To improve the lightweight level of auxiliary converters for urban rail vehicles, full SiC intermediate frequency auxiliary converters have become a trend. Aiming at its core component, a full SiC intermediate frequency isolated DC-DC converter using Boost + LLC topology is studied in this paper. The working principle and characteristics of the converter are analyzed in detail. Based on the operating characteristics of the LLC resonant converter and the realization mechanism of soft switching, an optimization design method of the resonant parameters is proposed, which can make LLC resonant converters exhibit the characteristics of DC transformers and realize soft switching in the full load range. Considering the power losses of the converter and the voltage stress of SiC MOSFET, the design method of the intermediate bus voltage is given. Finally, the correctness of the parameter design method is verified by simulation and experiment, and the performance of the full SiC intermediate frequency isolated DC-DC converter is analyzed. When the input voltage is 1500V and the output power is 15kW, the efficiency of the converter reaches 96.21%. Compared with the Si intermediate frequency isolated DC-DC converter, the power density is improved by 80.62%.