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).
 |
| 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.
