Biomedical Engineering Reference
In-Depth Information
Since even a modest ESD event can develop a current of 10 A in 1 ns, the voltage induced
across 1 cm of PCB track will be as high as 100 V! Similarly, high currents can
fl
ow
through capacitances on and across circuit components. Current
fl
flowing through a capac-
itor is given by
V
t
If an ESD event causes a change in potential of 1 kV within 1 ns, the current
d
d
I
C
fl
flowing
through an unprotected input with 10-pF capacitance would be as high as 10 A.
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 in
fi
fl
uence 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
fi
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 test-
ing 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 dis-
charge through air.
As shown in Figure 4.19, the modi
cation 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.,
fi
1
4
in.) to a conductive member of the device under test, a spark jumps, conveying
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