Valley-Preserving Spread-Spectrum Jitter for Quasi-Resonant Flyback Converters to Reduce Conducted EMI

This paper presents a spread-spectrum control method for a quasi-resonant (QR) flyback converter that reduces conducted electromagnetic interference (EMI) while preserving soft-switching in discontinuous-conduction mode (DCM). The proposed controller injects an adjustable and bounded jitter signal into the peak-current threshold, thereby modulating the switch on-time, varying the effective switching frequency, and dispersing the switching spectral energy. A direction-aware modulation algorithm is developed to limit cycle-to-cycle energy variation, mitigate interaction with the voltage-regulation feedback loop, and maintain output regulation. Both SIMPLIS simulations and experiments on a 65-W (20 V/3.25 A) QR flyback prototype validate the approach. Compared with a conventional frequency-spreading method, the proposed controller reduces the quasi-peak (QP) conducted EMI by up to 5 dBμV in the 400–700 kHz band, corresponding to an average broadband reduction of approximately 3 dBμV, while maintaining tight line and load regulation.