Digital Control Method of Synchronous Rectification for Bidirectional CLLLC Converter

Abstract

In order to solve the problems of low efficiency and high temperature of the switches under a wide range of working conditions of the bidirectional CLLLC resonant converter, this paper proposes a digital controller of synchronous rectification for the rectifier on the secondary side of the high frequency transformer. Based on the analysis of operating mode of different frequency, the relationship between driving time of the synchronous switch on the secondary side and the driving time of the active device on the primary side is obtained. In a wide operating frequency range, the delayed turn-on and leading turnoff time of driving signal for the synchronous devices is obtained respectively by calculation and piecewise linear function fitting methods. Compared with other synchronous rectification control, this method is realized by pure digital control, and can be flexibly applied to high-power applications with a wide range of bus voltage variation. Furthermore, this method is suitable for bidirectional converters. The shortcoming that the analog control chip is sensitive to bus voltage variation can be overcomed by using this method. Meanwhile, the proposed control method is simple to be implemented and is very cost-effective. Finally, a simulation platform and experimental prototype were built to verify the proposed method. The results of simulation and experiment show that this strategy can increase the efficiency of the CLLLC system by about 3%, and the temperature of the synchronous switches are greatly decreased.