Dc power distributions have been the trend to replace conventional ac distributions in data centers for their reduced conversion steps and higher efficiency, and 48 V VR has been proposed in recent years to reduce the large bus-bar I2R loss. However, conventional 12 V VR is still widely used for its mature design, and when replacing the 12 V VR with the 48 V VR, the converter before the VR usually needs to be redesigned for different output voltage, which results in a high cost during the update iteration. In this paper, a high efficiency high power density series resonant converter (SRC) dc transformer (DCX) with modular structure and different output voltage adaption is proposed. By using low voltage devices with low NFoM (NFoM=Rdson·Cotr), the conduction loss of the MOSFETs can be reduced. A chain series-parallel structure is proposed for current and voltage balancing and both 48 V and 12 V output voltage can be achieved with the same components. With the same key parameters, 1 MHz380V/48V and 380 V/12 V SRC DCX prototypes are built by only changing the connection structures. Peak efficiency over 98.0% is achieved and the power density attains more than 800W/in3under a full load of 1800 W.