CM Conducted EMI mitigation in flyback PFC converter

The switched mode power supplies offer many advantages in power conversion. But they offer low power factor and generate electromagnetic interference (EMI). The low power factor can be improved by a front-end power factor correction (PFC) unit. However, they also generate EMI in both differential mode and common mode (CM). The interwinding capacitance of the transformer and parasitic capacitance between the heat sink and ground are major contributors to CM noise. The traditional method to minimize the CM noise by using a line filter consists of CM choke and Y capacitor. But, CM noise causes leakage current to flow to the ground through the Y capacitor. This leakage current has to be restricted as per the standards for commercially used equipment and medical instruments. The technique proposed here reduces the values of Y capacitor thereby reducing the leakage current. This can be reduced by minimizing the predominant parasitic capacitances of the HF transformer used in the PFC converter. This paper presents a technique for reducing these parasitic capacitances by incorporating an additional parasitic capacitance noise cancellation winding (PCW). A flyback transformer winding capacitance model depicting the parasitic capacitances is considered, and a novel method to determine this parasitic capacitance is presented. Using this model, the CM Conducted EMI in a flyback PFC is determined by simulation and experimentation. These simulation and experimentation results are used to design an EMI line filter to reduce the Conducted CM noise to complaint levels. The value of Y capacitor now is reduced due to the addition of PCW thereby reducing the size and cost of the filter. This technique has been validated experimentally on a 30-W power factor correction flyback converter working in critical conduction mode.