An Improved Adaptive Synchronous Rectification Method with the Enhanced Capacity to Eliminate Reverse Current

Adaptive synchronous rectification (SR) is a good solution to solve duty cycle loss for LLC resonant converter. Usually, whether the synchronous rectifier is turned off prematurely or late can be detected by whether the body diode is conducted on or not. However, when the switching frequency is below the resonant frequency, both SR turning- off prematurely and late may generate body diode conduction (BDC), which will cause logic errors in the conventional adaptive SR strategy. Then, a large reverse current from load to source will occur, which damages the efficiency. To solve the problem, the time-domain modeling of the SR drain-to-source voltage is established to analyze the mechanism of BDC caused by SR turning- off late. Moreover, an improved adaptive SR driving scheme is proposed, which can identify the two different types of BDC and tune SR to be turned off exactly. Finally, a 280 W LLC prototype is built to verify the established model and the proposed SR method. The prototype proves that both the duty cycle loss and the reverse current are eliminated and efficiency improvement of 0.54% is achieved.