Passive Cancellation Method for Reduction of Common-Mode Noise Currents

To meet the requirements of electromagnetic interference (EMI) certification, ac-dc converters always require a common-mode (CM) filter to limit the amount of EMI emissions transferred to the power grid. However, CM filters based on the well-known LC low-pass filter are usually bulky and costly. Many solutions to this problem have been proposed based on active filter technologies, which have the major drawback of requiring the use of current detectors, active circuits, and auxiliary power supplies. Therefore, a passive CM current cancellation method is proposed that is composed entirely of passive components and has the advantages of cost and system reliability. Compared with conventional passive LC filters, the number of magnetic components is the same, and only one additional winding is required. Previous studies only presented an ideal model without any related derivation, and the impact of any nonlinear components was not mentioned. In this letter, a further derivation of the nonlinear model is provided, and a comparison of the differences based on different characteristics of inductors is conducted to improve the circuit design.