Soft Magnetic Materials for EMI Suppression

Abstract

In Chapter 15, the soft magnetic materials for low level applications in transformers and inductors were chosen for their high Q, i.e. low losses per unit of inductance. This characteristic gave them greater frequency specificity in LC filter circuits and low losses in transformers. Their use in analog circuits in telephony, radio and other telecommunications applications satisfied the requirements and had few problems. The introduction of transistors, IC’s and digital circuitry prompted changes in these magnetic materials again with few problems. However, in the past twenty years, with the proliferation of complex digital applications in computers, LAN and other networks, there has literally been a pollution of conducted and radiated electromagnetic interference (EMI) which has threatened to disturb severely the operation of much sensitive equipment. In fact, even earlier during World War II, there was a “critical mass” of delicate communication systems that could be tolerated before a breakdown in the operation would occur. The advent of computers, digital data transmission, cellular phones, Internet and satellite transmission have forced the governments of many countries to make restrictions on the EMI that could be generated by various electrical and electronic devices. In the early seventies, several companies started making ferrite and powder core components to assist manufacturers and designers of the offending equipment in taking steps to reduce these emissions. In addition, the victims of the disturbing emissions could also use the same components to attenuate the incoming EMI. One step for reducing the incoming radiated emi was discussed in Chapter 8 on magnetic shielding. A second is in the use of LC filters of several types. The third is in the use of ferrite beads and other shapes to encircle the wires and cables and by absorbing the offending “noise”, reduce it. This chapter will deal with the magnetic materials and components used to combat EMI.