC) converter for on-board charger (OBC) applications. The proposed AFPC dynamically adjusts the switching frequency based on real-time battery voltage and charging power data, effectively eliminating circulating current and improving efficiency. Furthermore, this article presents a comprehensive analysis of the current and voltage equations of the resonant tank components, providing estimations of the circulating interval (CI) and voltage gain across typical OBC charging profiles. A transformer design is introduced, optimizing both size and winding structure to achieve the desired leakage inductance while maintaining high efficiency and power density. The effectiveness of the proposed strategy is validated through experimental results from a 3LTLLC resonant converter prototype with a 400-V input and an output range of 220–380 V at a rated power of 7.4 kW. An efficiency improvement of 0.3% is observed during the most common charging intervals between 240 and 340 V.