A Switching Frequency Variation Detecting based Adaptive and Sensorless Synchronous Rectification Strategy for LLC Converter

To reduce the greenhouse gas emissions, the improvement of power electronic converters' efficiency is of great importance. LLC converter, as one of the most popular isolated and efficient DC-DC converter topologies, has been widely utilized in various industrial applications. The synchronous rectification (SR) technology is adopted to further improve the conversion efficiency due to the much lower ON-state resistance of SR MOSFETs when compared with diodes. Various SR driving strategies have been proposed for LLC converters and they can be divided into sensorless strategies and sensor-based strategies. Compared with sensor-based strategies, the sensorless strategies are low-cost and low-loss but model-dependent and less adaptive. To address these issues, this paper proposed a sensorless and adaptive SR strategy based on the switching frequency variation detecting. It adjusts the delay times of SR driving signals in fixed steps and detects the variation of their steady-state switching frequency. Based on the fact that the switching frequency variation is greatly different when the delay time is above or below the ideal delay time, the proposed algorithm can get the accurate SR signals in all LLC operation modes. The effectiveness and accuracy of the proposed SR strategy are verified by various simulations.