A Daily Verification Method for ESD Immunity Equipment

Varuzhan Kocharyan and Dave Tolman

Measurement of the near electric field of the horizontal coupling plane of the test station can turn up problems with ESD immunity equipment quickly and economically.

The management of electrostatic-discharge (ESD) simulators and standardized ESD test stations to ensure sound test results continues to be a major concern for test houses. Those facilities' quality-management systems should prescribe a daily equipment calibration check prior to performing tests. Early detection of malfunctioning ESD equipment is possible if a day-by-day check is performed.

ESD simulators and ESD test stations receive a lot of abuse. The simulators can malfunction, and the grounding cables and bleeder resistors can deteriorate. Annual calibration of the simulators and weekly checks of the bleeder resistors on the test stations generally are not sufficient. If a problem with a simulator is discovered during an annual calibration, the immunity tests that were done during the previous year must be investigated. This could result in costly retesting. A similar problem associated with the bleeder resistors could cause
inaccurate test results.

Figure 1. Measured near fields on the HCP 0.4 m from the discharge point.

The standard system for verification of ESD simulators is large, expensive, and impractical for performing an every-day check. It is a 1.5 X1.5-m metal sheet with a specially designed target, and requires a Faraday cage big enough to hold a 1-GHz-bandwidth oscilloscope.1 The method for verifying ESD simulators and standardized ESD test stations that is discussed in this article represents an attempt to find a simpler substitute for the standard technique. This new technique of ESD immunity equipment verification is based on the measurement of the near electric field of the horizontal coupling plane (HCP) of the test station. It is not intended to replace the annual calibration, but rather to supply an effortless method suitable for day-to-day checks in test houses.

The HCP of the test station was chosen as the target for discharges in order to simplify the set of devices necessary to conduct this express method of verifying ESD equipment. For the same reason, a method based on measurement of a discharge current was not used.

A 100- to 500-MHz-bandwidth oscilloscope can be used to measure the quasi-electrostatic field of the HCP after the package of discharges has been applied. The oscillograms and original tools presented in this article are believed to support claims that this method can assist in detection of potential problems with ESD equipment.

The HCP Near Field through the ESD Event

Figure 2. Near E-field of HCP between 100 and 700 nanoseconds.

The system developers, in analyzing the near field of the HCP after applied discharges, mainly investigated the first phase of development before the quasi-electrostatic condition was established. Figure 1 provides a typical sample of such measurement. That phase concludes when the near magnetic field fades and the electric field of the HCP has attained its maximum value. This takes a few tens of nanoseconds.

Figure 2 represents the electric field of the 0.7 X1.4-m coupling plane after the +4-kV discharge has been applied. It is evident that the quasi-electrostatic condition had been established at about 100-200 nanoseconds and during the next 0.5 microseconds had changed insignificantly.

The process of bleeding charges from the HCP takes much longer, tens of microseconds after the discharge had been applied. Figure 3 shows the final stage of development of the near electric field of the test station HCP.

Indeed, the oscillogram in Figure 3 indicates a capacitor's discharge process. The flat capacitor has the HCP as one lead and the ground reference plane of the test station as a second. The originality of the equipment verification process is that the charge bleeds not only through the bleeder resistor, but through space also.

Figure 3. Near E-field of HCP at final phase.

This last stage of development of the HCP electric field takes a comparatively long time, that is, a few tens of milliseconds. It is obvious that the HCP discharge rate depends upon the impedance of the bleeder paths.

The process discussed here has two phases: the rapid phase of discharge of the ESD simulator to the HCP, and the relatively slow phase of discharge of the HCP to the reference ground plane. The simple physics of this process allows making the following assumptions:

• The discharge rate from ESD simulator to HCP depends upon the parameters of all elements of the discharge circuit, particularly the discharge network of the ESD simulator; thus, the rise time seen in the oscillograms such as Figure 3 may qualify the parameters of the simulator discharge
  network.

• The quasi-electrostatic field of the HCP carries quantitative information about the discharge that caused the field; thus, the peak value of such a measurement depends upon the discharge voltage of the ESD simulator. (A low-sensitivity well-shielded transducer is able to measure the strong near-field event while rejecting the comparatively weaker facility-generated electromagnetic noises.)

• The discharge rate of HCP to ground plane depends upon the impedance of the bleeder resistor between the HCP and ground plane; thus, the minimum value of measurements similar to those depicted in Figure 3 is able to qualify the bleeder resistor's impedance.

An account of experimental results that prove these assumptions follows.

Figure 4. Experimental setup for measurements of near E-field of HCP.

Setup and Measurement Devices

The measurement setup used in the equipment verification method is shown in Figure 4. The relative positions of the oscilloscope, the discharge point, the point where bleeder resistors are connected to the HCP, and the grounding point of the ESD simulator return cable all affect measurement results. Any change in them may change a result. Therefore, for the method to succeed in its purpose, it is necessary to establish a distinct position for all of the elements involved. The method works well with any reasonable choices of position for these measuring system components, as long as the elements stay within the boundaries of the ESD test stations. Once the position of the oscilloscope and the discharge location have been selected, they should remain unchanged.

The oscilloscope setup is aimed mainly to capture the second phase of the event, the slowly changing quasi-electrostatic field of the HCP during the first 25-45 microseconds. The oscilloscope's horizontal scale is set to 4-5 microseconds per division. With this setup, the oscillograms carry enough information to allow detection of changes in the discharge network of the ESD simulator, changes in the discharge voltage, and changes in the bleeder resistor impedance. As shown below, three values are sufficient to draw diagnostic conclusions: the maximum voltage, the voltage at a few tens of microseconds later (the investigators selected 28 microseconds), and the rise time. The experiments employed the average acquisition mode, using 16 acquisitions, in order to minimize random or uncorrelated noise.

Figure 5. Shielded sensor for near E-field measurements. 1 = plastic case, 2 = inner shield, 3 = shield opening toward measurement area, 4 = wire antenna, and 5 = BNC connector.

Many publications discuss electric-field measurement and commercially available sensors or probes. One requirement is important in this case: the probe should be protected as well as possible against radiation from the ESD simulator. The investigators obtained good results using a self-manufactured sensor attached directly to the oscilloscope with no additional cables between them. Its design is shown in Figure 5.

For purposes of the research, it was not necessary to convert direct readings from the oscilloscope–volts on the input of the scope–into the actual volts-per-meter measurement of the quasi-electrostatic field of the HCP.
 
Experimental Results

Detection of Changes in the ESD Simulator Discharge Network. An ESD simulator discharge network contains a discharge resistance and a capacitance. Various ESD test standards require different values for these. Some models have variable networks that allow for adjustment of the ESD simulator for different standards. Because there is no immediately visible indication of which resistor-capacitor network is installed, with some models it is necessary to disassemble the simulator, check the discharge network, and put the simulator back together again. This is a potential source of mistakes.

It is also logical to assume that network parameters might change dramatically owing to physical strain on the equipment. Experimentation showed that the rise time provides an indication of major changes to the network parameters. Figure 6 compares two measurement results. Each test was taken at the same station, with the same ESD simulator and the same discharge voltage. However, a different discharge network was used in each measurement.

Detection of Changes in Discharge Voltage. The accuracy of the indicated value of the discharge voltage with respect to the actual discharge voltage is a very important calibration measurement for performing the ESD immunity test. In the method discussed, the maximum reading on the oscillogram is adopted as the quantity with which to appraise the discharge voltage levels.

Figure 7 shows two typical oscillograms. Except for the discharge voltage, all test parameters are the same for both. The simple link between the discharge voltage and maximum voltage thus is evident.

Detection of Malfunctioning of the Bleeder Resistor Circuit. The value of the bleeder resistor between the HCP and the ground plane–940 k‡–is established by IEC 61000-4-2 and is one of the parameters that ensures the proper performance of ESD test stations.1 Because of this, the bleeder resistors should be checked day by day along with the ESD simulators.

The principle on the basis of which this method detects any malfunction of the bleeder resistor is this: the rate at which the quasi-electrostatic field of the HCP decreases depends on the value of the bleeder resistor. When the bleeder resistor circuit is open, the quasi-electrostatic field of the HCP decreases significantly more slowly.

The oscillograms in Figure 8 display that difference. The point at the right of the last graticule on the oscillogram may be chosen for this measurement. In the method of the experimenters, the last time point is 28 microseconds. A comparison of the voltage readings at 28 microseconds reveals any possible malfunctioning of the bleeder resistor circuit. In the oscillograms in Figure 8, all parameters are the same except the value of the bleeder resistor.

When the equipment under test (EUT) is internally powered or contains circuitry isolated from protective ground, the applicable standard mandates that a measure shall be taken to prevent appreciable charge retention between individual test discharges.2 For that purpose, test stations have attached a ground-plane wire, the bleeder wire, for temporary grounding of the EUT through two connected-in-series 470-k‡ resistors. The bleeder wire might become a bypass circuit for the bleeder resistor if the open end of the wire accidentally touches the HCP. This diagnostic method will detect any reduction in resistance to ground, which would indicate a faulty test setup.

A pair of Excel spreadsheets were developed to implement this method. A statistical sheet was designated for computing arithmetical means and standard deviations of maximum voltage, voltage an interval (28 microseconds) after maximum, and rise time.The statistical information was to be obtained immediately after an ESD simulator was returned from calibration (within a period of 10 days).

The six numbers from the first sheet–three arithmetical means and three standard deviations–have to be transferred to a second diagnostic sheet, where they represent the baseline measurement. The results of daily diagnostic discharge measurements then are entered into the second sheet. If measurements are out of tolerance, the diagnostic sheet points to the most likely source. The statistical analysis of data obtained using this method is published.3

Conclusion

The method discussed here exposes any changes in the indicated discharge voltage or the HCP bleeder resistor impedance. Near-field measurement with a duration of about 30 microseconds after discharge to the HCP provides enough information to detect the disrepair of the discharge network of the ESD simulator and any fault in the condition of the bleeder resistor circuit. Consequently, that measurement has potential for use in quickly checking the performance capability of ESD equipment.

This verification method was developed to enable daily checking of ESD simulators and test stations before performing ESD immunity tests. The method is less time-consuming than standard methods for the verification of ESD simulators and involves less-expensive apparatus. It discovers deviations from baseline measurements that are taken immediately after the standard calibration. EMC test houses might use this method as a convenient and economically effective tool for managing their ESD test equipment.

References

1. IEC 61000-4-2, "Electromagnetic compatibility (EMC)–Part 4-2: Testing and measurement techniques–Electrostatic discharge immunity test" (Geneva: International Electrotechnical Commission, 2001).

2. IEC 60601-1-2, 2nd ed., "Medical electrical equipment–Part 1-2: General requirement for safety–Collateral standard: Electromagnetic compatibility–Requirement and tests" (Geneva: International Electrotechnical Commission, 2001).

3. V Kocharyan and D Tolman, "An Express Diagnostic Method for ESD Simulators and Standardized ESD Test Stations," in Proceedings of the 2003 IEEE International EMC Symposium (Boston: Institute of Electrical and Electronics Engineers, 2003):708-712.
 
Varuzhan Kocharyan is EMC engineer and Dave Tolman is quality manager for Northwest EMC Inc. (Hillsboro, OR). They can be reached via e-mail at vkocharyan@nwemc.com and dtolman@nwemc.com, respectively.