Partial Fluctuation Power Control of Resonant Converter in Solid-State Transformer

A solid-state transformer (SST) offers direct medium voltage to low voltage conversion, resulting in higher system efficiency and a smaller footprint compared to traditional transformers. Among the popular SST architectures, a two-stage system comprising a cascade H-bridge (CHB) as the first stage and a resonant converter as the second stage has gained prominence. However, the CHB circuit introduces power ripple at twice the line frequency, leading to poor power density due to the necessity of large bus capacitors for power regulation. To address this issue, a partial fluctuation power control method is proposed that allows a portion of the power ripple to flow through the resonant converter, thus reducing the size of the bus capacitors. Additionally, a loss evaluation is presented to calculate the losses related to fluctuation power, and a magnetic component optimization methodology is conducted to achieve an optimal tradeoff between loss and size for the resonant converter. The effectiveness of the proposed control method is demonstrated through a 15 kW 180 kHz CLLC converter module, achieving a peak efficiency of 98.8% and a power density of 3.1 kW/L. Furthermore, the bus capacitor size is reduced to 60% without compromising overall losses when compared to conventional regulated resonant converters.