I heard that a capacitor can protect a DC power line from a transient voltages by ESD event. Unfortunately I don't understand how it works in detail. It would be appreciated if someone could answer the following questions.
For the questions I selected an example product whose specs are 0.01uF, 25V.
(1) basic principle
The transient voltage is bypassed by the capacitor. This protection works for either positive or negative transient. A negative transient voltage is bypassed from GND to VCC through the capacitor so that the potential difference between VCC and GND remains small. Am I correct?
(2) maximum voltage that a capacitor withstands
The example product has a voltage rating of 25V. This would mean that the capacitor can withstand up to 25V. Then is it possible for the product to protect power lines from a 8kV voltage peak?
(3) resulting voltage on the power line
TVS diodes have a rating called clamping voltage. For a capacitor can you assume that the clamping voltage is just the DC power voltage?
(4) Would such capacitor protect 3.3V DC power line successfully against any kind of ESD events that include human body model, charged device model?
Best Answer
ESD will charge the capacitor. Here is an LTspice model for an ESD source.
The notable part of this model is that you are charging a 150pF and 8pF source then discharging through a R and L; the total amount of charge related to the ESD event is limited due to this. Which is why a capacitor that can respond with sufficient speed can absorb the ESD event without creating an excessive rise in voltage.
If you want to be careful, I would suggest simulating your ESD level (the link includes a initial condition for each IEC level) against the model for the capacitance you want to include. That should capture the total rise you will see.