Techniques for reduction of common-mode EMI based on the concepts of current balance on the power transformer windings

In this paper, effects of the transformer on the EMI performances of the converter system are first discussed. An equivalent circuit of a transformer incorporating the parasitic elements has been carried out to model the important dominant behavior of the high-frequency noise coupling mechanisms. To achieve the noise current balance, the technique of using the noise-reduction winding, to mitigate the common-mode (CM) EMI noise caused by the capacitive CdV/dt displacement current (i.e., caused by the voltage distribution in windings), is then proposed and implemented in the power supply system. It is found that if the induced dV/dt sources and loads are balanced, the electromagnetic energy (i.e., noise) circulates between the noise-sources and loads within the transformer, then so no CM EMI noise goes through LISN resistors, leading to substantially decreased CM EMI. The theory of CM EMI reduction based on the noise current balance technique is presented. Design examples are given in this paper and results are verified by experimental measurements.