Impact of Resonant Parameters on Common-Mode Voltage for Full-Bridge LLC-DCX Converter

Abstract

This article proposes a systematic method to analyze the resonant parameters, considering the common-mode voltage (CMV) and power density in full-bridge (FB) LLC-DC transformer (DCX) converters. A time-domain model of the CMV source triggered by high-frequency resonance is derived, and its frequency spectrum is obtained through Fourier decomposition. Furthermore, this study assesses the impact of resonant and parasitic parameters on CMV. In addition, for the selection of appropriate resonant parameters, two boundaries and their intersection are formulated. These boundaries encompass the consideration of CMV reduction and the maintenance of gain stability and resonant capacitor voltage. The importance of these proper resonant parameters is verified by a $2 \times 50$ kW prototype with 950-V input and 630-V output. Experiments show that with the proper resonant parameters, the LLC-DCX converter achieves CMV reduction in the high-frequency range. It provides the attenuation of up to 12.77 and 26.02 dB in the 0.15–0.5-MHz and 0.5–30-MHz frequency bands, respectively, when operating under rated conditions. Comparative analysis of the resonant element’s volume reveals an enhancement in power density.