ESD Cap Sizing

[perts]

I am trying to design a circuit with a 8KV contact rating (human body model). I want to use ceramic caps to provide esd protection. I want to see how I should size my caps. Is there a specific forumula/rule of thumb to use to determine the voltage rating and capacitance rating for my caps.


Thanks in Advance
Pert

 

[IRstuff]

Well, you start with determining what voltage your circuit can withstand. A typical example might be 2kV, because that's where the dielectric on the chip might break down.

however, it's not just caps, or only caps. You need devices that divert the discharge current safely to ground. A typical scenario starts with modeling the human component as a 100 pf capacitor discharging 25 kV through a 1500 ohm resistor. The combination of capacitors, resistors, zeners, diodes, triacs, etc., that form the protection circuitry try to keep the protected device below whatever voltage specification it can meet. see:

http://www.ecse.rpi.edu/~schubert/C...ody-model-and-electrostatic-discharge-ESD.pdf

A standard IC ESD protection uses the pad capacitance in conjunction with diodes, triacs,

http://www-tcad.stanford.edu/tcad/pubs/theses/sbeebe.pdf

http://www.nxp.com/documents/application_note/AN10753.pdf

http://www.ti.com/lit/ml/sszb130b/sszb130b.pdf

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[perts]

What about just looking at the charge calculation as a general rule of thumb? Qesd < Qcap

charge of the ESD gun =(8KV * 100pf) has to be less than the capacitance * voltage rating of cap

 

[IRstuff]

Is this for school?

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[zappedagain]

I had a coworker suggest this the other day. I guess the trick is that you need a cap that can withstand 8KV without breaking down. Vishay has a 0.01uF cap that can take 15KV, it is only 45.7mm x 25.9mm! I think I'd lean toward a diode with similar ratings in a 3mm x 2mm x 2mm package.

Z

 

[IRstuff]

Actually, it won't have to see the 8kV, for more than a few nanoseconds, perhaps, if at that, since the human body model (HBM) for ESD has a series resistance that limits the available charge. So, we'd be looking at the available charge stored in the parasitic capacitances associated with that series resistor, and the connection that goes to the resistor.

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[itsmoked]

limits the available charge

You mean limits the available current.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Well, both. The charge that is measured with an electrometer at the tip of the HBM doesn't necessarily go through the HBM resistor, so there is no current limit, other than the wiring resistance. Only after that charge is depleted will current start to flow through the 1.5-kohm HBM resistor.

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