A Peak Current Reducing Method for Input-Independent and Output-Series Modular Converters With LC-Branch-Based Power Balancing Unit

Input-independent and output-series (IIOS) modular converters are characterized by high step-up ratio and independent maximum power point tracking and can therefore be used to integrate renewable energy into medium voltage dc grids. However, the associated power mismatch can lead to voltage imbalance of the submodules(SMs). At present, the only solution using no extra active switch involves the insertion of an
LC
-branch-based power balancing unit (PBU) between every two adjacent SMs (the SM and PBU share the switches). However, the considerable peak current of the shared switches threatens the security of the switches and increases the power loss. Therefore, this article considered the peak-current problem. A dual active bridge (DAB) converter was employed as the SM of the IIOS converter. The basis for the large peak current associated with the shared switches was analyzed and a peak current reducing method using the DAB inner-phase-shift control was proposed. The contribution of this article is that the current stress can be reduced and the power efficiency can be increased. A two-SM and 1000-W down-scaled experimental prototype was developed for verification of the theoretical analysis results. The peak currents and efficiencies with and without the proposed method were compared with experimental results.