Analytic-Adaptive LLC Resonant Converter Synchronous Rectifier Control

Synchronous rectification (SR) is the key to achieve high efficiency for high output current LLC resonant converters. Recently proposed methods for SR control do not offer a complete solution for industrial applications. The drain-to-source voltage sensing based methods cannot utilize the full benefits of SR due to the field-effect transistor (FET) package stray inductance. The adaptive SR control methods suffer from the variable on time and cannot reach the minimum power loss. This research aims for a high-performance SR control strategy that not only has good steady-state performance but also works reliably during transients. This article introduces an analytic-adaptive method for SR control that delivers accurate adjustment for both turn on and turn off moments of SR FETs below and above resonance. The proposed method takes into account the stray inductance of SR FETs; hence, it enables full efficiency gain of SR. The feasibility of implementation for the proposed method integrated with closed-loop control is experimentally validated on a 300 W 390/12 V half-bridge LLC resonant converter with 93.66% peak efficiency.