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.

中文翻译：

---

一种改进的增强消除反向电流能力的自适应同步整流方法


自适应同步整流 (SR) 是解决 LLC 谐振转换器占空比损耗的良好解决方案。通常，可以通过体二极管是否导通来检测同步整流器是否过早关断或过晚关断。然而，当开关频率低于谐振频率时，SR过早关断和延迟关断都可能产生体二极管导通（BDC），这将导致传统自适应SR策略中的逻辑错误。然后，会产生从负载到源极的大反向电流，从而损害效率。针对该问题，建立了SR漏源电压的时域模型，分析了SR关断过晚引起BDC的机理。此外，提出了一种改进的自适应SR驱动方案，该方案可以识别两种不同类型的BDC并精确调整SR关闭。最后，搭建了一个 280 W LLC 原型来验证所建立的模型和所提出的 SR 方法。原型证明，占空比损耗和反向电流均被消除，效率提高了 0.54%。