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Low-Frequency Magnetic Field Shielding

RF shielding enclosure

Introduction

As compliance engineers and technicians involved in new product development, much of our time spent developing shielding (or consulting with others who do) is devoted to developing shielding that is suitable for high-frequency (HF) signals (those signals that have frequencies greater than 100 kHz). In case you need a refresher on how this type of shielding is accomplished, reference 1 covers this topic at a high level.

Occasionally, we are asked to help develop shielding effective for near-field low-frequency (LF) magnetic fields, perhaps in a situation where some regulatory agency has imposed limits on LF magnetic field emissions of our product, and we are forced to comply. In this situation, we find ourselves caught a bit off guard and may not know what to do since the shielding design techniques we know well (those used for shielding HF signals) will not work for LF magnetic fields. If you find yourself in this situation and are unsure what it takes to develop effective shielding for LF magnetic fields, please read on.

Reflection and Absorption at HF vs. LF

When working with HF fields, we can utilize the reflection and absorption properties of the shielding material (for instance, aluminum, copper, or steel) in our design to provide a reasonable amount of shielding effectiveness without too much difficulty. Our largest items of concern then become how to deal with penetrations in the shield (seams, slots, holes, etc.) necessary for most products that must do something with input and output signals. For HF shielding, it is the penetrations, not reflection and absorption properties of the shielding material, and how we deal with the penetrations, that contribute most to how effective (or non-effective) the shielding is.

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For LF magnetic field shielding, the amount of attenuation provided by the reflection and absorption properties of the shielding material is essentially nil. Therefore, we cannot rely on reflection and absorption to save the day, like it sometimes does when dealing with HF signals. Something else must be done. This “something else” has only two options:

  1. Use high-permeability shielding materials to divert the magnetic flux.
  2. Utilize the “shorted-turn” method.

Divert the Magnetic Flux

To divert the magnetic flux around an item that requires shielding, use a high-permeability, ferromagnetic material that provides a low-reluctance path that the magnetic flux can take. This path diverts the magnetic flux away from the shielded item in a controllable manner. Note that Mumetal is a material that has a high relative permeability (> 10,000 from DC to ~ 1 kHz).

Caution: High permeability materials are more prone to saturation than low permeability materials – more on this later.

Use the “Shorted-turn” Method

I call this the “fight fire with fire” method. Here, we utilize Faraday’s law to create a magnetic flux via a conductor loop placed such that the incident magnetic field (the one we intend to suppress) penetrates the loop, and this creates a magnetic field that is counter to the original magnetic field, thereby reducing the incident magnetic fields total net effect. You may have seen the shorted-turn concept in practice, where you noticed a band of copper (belly-band) placed around a transformer of a switch-mode power supply. This band utilizes the shorted-turn method to reduce the radiated magnetic fields of the leakage flux of the transformer.

Degradation of LF Magnetic Field Shielding Effectiveness Using Diversion Technique

Nothing in engineering is perfect, and just as we experience degradation of shielding effectiveness in HF shielding via penetrations in the shielding, we also experience degradation of shielding effectiveness in LF magnetic field shielding when we use the diversion technique. Except in LF magnetic field shielding, we do not have to concern ourselves too much with penetrations but must now to two other factors that degrade shielding effectiveness. These two factors are:

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  1. With increasing frequency, permeability decreases.
  2. With increasing magnetic field strength, permeability decreases, and saturation is likely.

Knowing about item 1, beware of the permeability specifications given for Mumetal at low frequencies, such as 1 kHz. The material does not have the same level of permeability at higher frequencies, such as 20 kHz, where cold-rolled steel is just as good as Mumetal but is less costly.

In the case of item 2, two layers of magnetic field shielding are required. The first layer is often a low permeability material with high saturation capability used to reduce the field strength of the initial magnetic field. The next layer with high permeability and low saturation capability then takes over and effectively diverts the magnetic flux as previously described using the diversion method.

Summary

Shielding for LF magnetic fields is not the same as shielding for HF fields, and what works well for the latter does not work well for the former and vice versa. Knowing the rudimentary concepts of LF and HF shielding is important in the compliance engineering profession. This article focused on LF magnetic field shielding using the divert magnetic flux and shorted-turn methods.

References and Further Reading

  1. In Compliance Magazine. (2018, August 2). What Every Electronics Engineer Needs to Know About: Shielding.
  2. Keller, R.B., Design for Electromagnetic Compatibility – In a Nutshell, Springer, 2023.
  3. Williams, T., EMC for Product Designers, 5th Edition, Newnes, 2017.
  4. Paul, C.R., Scully, R.C., Steffka, M.A., Introduction to Electromagnetic Compatibility, Third Edition, John Wiley, and Sons, 2023.
  5. Webinar: Omicron 12th Power Analysis & Design Symposium, March 15th, 2023 – Worldwide (Virtual), Shielding Low Frequency magnetic Fields by Arturo Mediano – University of Zaragoza.

 

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