Using an ESD-Pulse Validation System for Equipment Evaluation

A low-cost apparatus makes possible rapid, reliable validation of ESD guns. The system validates the peak current of ESD pulses, and the secondary current at 30 nanoseconds, as described in the IEC 61000-4-2 standard, and indicates via two LEDs whether these currents are within tolerance.

The apparatus is not the calibration equipment required by IEC 61000-4-2, involving an expensive test setup with a single-shot fast oscilloscope and a shielded room, or even an anechoic room. In addition to performing annual calibration of the ESD gun, EMC test houses need to quickly check the ESD waveform each test day. This requirement is imposed by EN 45001 accreditation rules which state that test equipment should be verified—not calibrated—each test day to ensure that it is working properly. The tool described here can fill that need for daily validation.

A simple but effective system to validate the shape of the waveform (see Figure 1) before each ESD test can stretch the interval between costly ESD-gun calibrations. A typical ESD calibration setup as described in IEC 61000-4-2 costs about 10 times as much as the validation system proposed in this article, owing particularly to the cost of the oscilloscope.

The economical ESD validation system described below evaluates the ESD waveform at two critical points:

Two go/no-go LEDs indicate if the waveform is within a set tolerance which is slightly larger than the tolerances given in IEC 61000-4-2.

Figure 1. An ESD pulse according to IEC 61000-4-2.

Functional Description of the Apparatus

The electrostatic-discharge waveform contains high-frequency components ranging up to several gigahertz. In order to preserve the steep rising and fall times of the pulse, the ESD-pulse validation apparatus is built from very fast electronic components. Figure 2 is a block diagram of the apparatus.

Figure 2. Block diagram of the ESD-validation apparatus.

ESD-Pulse Analysis. Before measurement commences, the apparatus is reset and armed manually. The ESD gun discharges through the Pelligrini target (see discussion below). The signal is divided between two evaluation circuits by a resistive splitter.

In the first circuit, the first peak current is evaluated. The ESD pulse spends 28 nanoseconds in a delay line, then is applied to a peak-hold circuit followed by an analog comparator and an analog-to-digital (A/D) signal convertor. As soon as the voltage level over the peak-hold circuit reaches a preset minimum value, the analog comparator triggers the A/D converter and measurement starts. The trigger pulse is delayed by 2 nanoseconds, the time required to set up a data latch. Data at the peak-hold circuit are converted into a digital signal and are held by the data latch at the trigger moment.

In the second circuit, the ESD-pulse signal is applied without delay to a second A/D converter, which continuously evaluates the signal. The trigger pulse that was generated in the analog comparator in the first circuit is used to hold the ESD-pulse signal on a data latch. Because the trigger pulse is delayed by a total of 30 nanoseconds, the current value of the ESD pulse is evaluated after that interval and latched as a digital signal on the second data latch.

A digital signal that is evaluated on the peak current appears therefore on the first data latch, while a digital signal that is the evaluation of the secondary current of the ESD pulse at 30 nanoseconds appears on the second data latch. The logic circuit converts the data-latch signals to TTL-level signals and supplies them via NAND gates to two indicator LEDs. The first LED will glow green when the peak-current value registers 30 A ± 15%; otherwise, it remains off. The second LED will light up when the secondary-current value corresponds to 16 A ± 35%; otherwise, it remains off. When both LEDs display green lights, the peak current of the ESD pulse and the current at 30 nanoseconds are both expected to be within tolerance, as mentioned in the IEC 61000-4-2 standard.

The ESD Target. In order for a reliable measurement result to be achieved, the ESD target needs to be linear up to a frequency level of at least 1 GHz. The system's ESD target is a Pelligrini target designed using recommendations from IEC 61000-4-2 and from recent research work. It incorporates low-inductance resistors that are capable of absorbing the 2-kW peak power of the ESD-pulse discharge without damage. The input resistance of the gun is 2 W, and the output resistance is 50 W.

Emerging ESD Standards for Commercial Equipment 1. ANSI C63 ESD Working Group This group has recently published its second draft, which addresses deficiencies existing in ESD standards by introducing: Limited control of peak current as a function of precharge voltage. A modified bandwidth (up to 18 GHz) of the calibration target (former Pelligrini in IEC 61000-4-2). Definition of discharge current derivative (di/dt), especially for the first peak. The limit of generated E/H fields. A modified and clearer test setup, especially concerning the position of the return-current strap. As with calibration of a compliant gun, a high-bandwidth target is needed, but also a tapered adapter (2/50 ).   2. IEC 61000-4-2 This standard dates from 1995. It is required by an IEC central office ruling to be revised within five years of that date, which is why Working Group 9 of IEC Technical Committee 77B was recently formed to revise the standard. The committee's chairman stated that the revised standard will probably be published in 2002. One of alterations to the standard is in the definition of test setups for nonearthed (Class II) equipment. For nonearthed equipment, the coupling phenomena will become very important. Test results differing by a factor of up to 200% are now possible owing to poor or missing test setup definitions in the existing standard.

Measurement of the transfer function of the Pelligrini target was conducted by means of the face-to-face method. (See Figure 3 for a presentation of results.) This method involves connecting two Pelligrini targets together by their 2-W target planes. The Pelligrini outputs are 50 W, which facilitates connection to a network analyzer or a tracking generator/ receiver.

Figure 3. Performance of the Pelligrini target (tracking generator output level = 70 dBµV).

If one builds the Pelligrini target as described in IEC 61000-4-2, its bandwidth is not more than approximately 700–800 MHz. With the version built for the system under discussion, a bandwidth of 3 GHz was obtained. In fact, a bandwidth of more than 10 GHz was calculated in simulations. This is not as high as the 18 GHz the ANSI C63 ESD working group proposes (see sidebar), but it is certainly a more cost-effective approach for now.

Test Setup

The lumped model presented in Figure 4 represents the complete ESD test bed. The model has to be used with care; it cannot accommodate for the traveling waves that are present in the natural ambient.

The first part of the lumped model represents the human body itself. Typical values for its components are 150 pF for capacitance (C_H), 0.4–2 µH for inductance (L_H), and 500 W for resistance (R_H). The second part represents the human arm and finger, with the following typical values: C_A = 3–25 pF, L_A = 0.05–0.2 µH, and R_A = 20–200 W. The capacitors C_ADUT, C_FDUT, and C_DUT form the interface with the device under test (DUT) and have typical values between 3 and 25 pF.

In the ESD-gun calibration procedure according to IEC 61000-4-2, a bulky 1.5 x 1.5-m metal reference plane in which the Pelligrini target is mounted is used to realize the capacitors C_ADUT and C_FDUT. The present apparatus avoids using such a plate, forming the capacitors by means of an L-shaped reference plane that is described in the IEC 60801-2 standard and which is also used in automotive testing.

Figure 6 diagrams the method used to adjust the validation system to the values required by IEC 61000-4-2, which are

Figure 7 shows the results of the adjustment for both the first peak and the second peak.

Figure 4. Diagram of the human body model.

Figure 5. The ESD-pulse validation system test setup.

Figure 6. Diagram of the adjustment of the ESD validation system in accordance with IEC 61000-4-2.

Figure 7. Results of adjustment of the ESD validation system

Conclusion

The low-cost apparatus described here is suitable for test setups according to the IEC 60801-2 standard. The system is portable, avoiding use of the 1.5 x 1.5-m plate called for by the ESD-gun calibration procedure in IEC 61000-4-2 to form the capacitor between the human body and the ESD target. It includes a high-bandwidth ESD target designed on the basis of recommendations in IEC 61000-4-2.

In addition to laboratory applications, the apparatus is now used as a round-robin evaluation tool in EMC-accredited test houses. Interesting results are being reported. Results of these comparative evaluations are expected to be available in the latter half of 2000.

Ivan J. Hendrikx is technical director of Hevrox EMC/Safety Services NV/SA (Beringen, Belgium). He can be reached at info@hevrox.be.