Effects of transformer structures on the noise balancing and cancellation mechanisms of switching power converters

The focus of this paper is to explore the possibility of minimizing capacitive CdV/dt displacement (common-mode, CM) currents present in the power cord of a switching converter, by use of two types of transformer windings, namely the balancing dV/dt winding (B-Winding) and core cancellation winding (C-Winding). In this paper, techniques for constructing both the B- and C-Windings are first discussed and measurements are given accordingly to verify the study results. Depending on the gradient of the voltage distribution on transformer windings, the noise balancing and/or cancellation conditions (loops) within the transformer, formed by the B- and/or C-Windings, can cause the CM displacement current to be circulated back and forth between the transformer windings and parasitic inter-winding capacitances, so that there should not be any current flowing into the LISN. Therefore, the CM EMI measurements should be zero in theory and extremely small in practice. Design examples are given and experiments are finally carried out to verify the proposed theory.