An Optimized Sensorless Synchronous Rectification Method for LLC Resonant Converter in Wide Output Voltage Range

In LLC resonant converters, synchronous rectification (SR), which replaces the secondary side diodes with metal-oxide-semiconductor field-effect transistors, is mainly mentioned to improve further efficiency. Among SR methods, the sensorless method has gained significant attention because it does not require additional circuits, which can maintain the cost and power density of system. However, conventional sensorless methods cannot achieve high efficiency in wide output voltage range due to their model inaccuracy. Also, these methods might cause an early turn-ON of SR in the step-down area, resulting in the circulating current that reduces efficiency and stability. To solve these problems, the proposed SR method is newly proposed by using the time-domain analysis method, which obtains high SR accuracy in wide output voltage range. Moreover, it can prevent the circulating current by introducing the accurate turn-ON timing of SR. First, the overall process descriptions and SR strategies of LLC resonant converter are described. After that, the proposed SR method is mathematically expressed by the time-domain analysis method. Finally, its effectiveness is evaluated by the simulation test.