Electromagnetic interference and its reduction in switched mode power supplies

Switched-mode power supplies (SMPS) generate conducted electromagnetic interference (EMI) noise due to its fast switching of high currents and voltages. The amount of this EMI noise which a product can generate is subject to various electromagnetic compatibility (EMC) standards published by organizations or governments. In recent years, power electronics research has continuously focused on high power density and high switching frequency solutions. This leads to severe EMI issues. The reduction of EMI noise plays a critical role in the development of high power density converters. This dissertation explores some major issues related with the reduction of EMI. By investigation of these issues, the power supply and EMI filter designs might be significantly improved and the size of EMI filters might be significantly reduced. First of all, for isolated power converters, a generalized common-mode (CM) current cancellation approach is proposed to attenuate the CM noise current generated by the parasitic capacitance of semiconductor switches iCM1, and the CM noise current generated by the parasitic capacitance of transformers, iCM2, simultaneously. This approach is more efficient than conventional techniques, most of which only focus on the suppression of either iCM1 or iCM2. A forward converter is used to demonstrate the effectiveness of the proposed approach. Secondly, due to the compact design of power converters nowadays, near field coupling has become a major concern in the design of EMI filters and it often results in costly iterations and suboptimal results. To address this issue, magnetic field distribution of planar transformers is analyzed and corresponding techniques to reduce near field coupling are proposed and verified through experiments. In addition, a CM inductor with external magnetic field immunity, low magnetic field emission and high differential mode inductance is proposed. Simulation and experimental results validate the advantages of the proposed CM inductor. Finally, as a promising alternative to bulky passive EMI filter, active/hybrid EMI filters are investigated. The major problem addressed is the modeling and stability analysis of active/hybrid EMI filter, which is very critical for the industrial application of active/hybrid EMI filters. A hybrid CM filter used for DC/DC power converters is used to illustrate the modeling and stability analysis process and similar procedure can be followed to analyze other types of active/hybrid EMI filters.