Startup Strategy for ISOP Hybrid DC Transformer Featuring Low Current and Voltage Stress

The input-series-output-parallel (ISOP) hybrid dc transformer (HDCT) combining series resonant converter (SRC) and dual-active bridge (DAB) converter has higher efficiency than the ISOP DAB converter while maintaining the controllability of dc voltage or power. However, the startup control of ISOP HDCT is quite challenging, and the incorrect and uncoordinated startup strategies may cause excessive voltage and current stress in SRC and DAB. This paper proposed a three-stage startup strategy to address these issues, which can cooperatively control two types of converter submodules, accurately limit the switching current, quickly eliminate the overvoltage on input capacitors, and set up the output voltage with negligible overshoot. Besides, through the proposed strategy, SRC and DAB can smoothly shift to serial resonant and single phase-shift (SPS) modulation mode, respectively, which will not affect the steady-state control performance. The proposed strategy is experimentally verified on a 500 W prototype.