Efficiency Comparison of Pulse Frequency Modulation and Phase-Shift Control in Light-Load Operation of CLLC Resonant Converter

This study evaluates the efficiency of a CLLC resonant converter during light-load operation using different control methods. Typically, the output voltage gain of a CLLC converter is controlled by adjusting the switching frequency through pulse frequency modulation (PFM). However, under light-load conditions, the output voltage gain increases significantly, which causes the switching frequency to rise excessively. This extreme increment reduces system efficiency. Phase-shift (PS) control is proposed as an additional method to limit the switching frequency for mitigating the aforementioned issue. In this study, an 11 kW CLLC converter is designed for rated output conditions based on fundamental harmonic approximation, and the switching frequency range is analyzed under light-load conditions. PS control is applied in four distinct cases to limit the switching frequency, and theoretical loss analysis is conducted through simulations to compare PFM-only control and PS control. Based on the loss analysis, the most efficient PS control case is identified, and experimental results confirm the high-efficiency performance of PS control. Experimental verification shows that PS control enhances efficiency by up to 2.2 percentage points in charging mode and 1.75 percentage points in discharging mode.