Capacitor guide of ESD (7): How can I choose an ESD protection capacitor?
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A. The first step is to confirm how much ESD protection is required. Keep in mind that a 12,000V module level requirement does not mean that the component requirement is also 12,000V. In many cases, the end customer sets a module level ESD requirement and lets the Design Engineer decide what component level requirements are necessary to meet the module level ESD.

ESD test rating alone is not the best way to select a capacitor value. Without looking at Vbd, DUT, and DC Bias affects, the Design Engineer could over-design or even under-design the protection circuit.

The relationship between the resultant voltage (Vx) across the capacitance (Cx)

As discussed earlier, DC bias causes a reduction in capacitance as much as 70% for Class II MLCCs. If we assume this as worst case,

This relationship tells us how much voltage (Vx) our capacitor (Cx) will really see. Therefore we make our capacitor selection by choosing a capacitor with the voltage breakdown level (Vbd) greater than Vx.

Vbd > Vx, where

Figure 1: Example of Vbd test results
For example, assume CO = 150pF, Cx = 1000pF and VESD = 12kV as we have been using in our previous examples. This results in a minimum Vbd of 4000V. The 1000pF capacitor we chose for Cx must have a minimum Vbd level of 4000V.

If we refer back to the example Vbd chart from question 10 above, we see that the average Vbd level is 4520V and the minimum is 4080V for this capacitor and therefore is a suitable choice.

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