A Bidirectional Fixed-Frequency Resonant Converter Using on Bridge-Leg Voltage Control

This paper proposes a bidirectional resonant converter with fixed-frequency LLC operation, in which the bridge-leg voltage is actively regulated to interface a DC bus and an energy storage system. The converter integrates a bidirectional buck/boost stage in series with one bridge arm of a bidirectional LLC resonant converter. By controlling the bridge-leg voltage via the buck/boost stage, the system enables output-voltage regulation while maintaining constant-frequency LLC operation. The buck/boost stage operates in a variable-frequency overcritical conduction mode (OCRM), which ensures zero-voltage switching (ZVS) and reduces circulating current–related losses, thereby improving overall efficiency. A 500-W experimental prototype with a 40- to 60-V low-voltage side and a 400-V high-voltage side was built to validate the concept. The converter achieves a peak efficiency of 96.8 (Formula presented.) in forward operation and 97 (Formula presented.) in reverse operation.