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A resonant converter has low switching losses, a small circuit volume and high power density and is also light. High-frequency switching converters have replaced conventional hard-switching converters. This study utilises a buck zero-current-switching (ZCS) pulse-width-modulated converter for battery chargers to manage resonant converters flexibly. An auxiliary switch is inserted into the resonant loop to control the resonant time. The operating modes of the circuit and the equivalent circuits are identified by analysing the operating principles of the charger circuit, based on the turn-on conditions of the active switches. The equations that are used to determine the circuit parameters are obtained from the equivalent circuits. The developed charger provides the advantages of hard-switching and resonant converters with constant-frequency control, reduced resonant time and the operability of all switching components in the charger under a ZCS condition, markedly reducing switching losses. Experimental results reveal the theoretical effectiveness of the developed novel battery charger circuit. The developed battery charger has a practical mean charging efficiency of over 90% and is highly suited to high-frequency operations with high charging efficiency.