(A) study on operation of two LLC series-resonant-converters for low voltage and high current applications

Abstract

Recently, the LLC series resonant converter is widely used for the front end DC-DC power conversion system because it has many advantages over other power converters. However, it is difficult to apply this topology to high current applications because the current ripple stress on the output capacitor is severely high. Nevertheless, with the interleaving technique, it is possible to reduce the output current ripple drastically. A novel and easier implementation of the interleaving control signals for two paralleled converters is proposed in this dissertation, which consists of the remarkably simple circuit of two D flip-flops and one logic inverter. The proposed method can utilize the characteristics of resonant controller so that it is possible to control the converter robustly. Besides on these simplicity and robustness, it is possible to control the number of phases at certain load condition with additional load detection circuit. With this circuit, the efficiency of light load condition can be increased as similar to single phase LLC converter. In order to validate the proposed method, two kinds of interleaved LLC converter are suggested. One has input-parallel and output parallel configuration and the other has input-series and output-parallel configuration. Furthermore, the laboratory experiments are executed and discussed with a 480W prototype.