A more thorough “in-between calibration cycle” check of the ESD simulator’s output waveform is something every EMC engineer and technician should consider.
Why is this check important?
In a busy EMC test facility, the ESD simulator (a.k.a. “ESD gun” or guns if there is more than just one at the lab) can get damaged by inadvertent dropping that goes unreported or the switching mechanism can stop working. The gun may still produce a “zap” or make a noise as if it were still outputting the correct waveform, when in fact it is not. Because of these issues, a calibration check should be utilized frequently in between regular calibration cycles in order to be sure the waveform produced by the simulator is still within the requirements of the standard.
Note: IEC 61000-4-2 is one standard that describes the output waveform characteristics of the ESD gun, details of which are described later in this article.
It’s not very fun finding out from the calibration lab that your ESD gun is out of spec, and you need to repeat testing on all the products tested with it since the last date that a known good calibration was performed. If the regular calibration cycle chosen is yearly, this would mean having to retest a year’s worth of products! Not something I’d want to do. If you’re a commercial EMC test lab, you will have to do this work free of charge while at the same time appeasing an upset customer who must return their product to your lab for retesting. If you have your own in-house EMC test facility, you’ll have to drop what you’re currently doing to address this issue, which could mean temporarily halting the development of new products, which is undoubtedly more important than retesting older products. Especially ones that should have already been compliance tested correctly.
The regular use of a current sensing transducer and high-bandwidth oscilloscope and an ESD gun in-house calibration check process will alleviate much of this anguish.
How is the Output Waveform Verified?
The output waveform of an ESD gun is verified with a current sensing transducer and a high bandwidth oscilloscope. It is highly recommended that the current sensing transducer, built as described in the latest version of the standard, is obtained and used for the check. This device and a detailed list of items needed to perform the ESD gun output waveform verification, along with a basic test procedure is described in Reference 1.
Contact Discharge Current Waveform Parameters
IEC 61000-4-2 describes contact discharge current waveform parameters. This standard indicates parameters for levels 1 through 4 corresponding to voltages (in kV) of 2, 4, 6, and 8. This standard specifies for each voltage level the required current of the discharge of the first peak (+/- 15%), the rise-time in nanoseconds (ns) of 0.8 (+/- 25%), the current at the 30 ns point, and the current at 60 ns point of the waveform.
All the IEC 61000-4-2 ESD output waveform points should be verified on a regular basis. You will have to perform your own risk analysis to determine how often this check is performed given your unique situation. If you performed the check before and after each test and included the before/after waveforms in a test report, you could easily prove the product was ESD tested correctly should proper testing ever be questioned.
The contact discharge current waveform parameters specified by IEC 61000-4-2 have only a couple of points to check: initial rise-time, the current at 30 ns, and the current at 60 ns. This leaves a lot of variability of the waveform that can occur, especially between the 30 ns and 60 ns points. This means that one ESD gun built by one manufacturer can have a different output waveform over one built by a different manufacturer and still meet spec. Different models from the same manufacturer can have different output waveforms, but all still meet the spec!
The different output waveforms produced by some guns can be “dirtier” in the areas not specified by the standard, than other “cleaner” guns. The “dirty” ESD guns output more aggressive, wider bandwidth signals that some equipment might be susceptible to when tested for compliance. This equipment will fail the compliance test and need re-design to pass. If this same equipment is tested with a “clean” gun, it might pass.
Some equipment suppliers elect to test their equipment with ESD guns outputting the more aggressive waveform, thinking that their product will be more robust to real-life ESD events that may occur in actual usage. Testing products with a “clean” gun is for “sissies.” Depending on who you talk too, testing with a “dirty” gun may be a misguided effort.
The other side of the camp wants the ESD gun to output the cleanest waveform possible, and each gun they use to be the same. This side of the camp is interested in greater test repeatability and less uncertainty.
Adding your own in-house capability to check your ESD gun’s output waveform will enable you to determine if you either have a “clean” or “dirty” ESD gun so that you can then take appropriate action.
References and Further Reading
- ESD Guns/The EMC Shop. Waveform Verification for IEC 61000-4-2 Waveshape.
- In Compliance Magazine. (2018, February 5). What Every Electronics Engineer Needs to Know About: ESD Simulators.
- IEC 61000-4-2:2009. Electromagnetic compatibility (EMC) – Part 4-2: Testing and measurement techniques – Electrostatic discharge immunity test