Optimization of DC-Link Capacitance for Single-Phase ISOP SST Considering the Second Harmonic Pulsating Power in LLC Converter

In solid-state transformers with input-series output-parallel configuration, the LLC converter operating at a fixed switching frequency is one of the solutions for the isolated DC/DC conversion stage to provide a fixed voltage gain with galvanic isolation. However, in single-phase AC/DC conversion, the pulsating power at the double line frequency will propagate through the LLC converter, causing increased current stress and ripple voltages. This paper studies the pulsating power transfer behavior of LLC converters with a passive equivalent circuit model. The conclusion indicates that the primary and secondary side DC link capacitors can participate differently in buffering the pulsating power with different DC link designs. Furthermore, an inappropriate design may cause an excessive harmonic power transfer, discontinuities in resonant operation, and reduced overall efficiency. Simulations and measurements of a two-stage solid-state transformer verify the analysis. The guideline for DC link design of similar applications is provided.