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Development of a Control Method for LLC Converter Utilized for Input-Parallel-Output-Series Inverter System with Solid-State Transformers

A multi-level power converter with series-connected solid-state transformer (SST) units for a power conditioning system for photovoltaic (PV) generation was developed. Each SST unit has an LLC converter driven by SiC-MOSFETs and a high-frequency transformer. By applying the developed converter, the system can be optimized and made lighter than a conventional system using a commercial transformer. A method for controlling the LLC converter so as to maintain its high efficiency over a wide voltage range of a PV panel is proposed. According to the results of an experimental evaluation of a prototype SST unit, the efficiency of the LLC converter is 98% or more even when the input voltage from the PV panel changes.