On-site, Quick and Cost-Effective Techniques for Improving the Performance of EMI Filters by Using Conducting Bands

In this work we describe a simple and effective set of techniques to decrease the parasitic effects that undermine the performance of EMI filters at high frequencies. These techniques involve the use of bands of copper tape conveniently placed around the components of the filter. These techniques are practically cost-free and can be easily and quickly implemented on-site during pre-compliance tests to provide an increase of the attenuation of the filter at high frequencies. In some cases this can make it possible to avoid a re-design of the filter or the device under test. Moreover, these techniques can be used to readily determine whether a noise is of common mode or differential mode nature without altering the measuring setup, thus helping to speed up the re-design of the EMI filter if required and to avoid overdesign. We have constructed and measured several low pass filters mounting typical polyproyilene (PP) film capacitors and/or common-mode chokes to show the effectiveness of the proposed techniques. We obtain an equivalent circuit model that provides a physical explanation of the reduction of the parasitic inductance of capacitors. Also, we offer an explanation of the improvement achieved for the rejection of the common mode of chokes. Our results demonstrate that increases in filter attenuation at high frequencies go from approximately 3-4dB for simple shunt-capacitor filter to 10-15dB for a typical EMI filter.