Study on a Modified CLLC Converter Paralleling a Resonant Branch With the Transformer for Wide Voltage Range Applications

In this paper, a modified bidirectional CLLC DC/DC converter is proposed to improve the efficiency in wide voltage range applications. The proposed converter adopts a LC series resonant branch parallel with the transformer, so that the equivalent inductance of the parallel branch and the equivalent transformer magnetizing inductor varies with the switching frequency. When the required gain is small and the operating frequency is high, the switch's turn-off current and the circulating power can be reduced through the large impedance of the LC resonant circuit; When the required gain is large, the switching frequency decreases and the auxiliary branch impedance decreases to obtain a higher voltage conversion ratio. The steady state operation and characteristics of the presented converter are analyzed, a design flowchart and a design example of the modified CLLC are also presented. Furthermore, the model of modified CLLC is established and is reduced for simple analysis. A 48V/400V/800W prototype was built and the experimental results verified the correctness of the proposed converter and the feasibility of the modeling results. Compared with the traditional CLLC resonant converter, the efficiency under wide voltage range application is significantly improved.