ESD Measurement Technology for Portable Products

The electrostatic discharge (ESD) robustness performance test of electronic systems usually adopts IEC 61000-4-2 as the standard. This standard defines the impulse current waveform at each voltage level, how to calibrate the ESD pulse source, the test environment for measurement, the criteria for test pass and fail, and also provides guidance on how to conduct the test.

But when performing ESD testing on electronic systems, one does not know how much stress the unit under test can actually withstand. This is especially true for portable products that are not grounded when subjected to a shock. Granted, it is possible to measure the actual surge current in ESD testing of portable battery-powered products, and it is even possible to show how to gain additional information from the measurement through simple calculations.

For small products, engineers often perform system-level ESD testing in a dedicated test environment (Figure 1). For IEC 61000-4-2 testing, these test environments include a metal ground plane on the floor, a wooden table, and a metal horizontal coupling plane placed on the table (with a 0.95-meter connection to the ground plane and a 0.5-mm insulation layer above the horizontal coupling plane).

Figure 1

Such a test environment will inevitably achieve repeatable results. First, place the equipment under test (EUT) on an insulating surface, and then apply shocks to the EUT from different directions. For example, apply contact discharge to conductive surfaces, which include all metal housings and the grounded metal shells of connectors. You can also perform air discharge to the surrounding insulating surfaces and focus on possible ESD paths, such as gaps in the equipment housing and all ventilation holes and keyboards.

Indirect discharge testing is also part of the job, specifically indirect discharges on horizontal and vertical coupled boards to simulate the electromagnetic interference (EMI) effects caused by ESD events in adjacent objects. Making things more difficult and complicated for the designer engineer is that the actual magnitude of the impulse applied to the EUT is not always apparent in these measurements.

Here we use a portable battery-powered personal digital assistant (PDA) as an example. First, apply a contact discharge mode shock to the metal grounded shell of the USB port, and then measure the current with a transformer-type current probe with a 1GHz upper bandwidth (preferably Fischer Custom Communications' F-65A) and connect to a standard 1GHz bandwidth oscilloscope. Note that the probe inner diameter needs to be large enough to fit the approximately 12mm diameter head of an IEC 61000-4-2 compliant ESD gun.

Figure 2

To simplify the description of this particular example, the measurement reference was a 0.6 square meter ground plane directly on the tabletop, rather than using a full IEC test setup (Figure 2). The ground lead of the ESD qiang was connected to one corner of the ground plane. All measurements were made at 8 kV. The first measurement was made directly to the center of the ground plane. These measurements were made using the extended time scale.

Figure 3

During the second measurement, the test engineer placed the PDA face down on the ground plane to facilitate contact with the metal shield of the micro USB connector. The current entering the PDA is much less than the current injected directly into the ground plane (Figure 3). To further reduce the current, a 0.9 cm insulation layer was inserted between the PDA and the ground plane.

Figure 4

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However, the reduction in current we see is not consistent. When the PDA is placed directly on the ground plane, the initial current spike is reduced by 10%. When the PDA is placed on a 0.9cm insulating layer, the initial current spike is reduced by 33% (Figure 4). At the 20ns point, the PDA placed directly on the ground plane experiences a 69% smaller shock, while the PDA on the insulating layer experiences a 93% smaller shock. The reduction in current is the result of charging the PDA during the shock state. After each shock test process, we must ground the PDA to return it to an uncharged state before taking the next measurement.

Figure 5

The schematic diagram in Figure 5 provides a good quantitative understanding of the above measurements. The 150pF capacitor and 330Ω resistor are standard circuit components for an IEC 61000-4-2 compliant ESD gun. The parasitic capacitance between the gun and the ground plane provides the initial current spike included in the IEC 61000-4-2 current waveform. This parasitic capacitance is only a few pF and can be ignored in the quantitative discussion.

The engineer placed the current probe around the tip of the discharge gun and began to discharge directly to the ground plane, as shown by the short line in the figure. Integrating the current discharged directly to the ground plane in Figure 3 over 300ns gives a charge of 1.21 μC. This capacitance is almost exactly the capacitance predicted by charging a 150pF capacitor to 8kV.

The discharge to the PDA is represented by a parallel plate capacitor, one of which is part of the PDA and the other is the ground plane. At the beginning of the pulse, the PDA capacitor provides a lower impedance, and the current injected into the PDA is similar to the discharge current to ground. As the pulse continues to be applied, charge accumulates in the PDA-to-ground plate capacitance, so the potential on the PDA rises until the potential between the PDA and the ground plane is equal to the voltage on the 150pF capacitor. At this point, no current flows, even though the capacitance of the ESD qiang has not been fully discharged.

By integrating the current charged to the PDA, we can get the amount of charge transferred from the ESD qiang to the PDA. The amount of charge transferred from the ESD qiang to the PDA can be used to calculate the voltage remaining on the 150pF capacitor of the ESD qiang, and then the voltage on the PDA. After understanding this voltage and the charge measured on the PDA, the capacitance between the PDA and the ground plane can be calculated (see Table 1).

The measurements show that the PDA placed directly on the ground plane charges to 6,253V, while the PDA placed on the 0.9cm insulating layer charges to 7,381V. These data correspond to 41.9pF and 12.6pF capacitance in the two cases, respectively. The capacitance of the PDA placed directly on the ground plane was measured using a 1kHz inductor, capacitor, and resistor meter, and the result was 34pF. This result is quite reasonable given the difference in measurement techniques.

Measurements have shown that the current from an ESD gun is measurable during an ESD strike on an electronic system. In this case, we found that the energy of the strike on a portable system like a PDA or cell phone is much smaller than the total energy from the ESD gun.

A simple circuit model and calculations based on the measured current can yield a lot of useful information, such as the charge transferred to the system under test, the voltage raised by the system, and an approximation of the system's capacitance to ground. The peak current in the initial current spike does not drop as much as the rest of the current waveform.

This result is consistent with the model in which the PDA's ground capacitance provides a low impedance path to ground for the initial current spike. This measurement technique can be used in the case of air gap discharges, as long as the accidental discharge to the current probe is not interpreted as a discharge to the device under test.