TVS for Protecting a 38Vmax DC-DC that needs to operate at 24Vin??...... - Page 1
Forum section: electronics / projects

author: Smokey
author_rank: Super Contributor
timestamp: October 20, 2023, 04:04:52 am
content: I have a design that's using a LMR33630-Q1 DC-DC converter.https://www.ti.com/lit/ds/symlink/lmr33630-q1.pdfThat chip has a 38V absMax. It will be running from nominal 24Vin, which is within the 3.8-36V range.--> Problem is that the TVS diodes to protect the input which have a max clamping voltage under 38V also have a Reverse Standoff voltage at or below 24V. So this means that these diodes will most likely be conducting normally with a 24Vin. Spice confirm this. If my Vin comes in high at 26V (which might happen) then there is like 1.something watts in that TVS diode all the time. That sucks.Example:https://www.diodes.com/assets/Datasheets/ds19002.pdfWhat can I do here? I was looing at a combination of the next bump up in TVS Reverse Standoff Voltage, so a 26V (or 28V), which has a Max Clamping voltage in the 41.6-45.5V range. If I use one of those in combination with a normal non-tvs zener at 35V (or even lower), spice says the voltage gets clamped below 38V for a load dump. Not sure if I should trust the spice here though.Thoughts?

author: johansen
author_rank: Super Contributor
timestamp: October 20, 2023, 04:24:55 am
content: The data sheet shows 5uA at the rated vrm?The max clamping voltage if im not mistaken is the voltage at the rated peak current, for a 24v rated part, which starts to conduct as low as 26.7 volts,, at 15 amps it will be 38 volts.If 15 amps isnt enough or 38 volts too high, parallel more of them.And yes you may need a 26v tvs to handle the nominal variations in 24v power supplies

author: Smokey
author_rank: Super Contributor
timestamp: October 20, 2023, 05:04:12 am
content: That's interesting. The two parallel TVS does drop the max clamping voltage (according to LTSpice at least). Cool.Unfortunately it's not enough to fix needing a 26V TVS and protect the DC-DC. I don't have room for like 4 of themIs the normal 35V zener in parallel with the 26V TVS that terrible of an idea?

author: jonpaul
author_rank: Super Contributor
timestamp: October 20, 2023, 07:53:31 am
content: supply is in vehicle? ALT load dump transient? What is the transient threat?We use MR2525, MR2535 Mot alt load dump for avaition and automotive bus 12/24/28VJon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 09:26:38 am
content: To some extent you can rely on abs. max. being somewhat conservative, i.e. it's rated 38V max continuous, in the same way the diode is rated 26V max continuous; breakdown is always some margin above there. You don't know by how much, of course, and they never specify breakdown of the regulator.Clamping is also at Ipp. What are your surge conditions? If much more relaxed (e.g. automotive pulses other than ISO-7637-2 pulses 1 and 5), expect tight clamping, closer to Vbr(max). Conversely, if you don't have much pulse to handle but it's very intense (e.g. IEC 61000-4-5 as might be applied to long DC lines), you might be closer to the 8/20us waveform (not defined for this SMBJ but others have it), where peak voltage can be several times higher (MOVs are competitive!), simply because the energy is relatively low at the short duration.If you do need to withstand pulses like load dump, consider a voltage limiter or electronic switch/fuse approach.Tim

author: Smokey
author_rank: Super Contributor
timestamp: October 20, 2023, 07:35:53 pm
content: Ya, I should have been more clear. It's automotive, and the concern is load dump.The vast majority of the time this thing will be running from 12V and that's what I originally designed it for (hence the 38V DC-DC and 24V TVS), but I picked up this application that needs to run at 24V and that sort of kills my design margin. I'm trying to see if I can salvage this, or if I need to redesign the DC-DC to use a 60Vmax Vin regulator (which I'm not looking forward to).

author: langwadt
author_rank: Super Contributor
timestamp: October 20, 2023, 09:02:52 pm
content: Quote from: Smokey on October 20, 2023, 05:04:12 amwhy do you need a 26V TVS? VBRis >26.7V for the 24V TVS

author: thm_w
author_rank: Super Contributor
timestamp: October 20, 2023, 10:44:42 pm
content: If its automotive, two series batteries, wouldn't the max continuous Vin be more like 13.5V * 2 = 27V?Depending on the float voltage.

author: Smokey
author_rank: Super Contributor
timestamp: October 20, 2023, 10:53:34 pm
content: Quote from: langwadt on October 20, 2023, 09:02:52 pmTVS can start to conduct at the Reverse Standoff Voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 10:58:26 pm
content: What level? 24V load dump can go up over 150V. You need way the hell more than an SMB TVS to handle that!Tim

author: Smokey
author_rank: Super Contributor
timestamp: October 20, 2023, 11:00:31 pm
content: Quote from: thm_w on October 20, 2023, 10:44:42 pmYa, this may be the case. The alternator is probably charging up to 28V. Either way I need some actual margin so it will probably end up being a 30V TVS.I think I'm going to just redesign the power supply to use a 60V max part. You guys had all day to come up with an idea and be a savior, but you let me down!.... I kid.. I appreciate the ideas.The SMB part was from the design when the requirements were 12V. There actually are two in parallel on that design though.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 21, 2023, 01:04:01 am
content: Quote from: Smokey on October 20, 2023, 11:00:31 pmIf you have room for two, a suitable resister between them will expand he clamping.I see TI claim to have 'better' TVShttps://www.ti.com/lit/pdf/slyy127

author: Smokey
author_rank: Super Contributor
timestamp: October 21, 2023, 01:30:25 am
content: What is going on with this part?https://www.diodes.com/assets/Datasheets/D30V0S1UG3LP20.pdfThe datasheet claims to have a Reverse Working Voltage of 30Vmax, AND an ESD Clamping Voltage at 70A of 30.5V?The plot looks like the clamping voltage dips to what looks like 23ish voltage at low currents.What gives?

author: Marco
author_rank: Super Contributor
timestamp: October 21, 2023, 01:51:58 am
content: Why don't they just make buck converters with back to back high voltage MOSFETs, which block both load dump and continuous/temporary reverse polarity, and some build in TVS to bridge the time for the MOSFETs to kick in? Lose a tiny bit of efficiency, lose a ton of external components at the same time. You'd think there's a market.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 21, 2023, 01:59:27 am
content: Quote from: Smokey on October 21, 2023, 01:30:25 amGet a part and measure it ?The numbers seem to contradict for a simple clamping part, but there are break-back thyristor trigger parts that drop to lower voltage.Imagine if a vendor combined (say) a 20V zener with a 10V breakback part, such a part could trigger > 30.5V and clamp at 25V 50A ?Addit : yes, see fig 5 in their data, that does not quite go through their claimed 25V/50Abut it does have a negative resistance action.

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 02:04:36 am
content: If more precision is required, a crowbar circuit made with a TL431 or some 30 V zener diode and PNP power transistor has a text book like knee voltage.Or a two step approach. A TVS and after the TVS a low value resistor and shunt regulator with a zener and a PNP transistor.

author: langwadt
author_rank: Super Contributor
timestamp: October 21, 2023, 02:14:37 am
content: Quote from: temperance on October 21, 2023, 02:04:36 amdepending on current why make it a crowbar? why not an (high voltage) NPN and a 30V zener, make a crude linear regulator

author: Smokey
author_rank: Super Contributor
timestamp: October 21, 2023, 02:29:57 am
content: I thought normal zeners aren't fast enough...

author: johansen
author_rank: Super Contributor
timestamp: October 21, 2023, 02:34:42 am
content: Quote from: Smokey on October 21, 2023, 02:29:57 amNanoseconds not fast enough?

author: Smokey
author_rank: Super Contributor
timestamp: October 21, 2023, 03:02:17 am
content: Nanoseconds! Talk to me again when you got some attoseconds!https://www.nobelprize.org/prizes/physics/2023/press-release/Either way, I think this actually solves my problem:https://assets.nexperia.com/documents/data-sheet/PTVS30VZ1UPA.pdf30V Reverse Standoff, 37Vmax Clamping Voltage. Tiny package.

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 03:05:37 am
content: QuoteThe difference between a zener and a TVS is the thermal capacity which has been enhanced by a copper slug for a TVS diode.QuoteThat's of course also possible. But considering the high output current (not stated but the buck reg in question is 3 A capable) the drawback is the power dissipation in the N type pass device. You might argue for A P type type voltage regulator. But those suffer from very bad power supply rejection because the base current or Vgs is referenced to the input and a transient will modulate the current / voltage while for a NPN or N-channel MOSFET the base current or Vgs is referenced to the output.

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 03:07:17 am
content: QuoteNice find. Noted.

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2023, 03:18:22 am
content: Quote from: Smokey on October 20, 2023, 11:00:31 pmYou are a lot more likely to see a nominal 24/28V system exceed 30V continuously (meaning for at least a few minutes) than you are to see a load dump actually hit 150V. And in the case of a regulator malfunction, you could see 32 volts or more for quite a while. Unless you have resistance in front of this device sufficient to allow your TVS to clamp continuously without melting, you're trading one vulnerability for another, and a more common one at that. Clamping a load dump requires either limitation of the input current or some really big parts. A 60V regulator would withstand almost every load dump short of a battery disconnect in a vehicle with a generator that lacks internal overvoltage protection.

author: langwadt
author_rank: Super Contributor
timestamp: October 21, 2023, 03:28:42 am
content: Quote from: temperance on October 21, 2023, 03:05:37 amguessing that the buck output voltage is much lower than the 24V input so the input current is much less than 3Aor with and extra transistor and a TL431 the pass element could be a P-fet for practically no loss in normal operation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 03:51:35 am
content: Quote from: Smokey on October 21, 2023, 03:02:17 am...But your problem was load dump? How is that dinky thing going to absorb 10+ joules?And yeah, they don't make any mention about what kind it is, but I would guess either a thyristor or snapback type. Snapback can actually have negative resistance comparable to internal resistance and therefore a pretty flat V(I) curve, with a fairly modest step as it latches on. Thyristor types I think only latch on hard (to a couple volts).Tim

author: Smokey
author_rank: Super Contributor
timestamp: October 21, 2023, 04:13:28 am
content: It's still two in parallel. Since this is smaller, I could add a third.But ya, it would be nice if they had a thermal impedance plot.

author: tszaboo
author_rank: Super Contributor
timestamp: October 21, 2023, 08:19:27 am
content: TVS is not the right part for surge protection. You use it to suppress eg. ESD events, not surge events. The energy in those are orders of magnitude different. So you TVS can be anywhere at 40-50V because having 35V on a 38V device for such a short amount of time is generally OK. Plus you have capacitors shunting the high frequency inputs.But your problem is with surge protection, that requires different circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 12:42:16 pm
content: Quote from: tszaboo on October 21, 2023, 08:19:27 amSo all those TVS sold for and specifically approved for surge application,...aren't?Oh, or do you meanthatTVS specifically? (https://assets.nexperia.com/documents/data-sheet/PTVS30VZ1UPA.pdf) Well, they clearly say 150A 8/20us, which is pretty damn good I'd say, for what it is. But it ain't gonna be much beyond that, and it'll be naught but a tiny crater from load dump.Tim

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 12:45:47 pm
content: QuoteYou didn't read what I wrote or didn't understand what I wrote.Of topic but some insight into LDO's with PNP, P-FET regulators.With a P-FET the pass device voltage is referenced to the input of the regulator and because of this the circuit is pretty bad at handling input transients. The transient will drive the P-FET further into conduction while the control circuit with it's limited bandwidth is taking it's time to respond. Making the control circuit faster is very difficult if not impossible. Early LDO regulators had difficulties dealing with input transients. Some newer types are much better but nowhere near the performance of a "normal" regulator.QuoteI think you are confused by the terminology used for those diodes. There are:TVS diodes with ratings for surges only also called tranzorb diodes by Vishay.TVS diodes suitable for ESD and to some extend surges. Example: SP0115-01ETGTVS diodes suitable for ESD only without surge rating specifications.

author: tszaboo
author_rank: Super Contributor
timestamp: October 21, 2023, 01:56:38 pm
content: Quote from: T3sl4co1l on October 21, 2023, 12:42:16 pmThis will survive 150A at IEC 61000-4-5, which is a surge standard. The lowest level of IEC 61000-4-5 is 500Ohm with 2Ohm source resistance for power supplies, which would be 250A current. It would be appropriate for surge protection for signal lines, but OP want to protect a power supply. And that it's rated peak pulse current, honesty I don't think it would survive that repeatedly. Or IEC 61000-4-4 which IMHO are much nastier, since it's repeated pulses. And these two standard are only applicable for household stuff, automotive standards are even more nastier.

author: langwadt
author_rank: Super Contributor
timestamp: October 21, 2023, 01:57:05 pm
content: Quote from: temperance on October 21, 2023, 12:45:47 pmwith pfet the idea was not to make a regulator but disconnect the load when the voltage it highhttps://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 02:06:59 pm
content: QuoteYou wrote earlier to to make a crude linear regulator. That what you've shown now is something different and probably more than fast enough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 02:17:15 pm
content: Quote from: tszaboo on October 21, 2023, 01:56:38 pm61000-4-5 is a supporting standard; typically it is adapted for particular use. It's used at full strength for most mains testing (2Ω differential), but reduced for common mode testing (22Ω mains-GND, or similar or higher for TNV circuits, etc.).This isn't an unusual confusion; another example:https://electronics.stackexchange.com/questions/682917/en61000-4-5-surge-protection-of-pcb-power-supplyNo way in hell that handles mains level surge, but the specifics of EN50121 match suspiciously close to the part rating, which seems a quite reasonable explanation for the product claims.Which is, of course, how they are able to test all those various voltage TVSs at 8/20us or 10/1000us or whatever, at specified current. Simply adjust the transient voltage, or use an attenuator (or both), to maintain specified waveform while delivering rated peak current. I'd be surprised if they did it by series resistance alone (as most other tests do), since they would have to specify 1.2/50us, or something inbetween, depending on rating, which would be either very unexpected, or very messy. But that is actually a good question, what exact waveform they test with and how much of it ends up clamped by the TVS (OCV peak vs clamping ratio).Automotive isn't really nastier, but it depends what it is. Ye olde ISO-7637 has two nasty pulses, one around 10A and some ~ms, which I'd want to model or test, but this device may well pass; and the notorious load dump, which this absolutely will not pass. All the other pulses are much lower level: EFT and ESD sorts of things at mere hundreds of volts, and impedances of 10s of ohms.There are others of course, like ISO-10605 ESD, which defines a more stringent test (ESD gun up to 330pF/330Ω), more or less because a seated occupant has more capacitance to surroundings than when casually walking by an office desk. Though the voltage can be whatever ("as defined in test plan"), presumably specified in other standards, or as agreed between supplier and customer. (The rail standards are generally similar, with more emphasis on agreement, since rolling stock isn't exactly a consumer commodity.)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 02:29:13 pm
content: Quote from: temperance on October 21, 2023, 02:06:59 pmAh, by mentioning "P-fet" I think they implied a different circuit. P-FETs are commonly used for fairly gross on/off limiting/switching application in automotive circuits like this. This wasn't exactly clear, of course.For my part, I didn't bother mentioning the other traditional protection methods (switch, pass limiter, shunt limiter), because I think OP is already well aware of them, and is concentrating on TVS for, well, unstated reasons. Or perhaps out of poorly made assumptions, such as "it worked for 12V"; but unstated in any case.The current level is a little high, enough that a pass limiter might not be attractive, or that the added bulk, or parts cost (especially if one contemplates a LT4363 "Surge Stopper" or the like $$$!), might not be acceptable. Though that doesn't leave much of any choice, as the pure-TVS solution is rather bulky and expensive itself. Perhaps the regulator must be changed after all.Tim

author: langwadt
author_rank: Super Contributor
timestamp: October 21, 2023, 02:49:11 pm
content: Quote from: T3sl4co1l on October 21, 2023, 02:29:13 pmthis is quite a bit cheaper,https://www.digikey.dk/en/products/detail/richtek-usa-inc/RT1720GF/6002895what is the current actually? with a 3A regulator making, say, 5V from 24V the input current is only going to be 500mA

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 02:58:16 pm
content: QuoteThat's something very different from what you wrote earlier stating TVS's are only applicable to ESD. A TVS able to survive a 150A surge pulses can be ok for some applications because the diode might not be the only component dissipating some energy.The automotive industry uses other standards for surge testing.https://www.vishay.com/docs/88490/tvs.pdfhttps://www.tdk-electronics.tdk.com/download/185840/e2cd27e63b1ece1bf787906a9474904a/ctvs-protection.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: October 21, 2023, 03:42:14 pm
content: Quote from: temperance on October 21, 2023, 02:58:16 pmIf it cannot be used for a power supply input that needs surge protection, but it could be used for something else, and we are talking about protecting a power supply input, then how exactly is it different from what wrote?Have you looked at the devices that are mentioned in that vishay appnote? They are not regular TVS diodes, they clearly have some sort of MOV or other device integrated into them, and are called "TVS devices". I mean, yeah, if someone names their MOV a TVS diode, and then that makes me technically wrong, congratulations, here is one internet point for proving me wrong: 1

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 04:06:42 pm
content: Here is what you wrote earlier:QuoteI don't see how I offended you. I only pointed out that the term TVS doesn't automatically imply ESD protection only.QuoteAs stated earlier but not explicitly, a 150 A 8/20 µs TVS can be used on a supply line if other components on the power supply line dissipate or smear out some part of the energy, which is more often than not the case. One small is enough to reduce the peak current trough the TVS. Of course the TVS on it's own will just go up in smoke.QuoteI don't think there are MOV's in there because MOV's deteriorate on every surge event. Those are probably zeners with very descent heat slug at least the package looks like that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 05:57:31 pm
content: Quote from: tszaboo on October 21, 2023, 03:42:14 pmThey... they give part numbers?e.g.https://www.vishay.com/docs/88382/sm5s.pdfIt's a diode.They don'tsaywhat it's made of, but "passivated junction" doesn't make much sense outside of planar semiconductors, and given the voltages, andas it was the style at the time, it's certainly silicon based.At worst, it's a diode in parallel with something, but absolutely nothing else but an avalanche diode will give you a 39V Vpp at 93A while also breaking down at 28V (5mA).They give waveforms too, and clearly the peak voltages are in line with the datasheet.They mention "EPI PAR" (and non-) types, without defining what those mean. And in fact PAR seems to only appear in this article, its alternative forms, and the product line itself. I'm guessing it's "passivated anisotropic rectifier", but they don't actually show the abbreviation side-by-side. Shoddy notation.Now, there are plenty of boners to critique in the application note -- it is as notoriously bad as any other appnote -- but,the ways in which it is badare more interesting than simply stating it's bad. I mean, they're all bad, that's kinda the starting point.......Aha, this is probably the patent:https://patents.google.com/patent/US5399901They must mean "anisotropic" as in part of the fab process, i.e. using KOH etch to make a pyramidal depression. Which is done to expose the edge of a depletion region, forming a mesa, which is then passivated (presumably with SiO2, hence "glass passivated"). I assume because the mesa can have more even current density / freedom from defects, and perhaps guard rings and surface states weren't well enough understood at the time (but I mean, this was the 80s, HV MOS was already well under production; maybe as a largely(??) bipolar/rectifier company they didn't have much knowledge/tech on those topics, who knows?), and this was done to make cheap robust rectifiers with reasonable avalanche ratings, indeed including load dump ratings. Probably modern Vishay parts are similar, though they could just be your average modern planar epitaxy something or other.Oh, and epi vs. non would just be how the PN junction is formed, epitaxy or diffusion, evidently the latter having higher internal resistance or built-in potential, whatever the case is.Anyway, I don't see where you get an MOV feel from this, or why it should be anything other than what it says it is.Again, we can formally critique the appnote if we like, but this extreme conclusion doesn't seem supportable in any way.Quote from: temperance on October 21, 2023, 04:06:42 pmAgain,whatpower line?If in present context, then OP never specified load dump parameters (peak voltage, duration, impedance), so at best all we can do is say whether it could work at all (which, I think is safe to conclude not; I'd be shocked if the survived even 12V load dump). Whether any particular level, who knows.And, load dump is slow enough that reactive components do little if anything, at least at realistic values; that said, maybe someone wishes to put one of those automotive amplifier "boost" capacitors in there, a couple/fraction of a farad across the line, which would do the job, actually surprisingly well; but for production purposes, that'd be an odd pick at least. (But an interesting one to see in a real product, hah.)If you're still stuck on IEC 61000-4-5 surge and 150A "not existing" in that standard, then, perhaps; but, again, there are numerous applications of that standard where the peak current is significantly reduced, so it's perfectly reasonable that a small device can handle something like that.QuoteMOV manufacturers give curves for infinite lifetime. Only undersized MOVs, or excessive surge conditions, cause significant deterioration.Tim

author: Marco
author_rank: Super Contributor
timestamp: October 21, 2023, 06:30:37 pm
content: Sure avalanche diodes are not as robust as MOVs, but a load dump ain't lightning either. The standard allows a fair amount of resistance, Vishay shows them surviving ISO 7637 testing. Likely there will never be a load dump bad enough to destroy the TVS and if there are, you point to Vishay's application note and run.To me blocking makes a lot more sense than dumping though. I'd just try SEPIC with a high voltage MOSFET and an extra input diode, it's going to be a little inefficient ... but I doubt it's completely impractical. (Vin for the controller can have a fair amount of added series resistance, so that can be easily protected with a TVS diode.)

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2023, 07:19:09 pm
content: Quote from: Marco on October 21, 2023, 06:30:37 pmI see discussion of 8/20µs pulses which is not relevant to load dumps which are 100-400ms. I think the Vishay reference to ISO 7637 testing is for nominal 12/14V systems, not 24/28V. This app note is a good read:www.ti.com/lit/an/snva681a/snva681a.pdf

author: temperance
author_rank: Super Contributor
timestamp: October 21, 2023, 09:25:22 pm
content: QuoteIt seems I should have made that more clear.Repetitive surge ratings for MOV's infinite lifetime are quite a bit lower than the usual surge pulse capability of those devices listed in data sheets. Most manufacturers don't even state infinite lifetime curves for their MOV's. I do have an old Harris application handbook (paper version) with such data. The peak power handling of those devices is extreme. But the number of allowed pulses is very low at the extremes. (I understood from literature that you actually burning tunnels trough the device at the extremes)For a 20 mm MOV:2000 A / 20 µs: 10 pulses500 A/ 20 µs: 10K pulsesInfinite life: 25A / 20µsThe data assumes enough time to cool down between pulses.That's how I conclude that there can't be a MOV in those Vishay TVS diodes and why I think your facepalm was inappropriate.Quotethat's the point and problem in this discussion which has been skipped by the OP.I found a PDF version of the Harris book:http://www.bitsavers.org/components/harris/1994_Harris_Transient_Voltage_Suppression_Devices.pdfPage 67

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2023, 09:37:36 pm
content: Thanks for the clarification.The other way you can tell is the peak voltage is low. Anything sold as a MOV, with comparable ratings, has a Vpp in the 100, 150V range. Usable with a voltage limiter circuit, you can get some pretty extraordinary robustness that way; but not alone, with a regulator like in question here.Tim

author: Marco
author_rank: Super Contributor
timestamp: October 21, 2023, 09:42:23 pm
content: Quote from: bdunham7 on October 21, 2023, 07:19:09 pmhttps://www.vishay.com/docs/87003/vishayloaddumptvsseriesfor24vpowertrains.pdf'PS. that's for higher clamping voltage than desired here though.

author: tszaboo
author_rank: Super Contributor
timestamp: October 21, 2023, 10:13:16 pm
content: Quote from: temperance on October 21, 2023, 09:25:22 pmNo you shouldn't have, and that's my point. People who spend their entire life online will find a way to find any teeny-tiny "mistake" in your generally true comment, because then they can point it out and then you are wrong sir.Quote from: T3sl4co1l on October 21, 2023, 09:37:36 pmHaha, you see Tim is wrong here. You are wrong sir. There are MOVs with lower voltages.https://www.bourns.com/docs/product-datasheets/mov14d.pdfSo you are wrong and I'm right because I found a teeny-tiny part number that is not according to your broad generalization.

author: bdunham7
author_rank: Super Contributor
timestamp: October 21, 2023, 10:53:25 pm
content: Quote from: Marco on October 21, 2023, 09:42:23 pmOK, thanks for the additional detail. To make those work completely reliably, you'd need quite a bit of source impedance, which means series resistance in this case as we've both mentioned previously. IIRC the OP's requirements were for 3A of current, so that's a big, warm 5R ceramic resistor. Or perhaps many parts in parallel.

author: langwadt
author_rank: Super Contributor
timestamp: October 21, 2023, 11:18:53 pm
content: Quote from: bdunham7 on October 21, 2023, 10:53:25 pmbuck converter rated for 3A _output_ current, it probably makes 5V so input current less that 1A

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 22, 2023, 12:25:46 am
content: Quote from: tszaboo on October 21, 2023, 10:13:16 pmI know you like to be argumentative for the sake of it, but if you would notice the pulse current, and scroll down to the plots, you'll see the point. You'd probably choose a 390 for 24V automotive use, which has a peak voltage around 88V at 10A, 110V at 100A. Which isn't taking much of a bite out of a say 150V load dump, and doing basically nothing at all at lower currents.Actually the 820 isn't much worse, clamping ~160V at 100A, despite more than double the nominal rating. The catch is, low voltage MOVs have even poorer performance, they're actually formulated differently; so it's all the more reason to prefer TVS here. Note also the test current in the table, all the low ones are a measly 10A.Voltages on the lowest rated parts are a bit better than I hinted, but leakage is pretty high as well, and the curve isn't any sharper, making TVS all the more attractive; you really don't use MOVs down here except in special cases.Tim

author: temperance
author_rank: Super Contributor
timestamp: October 22, 2023, 12:30:23 am
content: QuoteSorry to inform you but Tim is talking about a MOV's Vpp clamping voltages at comparable current ratings which are much higher than those of TVS diodes.However, I wholeheartedly apologize if what I wrote earlier made you nervous and revert to belittling.

author: Marco
author_rank: Super Contributor
timestamp: October 22, 2023, 12:58:55 am
content: I have never done it, but is it really not possible to make a reasonably efficient SEPIC converter with say LM3488 and say TPH5200 MOSFET and a 250V film capacitor for Cs?It won't care about load dump, it won't care about cold start (buck boost) and with a Schottky on the battery input reverse polarity is handled too.