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Aug 21, 2002

Keywords: EMI, EMI protection, electrical noise,
ESD, gas arrestor, varistor, suppressor diodes, chokes,
ferrites, capacitors, series resistors, thermocouple, angular
encodres, EMI immunity testing, motor control
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APPLICATION NOTE 1167

Practical Aspects of EMI Protection

Abstract: Article
discusses the sources of electromagnetic interference (EMI).
It describes how designs with gas arrestors, a varistor,
suppressor diodes, chokes and ferrites, capacitors, or series
resistors protect equipment. Methods for EMI testing are
described.

Introduction
More and more aspects of our lives depend
on the uninterrupted operation of electronic equipment and
appliances. "Drive by wire" and "fly by wire" have become
everyday realities. In automated plants and high-availability
systems especially, electronic equipment that fails to operate
correctly can cause enormous damage and disruption.

The
ideal electronic circuit should be free of active electromagnetic
interference (EMI, also called RFI or radio
frequency interference) sources and passively immune to
disturbances from outside.

The task of minimizing
active sources is probably easier than protecting against
interference. Common techniques to minimize EMI are line
filtering, power-supply design, proper layout, and shielding
the enclosure.

Electrical disturbances can be conducted
by the power lines or conveyed through the air by capacitive,
magnetic, or electromagnetic radiation. Typically, the
interference conducted over signal lines connected to the
equipment is the most difficult to manage. In any case, one
must distinguish between the need to protect against
damage or malfunction, and the need to prevent signal
or data distortion,
for example from a disruption in the sequence of a
microcontroller program. The first problem is attacked with
hardware design, the second with software
algorithms.

This article addresses the means for
protecting equipment against the intrusion of harmful voltages
and currents.

Guidelines for Regulating EMI
Recognizing the
importance of EMI, governmental bodies early undertook the
task of enforcing equipment compatibility through regulations.
IEC 61000-4, for example, standardizes test methods. Sections
relevant to this discussion include IEC 61000-4-2 for electrostatic
discharge (ESD), IEC 61000-4-4 for fast transients (FTB),
and IEC 61000-4-5 for high-energy transients (SURGE). All of
these test methods rely on realistic models for electrical
noise.

Before applying any protective elements,
consider these basic rules:

EMI protection should be considered while designing the
circuit, not added afterwards.

Block disturbances as near to the source
as possible, preferably before they enter the equipment;
redirect them to ground.

All sections that can be exposed to EMI disturbance,
even electrically isolated sections, should be located as
far as possible from sensitive circuitry.

Because
signal circuitry cannot withstand kilovolt-level voltages,
such disturbances must be excluded from the input, converted
to current, and then to heat. Ground-loop currents, which can
enter an interface and run throughout the circuit, are often
thwarted with galvanic
isolation. Isolation is especially useful for the longer
lines and high ground-loop currents that can occur in
industrial systems.

An ESD current pulse of 30A peak
may produce only tens of millivolts of resistive voltage drop
on a ground trace. However, its extremely steep rise time
(> 30A/ns) can induce hundreds of volts of inductive drop
on the same trace (assuming roughly 1nH/cm of wire
inductance)—more than sufficient to cause data errors. Skin
effect applies at these high frequencies; it increases wire
resistance dramatically by forcing current to flow only within
microns of the conductor surface. To counteract this effect,
ground connections need a large surface area to maintain low
resistance.

Fast rise times can enable capacitive
coupling of FTB and ESD disturbances from noisy sections to
supposedly quiet ones. In this context, a frequent mistake is
made when designers provide galvanically isolated supplies by
adding windings to the main power-supply transformer. This
arrangement allows "infected lines" (ground returns for the
external signal) to contaminate the entire circuit.

As
a compact, functionally clean and cost-effective alternative,
you can generate auxiliary voltage at the circuit periphery
using a forward
converter based on the MAX253 driver. The MAX253 offers a
small board space and an effective barrier between the clean
and noisy potentials. The tiny transformer required can be
manufactured with a coupling capacitance below 10pF, yet it
transfers up to 1W while isolating kilovolts (Figure
1).

Figure
1. A small transformer and driver IC derives an isolated 5V
supply from the 5V rail. For details about 1CT:1.3CT see Table
2 in the MAX253 data sheet.

Other commonly used
EMI-protection components are described below.

Gas
arrestor is a kind of dish-shaped capacitor filled with
gas, frequently neon. Overvoltage in excess of ~100V creates a
plasma that limits the voltage at low levels and carries high
currents. Gas arrestors absorb high-level transients, but are
not suitable for fast transients because the emergence of
plasma takes some time. They are not suitable for mains
protection and are difficult to use with low-source
impedances. Leakage currents in normal operation are very
low.

Varistor is an arrestor made with metal
oxide (mostly zinc), usually shaped like a tablet with two
connectors, one on each side. Behaving similar to a zener
diode, a varistor responds much faster than the gas arrestor,
but exhibits high leakage currents especially when the signal
approaches the clamping voltage.

Suppressor
(TransZorb®) diodes are used to limit fast transients at
low voltage levels. Their power-dissipation capability is
limited according to their form factor. As for varistors, they
exhibit significant leakage currents in the vicinity of their
breakdown voltage. Junction capacitance is also significant,
so in fast systems they are usually decoupled with diode
bridges.

ESD structures are novel designs that
behave somewhat like diacs. These designs are integrated in
the MAX202E, MAX485E, other
RS-232/RS-485 transceiver
ICs, and recently in analog multiplexers such as the MAX4558. Exhibiting
low capacitance and low leakage currents, they are suitable
for both ESD and FTB protection.

Chokes,
ferrites can attenuate high frequency and fast voltage
peaks, but do not absorb much energy. Beware of resonant
effects, and always use with attenuating capacitors (commonly
an LC filter in a T-structure). An inductor manufacturer
usually specifies the self-resonant frequency (SRF).
The SRF comes from the parasitic capacitance of an inductor
that cancels out the inductance when operating above the SRF.
For better performance, use an inductor as an RF choke below
its SRF, and never above the SRF. These devices are frequently
used to protect against common-mode
disturbances and for mains filtering.

Capacitors
are perhaps the most important protective element. A
capacitor's important features are equivalent
series resistance (ESR), inductance, high-current
capability, and voltage capability. Proper placement of
capacitors in the layout is crucial to minimizing EMI. When
used properly, capacitors create a high-frequency ground to
from a lowpass filter, bypassing the RF signal to ground.
Capacitors also have a self-resonant frequency, a point at
which it becomes inductive. Just like an inductor, a capacitor
should be used below its SRF. Another way to understand this
is to make sure that the SRF is much higher than the frequency
of noise that needs to be filtered.

Series
resistors are also among the most important and lowest
cost protective elements. Properly selected according to
resistance and power dissipation, series resistors can replace
more costly elements with comparable results.

Good
layout design can minimize the effects of EMI. Fast switching
currents generate the magnetic field, and fast changing
voltages create the electric field that can result in
undesired coupling. Electromagnetic coupling can be minimized
with the use of good grounds and shield cases. Proper
placement of vias is always a good practice. Have multiple
vias for bypass capacitors because multiple vias reduce
resistance and inductance. Also, avoid a ground that has an
unstable ground-voltage potential. Traces that carry
high-frequency signals generate a time-varying electromagnetic
wave that can propagate the cause of interference. Two traces
at a 90-degree angle result in the least interference between
the two signals. A good case ground also helps prevent outside
signals from entering the system. This, in turn, shields the
circuit.

The following examples illustrate use of these
components, some working principles, and possible
pitfalls.

Thermocouples
Thermocouple signals offer the advantage
of quasistatic processing. To guard against signal distortion
by ground-loop currents, most thermocouple applications
provide galvanic isolation between the signal-acquisition and
signal-processing circuitry. As Figure 2a shows, the
differential signal is fed through a multiplexer
to the input of an instrumentation amplifier,
and from there to an analog-to-digital converter (ADC).
Opto- or magnetic coupling transfers the ADC's digital output
across the isolation barrier.

Figure
2a. These components process the differential signal from a
thermocouple.

The thermocouple is easily protected
with a simple, lowpass RC network of 2kΩ and 100nF on each
electrode. An additional 1nF diverting capacitor with
high-voltage rating connects the circuit common and the
grounded frame of the equipment. This capacitor diverts ESD
disturbances to ground while maintaining galvanic isolation
for the DC currents. It also forms a capacitive
voltage-divider that limits peak voltages at the isolated
power supply. To further limit peak voltage, a high-voltage
varistor can be connected in parallel
with the diverting capacitor (Figure 2b).

Figure
2b. A high-voltage varistor (lower left) limits the peak
voltages.

The 2kΩ resistor must be large enough to
withstand high voltage (up to ±8kV ESD, and to dissipate
considerable energy during FTB and SURGE testing.
Unfortunately, leakage current at the inputs of the following
circuit (i.e., before the isolation barrier) can flow through
this protective series resistance to create substantial static
signal errors. Multiplexers, for example, can introduce
unacceptable errors; amplifiers added to buffer the
multiplexer can introduce additional input-offset and
input-current errors while adding cost and board space to the
system.

The MAX4051A
multiplexer, which is pin and function compatible with the
industry-standard MAX4051, offers a moderate-cost alternative.
Its leakage current is guaranteed at 5nA (max) over the
extended temperature
range and only 2pA typical at +25°C. Thus, the worst-case
leakage over temperature at the series resistors creates a
maximum error of only 2µV. This error is quite sufficient for
most thermocouples. If the application requires even less
error, an instrumentation-amplifier buffer based on the MAX4254 quad op
amp lowers the leakage to 100pA (max) over temperature and
to 1pA (typ) at +25°C. In addition, a low-input offset voltage
(VOS) drift (only 0.3µV/°C) makes this buffer
extremely effective for high-impedance, low-level
signals.

Another alternative is the monolithic MAX1402
signal-acquisition system, which includes a sigma-delta
ADC, buffer amplifier, multiplexer, current source for sensor
excitation, and so-called burnout current sources for signal
validation. It features very low input-leakage current, and
the flexibility for implementing applications such as
cold-junction compensation.

Angular Encoders
Industrial angular encoders determine
the rotor position in electrical machines. Precision
positioning systems feature dual-channel, orthogonal,
differential sinusoids (called "sincos" signals by some
manufacturers).Taken together, these sinusoids form a pointer
that allows a fine resolution of rotor position. Alongside the
analog-position signal lines, such systems often include RS-422
or RS-485 lines for initializing the encoder and setting its
parameters. In some cases these lines run for long distances,
carrying analog signals of low-kilohertz frequency and digital
data rates as high as megabits
per second (Figure 3).

Figure 3. Communication in this optical-encoder
system consists of analog "sincos" signals and bidirectional
digital-data signals.

Thus, protection in the form
of large series resistors and passive RC combinations is
excluded, but line-termination resistors (usually 120Ω) are
required to prevent reflections. The primary need is for
protection against ESD and FTB disturbances. In a system with
conventional data transceivers (Figure 4), the
differential transmitter's output voltage is limited by a
suppressor diode and decoupling diodes. (Decoupling diodes are
reverse-biased to decouple the wideband
data signal from the capacitance of the suppressor diode,
which otherwise would heavily attenuate the
signal.)

Figure
4. This diode/suppressor network prevents damage from ESD,
FTB, and SURGE faults. Optional PTC fuses protect against
fault connections to hazardous voltages.

Similar
protection is provided for the receiver,
but with an important difference: to guarantee an
unsymmetrical common-mode range (-7V to +12V for EIA-422A) the
limiting network must be unsymmetrical as well. Transmitter
outputs and receiver inputs comply with the same common-mode
range, so for convenience and economy both are protected by
the same suppressor diodes. However, this whole protection
network can be replaced by the MAX490E, an RS-422
transceiver that integrates the ESD- and FTB-protection
networks.

As a matter of good practice, the transceiver
ground should be tied to casing/earth with as short a
connection as possible. If the lines are shielded (which is
highly recommended!), the shield should also be tied to this
point with a short connection. If you expect large exchange
currents between separated ground potentials, you should
insert a 100Ω series resistor between the shield and earth,
preferably bypassed with a low-ESR capacitor.

If the
system requires SURGE protection, an external protection
network is unavoidable. In that case, it may be advisable to
split the line termination so it can function as a
current-limiting resistance as well. This is easily
accomplished on the receiver side with a moderate loss of
signal level. For the driver side, you must verify whether an
approximate 10Ω of series resistance is acceptable, given that
the MAX490E's differential output impedance
is about 40Ω. A possible circuit including PTC fuses in series
with the data lines is given in Figure 4 as well.

Standard Signal Interface
The mode of signal transfer
chosen for a system is affected by the enormous variety of
signal sources and transmission distances which the system may
present. For higher bandwidth
and longer lines, it is often necessary (especially in systems
prone to disturbance) to convert the signals to a higher
level: 0V to 10V; -10V to +10V; 4mA to 20mA; or in some cases
differential analog, as for an angular encoder. Other systems
prefer a digital format carried by wires, optical fiber, or wireless
radio links. In all cases, you should minimize noise effects
by placing the transducer as close to the signal source as
possible.

A ±10V interface, for example, is often used
to set the target position in motor-control applications.
Those environments are noisy, and the 24V industrial supply
(always present in the cabinet) can be a hazard when wiring
errors occur. Maxim's signal line protectors MAX4506/MAX4507, feature
60Ω channel resistances and a maximum 20nA leakage over
temperature. They also offer an elegant protection for this
interface (Figure 5). Signals within the supply-voltage
range pass through these ICs virtually unaffected.

Figure
5. This signal-line-protector IC protects a ±10V
interface.

If a disturbance causes the protected
terminal signal to exceed the positive or negative supply
voltage, the line-protector chip presents a high impedance to
the fault signal. It tolerates fault voltages to 36V, or to
±40V with the supplies turned off. Figure 6 shows the
input-to-output transfer function for these parts.

Figure
6. This transfer function illustrates the clamping effect of
MAX4506/MAX4507 signal-line protectors.

Using a
component such as the MAX4506 or MAX4507, Figure 5 shows a
possible protection scheme for ±10V standard signal outputs.
The output is generally more difficult to protect than the
input, although leakage introduced by protective elements at
the input can cause signal errors. It is sufficient to insert
a bidirectional suppressor diode, one providing a ±30V clamp
voltage, between the line terminal and ground/earth at each
receiver input or transmitter output.

Because the
suppressor diode's clamp voltage is far beyond the standard
signal range, any distorting leakage currents that originate
in the suppressor diode are quite low. The line signal is
regulated by feedback, while series resistance for the
low-signal-line protector places minimum requirements on the
driver swing. (It is just 100mV dropout voltage, assuming a
10kΩ minimum load resistance at the receivers.) This
protection holds even for continuous faults (to 24VDC, as
mentioned above), without endangering either the suppressor
diode or the circuit.

Similarly, signal inputs at the
receiver side can be protected by inserting a signal-line
protector in front of the buffer amplifier. The MAX197, a
specialized A/D
converter with internal fault protection to ±16V, allows
simple and effective protection by connecting bidirectional
suppressor diodes (which provide a ±15V clamping voltage) in
parallel at the inputs. Software-programmable input ranges for
each input channel enable the MAX197 to handle the standard
voltage ranges discussed. If higher levels of protection are
required, you can place the MAX4507 octal signal-line
protector in front of the MAX197.

EMI Immunity Testing and Measuring Techniques

Electrostatic Discharge Immunity Test (IEC
61000-4-2)
The test generator consisted of a high-voltage,
150pF capacitor that discharged through a 330Ω series resistor
(Figure 7); a switch; and an electrode, shaped to
simulate a finger, that discharged into the equipment under
test (EUT).

Figure
7. A simple test circuit applies controlled ESD pulses to a
test device.

The test specification distinguished
between contact
discharge and air
discharge. Contact discharge was applied by touching the
equipment enclosure with the test finger while closing the switch.
This procedure exposed the equipment surface to the test
voltage (±2kV, ±4kV, ±6kV, and ±8kV), minus a voltage drop
caused by current flowing through the series resistor. The
resulting short-circuit currents are depicted in Figure
8.

Figure
8. An ESD "zap" from the circuit of Figure 1 produced this
current in the device under test (DUT).

Air
discharge was applied to insulating surfaces by bringing the
test tip toward the surface as rapidly as possible. The test
required at least 10 measurements, spaced at least one second
apart at each level of standard test voltage (±2kV, ±4kV,
±8kV, and ±15kV). The objective was to find the weakest point
within the equipment's enclosure by testing to the limit
specified for that class of equipment. Using the same test
generator, an additional capacitive-coupling test was
implemented by connecting the test tip to a metal plate
positioned close (i.e., about 10cm) to the equipment casing.
This test applied only to those parts of the equipment that
users touch during normal operation and maintenance.

Fast Transient Burst Test (FTB) (IEC 61000-4-4)
An FTB
test capacitively coupled a test signal onto the mains line or
the signal line. It employed a coupling device that was shaped
like a tube and enclosed the line under test for about one
meter. The test signal consisted of approximately 75
high-voltage pulses, delivered in bursts at a 3Hz repetition
rate. Each pulse had an approximate rise time of 5ns, and a
fall time (to 50% of peak value) of 50ns with a 50Ω load or
100ns with a 1000Ω load. The time between pulses was 10µs or
200µs (Figure 9).

Figure
9. The test pulses in an FTB burst are specified as
shown.

To create the high voltage required for low
load resistance, the output resistance of this generator was
much lower than that of an ESD generator. Peak amplitudes for
the test pulses ranged from ±0.25kV to over ±0.5kV, and at a
50Ω load from ±1kV to ±2kV. At 1000Ω the load amplitudes are
±0.5kV, ±1kV, ±2kV, and ±4kV. A decent cable
shield, connected properly to ground/earth on the equipment
side, can remove a lot of trouble from this test.

SURGE Immunity Test for High-Energy Transients (IEC
61000-4-5)
With considering potential damage hazards to
equipment, the most stringent test is for high-energy
transients, the SURGE test. In this test, the mains supply
lines, unshielded signal lines, or cable shields were driven
by low-value series resistors with long-duration, high-voltage
pulses. The pulses had approximate 2µs rise times and 50µs
fall times (to 50% of peak value);amplitudes ranged from
±0.5kV to ±2kV for mains lines and to ±1kV for signal
lines.

For supply lines, the SURGE test galvanically
coupled the test signal using a 9µF DC decoupling capacitor in
parallel with a series resistance as low as 10Ω (and in some
cases, with no resistor at all). Unshielded signal lines were
tested with a series resistance as low as 40Ω, line-to-line or
line-to-earth. Adding 90V gas arrestors in series prevented
capacitive loading of the signal lines between tests
(Figures 10a, 10b).

Figure
10a. Test setup: Capacitive coupling of a test signal to AC or
DC lines.

Figure
10b. Test setup: Coupling to unshielded, asymmetrically
operated lines.

TRANSZORB
is a registered trademark of Vishay General Semiconductor,
LLC.

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APP 1167: Aug 21, 2002

AN1167, AN 1167, APP1167,
Appnote1167, Appnote 1167

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