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Test & Measurement World: Check ESD simulators before testing

When you are designing products for compliance with EMC (electromagnetic- compatibility) standards, testing circuits for immunity to ESD (electrostatic discharge) is mandatory. Standards such as IEC 61000-4-2 and ANSI C63.16, which specify how to set up and perform these ESD tests (references 1 and 2), require that you use an ESD simulator to produce test pulses. The standards also specify the shape and timing of the current pulse you must inject into your EUT (equipment under test), so before running an immunity test, you must verify that your ESD simulator produces a current pulse with the proper shape and rise time; you do so using a calibrated ESD target and a high-bandwidth oscilloscope.

A typical human-body ESD event creates a high-current discharge into any metallic object as a person’s finger approaches it. The resulting current pulse may amount to several amperes at a very high leading edge with a rise time of less than 1 nsec (Figure 1). You can model the human body as a simple series RC network. As the electric charge builds, the capacitor charges to several thousand volts. A switch models the instant of electrostatic discharge; the charge is transferred rapidly into the EUT. Several manufacturers offer simulators that reproduce current waveforms very close to this humanbody model. The wave shape that these simulators must generate is specified in IEC 61000-4-2, which also requires that you verify the ESD simulator’s tip voltage before testing your EUT, and that you verify several characteristics of the resulting current waveform, such as current peak, current reading at 30 nsec, and current reading at 60 nsec.

You must measure the simulator’s tip voltage with an electrometer or gigohm meter (Reference 3). I’ve found that for pre-compliance tests, you can use a simple high-impedance, high-voltage, 25-kV-rated voltage divider (a resistive divider of 100 MΩ in series with 1 MΩ) and a digital voltmeter.

To check an ESD simulator’s output, you must measure the waveform of the resulting current across a low-impedance, high-frequency resistive shunt connected to ground. This shunt, or ESD target (Figure 2), emulates a discharge into a large metallic object such as an equipment enclosure. The IEC and ANSI standards currently specify a shunt impedance of less than 2.1Ω, but that specification will change in future revisions. To help engineers more accurately verify ESD simulator performance, draft standards now specify a higher-bandwidth, lower-impedance calibrated ESD target. The new target has an impedance of about 1Ω. Today, the IEC and ANSI standards specify a 1-GHz bandwidth target; the draft standards specify a 4-GHz bandwidth. ANSI C63.16 target specifications include a reflection coefficient of less than 0.1—equivalent VSWR of less than 1.22—and an insertion loss of less than 0.3 dB up to 4 GHz. To complete the test setup, you’ll need cables, attenuators, and an oscilloscope. You’ll need attenuators to protect your oscilloscope’s input preamplifiers; your attenuator must be capable of handling up to 50V spikes, and its bandwidth must accurately pass frequencies up to 6 GHz. When choosing an oscilloscope, look carefully at an instrument’s bandwidth, rise time, noise, and—in the case of a digital ’scope— sample rate. Some ESD simulators can produce rise times of 50 psec and thus require an oscilloscope bandwidth of as much as 8.6 GHz.

In the full version of this article at www.tmworld.com/article/CA6582582, you will find further detailed advice on selecting the test equipment, and on carrying out and interpreting the ESD tests.

REFERENCES

IEC 61000-4-2, “Electromagneticcompatibility (EMC) - Part 4-2: Testingand measurement techniques - Electrostaticdischarge immunity test,” IEC,Geneva, Switzerland, 2001, www.iec.ch. ANSI C63.16-1993, “AmericanNational Standard Guide for ElectrostaticDischarge Test Methodologiesand Criteria for Electronic Equipment,”ANSI, New York, NY. www.ansi.org. Senko, Greg, and Ken Wyatt, “ESDSimulator Verification,” IEEE EMCSociety, 2003, www.ewh.ieee.org.