Adaptive Synchronous Rectifier On-Time Control Within Dead-Time for Improving Light-Load Performance of LLC Resonant Converters

LLC resonant converter exhibits non-monotonic voltage gain at light-load, which could lead to the malfunction of traditional pulse frequency modulation (PFM) strategy and degraded efficiency due to excessive increase in switching frequency. To address this issue, an adaptive synchronous rectifier (SR) on-time control within dead-time is proposed for LLC resonant converters to normalize the voltage gain and enhance the light-load efficiency. First, the root cause of non-monotonic voltage gain is analyzed and revealed as the energy accumulated in resonant inductor due to mismatched output capacitor discharge between primary and secondary side switches within dead-time. To suppress the energy accumulation, an adaptive SR on-time control within dead-time is developed to synchronize the output capacitor discharge processes of primary and secondary side switches, which is characterized as: 1) SR is turned on within dead-time to accelerate its output capacitor discharge; 2) the turn-on rate of SR is controlled by designing its gate resistor to match the output capacitor discharge rate of primary side switches; 3) the turn-on instant of SR is adaptively tuned according to the switching frequency. With the proposed SR control, the energy accumulation of resonant inductor during output capacitor discharge of switches is fully suppressed, resulting in a monotonic voltage gain at light-load, which facilitates voltage regulation and improves light-load efficiency. A 360 $\sim$ 440-V input, 50-V/1-kW output LLC prototype is built to verify the effectiveness of proposed SR control.