Spread Spectrum Modulation under Second Harmonic Distortions in Two-Stage Converters

A spread spectrum modulation (SSM) using the second harmonic distortion of a boost power factor correction (PFC) rectifier is proposed. In a system where a boost PFC is connected to an LLC resonant converter in series, the LLC resonant converter employs the SSM to mitigate electromagnetic (EM) noise without control. Operating the SSM near the resonant frequency enhances efficiency while the boost PFC manages the output voltage. However, SSM-induced frequency variations can lead to significant output voltage fluctuations. The proposed method can compensate for those fluctuations by synchronizing the SSM with the second harmonic ripple of the boost PFC. This approach is cost-effective and simpler than conventional methods. In addition, the proposed SSM can reduce the EM noise. The proposed method will be verified by using an 800 W prototype converter.