TVS protection on 48V input? - Page 1
Forum section: electronics / beginners

author: Tantalum
author_rank: Regular Contributor
timestamp: December 15, 2022, 11:06:51 am
content: HelloI’m designing a circuit using a led driver circuit (for outdoor application) and I need to protect the 48Vdc input from transients.I have decided to add a huge 15KPA048C TVS diode + fuse, but I’m not sure it will be enough to protect the circuit because of its “maximum clamping voltage” that seems to high to me. It’s actually 71.5V.The problem is that my TPS92511 LED driver has an abs max of 67V, and the TPSM265r1 voltage regulator 68V.That’s below the maximum clamping voltage of the TVS.I was think about using a 80V hot swap controller after the TVS diode, that would shut-down or clamp the voltage to protect the circuit in case the TVS doesn’t clamp the voltage enough. But I think it’s a little bit overkill and would increase the bill.Would the 15KPA048C TVS diode be enough?I mean, that “maximum clamping voltage” is actually only a MAXIMUM. When the diode starts conducting, the voltage should drop except if there is a really high surge event.What’s your opinion?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2022, 11:45:05 am
content: Againstwhat?!Induced lightning strike? Nearby strike? Probably not direct strike, it's not quitethatbig, but I can't imagine what else you'd need such a large TVS for other than the first two, and rather severe levels of them at that.And what is the circuit? Why are you designing a system (assumed intent) which can't withstand the 71V peak of the protective device?Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2022, 12:28:25 pm
content: If you know your surge won't be I_PP but smaller, then the maximum clamping voltage goes down, too. It's just they rarely rate this. A curve would be nice, but all you get is two points, working voltage @ I_leak, and max clamping voltage @ I_PP. You can try to interpolate some line between the points, but be very conservative. For example, if I had this juicy, otherwise perfect downstream part which is only rated to 70.0V max recommended, then I could consider using the TVS rated at Vclamp = 71.5V, knowing that my input pulse current is always significantly smaller than I_PP (say, half of it).But stricly, for a robust design, choose so that downstream parts are rated for at least 71.5Vrecommended, so probably even more absolute max.

author: Tantalum
author_rank: Regular Contributor
timestamp: December 15, 2022, 01:01:55 pm
content: Quote from: T3sl4co1l on December 15, 2022, 11:45:05 amIt's a personal project, not for a commercial product.The problem is that I have no idea what kind of surges something placed outdoor, has to face. It's out of my skills :SQuote from: Siwastaja on December 15, 2022, 12:28:25 pmYes. But I couldn't find a led driver and a voltage regulator module with better specs (including low price range).Would the hotswap controller that clamps the voltage be a solution?There is a TVS diode with 48V working voltage and 64V max clamping. But it has only 3.5A IPPM, that suites signal lines, not for power lines.https://www.nexperia.com/products/automotive-qualified-products-aec-q100-q101/automotive-esd-protection-and-tvs/lin-can-fd-flexray/PESD2IVN48T-Q.htmledit:Well, actually the LM3402HV doesn't look to bad (76V abs max)

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2022, 04:31:34 pm
content: Where is this 48V DC coming from? A standard off-the-shelf switch-mode power supply? Maybe a battery? Maybe a system with mechanical generator (like car alternator)?How is it delivered? Over a long DC cable? 10 meters? 100 meters? 1000 meters?That TVS definitely helps with transients, even if non-optimal with the downstream devices rated for voltages too low. It's still significant improvement over nothing. The question just is, how bad will the transients be? Chances are, you are fine with nothing at all, or just a 470uF 63V electrolytic capacitor.And BTW, for DC supply, you would usually want the unidirectional version. It would protect from wrong polarity (by blowing a fuse, etc.), where a bidirectional device will happily allow -48V supply.

author: Tantalum
author_rank: Regular Contributor
timestamp: December 15, 2022, 04:54:03 pm
content: Quote from: Siwastaja on December 15, 2022, 04:31:34 pmDIN rail Power Supply (Meanwell SDR-240-48). Max 20m buried cable.I just found a better led driver (ILD8150), 90V abs, that should be fine.I couldn't find a dc-dc regulator with integrated inductor with the same voltage rating. So I will probably need to do it my self.It's ok.Quote from: Siwastaja on December 15, 2022, 04:31:34 pmOk thanksI will take this one: 8.0SMDJ48A

author: Faranight
author_rank: Unknown
timestamp: December 16, 2022, 06:03:21 am
content: Quote from: Siwastaja on December 15, 2022, 04:31:34 pmHey, I'm curious about this. How does one choose a TVS diode for reverse polarity protection so that the fuse blows before the diode does?Does it have to be a TVS diode or can we just stick a normal junction diode in there that has a higher continuous current rating (with some margin) than the fuse?-Far

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2022, 04:08:59 pm
content: The fuse has a clearing (I^2t) rating. Rectifiers do as well (or, usually do). TVS usually don't, I think, but should be comparable to a rectifier of similar size (e.g. SMC package TVS should be analogous to a 6-10A rectifier).I^2t means to square the current of a given waveform, integrate it over time, and that figure is representative of surge energy the device can handle. Namely, for fuses, the maximum required to clear it; for diodes, the maximum it can handle safely without failure. (This figure would be energy exactly, if it were V*I*t. Under surge conditions, V can be replaced with average resistance. We don't actually care what the resistance is, so only I and t are used in the figure, hence I^2t. Note that the average resistance of a diode varies with current, so the I^2t rating may depend on surge amplitude/duration; it's usually for 8.3 or 10ms, i.e. half a line cycle.)So, choose a fuse with lower I^2t than the rectifier, and you're set.From this description, you might reasonably anticipate that the reverse (zener/avalanche breakdown) fusing rating of a TVS is simply the forward figure divided by the ratio of reverse to forward voltage. And you'd be quite correct. Unfortunately, since this ratio is rather large (using this thread for example: about 50!), it's hard to find fuses fast enough to be able to use a TVS to protect against overvoltage.But if you aren't trying to use it against sustained overvoltage, that's not a problem, anyway.Which still isn't to say it's a bad idea -- for low voltage TVSs, the ratio is less extreme, and low-I^2t fuses are available at low voltages and currents. Or, perhaps the TVS's heat could be shared with a Polyfuse connected nearby. Indeed, combination polyfuse-TVS components have been made before -- AFAIK they're either boutique or obsolete nowadays, but it was an interesting combination, anyway.Tim

author: Infraviolet
author_rank: Super Contributor
timestamp: December 16, 2022, 06:41:21 pm
content: Could you use the TVS diodes to clamp to 71V for fast events, then a further zener diode, this time one which clamps to only just above 48V to further reduce voltage spikes. Perhaps with some sort of low pass filtering after the TVS and before the 48V+delta zener?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 16, 2022, 07:56:11 pm
content: Quote from: Infraviolet on December 16, 2022, 06:41:21 pmThe key problem is the slope of the curve, and a cascade design does not help with that. Problem is, such zener which clamps "just above 48V" is not available as a simple passive component; if it was, one could just use it in the first place.For sharp on-off curve, one needs to design an active circuit. Something based on TL431 would be absolute classic.But usually, one just accepts the TVS or zener curve and makes the downstream circuit tolerant to the highest clamp voltage instead.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2022, 04:49:30 am
content: Only difference between a TVS and zener is the zener is made for low currents and modest precision, and the TVS is made for surges and looser tolerances. The physics are identical.Mind, that doesn't mean all zeners can be used as TVS, they may not handle surge conditions. But, there are brands or families with ESD and surge ratings, so, check the datasheet.Also, mind the distinction between low voltage and regular TVSs; there are "diode" type TVSs that use other structures/designs, like snapback diodes. These are distinguished by their lower voltage ratings and stiff clamping voltage (basically, because they put some negative resistance in series with the positive resistance of the bulk material). Avalanche types don't go below 5V (nominal), and zener (breakdown < 6V) isn't useful (high dynamic impedance), hence the other types for lower voltages. There are also non-diode TVSs like SIDACs, which are basically low voltage (and somewhat slower) GDTs. And composite types, like clamp diode arrays with common  TVS. So to be clear, I mean avalanche type TVS above, which is the most common kind.Anyway, as far as using a zener, you could add a series resistor and clamp zener, which works fine for small loads. I think you'll understand the problem with doing it at any kind of power.Power can be saved if you add a depletion mode NMOS source follower after the zener; the resistor then only needs to supply gate leakage, which is negligible. This is basically a crude LDO, handy for limiting excess voltage when low dropout is needed otherwise. Of course, the transistor has to handle the power of (Vin - Vout) * Iload during the excursion. This can be handy for automotive load dump, for instance.Tim

author: Tantalum
author_rank: Regular Contributor
timestamp: December 17, 2022, 12:31:25 pm
content: Quote from: Faranight on December 16, 2022, 06:03:21 amDave did an interesting video about it (about multimeter input protections, GDTs, MOVs I think).Unfortunately I can't remember which video exactly it was.

author: Infraviolet
author_rank: Super Contributor
timestamp: December 18, 2022, 01:51:23 am
content: Zeners might not have a completely sharp transition between off and on, but having them being ever so slightly on and leaking a small current during normal 48V operation should probably be ok. For something like this you're not running of a tiny battery where a few milliamps wasted actually counts for something. Have a max reverse standoff somewhere below 48V and a min breakdown above 48V, I guess you'd still find some options able to clamp to <67V?I can see a few options filtered which might do as secondary TVS diodes after the ones you've already got:https://uk.rs-online.com/web/c/semiconductors/discrete-semiconductors/tvs-diodes/?pn=1&applied-dimensions=4294767590,4291051452,4291604869,4294293336,4294736786,4293169266,4290903364,4294739647,4293169268,4290903366,4294436787,4294767586,4294863281,4291649775,4294341691,4294708841,4290552789,4290592733,4291051451,4294806547,4291220155,4293169265,4294708838,4293588943,4294690976,4294356404,4294762170,4294768824,4293588946,4291220153,4294708831,4291649772,4294329270,4293169256,4291209029,4294723029,4290903361,4294767611,4291051450,4294282095&sortBy=Maximum%20Clamping%20Voltage&sortType=ASCalthough depending on the quantity you want you might need to buy elsehwere for smaller numbers.

author: Faranight
author_rank: Unknown
timestamp: December 18, 2022, 11:59:24 am
content: Quote from: T3sl4co1l on December 16, 2022, 04:08:59 pmAnd what kind of device would be appropriate for use against sustained overvoltage, if not a TVS diode?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 18, 2022, 02:19:18 pm
content: Quote from: Faranight on December 18, 2022, 11:59:24 amPass element. I.e., instead of trying toconsumeeverything the source can supply, just... disconnect it. Or if the device needs to operate,regulateit down (which boils down to just designing a power supply that can operate with high enough input).You of course need a pass transistor with sufficient voltage rating to do that, but a shunting route is no-go if the input can produce a lot of power over a long time - i.e., a lot of heat in TVS.But TVS have excellent peak power handling ratings, so great for short surges.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2022, 03:10:28 pm
content: Yes, "sustained overvoltage" is what you call the situation at the input to a voltage regulator.And if that would dissipate too much power, then switch it off within some time frame -- now it's merely an energy problem again. That can be a pass switch, or a crowbar to blow a fuse.Note that the case can be made for seriesorshunt regulation, under pulsed or continuous conditions, depending on the source. But low impedance / voltage sources are more common than high impedance / current sources, so we tend to prefer switching off, or blowing a fuse, versus shunt regulation or crowbar.Tim

author: Faranight
author_rank: Unknown
timestamp: December 18, 2022, 05:18:01 pm
content: No, I was referring to a case where a user would accidentally connect a 24VDC power supply to a device rated for 12VDC, thus causing overvoltage.There is of course a fuse on the input, and there needs to be a shunt element after it that allows sufficient current under overvoltage condition (without blowing) for the fuse to melt.I originally wanted to use a TVS diode for the purpose, but I see now they are unfit. TheSCR crowbar circuitis the next interesting thing I found after googling for a bit.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 18, 2022, 06:59:23 pm
content: A large TVS + a fast fuse is fine as a passive crowbar, but the TVS will have a modest curve which is why you can't get away with say 20V abs max downstream parts.So you have two options, make the downstream power supply accept the maximum clamping voltage of the TVS (so maybe 25V), or add an active crowbar circuit which will have an almost ideal brickwall curve unlike the TVS; you can make it trigger at e.g. 15.0V and choose downstream parts rated to 16V!Whichever is suitable depends on the case. IMHO, it's usually simpler to just make the downstream supply accept wider input range (and leave most of it unused during "normal" operation), but sometimes you have multiple supplies and other branches directly off the Vin and you want to protect them all in one go.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2022, 06:15:45 am
content: I wouldn't say... "fine", there's a few problems with it -- a slowly rising voltage (or, really, a "just right" voltage, rate doesn't matter) will cook the TVS, eventually it melts and fails shorted and the fuse opens. Or under excess or surge conditions, the TVS just doesn't handle as much fusing I^2t as the fuse does, and shorts out and that opens the fuse. "Problem" may be overstating it, depends what you want of course.Anyway, if you're using the TVS as a (shorting) fuse, then you need to replace both when a failure occurs. (Though maybe the TVS leaves a burn mark or something, it'll be pretty hot by the time it fails short. Might need to replace the board anyway..)And maybe that's not a problem; if you're using a board-mounted fuse, any failure might mean board replacement, so who cares. Though, by extension, why bother with a large TVS, or one at all, if the board is dead anyway? So, your priorities may change if that's the kind of thing you're after.Again, fusing being what it is, maybe a sufficiently large, and low voltage, TVS will clear a fuse alright, but I wouldn't count on it beyond... maybe 5V (nominal)?The nice thing about the crowbar is, even just a zener into SCR can be made to not cook itself, but to latch on by then. And hysteresis can be added, you can even use a TL431 for very accurate threshold (at expense of a few µs delay, which a small TVS can deal with easily).To implement hysteresis, you need an inverter, and need one anyway to drive the SCR, hence the PNP here. Exact resistor values might vary, but thats the general idea.Tim