DC power distribution system has become an important development trend of urban power distribution network due to its advantages of high power density, high conversion efficiency and flexible networking. Medium-voltage high-power solid-state transformer play a more crucial role in the interconnection of medium-low voltage DC systems. In this paper, a series-parallel connected four-transformer LLC resonant converter based on series-connected IGBTs is designed. A half-bridge module with six IGBTs in series is used on the switch network side to improve the voltage stress tolerance level of the switch network and increase the power density of the system. The voltage and current stress on the primary and secondary sides of the transformer can be reduced at the same time by combining four transformers in series and parallel, which is convenient for the optimal design of the transformer, and the output power of the four transformers can be naturally balanced.