Reduction of common-mode electromagnetic interference in isolated converters using Negative feedback

In the field of power electronics, there is a trend for pushing up switching frequencies of switched-mode power supplies to reduce volume and weight. This trend causes an increasing level of electromagnetic interference (EMI) emissions. It is well known that common-mode EMI is resulted in by the switching of a parasitic capacitance of transistors, diodes, and transformers the power circuit consists of, which implies current flow to ground (actually the shielding) of the circuit. In this paper a method of common-mode EMI reduction, based on a negative feedback which implies compensating current flow is presented. More specifically, desired EMI reduction in the family of isolated DC/DC converters is achieved by means of a compensating transformer winding and a capacitor. An isolated flyback converter was constructed and tested in the Power laboratory in Tel-Aviv University. Using a spectrum analyzer, it was found that the common-mode EMI created by the parasitic capacitance of the switching transistor decreased between 10 and 15 dB in the low frequency harmonics of the converter.