LLC Synchronous Rectification Using Resonant Capacitor Voltage

Synchronous rectification (SR) for LLC resonant converters has been developed to enhance the power conversion efficiency and achieve high-power-density design. Conventional SR driving strategies can be categorized as current-driven methods, VDS-ON sensing methods, and alternative approaches. The current driven methods widely adopt current transformers (CTs) to sense the rectifier current and generate the SR driving signals. The CTs are lossy and bulky, which is unfavorable to high-power-density design. The VDS-ON sensing methods remove the current sensors by sensing the voltage across the ON-state resistor of the SR MOSFET, yet the sensed signal is small and prone to be offset by the inductive voltage induced from parasitic components. Most alternative approaches avoid sensing noise-sensitive signals; however, the operating range is narrow due to the limited information from the converter. This paper proposes an SR driving strategy based on the resonant capacitor voltage (RCV) to address those issues. The RCV SR driving strategy does not require current sensors. The sensed RCV is insensitive to the parasitic effects. In addition, the RCV strategy controls the SR ON-time effectively over a wide range of operating frequency and loading conditions. Simulation and experimental results of a 650-W/24-V LLC converter are presented to validate the effectiveness of the proposed RCV strategy. Compared with the conventional VDS-ON sensing method, the ON-time error caused by the parasitic effect is greatly reduced, which improves the power conversion efficiency and reduces the SR MOSFET temperature.