Analysis of LLC converter with series-parallel connection

A parallel LLC resonant converter with series-connected in primary side and parallel-connected in secondary side is presented for server power supply systems. Based on the series resonant behavior, the power MOSFETs are turned on at zero voltage switching (ZVS) and the rectifier diodes are turned off at zero current switching (ZCS). Thus the switching losses on power semiconductors are reduced. In the proposed converter, the primary windings of two LLC converters are connected in series. Thus two converters have the same primary currents to ensure that two converters can supply the balance load current. In the output side of two LLC converters are connected in parallel to share the load current and to reduce the current stresses on the secondary windings and the rectifier diodes. In this paper, the principle of operation, steady state analysis and design consideration of the proposed converter are provided and discussed. Experiments with a laboratory prototype with 24V/21A output for server power supply were provided to verify the effectiveness of the proposed converter.