An Enhanced Adaptive Synchronous Rectification With Turn-ON Optimization for LLC Converters Based on Stepwise-Plus-Feedforward Control

Synchronous rectification (SR) is an essential aspect for LLC resonant converters to achieve high-efficiency design, and the mainstream SR method is the adaptive directly voltage-sensing-based method (ADVS-SR), where the drain–source voltage of the SR switch is sensed. However, due to the presence of parasitic capacitor of the transformer and output capacitors of SR switches, undeserved bode-diode conduction (BDC) caused by intricate voltage ringing across the SR switch will invalidate the conventional ADVS-SR turn-on time tuning process under light-load condition, resulting in the dissipation of power efficiency. To solve this problem, in this article, the capacitive current spike, which is the main root of the voltage ringing in terms of turn-on, is modeled and analyzed, and then different SR turn-on candidates are compared. Based on that, this article proposes an enhanced adaptive SR strategy based on a novel stepwise-plus-feedforward (SPF) control to optimize SR turn-on. The SPF-SR strategy can eliminate not only large reverse current caused by premature turn-on but also large BDC interval caused by belated turn-on over the whole operating range. On the basis of ADVS, the SPF-SR only need to sense extra V_in, V_o, and I_o, which are easily sensed dc signals, and can be easily implemented in a cost-effective digital controller. Finally, a 300 W LLC prototype is built where experimental results show that the SR turn-on issue is optimized by the SPF-SR strategy, and compared with commercial SR IC and conventional ADVS-SR, the SPF-SR strategy improves power efficiency up to 6.99% and 5.52%, respectively.