DC-DC converter is the core equipment for connecting distributed photovoltaic power generation to be connected to the medium voltage DC grid. A circulating current free medium frequency DC-DC converter with auxiliary transformer is proposed in this paper. This converter comprises a main power circuit and an auxiliary control circuit, consisting of one three-level bridge arm and two two-level bridge arms in total. In this converter, the main power circuit transmits approximately 90% of the power, while the auxiliary circuit transmits the remaining small portion of the power. By utilizing high frequency chopping of some switching devices in the auxiliary circuit, the current waveform is approximated to a trapezoidal wave, thereby reducing the peak current. Due to the small current flowing through the auxiliary circuit, the switching losses generated by high frequency chopping are also very small. The switches that flow large currents only operate at a medium frequency of 500 Hz and can realize zero current switching (ZCS). In addition, the loop current in the auxiliary branch is eliminated by introducing a blocking capacitor in the auxiliary circuit, so that the loss of the whole converter is small. The paper analyzes the working principle of the converter, discusses the parameter design, and builds a 160 V/4 000 V/4 200 W principle prototype for experimental verification,The experimental results indicate that the proposed converter achieves ZCS with low switching losses and a peak efficiency of 98.5%.