Control Strategy for Wide Input Voltage Range in Series Half-Bridge LLC Resonant Converters

In recent years, the demand for high-efficiency and high-density power conversion in transportation and industrial sectors has increased significantly. Resonant converters, particularly the LLC topology, have gained widespread adoption due to their inherent soft-switching capability and suitability for wide input variations. The series half-bridge LLC resonant converter features advantages such as compact size, fewer switching components, and high voltage stress tolerance of switches. In urban rail transit auxiliary power systems, the use of soft-switching techniques in such converters helps reduce switching losses and enhance power density. However, during actual train operation, the converter faces challenging conditions, including a wide input voltage range and significant load fluctuations on the output side. To ensure stable operation under these complex scenarios, this paper proposes a closed-loop pulse frequency modulation (PFM) control strategy. By dynamically adjusting the switching frequency, stable output voltage regulation is achieved. The effectiveness of the proposed strategy is verified through MATLAB simulations, demonstrating that the output voltage deviation remains within ± 2% under varying operating conditions.