Voltage and Power Sharing in $LLC$-Based DC/DC Stages of Solid-State Transformer Architectures

This letter investigates the power sharing behavior of the DC-DC stage in two main modular solid-state transformer architectures: modular multilevel converter (MMC)-based and cascaded H-bridge (CHB)-based systems. The DC-DC power module is considered to be an LLC resonant converter operating as a DC transformer. An analytical approach is developed to characterize the impact of component mismatch on module input voltages and power distribution. The analysis enables direct evaluation of the influence of DC-DC stage parameters and identifies the transformer turns ratio nT as the dominant factor governing imbalance. In MMC-based systems, all modules remain active under nT deviations, although voltage and power imbalances occur. In contrast, CHB-based systems are more sensitive to variations in Vi and nT, and adverse conditions may block power transfer. Experimental results validate the analysis and confirm the accuracy of the developed study.