Electrostatic discharge (ESD) robustness performance testing of electronic systems typically uses IEC 61000-4-2 as a standard. This standard defines the inrush current waveform at each voltage level, how to calibrate the ESD pulse source, the test environment used for the measurement, the criteria for test pass and failure, and a guide to how to test.
However, when performing ESD testing on electronic systems, people do not know how much stress shock the unit under test can actually withstand. This is especially true for portable products that are not grounded in the event of an impact. It is true that in the ESD test of portable battery-powered products, it is possible to measure the actual inrush current, and it is even possible to show how to obtain additional information from the measurement by simply calculating it.
For small products, engineers often perform system-level ESD testing in a dedicated test environment (Figure 1). For the IEC 61000-4-2 test, these test environments include: metal grounding plates on the floor, wooden tables, metal horizontal coupling plates placed on the table (to the grounding plate, there is a 0.95 meter long connection, at the level A 0.5mm insulation layer above the coupling plate).
figure 1
Such a test environment is bound to achieve repeatable results. First, place the device under test (EUT) on an insulating surface and then apply an impact to the EUT from different directions. For example, a contact discharge is applied to the conductive surfaces, including all metal housings and grounded metal shells of the connectors. It is also possible to air discharge the surrounding insulating surfaces and focus on possible ESD paths, such as gaps in the device housing and all vents and keyboards.
Indirect discharge testing is also part of the work, especially indirect discharges on horizontal and vertical coupling plates to simulate electromagnetic interference (EMI) effects caused by ESD events in adjacent objects. For designers engineers, making things more difficult and complicated is that the actual impact size applied to the EUT is not always obvious in these measurements.
Here is a portable battery powered personal digital assistant (PDA) as an example. First, apply a contact discharge mode shock to the metal grounded case of the USB port, then measure the current with a transformer type current probe (preferably Fischer Custom CommunicaTIons F-65A) with an upper bandwidth of 1 GHz and connect to a standard 1 GHz bandwidth. Oscilloscope. Please note that the probe inner diameter needs to be large enough to fit the IEC 61000-4-2 compatible ESD qiang's approximately 12 mm diameter head.
figure 2
To simplify the description of this particular example, the measurement reference is a 0.6 m2 ground plane directly on the table, eliminating the need for a complete IEC test set (Figure 2). The ESD qiang ground wire is connected to one corner of the ground plate. All measurements were made at 8kV. The first measurement is taken directly to the center of the ground plane. These measurements are based on an extended time scale.
image 3
During the second measurement, the test engineer placed the PDA face down on the ground plane to facilitate access to the metal shield of the micro USB connector. The current entering the PDA is much smaller than the current directly injected into the ground plane (Figure 3). To further reduce the current, a 0.9 cm insulating layer was inserted between the PDA and the ground plane.
Figure 4
However, the currents we see are not consistent before and after the current is reduced. When the PDA is placed directly on the ground plane, the initial current spike is reduced by 10%. When the PDA was placed on a 0.9 cm insulation layer, the initial current spike was reduced by 33% (Figure 4). At 20ns, the PDA placed directly on the ground plane is 69% less impacted, while the PDA on the insulation layer is 93% less impactful. The current reduction is the result of charging the PDA under impact. At the end of each impact test, we must ground the PDA to return it to an uncharged state for the next measurement.
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