ESD Considerations

[Rusttree]

I'm choosing a TVS diode for ESD protection in my USB circuit. I'm noticing that the datasheets for a lot of parts recommend grounding the TVS diode to the chassis, not the signal ground. I don't anticipate ever having a metallic chassis for my device, so is signal ground good enough?

Also, I'm gathering that reverse standoff voltage and capacitance are the most important qualities (for high-frequency signals, at least) to look for in a TVS diode. Are there any other qualities I should take into account? For example, I see the power rating on TVS diodes, but how do I choose an appropriate value for power? Aren't ESD events erratic and unpredictable? How would I know how much power to design for?

 

[Mikebits]

Take a look at this article.
**broken link removed**

 

[Mosaic]

Ok, a USB circuit won't see very high energy spikes like those found in automotive load dump events. Notably the bidirectional TVS are slower than the unidirectional . If u expect to see negative going spikes u can add a 2nd unidirectional TVS & series diode to block shorting the regular supply . The series diode should be a fast swithcing type like a schottky, a GP 1n4001 works reasonably well though. Also for multi input devices u can use routing diodes with a single TVS rather than a whole bunch of TVS or buy a TVS array chip.

When I design heavy duty spike suppression , I use a TVS backed by a MOV of the correct rating as the MOV is much slower (but much higher power). So the TVS catches the fast transients and the MOV kicks in if the transient is long lasting and prevents the TVS from frying. U have to make sure the max clamping voltage of the MOV is a bit less than that of the TVS. For very, very fast TVS response, use one with very short leads or an SMD version to reduce inductance.

Use the biggest TVS u can....no such thing as too much. Cost & space are the constraints.

The max clamping voltage is the most important spec followed by the power dissipation and the reaction time

 

[Rusttree]

Thanks for the replies. Good info.

I landed on this part as a general purpose TVS for my USB device:
https://www.electro-tech-online.com...275537FA7Fpdf7FEnglish7FENG_CD_SCD27553_A.pdf

The ESD environment I'm designing for is pretty minimal - just around my house. I will handle it a lot, though, and I have carpet in the room I'll use it the most. Back to one of my original questions, is it ok to use the signal ground if I don't have a chassis ground?

 

[lebevti]

wear heel straps or a ground strap when handling electronic boards or components

 

[Rusttree]

Well, I'm not about to implement industry-standard ESD compliant practices to do hobby electronics in my bedroom. But thank you.

 

[lebevti]

lolol

 

[lebevti]

yes, connecting to signal ground is OK

u would do that anyway even if there were a chassis

 

[Rusttree]

Ok, thanks lebevti.