With the development of electric vehicles, LLC converter for bidirectional energy transmission has received more and more attention. In order to achieve bidirectional boost operation, the L-LLC converter is applied. However, the forward operation efficiency of L-LLC converter is reduced compared to the traditional LLC converter due to the addition of an extra inductor. This paper proposes an improved bidirectional variable structure LLC (VS-LLC) resonant converter to improve forward operation efficiency. Compared with L-LLC converter, the VS-LLC converter improves the efficiency in forward mode by changing the resonant network. The working principle of the proposed converter is introduced, and the parameters of a VS-LLC converter are designed. Bidirectional boost operation and soft switching capabilities of VS-LLC converter are validated by the simulation results. The comparison results show that VS-LLC converter has higher efficiency than L-LLC converter in forward mode, and the backward operation efficiency of two converters is approximate.

efficiency; LLC; soft switching; variable structure