High-Frequency Transformer Design With Medium-Voltage Insulation for Resonant Converter in Solid-State Transformer

Abstract

A Solid-state transformer (SST) can directly convert medium voltage to low voltage (e.g. 400 V) with minimized power-conversion stages. The transformer in the DC-DC module not only provides medium voltage insulation but also significantly affects the power density of the module. In this paper, an innovative compact transformer structure is proposed to handle medium voltage insulation using an insulation coordination with epoxy, bobbin, shielding layer and stress-control layer. A brand-new bobbin design and related fabrication process ensures a partial-discharge-free insulation. Besides traditional core loss and winding loss, high-frequency related core loss and shielding loss is modeled in detail for an accurate loss evaluation. A simple and comprehensive multi-parameter, multi-objective optimization process is proposed to achieve the size and loss trade-off. The transformer prototype passes the related insulation tests, following the standard IEEE Std. C57.12.01. Finally, the design is demonstrated on a 15 kW, 200 kHz CLLC converter with 98.9% peak efficiency and 3.8 kW/L power density.