Optimal ZVS Control of an LCL-T Resonant Converter Using a DC-Biased Variable Resonant Inductor and Dead-Time Charge Feedback

The main purpose of this study is to overcome the difficulty of maintaining high-quality zero-voltage switching (ZVS) in resonant converters over wide operating ranges. Conservative designs ensuring light-load ZVS inevitably generate excessive reactive current, causing severe circulating losses and body diode conduction under heavy loads. To resolve this intrinsic trade-off, this paper proposes an active closed-loop optimal ZVS control strategy utilizing a DC-biased variable resonant inductor. The core mechanism actively shifts the tank impedance to dynamically reshape the primary current. By integrating the primary current during each dead time, the real-time integrated charge is actively regulated to track an optimal reference limit defined by the switch parasitic capacitance and dc-bus voltage. Consequently, the variable inductor continuously regulates the tank current toward the optimal ZVS boundary, ensuring the parasitic capacitance is completely discharged just before the turn-on instant. Validations via simulation and a 192 W hardware prototype confirm the method’s efficacy. The strategy completely eliminates light-load hard switching and significantly suppresses heavy-load body diode conduction without compromising output voltage regulation. Compared to conventional active schemes, this approach achieves full-range optimal ZVS without requiring additional high-frequency switching devices, establishing a highly efficient shift from passive parameter design to active boundary tracking.