A Secondary-Side Phase-Shift-Controlled LLC Resonant Converter With Reduced Conduction Loss at Normal Operation for Hold-Up Time Compensation Application

A novel secondary-side phase-shift-controlled (SS-PSC) LLC resonant converter is proposed for applications requiring hold-up time operation, such as distributed power systems and server power supplies. High efficiency at the normal input voltage is achieved because the proposed SS-PSC LLC converter always operates at the series-resonant frequency of the resonant tank. The magnetizing inductor of the proposed converter does not need to be reduced to provide desired voltage boost ratio for the hold-up time operation, which results in reduced conduction losses and improved efficiency. Sufficient voltage boost ratio for the hold-up time operation is provided by using SS-PSC strategy. In comparison with the conventional variable-frequency-controlled (VFC) LLC resonant converter, the main advantages are that the circulating current caused by the magnetizing inductor is effectively suppressed and the efficiency of normal operation is significantly improved. The operation principles, output characteristics, and design considerations of the proposed converter are presented in detail. Experimental results are given to verify the effectiveness and the advantages of the proposed solutions.