It is imperative that test
variables be controlled to avoid testing errors when looking for design
problems or during compliance tests. Making measurements to characterize
the test setup is one way to accomplish this. Using an ESD
example, the use of current probes during testing is demonstrated as a
way to monitor and control variables in the ESD test setup and equipment.
: The importance of controlling variables during
tests that involve high frequencies, such as ESD, cannot be overstated.
An example of ESD testing is used in this article to illustrate the
point. Figure 1 above shows a current probe being used for this
purpose. By measuring the injected ESD current, uncontrolled variables
in the test can be exposed and corrected.
Since ESD, either simulated or real, is a wide bandwidth event
with the high end of the frequency spectrum often extending into
hundreds or thousands of megahertz, small parasitic capacitances and
inductances can have significant effects. If these stray impedances
vary during the test, the results of the test may be difficult to reproduce.
In Figure 1, ESD is being applied to a disk drive through a Fischer F-65
current probe. Seemingly small changes to the test setup can result in
a large differences to the ESD current delivered to the drive. For
instance, if the insulation (visible in Figure 1) between the drive and
the metal horizontal
coupling plane (metal tabletop) used in the test differs in thickness
from test to test,
the capacitance of the drive to the metal plane will vary as well. The
variation will be approximately
in inverse proportion to the thickness as the drive and plane form a
parallel plate capacitor. This results in significant differences in
the current applied to the drive for different insulation thickness. I
have seen variations of more than two to one in ESD current because
attention was not paid to the parameters of the insulation on the
horizontal coupling plane. IEC 61000-4-2 calls for a thickness of 0.5
mm, and yet I have seen insulation thickness used that was many times
this figure. I have even seen thick ESD static dissipative mats used
for the purpose. For many reasons, ESD mats should never be used this
way in ESD testing.
If an air discharge is made to the EUT (equipment under test) from a piece of metal that is
plated, the plating material will also significantly change the current
delivered to the EUT. In actual lab tests, I have observed
differences of nearly ten to one in the ESD current due to this effect. The sharpness
of the edge of the metal also affects the voltage breakdown in air and
therefore the discharge current. All of these effects can be easily
seen and corrected by monitoring the ESD current as shown in Figure 1.
Be sure to keep the probe spaced far enough from the target equipment
so that the probe itself is not subjected to ESD testing. In Figure 1,
the probe is getting a little close to the disk drive for comfort.
Once the test is stable and the desired ESD current is well controlled,
it is not necessary to monitor the current on every discharge. It is a good
idea to remeasure a previous discharge periodically to make sure everything is still
working, especially at the
beginning of a test that has been continued after a break.
After test variables are controlled through measurements, the ESD
current waveform delivered into a
piece of equipment by an ESD simulator can still vary significantly
waveform specified in IEC 61000-4-2. Knowing the actual waveform can
help one troubleshoot problems as well as understand test
results. Measuring the actual ESD current waveform and making sure
variations are kept to a minimum has real benefits.
Consider Figure 2 which shows the suggested waveshape of an IEC
61000-4-2 ESD simulator when the simulator is discharged into a measurement test setup specified in the standard. The
specification calls out the initial risetime, the first current peak and
the value of current at 30 and 60 nanoseconds, but the picture in
Figure 2 suggests what the actual current waveform should look like.
Suggested IEC 61000-4-2 Waveform
The actual ESD current that results when a simulator is applied to
real equipment can be quite different. Figure 3 shows a test setup
where ESD is applied either directly or through a multi-turn
ferrite core to the chassis of a piece ot electronic equipment. This
test was described in last month's Technical Tidbit for September 2005,
"Multiple Turn and Single-turn Ferrite Chokes Compared
Measurement of ESD Current into Chassis with Multi-Turn Ferrite
The resulting ESD current waveform for both connections in Figure 3 are
shown in Figures 4 and 5. The simulator as used in this configuration has
a risetime faster than what IEC 61000-4-2 calls for, but on the time scales
of Figures 4 and 5, the difference would be difficult to see.
In Figure 4 there is a double peak and more pronounced dip after the
peaks than in Figure 2. Figure 5 bears little resemblance to the IEC
Figure 4. ESD Current Resulting from Figure 2 Setup
(Vertical scale = 500 mA/division)
ESD Current into Chassis Through Multi-Turn Ferrite
Figures 6 and 7 show a test setup for measuring ESD current through
a small inductor connected to a ground plane (chassis) and the resulting current. This test configuration
is described in the May 2000 Technical Tidbit, Measuring Inductor Performance
. The ESD current through the small 10 microhenry inductor shown in Figure 7 again varies substantially from the IEC waveform.
Figure 6. Setup for Measuring ESD Current into Inductors Connected to a Chassis
V = 0.5 Amp/div H = 10 ns/div
ESD Current Waveform Through Inductor of Figure 6
The point to be made is that the ESD current varies dramatically with the
specific equipment being tested. Knowing what the ESD current waveform
is can help one understand the test results as well as fix any problems the
test uncovers. The May 2000 Technical Tidbit showed that inductors
don't always behave as we would expect under ESD conditions.
Controlling test variables in tests involving high frequency signals is
imperative. Measurements should be made to verify test consistency. The
example of ESD discussed showed clearly for several cases the value in using measurements
to verify test procedures and in understanding test results.
tutorial on this subject, covering background as well as more technical details, is available at:
If you like the information in this article and others on this website, much more information is available in my courses.
to see a listing of upcoming courses on design, measurement, and
troubleshooting of chips, circuits, and systems.