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Electro-Magnetic
Interference
EMI
General Information
With an increasing awareness
of the problems that can be caused by Electromagnetic interference, government agencies
and the electronics industry in general are paying more attention to the interference
characteristics of electrical and electronic components, devices and systems.
Electromagnetic interference (EMI) Specifications have been issued by various
organizations. In the United States, the DOD has issued MIL-STD-461, while the FCC
has issued Part 15, sub-part J, for Class A and B devices. Verband Deutscher
Elektrotechniker has developed VDE 0871 for level A and level B. The FCC and VDE
specifications are similar in that Class A and level A describe industrial equipment,
while Class B and level B are applicable to consumer equipment. The worldwide
defense electronics and aerospace community recognizes and generally accepts MIL-STD-461.
Abbott has, therefore, used this specification as its standard for EMI performance.
There are various types of EMI. The following definitions are provided to
clarify how MIL-STD-461 applies to power supplies.
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Interference
Interference is defined as
any electromagnetic disturbance, phenomenon, signal or emission which causes or can cause
undesired response, malfunctioning or degradation of performance of electrical and
electronic equipment. There are two basic types of EMI associated with power
supplies, BROADBAND and NARROWBAND. |
Broadband
Interference
Broadband Interference is
interference which has a broad spectral energy distribution. This type of
interference usually exhibits energy over a wide frequency range and is generally a result
of sudden changes in voltage or current. It is normally measured in "decibels
above one microvolt (or microampere) per Megahertz" e.g., dBuV/MHz or dBuA/MHz.
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Narrowband
Interference
Narrowband Interference is
interference which has its principal spectral energy confined to a specific frequency or
frequencies. This type of interference is usually produced by an oscillator circuit
which contains energy only at the frequency of oscillation and harmonics of that
frequency. It is normally measured in "decibels above one microvolt (or
microampere)", e.g., dBuV or dBuA. In normal usage, the magnitude of the
interference will depend on the bandwidth of the receiver used to detect the signals.
However, what is important to know is if the interference energy will degrade the
performance of a device or system. Thus it would be useful to classify these
interference signals in terms of the bandwidth of the device or system.
Unfortunately, for general interference measurements, this bandwidth is not known in
advance, so we are restricted to the above definitions.
Although the boundary between narrowband and broadband signals is somewhat hazy, this
useful generalization can be made: Signals that can be represented on a frequency
amplitude plot as a single line or group of single lines spaced at wide intervals are
narrowband signals, and signals that appear as continuous bands of energy are broadband
signals. There is another aspect of EMI that is important to you as a power supply
user and that is the method of propagation of the interference signals. EMI is
propagated by either of two methods, conduction or radiation. |
Conducted
Interference
Conducted Interference is an
undesirable electromagnetic energy appearing on the external conductors (input and output
leads of a power supply) which may be conducted into other sensitive equipment and cause
malfunction or otherwise affect the operation of the equipment.
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Radiated
Interference
Radiated Interference is the
propagation of electromagnetic energy without a direct conducting path, and is generally
the result of electromagnetic fields around external wiring or inadequate shielding
afforded by the power supply case.
Besides the Electromagnetic Interference generated by a power supply, you will also be
concerned with the degree that a power supply will be affected by interference generated
by other external sources. This is known as susceptibility.
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Susceptibility
Susceptibility is defined as
that characteristic which causes equipment to malfunction or exhibit an undesirable
response when its case or any external lead is subjected to electromagnetic interference
voltages or fields.
It is important to note that virtually all EMI specifications, including MIL-STD-461, are
systems specifications and power supplies are usually only one of many components in the
total electronics system. Even if all the components used in an electronics system
meet all of the provisions of the specification individually, the system taken as a whole
may not meet the required specification, and additional EMI suppression measures will be
required. System layout, wiring, grounding, shielding and construction techniques
have definite effects on the EMI emission and susceptibility of modern systems and may
require "fine tuning" in order for your system to meet the required
specifications.
MIL-STD-461 describes EMI requirements for specific branches of the Department of Defense
(DOD) and for classes of equipment. This specification has been revised several
times. MIL-STD-462 defines the test procedures for measuring EMI. |
Martek Power Abbott's power supplies meet the EMI requirements of most systems
due to their design and construction.
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Application
Issues
In most cases, MIL-STD-461
is imposed on a system which contains at least one power supply. The system designer
must be aware that, although narrowband noise is rarely additive, broadband noise is
always additive. Therefore, there is no guarantee that a system will meet
MIL-STD-461 even though all power supplies in the system meet the specification.
External filtering of input power may be required. As a final note of caution,
filters must be designed for specific applications and merely cascading filters may
aggravate the situation rather than improve it. |
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© 2007 Martek Power Abbott, Inc. All Rights Reserved.
This page last updated on
March 19, 2008 06:58 PM. |
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