Spread Spectrum Technique to Reduce EMI Emission for an LLC Resonant Converter Using a Hybrid Modulation Method

A spread spectrum technique has been introduced to mitigate electromagnetic interference (EMI) in power converters. However, this technique is difficult to apply to resonant converters that use a pulse frequency modulation (PFM), because the spread spectrum can induce large output voltage fluctuations by PFM's switching frequency variations. In this letter, a hybrid control method using the PFM and phase shift modulation is proposed to obtain tight output voltage regulation under the spread spectrum operation of an LLC resonant converter. The performance of the proposed hybrid control method is experimentally verified using a 500 W prototype LLC resonant converter.