Fast Soft Startup Strategy for DAB-SRC ISOP Hybrid Modular DC Transformers based on SRC Resonant Tank State Observing

The input-series-output-parallel (ISOP) hybrid modular dc transformer (HMDCT) consisting of series resonant converter (SRC) and dual-active bridge (DAB) converters combining the advantages of both the superior regulating capability of the DAB and the extremely high efficiency of the SRC. However, the soft start strategy of ISOP HMDCT is quite challenging due to differences in SRC and DAB submodule modulation methods and power transfer characteristics. To address this problem, this paper proposed a soft startup strategy based on SRC resonant capacitor voltage observing which can accurately limit the peak current of SRC and DAB as well as maintain the input voltage balance among different submodules during the startup procedure. Besides, using the proposed observation method, the use of any resonant tank current or voltage sensors can be avoided, and it can also be applied to the startup control of a single SRC converter. The proposed strategy is experimentally verified on a 500 W prototype.