Download EMI EMC unit 1

EMI EMC
Introduction :
• Widespread use of electronic circuits for
communication, computation, automation,
and other purposes makes it necessary for
diverse circuits to operate in close proximity
to each other
• These circuits affect each other adversely.
EMI EMC
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EMI …..A major problem.
Large no of electronic devices.
ICs and LSI to reduce size.
More circuits crowded in less space.
Increase in clock freq. upto 1 Ghz.
EMC EMI
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Not merely to operate in lab condition.
To work in real world environment.
Comply with EMC regulations of Govt.
This means that equipment should not be
affected by external electromagnetic sources
and should not itself be a source of
electromagnetic noise that can pollute the
environment.
• Electromagnetic compatibility should be a major
design objective.
• An electronic system that is able to function compatibly with
other electronic systems and not produce or be susceptible
to interference is said to be electromagnetically compatible
with its environment.
• The objective is to learn how to design electronic systems
for electromagnetic compatibility (EMC).
• A system is electromagnetically compatible with its
environment if it satisfies three criteria:
• 1. It does not cause interference with other systems.
• 2. It is not susceptible to emissions from other systems.
• 3. It does not cause interference with itself.
Noise & Interference
• Noise is any electrical signal present in a circuit
other than the desired signal.
• This definition excludes the distortion products
produced in a circuit due to nonlinearities.
Although these distortion products may be
undesirable, they are not considered noise
unless they are coupled into another part of the
circuit.
• It follows that a desired signal in one part of a
circuit can be considered to be noise when
coupled to some other part of the circuit.
Noise
• Noise sources can be grouped into the following
three categories:
• (1) intrinsic noise sources that arise from
random fluctuations within physical systems,
such as thermal and shot noise;
• (2) man-made noise sources, such as motors,
switches, computers, digital electronics, and
radio transmitters; and
• (3) noise caused by natural disturbances, such
as lightning and sunspots.
Noise..Contd.
• Interference is the undesirable effect of
noise.
• If a noise voltage causes improper
operation of a circuit, it is interference.
• Noise cannot be eliminated, but
interference can.
• Noise can only be reduced in magnitude,
until it no longer causes interference.
EMC
• Electromagnetic compatibility (EMC) is the ability
of an electronic system to
• (1) function properly in its intended
electromagnetic environment and
• (2) not be a source of pollution to that
electromagnetic environment.
• The electromagnetic environment is composed
of both radiated and conducted energy.
• EMC therefore has two aspects, emission and
susceptibility.
• Susceptibility is the capability of a device or
circuit to respond to unwanted electromagnetic
energy (i.e., noise).
• The opposite of susceptibility is immunity.
• The immunity level of a circuit or device is the
electromagnetic environment in which the
equipment can operate satisfactorily, without
degradation, and with a defined margin of safety.
• One difficulty in determining immunity (or
susceptibility) levels is defining what constitutes
performance degradation.
Definitions
• Electromagnetic Compatibility : (EMC)
The capability of electrical and electronic
systems, equipment, and devices to operate in
their intended electromagnetic environment
within a defined margin of safety and at design
levels or performance without suffering or
causing unacceptable degradation as a result of
electromagnetic interference [American National
Standards Institute (ANSI) C64.14-1992)].
Electromagnetic Interference : (EMI) :
• The process by which disruptive
electromagnetic (EM) energy is
transmitted from one electronic device to
another via radiated or conducted paths
(or both).
• In common usage, the term refers
particularly to RF signals; however, EMI is
observed throughout the EM spectrum.
• Radiated Emissions.
• The component of RF energy that is emitted through a
medium as an EM field. Although RF energy is usually
emitted through free space, other modes of field
transmission may be present.
• Conducted Emissions.
• The component of RF energy that is emitted through a
medium as a propagating wave generally through a wire
or interconnect cables. Line-conducted interference (LCI)
refers to RF energy in a power cord or alternatingcurrent (AC) mains input cable. Conducted signals
propagate as conducted waves.
• Susceptibility.
• A relative measure of a device or a system’s
propensity to be disrupted or damaged by EMI
exposure to an incident field. It is the lack of
immunity.
• Immunity.
• A relative measure of a device or system’s ability
to withstand EMI exposure while maintaining a
predefined performance level.
• Radiated Immunity.
• A product’s relative ability to withstand EM
energy that arrives via free-space
propagation.
• Conducted Immunity.
• A product’s relative ability to withstand EM
energy that penetrates through external
cables, power cords, and input–output
(I/O) interconnects.
EMC CONTD
• Control emission
• Raise the level of immunity ( Less
susceptible)
• EMC regulations of the Govt. (control
allowable emissions and define the degree
of immunity required)
• Crisis approach & Band aid approach.
• System approach….Not an afterthought.
FCC
• FCC Regulations
• In the United States, the Federal
Communications Commission (FCC)
regulates the use of radio and wire
communications. Part of its responsibility
concerns the control of interference.
FCC..Contd
• If a product uses digital circuitry and has a
clock greater than 9 kHz, then it is a digital
device under the FCC definition. This
definition covers most digital electronics in
existence today.
• Computer terminals and peripherals,
which are intended to be connected to a
computer, are also considered to be digital
devices
FCC contd
• Digital devices covered by this definition are divided into
the following two classes:
• Class A: A digital device that is marketed for use in a
commercial, industrial or business environment
• Class B: A digital device that is marketed for use in a
residential environment, notwithstanding use in
commercial, business, and industrial environments
• Because Class B digital devices are more likely to be
located in closer proximity to radio and television
receivers, the emission limits for these devices are about
10 dB more restrictive than those for Class A devices.
Mechanism of Interference
Mechanism …contd
Mechanism..contd
Transfer of EM energy into 4
subparts :
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Radiated emission ( RE)
Radiated susceptibility ( RS).
Conducted emission (CE)
Conducted Susceptibility ( CS).
CE
• Conducted emissions are those currents
that are passed out through the unit’s ac
power cord and placed on the common
power net, where they may radiate and
thus cause interference with other devices.
• The frequency range for conducted
emissions extends from 150 kHz to 30
MHz
RE
• Radiated Emission concern the electric and
magnetic fields radiated by the device that may
be received by other electronic devices, causing
interference in those devices.
• FCC, as well as other regulatory agencies,
requires measurement of the radiated electric
field, and the regulatory limits are given in terms
of that field in dBmV/m.
• The frequency range for radiated emissions
begins at 30 MHz and extends to 40 GHz
FCC regulations
• The FCC specifies that for digital devices,
measurements to show compliance with
Part 15, must be performed following the
procedures described in measurement
standard ANSI C63.4–1992 ( 9 Khz to 40
Ghz)
FCC reg.on CE & RE
• The FCC Part 15 EMC Regulations limit
the maximum allowable conducted
emission, on the ac power line in the
range of 0.150 to 30 MHz, and the
• maximum radiated emission in the
frequency range of 30 MHz to 40 GHz.
Canadian Standards
EU standards
• The EU’s conducted emission requirements are
now the same as the FCC’s (see Tables 1-5 and
1-6 as well as Fig. 1-4).
• The Radiated emission standards are similar but
not exactly the same. Table 1-8 shows the
European Union’s Class A and Class B radiated
emission limits when measured at 10 m.
CISPR RE limits
RF & Transient Immunity
Transient Immunity
• There are basically three types of high
voltage transients that electronic
equipment designers need be concerned
about.
• ESD
• Electrical fast transient (EFT)
• Lightning surge
Transient Voltage Suppression
Diode (TVS Diode)..Signal line
suppression.
• A TVS diode is similar to a zener diode but with a
larger p-n junction area that is proportional to its
transient power rating
• The three most important parameters of TVS diodes
are as follows:
• Reverse standoff voltage
• Clamping voltage
• Peak pulse current
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Power Line Transient Suppression
• Power-line filters can handle the low
energy transients such as ESD and EFT.
• Additional EFT or ESD suppression, if
required, can often be obtained on the
power line by the addition of a commonmode ferrite choke on the input cable to
the power-line filter or power supply.
• Surge, however, has a pulse energy level
that is a thousand or more times greater
than that of ESD or EFT and is another
problem altogether.
• Transient protection for these high-energy
pulses is often required prior to the powerline filter. Three types of nonlinear
transient protectors are commonly used
for high power transient.
• TVS diodes
• Gas discharge tubes
• Metal oxide varistors (MOVs)
• Transient voltage suppression diodes and MOVs
are voltage-clamping devices. They operate by
limiting the voltage to a fixed level. Once they turn
on, they must dissipate the transient pulse energy
internally.
• Gas discharge tubes, however, are crowbar
devices. Once they turn on, the voltage across
them drops to a very low value; therefore, their
power dissipation is much reduced. They can
handle extremely large currents.
ESD control
Human body model as a prime
source of discharge
• Electrostatic discharge is normally a threestep process as follows:
• 1. A charge is generated on an insulator.
• 2. This charge is transferred to a
conductor by contact or induction.
• 3. The charged conductor comes near a
metal object and a discharge occurs
• Decay time…The time it takes for the
charge to be reduced to 37% of its initial
value. The decay time (sometimes called
the relaxation time) is equal to
I/O Cables Treatment
Approx values
• L1 + L2 = 0.7 micro
• R1 + R2 = 1 to 30 K
• C1 + C2 = 150 pf.
Biological effects