Passive cancellation of common-mode noise in power electronic circuits

A primary source of common-mode EMI in switching power converters is the parasitic capacitance between a switching element and its heat sink. A method of canceling this common-mode EMI with a compensating transformer winding is introduced. Compared with active cancellation techniques, it is much simpler and requires no additional transistors and gate-drive circuitry since it merely adds a small copper winding and a small capacitor. By using this technique, the size of the EMI filter can be reduced, especially for applications requiring high currents. This technique has been applied to an isolated half-bridge power converter and a forward power converter