In view of the problem of traditional single-stage DC-DC converter transform difficult in the 380V DC micro-grid composed of distributed power, this paper proposes a novel topology, achieving high-step-down ratio and soft-switching. Based on the analysis of its basic principle, this paper gives the soft-switching conditions, and discusses the stability of the circuit in sliding mode variable structure control (SMVC) by building small-signal model. Finally, the simulation proves the correctness of theoretical analysis.