Transient voltage protection where Vtyp is close to your IC Vmax - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: September 13, 2020, 03:21:01 am
content: I have an application where the circuit absolute max voltage is 42V but the operating voltage will be around 30VI'm quite used to using 3kW TVS diodes but all the ones i can find have their working voltage and clamping voltage further apart than that.So my question is, what other transient voltage protection approaches are there for dealing with this sort of situation?

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2020, 10:47:28 am
content: Quote from: Psi on September 13, 2020, 03:21:01 amA series regulator or shunt regulator or both can work. The problem then becomes achieving a quick enough response.

author: Alti
author_rank: Frequent Contributor
timestamp: September 13, 2020, 11:48:07 am
content: Quote from: Psi on September 13, 2020, 03:21:01 amDo you want the device to work undisturbed at 42V supply and die at 43V or do you want it to shut down at 31V and die at 1kV 8/20us? Just describe the test this device is intended to survive (and die at +1V) and then we can propose some solution.As for TVSes you can use TVS for 14V nominal voltage. This will clamp to 43V (max) exactly one 8/20us transient (I took some random TVS datasheet). At 30V it is going to dissipate some power, now the question is do you care. If this is a uA application then 1W dissipation might be a problem but if it s 1kW then does it matter? Is it ok to survive exactly one 43V 8/20us pulse or maybe 1000 pulses is more adequate for this application? Because then you have to use TVS for higher voltage, etc.

author: Nauris
author_rank: Regular Contributor
timestamp: September 13, 2020, 02:33:12 pm
content: Try how big transients it can actually take. It might be that ordinary TVS is good enought.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 13, 2020, 05:06:03 pm
content: Quote from: David Hess on September 13, 2020, 10:47:28 am... and adding capacitance makes the "quick enough response" problem disappear.Of course, with proper layout and minimized inductance. And preferably damped with higher ESR capacitance, say a smaller amount of ceramic with larger amount of electrolytic in parallel.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2020, 07:38:31 pm
content: Guessing you aren't protecting against IEC 61000-4-5 surge, on a DC bus. More pedestrian surges may be more likely: ESD, perhaps automotive surge (fast, positive and negative pulses), perhaps hot plugging/inrush (an LC resonant peak waveform). Or just overvoltage in general, say from using the wrong power supply.In that case, you won't see the peak clamping voltage -- it is measured at a peak current much higher than these cases will see. The clamping voltage will be somewhere between breakdown and that. Exactly where, is hard to say without a plot, and without knowing the expected peak current.Absolute maximum ratings are the highest point of survival, with survival not guaranteed beyond there. The failure mode could be gradual breakdown due to a stress condition, or it could be acute. The real breakdown voltage is some fraction higher than the abs. max., typically 10-30% (in the same way that the TVS's breakdown is somewhat above its nominal rating, for the same reason!), though of course you don't know which end of that range a particular part will fall in.Breakdown may be absolute (the tiniest current causes failure), or it may be robust (able to withstand significant avalanche current, until it fails thermally). The avalanche characteristic can be anything from positive to negative resistance; a robust avalanche breakdown will have some modest resistance, and a positive tempco (so the breakdown voltage rises as it heats up; avalanche devices of matched voltages will share current). Avalanche can have negative resistance and even weird snappy discharge behavior (an avalanche pulse discharge circuit is a preferred way to generate relatively large, quite fast (sub 1ns!) edges; even an ordinary 2N3904 can exhibit this phenomenon), though I would not expect an IC to exhibit this behavior.You'd have to ask the manufacturer about this, or measure a range of parts yourself. Note, the manufacturer may not be interested in telling you about any of this, instead taking the safer route of simply saying: don't do, that, use this nominal supply voltage with this TVS, nevermind if it's lower than your required 30V operating range.Which brings me to the final point: the safest design is not to worry about any of this. Shop for a 60V or higher part instead. Then you will have more than enough headroom to choose a TVS of adequate rating.Personally, I wouldn't have a problem with using the 42V part, with a SMAJ30A, for purposes of inrush and ESD. For lightning induced surge, or automotive load dump, I would look for another solution, perhaps the higher voltage rating, perhaps a LDO to afford ride-through, or a switch to disable or crowbar the input when such voltages are applied. (There are load dump protection devices which do exactly this.)Tim

author: Psi
author_rank: Super Contributor
timestamp: September 14, 2020, 02:26:52 am
content: Thanks for the detailed answers!The protection is for an automotive application, so doesn't need to handle lighting or anything excessive.Maybe i should do a test with an ignition coil to see where some TVS diodes clamp to when hit by that.That is probably the absolute worst case for automotive use case, and even that is likely excessive.If i can get a TVS that has a working voltage above 30V and it clamps to <42V when hit by an ignition coil then i would be happy to use it.Yeah, i'm still looking for any other parts that might be suitable with a higher absolute max than 42V.This is the 42V max part that i'm looking at currently.https://www.ti.com/lit/ds/symlink/lmzm33606.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe&ts=1600050385078

author: exmadscientist
author_rank: Frequent Contributor
timestamp: September 14, 2020, 03:05:28 am
content: TI's "FlatClamp" lineup, specifically theTVS3300, may do what you want. They've advertised this line heavily, so I've looked at it a few times, but it's never been right for what I need so I haven't bothered with it. I believe other vendors have similar parts, they just haven't been advertised as well. And I'd be particularly alert for specsmanship, given the advertising....Do be mindful of what you actually need to protect against, as others have advised. If load dump is on the table, a TVS or TVS-like alone probably won't cut it.

author: maginnovision
author_rank: Super Contributor
timestamp: September 14, 2020, 03:35:12 am
content: For a similar issue I've just started testing my TVS. For low volume the cost isn't an issue to simply toss aside parts that don't meet the required spec and use those that do. Not sure if that'd be an option for you.

author: Psi
author_rank: Super Contributor
timestamp: September 14, 2020, 03:36:05 am
content: Thanks!I missed seeing that one in my digikey searches because i was looking in TVS Diodes not TVS Mixed Technology