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Brainstorm: EMI Shielding

Fri, 07/18/2014 - 2:41pm
Donald L. Sweeney, Senior EMC Engineer and President, D.L.S. Electronic Systems, Inc.

In what ways are EMI shielding components ideal for replacing metal or conductive coated plastic housing in sensing systems?

An EMI shielding or filtering component can be used in two ways to correct an area that is prone to high levels of emissions or susceptibility. It can be placed over an area of the circuitry (shielding), or it can become part of the circuit (filtering). When shielding is installed and bonded or grounded properly, or if filtering is used, it confines the energy closer to its source and replaces the need for overall shielding.

Reducing the total amount of shielding also reduces the product’s weight and cost. Reciprocity prevails when the emissions and susceptibility of the system are reduced.

There are many kinds of components to choose from, such as ferrites placed on cables, small filter components added to a noisy wire leaving a system, and shields placed over noisy integrated circuits or noisy areas on a circuit board.

The goal in using an EMI shielding component is to reduce the energy as much as possible and contain it close to its source. With this confining of energy, there is less opportunity for the interference to contaminate other areas or escape from the system; reducing the area where shielding might then be needed.The following photos show examples of how components can be installed as mini shields or filters. The type of protection is noted, using the following abbreviations: CM = Common Mode; DM = Deferential Mode; and OS = Over all Shield.

1. A 1 centimeter ferrite added to a 1 meter cable increases the series inductance from 1uH to 300uH because of the permeability of the ferrite material.  (CM)


2. When more than one turn is added to a ferrite on a cable, the impedance increases as the square of the number of turns. Two turns gives 4 times the impedance (Z) of one turn and three turns gives 8 times Z.  (CM)


3. Longitudinal transformer (non-isolating) reduces the Common Mode energy from signal wires leaving a system.  (CM)


4. A filter connector removes unwanted energy from wires leaving a system. No shield can solve this problem; it requires filtering.  (DM and/or CM)


5. Both commercial and custom made common mode chokes used to reduce energy on wires.  (CM)


6. Line to ground capacitors used to filter power leads; shielding is of little value if this is not done first.  (CM)


7. Line to line capacitors used for differential filtering of power leads.  (DM)


8. Small circuit board mounted shields used to shield a specific circuit area.  (OS)

 

References: 

Controlling Radiated Emissions by Design, 2014, by Michel Mardiguian, Donald L Sweeney and Roger Swanberg. Springer. ISBN 978-3-319-04770-6.
Design enclosures to meet EMC requirements by Donald L. Sweeney, Test and Measurement World, 2012.
EMC by Your Design, a practical application seminar taught by Donald Sweeney, Roger Swanberg and Tim Lusha, Northbrook, IL

For more information visit www.dlsemc.com.

This article originally appeared in the January/February print issue. Click here to read the full issue.

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