With the increasing proportion of new energy power generation, it is necessary to design a high-power converter with a high efficiency and a wide input range. In this paper, a Sigma converter composed of a Buck-boost LLC converter and an LLC based on DC trsnsformer(LLC-DCX) converter is proposed based on the existing research, which can meet the requirements of a high efficiency and a wide input range. The Buck-boost LLC converter can effectively decrease the number of components and reduce loss through bridge arm integration. However, all the input current flows through its Buck-boost inductor, which will cause a lot of loss in high-power scenarios. Therefore, the Sigma structure and the concept of partial power transmission are adopted to reduce the loss caused by the Buck-boost LLC converter. Finally, experimental results demonstrated the superior voltage regulation characteristics and high efficiency of the proposed topology.