Synchronous Rectification for LLC Resonant Converter: An Overview

LLC resonant converter is one of the most popular isolated dc/dc converters owning to its advantages of simple control, wide voltage gain regulation capability, and soft switching operation. Nevertheless, high efficiency and high power density are always the goals for LLC converters. To further increase the conversion efficiency, the synchronous rectification (SR) technology is adopted, where the uncontrolled diodes on the secondary side are replaced with SR MOSFETs. Due to the extreme low on-state resistance value for SR MOSFET, considerable secondary rectifier conduction loss reduction can be achieved; thus, the system efficiency is improved. Tremendous SR driving strategies for LLC converter have been proposed. However, comprehensive overview and insight comparisons among these different driving strategies are lacking. Therefore, the main purpose of this article is to present a comprehensive overview for SR driving strategies and answer the following four questions: 1) Why is SR important for LLC converter? 2) How to generate the SR MOSFET driving signal? 3) What are the advantages and disadvantages? 4) What is the future work regarding SR driving strategy?