EMC Question of the Week: March 25, 2024
An ESD generator has an energy-storage capacitor, Cs, a distributed capacitance, Cd, a discharge resistor, Rd, and a ground strap. Which component is primarily responsible for producing the initial current spike in the calibration waveform?
- Cs
- Cd
- Rd
- the ground strap
Answer
The best answer is “b.” The distributed capacitance is essentially the absolute capacitance of the generator body. It is relatively small (~20 pF), but charge is easily pulled from it due to its low connection inductance to the energy storage capacitor. When the generator is triggered, charge flows out of the energy-storage capacitor, through the discharge resistor, through the calibration test fixture to its mounting plate and back through the distributed capacitance to the energy-storage capacitance. Once the distributed capacitance has provided all the charge it can, the current drops until the current can be pulled in through the relatively high inductance of the ground strap.
The width of the initial spike is determined by the size of the distributed capacitance. Without the distributed capacitance (and its relatively low path inductance), there would be no initial spike in the current. The current would simply rise as fast as the strap inductance would allow and then fall as the charge stored in the energy storage capacitor was depleted.
Note: The generator body can be connected to the high-voltage side of the energy-storage capacitor instead of the ground side. In this case, the distributed capacitance provides its own charge rather than facilitating the flow of charge from the energy-storage capacitor. Most ESD test standards do not specify the construction of the ESD generator or its component values. Instead, they specify the current waveform that should be obtained when discharging to a specific calibration fixture.
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