Modelling the performance of ferrite based common-mode filters: an educational approach using spice and mathcad

It is well known in the industry that ferrite-based common mode filters can produce mitigation of common mode noise. Previously published research in this field has disclosed a wealth of information regarding how to design this kind of filter [1-5]. This information, however, is not normally encountered in the classroom by electrical engineering students, and can appear to be difficult to understand at first glance. This paper discusses the process of designing large common mode ferrite-based filters that are also based upon a cylindrical geometry, and that can be readily understood by students through the use of easily available tools such as SPICE and Mathcad®. Concepts such as lossy ferrite materials, magnetic field illumination patterns, leakage flux, inductance, Ampere's Law, 3dB bandwidths, crosstalk, and other relevant concepts that relate to the design of such filters, are important to obtaining a firm understanding of this class of filters. After such a filter is designed, the common mode and differential mode attenuation performances need to be obtained from the filter. From this information, it can then be determined if the filter meets the requirements. This paper presents results from a Mathcad based numerical model that was derived by the author, and that can be used in an educational program for designing these kinds of common-mode filters. The model first extracts the inductance for one differential pair of conductors that are passed through a large toroid that is comprised of Manganese-Zinc. These results include the following cases: (1) small leakage inductance, (2) medium leakage inductance, and (3) large leakage inductance. From these results, it can be determined that leakage inductance can be used as a means of providing some mitigation against common mode noise, while also increasing the differential 3dB bandwidth of the filter. These results are then extended to more realistic cases such as the inclusion of several differential pairs of conductors, which is similar to the case of a CAT 5 cable that is used for Ethernet applications, or a cable that is used to carry multiple low-frequency differential telephony signals, such as the class of T1 or E1 trunk signals. Finally, the crosstalk performance of such filters is addressed.