Testing for Immunity against ESD Two ESD testing techniques are used to check medical
devices. The first is air discharge; the second is contact discharge. Testing by air discharge
consists of charging the ESD simulator to the required test voltage and slowly moving the
simulator’s discharge electrode toward the device under test until discharge occurs. This is
very similar to what happens when a charged human approaches a device. However, test
results obtained through this technique are notoriously unrepeatable, since the tester’s rate
of approach, exact angle of approach, conditions of the air around the device, and other
variables influence the magnitude and path the discharge will take through the device under
test.
The contact test technique was developed in an attempt to improve repeatability. In this
test, the discharge electrode of the ESD simulator is held in contact with a metallic surface
on the device under test when the discharge switch closes. The actual discharge occurs
within the ESD simulator in a controlled environment, and the current can be injected at
the same contact point each time. The test requires an unpainted conductive contact area
on the device under test. As such, this test applies only for devices that have a conductive
surface from which paint can be removed and is not applicable when no metallic surfaces
are directly accessible.
Testing to EN-61000-4-2 involves delivering air discharges of up to ±8 kV (using an
8-mm round tip to simulate a human finger) to everything nonmetallic that is normally
accessible to the operator. Contact discharges of up to ±3 kV (using a sharp tip that is
touched against the product before the discharge) are applied to operator-accessible metal
parts. Test voltages are increased gradually from low values, often using the settings 25%,
50%, 75%, and then 100% of the test voltage. This is because ESD failures are sometimes
seen to occur at lower voltages but not at the maximum test level. The highest test level on
an ESD test is not necessarily the one most likely to cause a failure.
It must be noted that the contact test is more severe than the air-discharge test. This is
because the former yields faster rise times than the latter. In turn, faster rise times yield
higher bandwidth for the EMI generated by the ESD event. An 8-kV air discharge is in the
same category as a 6-kV contact discharge, and a 15-kV air discharge is as severe as an
8-kV contact discharge. Note the nonlinear relationship. European regulatory agencies are
considering increasing the 3-kV contact test level to 6 kV, so keep yourself up to date with
the standards.
Despite the simplicity of the human discharge model, ESD simulators are not all that
simple, and commercial units are certainly expensive. However, for development-time testing
meant to give you a good “gut feeling,” there are some simple alternatives to buying a
fully compliant ESD test system. Tiwari [1996] proposed modifying a piezoelectric type
of kitchen gas lighter as a fast-static-charge generator which can produce an ESD-like discharge
through air.
As shown in Figure 4.19, the modification involves removing the gas reservoir and
replacing the gas feed line by a pin which extends beyond the gas lighter’s tip. When the
handle is squeezed and the tip of this makeshift ESD gun is placed in close proximity
(e.g., 1/4 in.) to a conductive member of the device under test, a spark jumps, conveying
approximately 0.2 μC within a total ESD pulse of 100 ns to 1 μs. To assess the current and
waveform delivered by the discharge, use a 50-Ω resistor in series with the ESD gun’s
ground terminal as a current shunt.
A simulator which produces waveforms that are closer to a professional unit compliant
with IEC-801-2 [1991] can be built for under $100 using surplus high-voltage components.
In the circuit of Figure 4.20, a TDK model PCU-554 dc-to-ac inverter is used to drive a
Cockroft–Walton quintupler. The dc-to-ac inverter is originally sold as a cold-cathode fluorescent
lamp driver for LCD screen backlighting and may be substituted by any similar part
capable of delivering at least 1.2 kVRMS at 10 mA. The module produces a high-voltage output
that is proportional to its dc input. Dc power for the module is supplied by a variable power
supply built around IC1, a LM317 adjustable voltage regulator. The PCU-544 operates well
for input voltages in the range 1.5 to 5V.
The output polarity of the Cockroft–Walton multiplier depends on the way in which its
diodes are oriented. Since ESD standards call for testing with discharges of both polarities,
the multiplier was designed to yield either positive or negative output. If the high-voltage
ac output of the dc-to-ac inverter is connected to point A of the voltage multiplier and point
B is connected to ground, the output at point D will be positive. If, however, point C
receives the high-voltage ac and point D is connected to ground, point B will be negative.
The multiplier can be built on a piece of perfboard, with square-pin connectors at points
A, B, C, and D. Ideally, the multiplier assembly should be potted in RTV silicone rubber.
This board can then be disconnected from the main circuit and turned around to change
polarity.
Switching C5, the ESD model capacitor, between the output of the voltage quintupler and
the output is accomplished by K1, a vacuum relay. Vacuum relays are much better at generating
fast-rise-time waveforms than most other switches (e.g., firing thyratrons) and yield
more reproducible waveforms than those of spark gaps. Vacuum relays can be expensive (a
few hundred dollars), and it is better to search the inventory of electronic surplus stores such
as Surplus Sales of Nebraska and Fair Radio Sales for a suitable SPDT relay with at least a
10-kV contact rating and a 12-V dc coil. If you cannot find a suitable SPDT relay, you can
use a 50-MΩ resistor (with at least a 15-kV rating) to charge the 150-pF capacitor constantly,
and use a SPST vacuum relay to deliver the ESD to the ESD resistor network.
Use carbon-composition (noninductive), high-voltage resistors for R4–R7 and build the
high-voltage discharge path with the shortest possible lead lengths. This will ensure low
path inductance and fast ESD pulse rise times. When pushbutton switch SW2 energizes
K1, high voltage will be present at the ESD probe. A piezoelectric buzzer is used to warn
the user that the probe is potentially charged. To comply with the standard, the probe must
have specific dimensions. However, good results are obtained using a 3/8-in. smooth rounded-
head bolt as the probe tip for air discharge. For contact-discharge tests, use the pointed
edge of a 1/4-in. steel nail.
During operation, the discharge return ground cable of the generator must be connected
to earth ground. The ground cable should be at least 2 m long and have insulation rated at
12 kV or more. Dc power for the ESD gun can be obtained from a 12-V battery pack or a
12-V dc adapter with a current rating of at least 400 mA. Finally, make a calibration dial
to be placed around the shaft of potentiometer R2 by measuring the voltage across C5
using a high-impedance high-voltage probe and a digital voltmeter.
The standards call for a ground reference plane to be placed in the floor of the laboratory.
The plane should be an aluminum or copper sheet no thinner than 0.25mm, covering
an area no smaller than 1m2, and projecting at least 0.5 m beyond all sides of the device
under test. This is the ground reference to which the ESD simulator should connect. The
plane must also be connected to the protective earth ground. The device under test should
be placed on a nonconductive test table 0.8 m high. All nonconductive construction (e.g.,
all wood) is necessary because metal objects in the table construction would distort the RF
fields radiated by the ESD event field. The table should be placed no closer than 1 m to the
walls of the laboratory or any other metallic object. A 1.6 m × 0.8 m metallic sheet horizontal
coupling plane covered with a 0.5-mm insulating support is placed between the
tabletop and the device under test. This coupling plane must be connected to the reference
ground plane via two 470-kΩ resistors in series.
Indirect Injection of ESD Fields Since ESD events generate large amounts of RFI, it is
not always necessary for the ESD event to happen between a charged body and the medical
device itself. A discharge between two bodies in the vicinity of the medical device may
suffice to cause a failure. For this reason, IEC-61000-4-2 specifies that testing shall also be
done by generating EMI fields through ESD between the ESD simulator and the isolated
horizontal coupling plane, as well as between the ESD simulator and an isolated vertical
coupling plane. The vertical coupling plane is effectively an antenna of dimensions
0.5 m × 0.5 m that is placed on the horizontal coupling plane but is isolated from it. An
ESD generator is then placed in the center of the vertical edge, and at least 10 impulses of
either polarity are applied. The vertical coupling plane must also be connected to the reference
ground plane via two 470-kΩ resistors in series.
Testing for Immunity against ESD Two ESD testing techniques are used to check medical
devices. The first is air discharge; the second is contact discharge. Testing by air discharge
consists of charging the ESD simulator to the required test voltage and slowly moving the
simulator’s discharge electrode toward the device under test until discharge occurs. This is
very similar to what happens when a charged human approaches a device. However, test
results obtained through this technique are notoriously unrepeatable, since the tester’s rate
of approach, exact angle of approach, conditions of the air around the device, and other
variables influence the magnitude and path the discharge will take through the device under
test.
The contact test technique was developed in an attempt to improve repeatability. In this
test, the discharge electrode of the ESD simulator is held in contact with a metallic surface
on the device under test when the discharge switch closes. The actual discharge occurs
within the ESD simulator in a controlled environment, and the current can be injected at
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