Shielding Low Frequency Magnetic Fields

by Arturo Mediano - University of Zaragoza
It is common to consider EMI problems appear at high frequencies. Higher frequencies are usually responsible for parasitic couplings, undesired effects of non-ideal components, and other unexpected results for electronic designers. But, there are a lot of situations where low frequencies can create serious problems for electronic systems. Any sector can be affected by this problem: medical, instrumentation, consumer, audio, home appliances, communications, etc.
When EMI problems are conducted, filters are a common solution. When EMI problems are non-conducted, we use shielding. And with the shield we prevent aggressive systems from interfering with sensitive circuits. Shielding uses two main mechanisms: absorption and reflection. And the effectiveness of a shield is dependent of frequency and nature of the field (electric or magnetic). As the frequency of EMI is reduced both reflection and absorption losses decrease for the magnetic field so low frequency magnetic fields are harder to shield than medium and high frequency EMI signals. Then, we will need to apply special shielding techniques as reorientation or using highly permeable materials offering low reluctance (i.e. mu-metal or steel).

In this presentation we will …
1) ... review (briefly) the theory of shielding,
2) … understand why electric/magnetic fields work in a different way for shielding,
3) … discover what is “low frequency” for shielding,
4) … review common techniques to solve this problem.