Optimal Design of a LLC Resonant DC-DC Converter Based on the Stacked Half-Bridge Concept for Solid-State Transformer

This paper investigates the DC/DC stage of a two-stage solid-state transformer (SST) system, implemented using a modular approach with multiple power electronic building blocks (PEBBs) arranged in an input-series, output-parallel (ISOP) configuration. Each PEBB is based on an LLC resonant converter operating as a DC transformer. The study provides an in-depth analysis of the modular DC/DC stage, evaluating two-level (2 L) and stacked half-bridge (HB) converters as the PEBB. It presents the first multi-objective design methodology for the modular DC/DC stage and includes a comparative study between the stacked HB and 2 L solutions. Additionally, the advantages of the stacked HB concept over the 2 L solution are demonstrated and quantified. To validate the theoretical analysis and demonstrate the converter's feasibility, experimental results are presented using a down-scaled laboratory prototype of the LLC resonant converter based on the stacked HB using two series connected HBs, rated at 750–1000 V and 5 kW. The prototype achieved an efficiency of 98.6<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula>.