Machine Design

# Fencing in Magnetic Fields: EMC and Interference

Electronic devices are said to be in a state of electromagnetic compatibility (EMC) when they do not conduct or radiate excessive electromagnetic energy, nor are susceptible to such energy from internal or external sources. Interference from a combination of radiated or conducted electromagnetic fields or radio frequency waves (EMI/RFI) can easily disrupt the EMC of unshielded circuits. Establishing basic EMC in any electronic device generally requires two distinct steps.

Designing an electronic circuit or device so it inherently generates little EMI is the best first step. Filtering cables at the entrance or exit of enclosures may suppress or contain conducted EMI within the enclosure. Also, shielded enclosures and materials substantially reduce most radiated EMI.

Improving device immunity (or reducing its susceptibility) to interference from external EMI sources is the second method for establishing EMC. Designing circuits and choosing components which are inherently less sensitive to interference largely reduces or eliminates susceptibility. As in the case of internal sources, filtering devices on incoming and outgoing leads reduces conducted EMI, and effective shielding reduces susceptibility to externally radiated EMI.

Electronmagnetic Fields
Radiated electromagnetic waves consist of an E-field (electric) and an H-field (magnetic) oscillating at right angles to each other. Voltage sources such as logic chips or clocks switching between 0 and 5 V states generate E-fields. On the other hand, current sources, such as motors and transformers, generate H-fields.

The ratio of E-field (V/m) to H-field (A/m) intensity is called the wave impedance Zw, and is expressed in ohms. A device operating on only a small amount of current (mA or less) generates waves with high impedance. These are considered E-fields. Conversely, if a device contains a large current flow compared to its voltage, it generates a low impedance H-field.

At very large distances from the radiating source the ratio of E to Hfield strength approaches unity regardless of their origin. EMI waves become plane waves and their impedance equals 377 Ω, the intrinsic impedance of free space. Beyond this, all waves essentially lose curvature and the surface containing the two components becomes a plane instead of a section of a sphere, as in the case of a point source of radiation.

EMI Shielding Principles An electromagnetic wave encountering a metal shield shows the effect of wave impedance. When the impedance of the wave differs greatly from the natural impedance of the shield, much of the energy reflects and the rest travels across the surface boundary where absorption in the shield further attenuates it. Because most materials have an intrinsic impedance of only milliohms, less lowimpedance H-field energy reflects, and more absorbs. The metal more closely matches the impedance of the field, and this is the reason it is difficult to shield magnetic fields. On the other hand, the wave impedance of electric fields is high, so most of the energy reflects. At higher frequencies, particularly over 10 MHz, absorption principally determines the EMI shielding.

Metallic enclosures do not make perfect shields because all metal conductivity is finite. They can, however, approach very large values. Because metallic shields are less than perfect, part of the field travels across the boundary and supports a current in the metal. The amount of current at any depth in the shield and the rate of decay depend on the conductivity of the metal, its permeability, and the frequency and amplitude of the field source. The residual current appearing on the opposite face generates a field on that side.

Thickness does not usually affect current density in a metal shield. A secondary reflection appears at the far side of the shield for all thicknesses. The only difference with thin shields is that a large part of the re-reflected wave may appear on the front surface. This wave can add to or subtract from the primary reflected wave, depending upon the phase relationship between them. For this reason, a correction factor in shielding equations accounts for reflections from the far surface of a thin shield.

A gap or seam in a shield lets EMF pass through unless the current path can be maintained across the gap with an EMI gasket. When the gasket material matches the walls of the shielded enclosure, the current density in the gasket is the same. This assumes it could perfectly fill the slot, but in practice, this is not possible due to mechanical limitations.

Current through a shield including a gasket interface is shown in the figure. Electromagnetic leakage through the seam can develop in two ways. First, the energy can leak through the material directly. The gasket material shown is assumed to have lower conductivity than the material in the shield. The rate of current decay, therefore, is less in the gasket, producing more current flow on the far side of the shield. This increased flow generates a larger leakage field on the far side.

Second, leakage can develop at the interface between the gasket and the shield. When an air gap appears at the interface, current diverts to the points or areas in contact. A change in current direction alters the current distribution in the shield as well as in the gasket, which lowers shielding effectiveness. A high-resistance joint does not behave much differently than an open seam. It simply alters the distribution of current. Current distribution for a typical seam is shown in the figure. Lines of constant current spaced at larger intervals indicate less current flow. The electrical properties of the gasket must be as close to the shield as possible. Also, maintain low impedance interface surfaces, and avoid air gaps which increase joint resistance.

EM waves impinging upon a discontinuity are partially reflected and partly absorbed by the material. The effectiveness of the shield is the sum of these two effects plus a correction factor to account for reflections from the rear surfaces of the shield.

The frequency, shielding-material conductivity, thickness and permeability, and distance between the radiating source and the EMI shield determine the reduction in field strength. How well a shield attenuates the energy of a radiated EM field is referred to as its shielding effectiveness or SE. The standard unit of an SE measurement is the decibel or dB. The decibel value is the ratio of two measurements of EM field strength taken before and after shielding. Each 20-dB increase in SE represents a tenfold reduction in EMI leakage through a shield. A 60-dB shield reduces field strength by a factor of 10,000 times (such as from 5 V/m to 5 mv/m).

The expression for shielding effectiveness is

SE = R + A + B

where SE = shielding effectiveness, R = reflection factor, A = absorption factor, and B = correction factor to account for reflections from the far boundary. All values are expressed in dB units.

Reflection loss R includes reflections at both surfaces of the shield, and depends on the relative mismatch between the incoming wave impedance and the frequency of the impinging wave, as well as upon the electrical parameters of the shielding material. The equations for the three principle waves are

where RE, RH, and RP= reflection terms for the electric, magnetic, and plane wave fields, dB; G = relative conductivity referred to copper; f = frequency, Hz; μ = relative permeability referred to free space; r1 = distance between source and shield, in. The absorption term A is the same for all three waves and is A=3.338 X 10-3 X t√μfG, where A = absorption or penetration loss, dB; and t = shield thickness, mils.

The correction factor B can be mathematically positive or negative (in practice, it is always negative) and becomes insignificant when A is greater than 6 dB. It is usually only important when metals are thin and are at low frequencies, below about 20 kHz.

It is found from

A plot of reflection and absorption loss for copper and iron gives a good physical representation of the behavior of the component parts of an EM wave. It also illustrates why it is so much more difficult to shield magnetic fields than electric fields or plane waves. Copper offers more shielding effectiveness than iron in all cases except for absorption loss because the permeability of iron is high.

When only electric-field or plane-wave protection is required, reflection is the main factor to consider. If magnetic shielding is required, particularly at frequencies below 10 kHz, it is customary to neglect all terms in the equation except absorption A.

Information for this article supplied by Norman Quesnel, Chomerics Div., Parker Hannifin, 77 Dragon Court, Woburn, MA 01888.