Surge Current Reduction in LLC Resonant Converter with a Hybrid Control Strategy of PFM and PSM for Expansion of Output Voltage Range

Increased disturbances in the AC power system are caused by the rapid charging of high-capacity battery packs, hence increasing the requirement for efficient, low-distortion smart chargers. Therefore, a battery charger that can react to the battery state and adjust its charging process accordingly is essential. In this context, the application of the LLC resonant converter employed in battery chargers is considered in this paper. A hybrid control strategy that integrates both the pulse frequency modulation (PFM) and the phase shift modulation (PSM) is proposed, which can handle a wide voltage range of the battery. This control method includes non-overlapping (NOVL) mode of operation. In this study, the input voltage of the LLC converter is assumed to be fluctuated. When the control state changes due to the fluctuation of the input voltage in the NOVL mode, a surge current is observed in the output current. This surge causes heat generation and deterioration of the battery. Therefore, an alternative overlapping (OVL) mode of operation is further proposed as a new control state change method to curtail the surge current in the charging process. Finally, the obtained experimental results confirmed the surge current reduction are introduced, which also demonstrated the effectiveness of OVL mode over NOVL mode control strategy.