Synchronous Rectification of LLC Resonant Converters Using Homopolarity Cycle Modulation

Abstract

In order to further reduce losses in LLC resonant converters, the use of synchronous rectifiers (SRs) in the output rectifier is desirable. Analysis and detection of the conduction angles of the SRs used in an LLC converter are the main challenges in synchronous rectification. This paper develops a new time-domain theoretical analysis, called homopolarity cycle, enabling synchronous rectification in LLC resonant converters with the use of a low-cost polarity-based sensing technique. The homopolarity cycle considerably reduces the complexity of the LLC converter's analysis, relates the conduction angles of the SRs to the gate driving signals of the inverter switches and the polarity of the rectifier voltage, and finally introduces a low-cost polarity-based sensing technique. A synchronous rectification control algorithm, called homopolarity cycle modulation (HCM), is proposed, which only needs to sense the polarity of the rectifier voltage, which is simple to measure and immune to noise. A simple graphical plane, named the homopolarity, is introduced to provide information about the SRs’ conduction angles. The proposed HCM method is validated by experimental and simulation results. Unlike the conventional synchronous rectification technique, the results have shown that the proposed HCM synchronous rectification method has a flat synchronous rectification coverage from light to full loading conditions while using a simple sensing strategy.