Just like the AC transformer in the AC power system, DC transformer interconnects different levels of DC grids, and its conversion efficiency and power density have important influence on the future DC power systems. This talk introduces a DC solid state transformer based on series-connected SiC MOSFETs. The DC transformer interconnects medium and low DC grids and can achieve a peak efficiency 98.4% with high power density. The balancing technique of the series-connected SiC MOSFETs will be detailed. 6 SiC MOSFETs are series-connected with a switching frequency 10kHz and the imbalance degree is less than 5%.