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bipolar tvs

bipolar tvs

(OP)
I'm looking for a bipolar TVS solution for ESD protection. While there are many bipolar TVS solutions available, are there any drawbacks to using two unipolar TVS in series with opposite polarity? I've seen three pin devices where you can hook the two devices in parallel (anodes tied together and cathodes tied together) or in series (only anodes tied together), so it seems two devices could work as well.

Other than more space requirements, are there any other gotchas? Both diodes are off until a pulse event happens, and then one has forward current and the other has reverse (avalanche) current while the pulse energy is dissipated. Turn on time for forward conduction is the only parameter that I can think of that may be an issue.

Thanks,

Z

RE: bipolar tvs

That's exactly what the bipolar ones are - two in series. Why would you want two parts when one would be less inventory, space, and cost? Availability?

Keith Cress
kcress - http://www.flaminsystems.com

RE: bipolar tvs

(OP)
Availability, odd clearance requirements, and specs. This device also needs to withstand fault conditions, so I need a TVS with a steady state power rating, not just peak power. Vishay and Microsemi have pretty good specs, but most other manufacturers skip that parameter on the data sheet. I'm finding many more unipolar options than bipolar that meet my requirements.

Z

RE: bipolar tvs

Ah yes, the spec issue, I see your rational now. I actually end up with unies most the time too.

Keith Cress
kcress - http://www.flaminsystems.com

RE: bipolar tvs

Well what all specs are you looking for? How much steady state power will you be running through it, what clamping voltages do you need, and what do you expect your transient power to be?

Have you checked out the 1.5KE series from littelfuse? They have a whole bunch of clamping voltage rated options, and are all rated for 6.5W steady state and 1500W peak power dissipation:

http://www.littelfuse.com/~/media/electronics/data...

Forgive me if this is old news for you since I think that vishay might make its own version of the 1.5ke series, which you might have seen already...

And just out of curiosity, may I ask you what your application is?

-Rob

RE: bipolar tvs

(OP)
My application is for ESD protection, except the sub-circuit is being used in an Intrinsically Safe (IS) application, where devices must be able to sustain constant power (i.e. static power dissipation) and meet the IS temperature requirements during a fault event. If a device isn't specified for steady state power, it must be tested during the approval process. Using properly specified parts reduces the amount of testing required.

I only need about 0.3W steady state, but i have some very tight size restrictions.

Z

RE: bipolar tvs

I have 2 comments about this

1. Using two packcages in series, compared to 1 bi-directional package, may result in a bit more lead inductance so your peak voltage during ESD discharge may be a bit higher. Ultimitely, your test will confirm the pass/fail result.

2. Generally, ESD is a CE and/or EMC requirement only, not for IS. Therefore, as long as the failure of the ESD diode (open or short) does not result in an unsafe condition, the part does NOT need to be rated according to the IS standard (I assume 60079-11 or similar). Remember, those component sizing rules and tests only apply for components which intrinsic safety relies. Of course, if you are using the same diodes to maintain an intrinsically save voltage, then you need to go through the analysis.




RE: bipolar tvs

(OP)
1) That makes sense.

2) The feedback I'm getting from my Intrinsic Safety expert is that the TVS needs to be rated to withstand the steady state fault power without letting the smoke out. I believe this is because uncountable faults could apply the fault power directly to the TVS leads. Encapsulating the TVS may be a solution but I'm hoping to avoid encapsulation.

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