Improving Performance of Compact EMI Filters by using Metallic and Ferrite Sheets

Magnetic coupling between components in compact electromagnetic interference filters is one of the main effects that undermine their ability to reduce conducted emissions of power converters at frequencies above several hundreds of kilohertzs or a few megahertzs. This work analyzes and compares the performance of two different shielding strategies based upon the use of copper bands (high-conductivity material) and flexible ferrite sheets (high-permeability material) to improve the attenuation provided by electromagnetic interference filters. We make use of electromagnetic simulations, measurements and modelling with a circuit model of the filter to identify the causes of the limited shielding performance of high-conductivity materials and to quantify the reductions in the mutual inductances between the components of the filter that can be obtained by using a smart arrangement of the shielding structures that is able to overcome these limitations while preserving the compactness of the filter. We also describe a design strategy for electromagnetic interference filters which maximizes the benefits of the proposed shielding techniques. Measurements on several filter prototypes demonstrate that the shielding techniques proposed here can provide 20 to 30dBs increase in the attenuation provided at high frequencies by the electromagnetic interference filter for differential mode noise.