Analytical Modeling and Worst Case Analysis of Common-Mode EMI Spectrum in Quasi-Resonant Flyback Converters

Modern quasi-resonant (QR) flyback converter employs advanced control strategies to optimize efficiency across all load conditions, resulting in a complex electromagnetic interference (EMI) spectrum. This article proposes an analytical modeling method to facilitate the prediction and suppression of the common-mode (CM) EMI noise of QR flyback converter. By incorporating practical considerations such as the second-harmonic ripple on the input, valley-switching techniques, and varying load conditions, an accurate CM EMI (CE) model for QR flyback converters is developed, serving as a predictive tool for CM noise. Furthermore, a worst case analysis (WCA) is carried out, which establishes the design criteria for the CM filter. The proposed noise-prediction method is validated by experiments on a 90-W QR flyback charger, demonstrating an exceptionally low prediction error of less than 3 dB under different input and output conditions. Additionally, a CM filter is designed according to the WCA-based guidelines, which shows effective suppression of CM noise, yielding satisfactory results by achieving compliance with EN55032 Class B standards across the entire test frequency range.