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Douglas C. Smith

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Technical Tidbit - March 2011
Using High Frequency Measurement of ESD Current to Find Problems with an ESD Simulator

ESD waveform

Figure 1.
Current Waveform from ESD Simulator
(Vertical = 1/2 Amp/div, Horizontal = 50 ns/div)

Abstract: ESD Simulators, as with all equipment, can exhibit degraded performance. A short video is presented of sequential measurements of the contact discharge current of an ESD simulator that shows unacceptable variation from one discharge to the next. Possible causes are discussed for the example presented and recommendations are made.

Discussion: Figure 1 shows one frame of a short video taken of sequential contact discharges from an ESD simulator. The measurement was made in a fashion similar to that shown in Figure 2, although the ESD simulator used to generate the data for this article was not the one shown in Figure 2, but a different one measured at a later time.

In Figure 2, the ESD simulator is discharged in the contact discharge mode into the Horizontal Coupling Plane, HCP, of an ESD test table through a Fischer Custom Communications F-65 current probe. To get maximum current, the simulator ground lead was connected to the HCP (as done in the ISO10605 ESD standard) rather than the ground reference plane on the floor to which the simulator ground lead is normally attached for IEC testing. The test was performed near the edge of the table for convenience, so the waveform will not be exactly a calibration waveform, but similar. The object was to look at variations from discharge to discharge.

discharge through a current probe

Figure 2.
Current Waveform Measurement of ESD Simulator

Figure 3 is identical to Figure 1 except if you click on Figure 3 a Flash video should play in your browser. You can see significant variation from discharge to discharge. The scope was sampling at 8 GSa/sec, so the variation was not due to sampling errors. That sampling rate gives 400 samples per horizontal division, more than adequate for this test.

ESD waveform

Figure 3.
Current Waveform from ESD Simulator
(Vertical = 1/2 Amp/div, Horizontal = 50 ns/div)
Click the picture or here to see the video.

I suspect a defective relay in the simulator may be the root cause of the variable discharge waveforms. Also possible are simulator design problems such as using an inadequate discharge relay. An unintended discharge within the simulator itself cannot be ruled out at this point.  The waveforms in the video point out that ESD simulators should be checked frequently, especially if the simulator is rented or you don't know the service history of the simulator. Being calibrated once per year is not nearly enough for ESD simulators. I personally check the simulator waveform at the start of every test!

Summary: Measuring ESD current from a simulator before ESD testing starts is critical in order to reveal problems with the ESD simulator itself. This is especially important if you are using a rented ESD simulator.

I would like to thank RMV Technology Group at NASA Ames Research Center for use of their facilities to generate the data for this Technical Tidbit.

Additional articles on this website related to this topic are:
  1. December 2010 Technical Tidbit, Comparing "IEC 61000-4-2 Compliant" ESD Simulators
  2. November 2010, Comparison of Current Waveforms from 150 Ohm and 330 Ohm Networks in an IEC 61000-4-2 Simulator
Equipment used in this Technical Tidbit:
  1. Fischer Custom Communications F-65 Current Probe
  2. Agilent Infinium 54845a scope
I would like to thank RMV Technology Group at NASA Ames Research Park for the use of their facilities to perform the tests for this article.

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EMC Lab Techniques for Designers
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Copyright 2011 Douglas C. Smith