Mitigation of High-Frequency CM Conducted EMI in Offline Switching Power Supplies

This paper presents a new technique to minimize high-frequency CM conducted EMI noise in offline flyback SMPS and Near Unity Power Factor (NUPF) converter. In this technique, separate ground paths are provided for noise sources on the primary and secondary side of the high-frequency transformer. This is achieved by splitting the Cz capacitor of line filter into Cm1 and Cm2 with their mid-point grounded. By this method, a considerable reduction in CM conducted EMI in the high-frequency range can be achieved. Hence the size of the EMI line filter can be reduced. Experimentation and simulations are carried out with Cm1, Cm2 and Cz capacitor for offline flyback configured NUPF and SMPS in the critical conduction mode. The experimental and simulation results are presented and are found to be in close agreement.