Abstract: DC transformers are key components in DC grids. To overcome the drawbacks of existing non-isolated modular multilevel converter DC transformer（MMDCT） such as high circulating current and large filtering requirement, a step-up modular multilevel DC transformer along with its control strategies was proposed. It has the following features: 1) Power transfer between the cells is achieved by the resonant circuits, which avoids the circulating current through the arms. 2) Using the proposed modulation method to insert and bypass the adjacent cells in a complementary manner ensures self-balancing of the cell voltages. 3) Voltage stress across the input inductor is limited to the cell voltage, thus reducing the input inductor size significantly. 4) Soft-switching operation is achieved for the majority switches, thus alleviating the switching losses and electromagnetic interference. Based on Matlab/Simulink, simulation and analysis was carried on. Correctness of the operation principles and feasibility of the control strategies was confirmed.