A Synchronous Rectifier Control Scheme to Realize ZVS Considering the Secondary Parasitic Capacitance in LLC Resonant Converter

In LLC resonant converter, zero voltage switching(ZVS) will be influenced or even fail to be realized considering the parasitic capacitance of the secondary side synchronous rectifiers(SRs), because magnetic inductor current i Lm does not wholly flow to charge the parasitic capacitance of the primary side switches during dead time, part of which flows to charge the secondary parasitic capacitance. In this paper, the mechanism of the failure of ZVS considering the secondary parasitic capacitance is analyzed, then an SR ON-time extension(SOE) control scheme is proposed to realize ZVS without hardware re-design. A simplified calculation method for the extension length of SR ON-time is also presented. Finally, the effectiveness of the SOE control scheme is verified by a 1.5kW 1MHz LLC converter prototype with a maximum efficiency increase of 0.9%.