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First Published in
EOS/ESD Technology Jan/Feb 1993
Developing ESD-Immune Electronic
Systems
Here is how two different systems
were affected by ESD and how they were designed to be ESD-immume.
Michael
J. Smith
Chilworth Technology Ltd.
Beta House
Chilworth Research Centre
Southampton, SO1 7NS UK
Electronic systems
can respond erratically to any high-power switching operations that
occur during normal functioning. Systems can also malfunction due
to the small current produced by electrostatic discharges on products
being handled for from charged structures in computer-controlled
plants. Electrostatic discharges can cause unstable operation or
even damage equipment if measures are not taken to ensure immunity
form this type of problem.
The two case studies presented her show the connection between system
malfunctions and electrostatic discharges. Also discussed are the
methods developed to ensure that the systems would be able to operate
normally and with immunity to ESD.
ESD Effects of
Equipment
There are several ways
equipment can be affected by electrostatic discharges. According
to the document IEC 801-2, equipment subjected to electrostatic
discharges can exhibit the following behavior modes. (3)
 Normal
performance within specified limits.
Temporary
degradation, loss of function, or performance which is recoverable.
Temporary
degradation, loss of function, or performance requiring operator
intervention or system reset.
Degradation
or loss of function unrecoverable which is not recoverable due to
damage to equipment, components, software, or loss of data.
In the following two
case studies, the equipment experienced degradation or loss of function
and data when measures were not taken to make the systems ED safe.
Efforts to suppress ED should enable the equipment to operate normally
without operator intervention.
CASE STUDY I
a PC-based data-acquisition
system was used to determine how susceptible to ignition dispersed
dust clouds would be when subjected to electrostatic spark discharges.
Computer-Controlled
Test Equipment
The computer-controlled
test equipment creates interference signals as a by-product of its
design requirement to produce high-voltage sparks. Using an operator-guided
program, the computer configures a high-voltage discharge circuit
consisting of a power supply, charging resistor, variable capacitor
bank, waveform characterizing inductor and spark gap assembly (Fig.
1).
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| Figure 1: MIE measurement circuit.
Switch open for VDI method. Switch closed for BS test method. |
Powder is dispersed
around the electrodes, between which sparks of measured energy are
passed and the ignition effects recorded. The minimum ignition energy
(MIE) is defined as the highest quantum of energy which does not
give rise to an ignition.
Interference Problems
The system generates
electrostatic discharges across the spark gap, with energies in
the area of 1mJ to 1000mJ. These spark discharges can generate cable-borne
and aerial interference, causing the computer to lose control of
the system.
A current probe was
used to observe the transient and oscillations present on various
cables within the system network. Unfortunately, no accurate quantitative
data can be presented to describe the magnitude of the interference
levels detected. The digital storage oscilloscope showed large interference
signals occurred at the instant of sparking, but with such fast
rise times that the trace featured very few digitized data points.
Similar results were obtained when using an oscilloscope probe in
contact with individual circuit screened enclosures.
Solutions and
Precautions
To prevent the potential
problems caused by high-energy electrostatic discharges, many of
the following measures and precautions were incorporated as part
of the system design.
Screening. Earth-bonded metal chassis cases for the individual circuit
modules were used to screen against capacitively-coupled interference.
Special care was taken to ensure that the screen was not interrupted
by the presence of paint or lacquer or abutting surfaces. Screened
cables were used throughout the system.
Woven screens tend to offer on 70-85% optical coverage of the signal
lines and interference may couple through the gaps. Interference
signals flowing in the braid may also be carried from the outside
to the inside of the woven form. An alternative to woven screens
are foil-screened cables. Although foil-screened cables are less
flexible than woven screens, a retro-fitted screen made by lapping
copper-foil tape around the outside of a cable provided a solution
for the apparatus under discussion.
Connectors. The cable screen should not be rendered an isolated
conductor by intermittent contact at the connector and those with
earthing continuity fingers are highly recommended.
Cables. The routing of cable through the instrument was controlled
to ensure that interference emitting cables (e.g. HT lines) did
not run paralleled to any control, data or low-voltage power lines.
(High-tension cabling could not be screened in this example since
coaxial cable would increase the system capacitance and raise the
lowest-producable spark energy.) The use of ferrite sleeves fitted
over cables can provide EMI attenuation of 10dB. (1) Such devices
were fitted near source oscillators to attenuate EMI emission and
suppress interference caused by the cables.
Earthing and filtering. Cable-borne interference can be understood
and controlled by examining the electrical earthing system. A diagram
showing circuit modules and interconnects can be made to determine
where earth loops might exist. Isolated conductors can also be determined
in this manner.
Power filtering is a two-way process: it bars interference from
a"dirty" utility supply entering the equipment, and simultaneously
prevents high-frequency or noise signals generated within the instrument
from spreading into the power supply. The filter must be correctly
installed for optimum efficiency. Those used in the minimum ignition
energy apparatus were the screened chassis Euroconnector-type. With
these filters, there is no possibility of capacitance interference
coupling between input and output cabling once the filters are mounted
to metal paneling.
Switching. Switching operations generate cable-borne interference.
Although a relay provides isolation between input and output, there
is a reaction in both circuits when the device is operated. The
coil circuit is inductive and a free-wheeling diode connected in
reverse bias will dissipate the residual energy when the relay is
switched off. When the contacts are opened and closed, an arc can
be generated that may affect the circuit being switched. This arc
may also reduce the relay contact life. Installation of a quencher
(typically a 0.1uF capacitor in series with a 100 ohm resistor)
suppressed this effect in the MIE apparatus. An alternative to the
quencher is the metal oxide varistor (MOV). These voltage dependent
devices can also be used in a power supply smoothing role when connected
between live and neutral.
High-speed silicon switches. In addition to protecting the mains
supply, the computer and the control module circuits must be protected.
High-speed silicon switches times of order lps are applied to the
main power supply rail voltages to prevent damage in the event of
HT discharge fault conditions.
DC power supplies. A switch-mode device containing MOSFETs switching
at frequencies in excess of 100 kHz has a very high noise content.
Although noise is not a problem in computers where all processing
is digital, the MIE control apparatus had some analog circuitry,
making low-noise supplies more appropriate. Despite their lower
power-to-weight ratio, linear power supplies were chose for this
application. Large capacitors (10000uF) were used to ballast the
outputs of the 24V and 12V supplies.
Decoupling. The prototype apparatus involved both analog input (capacitor
bank voltage via potential divider) and analog output lines (HT
power unit control signals). This technology was found to admit
interference causing system malfunction, so all output controls
were decoupled using relays. Measurement of the noisy high voltage
signal was affected using a rotating vane fieldmill: the non-contact
approach inherently decouples and filters the signal.
At the time for writing, the apparatus has undergone evaluation
and calibration tests at Chilworth Laboratories and has been delivered.
CASE STUDY 2
The second study involves
a microprocessor-based machine designed to sort metal discs into
size and weight categories. Electrostatic discharges within the
system often caused the machine to reboot the computer program,
losing information in the process.
Sorting Equipment
The sorter mechanism
consists of a compartment wheel rotating in a plane at 60 deg. to
the horizontal. The discs are collected from a feed hopper at the
bottom and carried one per compartment to the top of the wheel.
At this point, a magnetic coil is used to induce a signal in the
disc- the signature generated is recognized by the microprocessor
which energizes a relay to eject the disc into one of a number of
sorting tubes.
Equipment Problems
The equipment often
rebooted during operation and lost disc count registers. This problem
would manifest itself by freezing the on-screen real time clock
for about two seconds prior to performing a system reset.
The rebooting of the system was found to be initiated by electrostatic
discharges to various points of the machine. contact-injected electrostatic
discharges were produced form a 100 pF capacitor discharging through
a 150 ohm resistance. The capacitor was initially charged from a
high-voltage power supply via a 1 megohm decoupling resistor. Discharges
of about 3.5 kV could induce rebooting.
The sorting machine consisted of many unenclosed, distributed electronic
assemblies interconnected by unscreened ribbon cabling. To examine
the ESD tolerance levels of the system, as many cables as possible
were disconnected while the display system was left intact. When
this was done, the system responded once to a 4kV. Reconnection
of just the keyboard produced a sensitivity threshold of 4kV with
regular failures at 6kV. Other circuit modules reconnected one-at-a-time
to the display system produced ESD tolerance levels of 4.5kV and
3kV respectively.
Having established the fault condition thresholds, the reasons for
in-operation rebooting were investigated. For electrostatic discharges
to cause rebooting, a charge generation mechanism must exist. Various
components of the disc sorting mechanism were found to be highly
insulating. Specifically, the disc transfer wheel, the disc loading
chute and the disc reservoir hopper generated fields of greater
than 100kV/m and have charge relaxation periods of greater than
30 minutes when tribo-charged by rubbing with a metal disc. These
measurements confirmed that charge accumulation during ordinary
operation could result in a matter of seconds.
ESD-Protective
Measures
Material resistivity.
Replacing highly insulating system components with conductive ones
will dramatically reduce the event of electrostatic discharge. For
engineering reasons, this may not always be possible and alternative
static-dissipation measures may be considered.
Air ionization. The use of air ionization will provide a conductive
path to earth by way of ionized particles in the atmosphere. Such
devices are available with either radioactive or high-voltage corona
discharge sources. When evaluated with an AC corona ionizer, the
test case showed a significant reduction of measured fields to below
-10kV/m. (AC votons ionizers have a negative offset bias voltage.
Use of a DC corona ionizer can overcome this problem).
Brushing. A conductive earth-bonded brushing arrangement can be
used as the metal discs slide across insulating surfaces, any static
charge generated can be dissipated to earth. Successful implementation
of this solution requires that intermittent contact must be avoided.
Conclusion
Two equipment case
studies have been examined to show ESD fault investigation methods
and possible solutions. Both cases showed how electrostatic discharges
could adversely affect the normal functioning of the systems. To
prevent any danger or loss of function, measures were taken to ensure
that the equipment would have a high degree of ESD immunity.
In the first case study, potential problems caused by high-energy
electrostatic discharges were averted by proper screening, filtering,
cabling, decoupling, and suppression techniques.
The second case example showed that to prevent data loss due to
computer rebooting brought on by electrostatic discharges, conductive
components should be used instead of highly insulating ones. While
this solution may not be appropriate in all cases, other alternatives
included use of air ionization or conductive earth-bonded brushing
equipment.
Michael J. Smith is
an Electrostatics and Computer Control Techniques Specialist at
Chilworth Technology Ltd., Southampton, UK. Chilworth Provides independent
testing and consulting services in electrostatics, and will soon
have offices in New Jersey.
References
1. Chomerics Incorporated,
Technical Bulletin 75, Chomerics 1987.
2. RD Data sheet 6200: "Suppression", RS Components Limited,
November 1985.
3. B. Jones, British Telecommunications PLC, ESD 90-Electrostatic
Damage to Electronics Conference Proceedings, April 1990.
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