Common-Mode EMI Reduction in Switching Flyback Power Supply by Implementing Cancellation Method of Heat-Sink Capacitor

Electromagnetic compatibility in switching power converters which are noise sources themselves، has a special importance. Electromagnetic interference (EMI) in the form of conducted or radiated reaches to sensitive sections and interferes with their operation. On the other، electromagnetic compatibility (EMC) standards causes another forces to reduce noise in switching converters. Major part of noise is due to the common-mode (CM) current passes through parasitic elements to the circuit ground (Earth). One of the important parasitic elements from the CM noise viewpoint is the switch heat-sink capacitor (common-mode capacitor). In this paper، a cancellation method of the heat-sink capacitor via a passive circuit is proposed in a 50W isolated flyback converter and is also modeled in OrCAD software. Also، experimental measurement results of the CM electromagnetic interference in regular and proposed flyback converter prototypes are presented to examine the modeling accuracy. Based on the experimental results، significant reduction of CM-EMI is verified after applying the cancellation method of the heat-sink capacitor.