How to choose capacitor voltage rating for ESD protection?

Question

I understand the basic calculations on capacitors for ESD protection. In a nutshell the higher the capacitance the lower the final voltage will be because the charge of the ESD event will be "distributed" among the capacitors...

However, I'm still confused about the voltage rating...

Is it enough to choose a capacitor that can withstand the final voltage? I suspect not, because I have seen capacitors specifically designed for ESD protection... Example ESD Capacitor

What ESD-Levels can "normal" (not specifically designed for ESD) MLCC caps withstand? Where can I find this information for a given package size, capacitance and voltage rating? I've looked at some MLCC data sheets but couldn't find a satisfactory answer...

Any clarification will be appreciated.

Answer 1

Is it enough to choose a capacitor that can withstand the final voltage?

It absolutely is but, I think where you might be getting confused is the definition of the "final voltage". It's not the 8 kV initial voltage that the ESD generator can produce into an open circuit; it's the voltage that appears across the capacitor (much much less than 8 kV) due to the internal series resistance of the ESD generator/gun. That internal resistance (and the ESD capacitor in question) form a low-pass RC network that significantly limits the build-up of voltage across that ESD capacitor.

In the picture below, the 150 pF capacitor is charged up to (say) 8 kV. These are internal ESD gun components and will be subject to the full kV rating but, that's not your problem. The important component (still internal to the ESD gun) is the 330 Ω resistor. It will limit the peak voltage that can be produced across your ESD protection capacitor. And, of course, you have to choose a value of ESD capacitor that limits the voltage sufficiently to protect your vulnerable circuit AND is limited below the maximum voltage rating of the ESD capacitor.

(Image source: Keith Armstrong - Design Techniques for EMC – Part 6)

Where can I find this information for a given package size, capacitance and voltage rating?

You won't find it because you have to calculate it. Manufacturers don't know what your victim circuit might be able to safely withstand and therefore they cannot tell you what value capacitor to choose. Given also that there are a few different ESD requirements specifications, this makes definitive recommendations improbable.

But, if you have a simulator (highly recommended these days for any level of designer) it's easy to set the initial conditions on the 150 pF capacitor to be 8 kV and see what the voltage rises to on your prospective ESD capacitor based on capacitance value.

Regarding the capacitors proposed in the question, the data sheet is a little ambiguous over the part numbers but, nevertheless, I think they tie in with this range on the Kemet website. They are described by Kemet as being "SMD ESD Rated Commercial Grade" and, if I focus on a 10 nF 0603 part, it has an ESR curve like this: -

Basically the ESR is 100 mΩ and this ought to be considered when making calculations or, just make a simulation using that ESR value. You can also check the ESL: -

It's pretty good; it doesn't rise about 500 pH (pico henries) but, you should still use it in your simulation just to ensure there are no extravagant ringing artefacts that might cause voltage overshoot problems.

You should also check out what Kemet say about the peak voltages and currents. This just doesn't only apply to ESD but normal operation as well: -

Factor all these things in and you should be good to go. Anyway, here's a quick simulation of a 10 nF capacitor being discharged into. I've taken the liberty of speeding up the charge process into the 150 pF capacitor just to make things easier to see: -

And here are the waveforms: -

Under these circumstances, the ESD capacitor rises towards 195\$\color{red}{\boxed{^1}}\$ volts for a closed switch duration of 10 μs (that begins at 0.5 ms). I'm sure, if you had a simulator you could reproduce this and add in ESR and ESL to the ideal capacitor model used.

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\$\color{red}{\boxed{^1}}\$ 195 volts is a little high due to the liberty I took when reducing the 50 MΩ resistor to 100 kΩ. I took this liberty to speed up the 150 pF charge time so that the sim result was more easily viewable. In reality, with basic charge redistribution from the 150 pF capacitor to the 10 nF capacitor, the peak voltage would be: -

$$\text{150 pF}\times\text{ 8,000 volts} = \text{(10 nF + 150 pF)}\times\text{ final voltage}$$

Answer 2

The capacitors will need to have a voltage rating that is higher than the highest DC voltage that will ever be across that capacitor during normal operation.

The idea of proper ESD protection using capacitors is that the voltage will never become very high in the first place. The capacitors are supposed to absorb the charge from injected by the ESD event.

If your ESD event would still give, for example 100 V across the capacitors (and you would use 150 V capacitors because of that) then **you would be missing the point of those capacitors providing ESD protection. That 100 V would already have damaged your IC!

So you need to add capacitors that are large enough to absorb enough charge so that the voltage doesn't exceed the damage level.

Do realise that all ICs also have internal ESD protection. However it is good practice not to rely on that and add your own protection where possible.

Answer 3

That depends on the clamping diode(s) that are around on the same node. For discrete TVS diodes you can usually find graphics of clamping voltage versus current. If you rely on diodes internal to ICs this information is probably not well documented.

The situation that the full ESD charge flows into the capacitor can happen when you have a large series impedance after the ESD cap. In this case, you can calculate the final voltage rise as the full ESD charge divided by the capacitance. However, this situation is rather unlikely in my opinion, as the series impedance should be sufficient to prevent ESD damage in conjunction with IC-diodes.