The input-series/output-parallel (ISOP) configuration is widely adopted in the DC/DC stage of solid-state transformer (SST) systems due to its modularity and scalability. However, unavoidable parameter deviations among power electronic building blocks (PEBBs) can cause voltage and power imbalances. This paper presents an analytical investigation of the voltage and power sharing mechanisms in ISOP-based DC/DC stage using LLC resonant DCX converters. Analytical expressions are derived to quantify input voltage distribution and power imbalance under parameter mismatches. For a LLC resonant DCX converter operating with Ln > 100 and Qe < 0.1, the analysis demonstrates that resonant tank variations have a negligible effect, whereas small deviations in the transformer turns ratio critically affect voltage gain, output voltage, and power sharing among modules. The proposed analytical approach is validated through experimental results obtained from a 1 kV / 5 kW two-module ISOP prototype, showing close agreement with theoretical predictions.
