Research on Symmetrical LLC Converter Applied to DC Transformer

This paper analyzes the advantages of asymmetric LLC converter applied to a bidirectional DC transformer. The symmetric LLC converter has the same voltage gain curve in the forward and reverses directions due to the symmetry of the resonant tank, and its voltage gain is one at the first resonant frequency. Through the reverse current generated by the transformer's magnetizing inductance and the external inductor, although the switching frequency sets the first resonant frequency, the primary and secondary switches can achieve zero voltage switching under any load conditions. The controller only needs to use the open loop circuit to synchronize the switching signals of the primary side and the secondary side to complete the automatic bidirectional power flow action, which has the advantage of the simple control. This paper analyzes the above advantages by theory and verifies the correctness of the arguments by simulation and implementation. The implementation of an 18kW prototype symmetric LLC bidirectional DC transformer as a verification, its efficiency performance is more than 97%.