High-Frequency Transformer Design with High-Voltage Insulation for Modular Power Conversion from Medium-Voltage AC to 400-V DC

CCG Facility Integration proposed that the 4.16-kVAC power distribution in a data center greatly reduces the loss on cable, especially when compared to 480-VAC distribution. Also, it is suggested that the minimum stage of power conversion is obtained by powering each server rack with a 400-VDC bus that is directly step-downed from the 4.16-kVAC. An input-series-output-parallel (ISOP) structure becomes a promising topology for this power conversion. In addition to the modularity, the medium voltage (MV) is handled by efficient low-voltage devices connected in series, leading to high efficiency and high power density. The transformer in the DC/DC converter is crucial, since it is responsible for the MV insulation. Since insulation occupies a significant space in the transformer, the conventional optimization approach needs to be re-evaluated, especially in regard to switching frequency. The increase in spacing provides the opportunity to integrate the resonant inductor into the transformer. In this paper, a new optimization method for switching frequency is proposed with a better insulation structure. The design is demonstrated on a 200-kHz CLLC converter with 98.7 % efficiency and 61 W/in³ power density.