Spread-Spectrum Technique Employing Phase-Shift Modulation to Reduce EM Noise for Parallel-Series LLC Resonant Converter

A spread-spectrum technique (SST) has been developed to mitigate an electromagnetic interference in power converters. However, for resonant converters using a pulse-frequency modulation, it is difficult to regulate its output voltage with the SST, because the switching-frequency variation according to the spread spectrum induces large output-voltage fluctuation. In this letter, an enhanced phase-shift algorithm of the frequency modulation of the SST and a parallel-series structure for an LLC resonant converter are proposed to obtain a tight output-voltage regulation under the spread-spectrum operation. The performance of the proposed methods is experimentally verified using a 600-W prototype parallel-series LLC resonant converter.