Mains Transient protection circuit looks very bad.....should we bin it? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 12:45:40 am
content: Hello,Our contractor has put forward a Mains Transient protection circuit (for our power factor corrected 40W offline LED driver) which looks so disastrous that please may I put it here for judgement? (it is as attached).The idea is that any transient gets shunted by both the TVS and the controlled NFET , M3. The Current clamp based around M1 ends up taking some of the transient voltage, so that the LED driver and LEDs don’t get exposed to the transient overvoltage spike.The problem I see is that the resistive divider which connects to the ZR431 has extremely high value resistors, and so noise tripping of the ZR431 is likely.Also, the NFET M3 has a 100k resistor connected Gate-Source. This is very high value, and so the NFET M3 may noise trip, and thus turn ON when it shouldn’t and disastrously shunt the high voltage DC Bus and get blown up.What do you think? Should we kick this circuit out? In theory it seems fine, but in practice…ummmmm.ZR431 datasheet:https://www.diodes.com/assets/Datasheets/ZR431.pdf(This PCB has very little room for components, and so we can’t fit more conventional Mains Transient protection circuitry on it. –This is why the attached circuit has been put forward.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2018, 12:53:28 am
content: Ooh, I didn't realize there were 431 variants with lower bias current (ZR431 needing max 50uA versus the the original version needing 1mA; even TLV431 needs 70uA but doesn't handle much voltage).Afraid to say, your contractor isn't doing any better work than you guys have already done...Tim

author: wraper
author_rank: Unknown
timestamp: January 07, 2018, 01:00:04 am
content: This circuit is made to blow up if it ever has a chance to act. 1A mosfet supposed to stop basically unlimited current mains can supply. Also transient protection after diodes. To blow them up on impact, together with the rest of the circuit? Why not do it a standard way (MOV)?EDIT: my judgement is that contractor shall be quartered.

author: wraper
author_rank: Unknown
timestamp: January 07, 2018, 01:11:57 am
content: Just compare with the peak current usual MOV is supposed to survive:http://www.farnell.com/datasheets/2243464.pdf?_ga=2.12051825.412457660.1515153068-1354045019.1503601835

author: tszaboo
author_rank: Super Contributor
timestamp: January 07, 2018, 01:17:44 am
content: No varistorNo fuseThe rest doesn't even matter after this.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 01:51:26 am
content: Sorry...damned me...i forgot to put the 3n3 cap across the 10V zener in the schem...now included as attached...sorry

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 02:01:24 am
content: QuoteThe current clamp M1 is supposed to limit the current to 700mA…….so the NFET M3 should never be able to pass more than that, so it should be OK?Quote..Sorry there is a littelfuse LSP05 MOV module at the AC mains input, then a fuse just before the bridge rectifier (AC side). However, the MOV module “lets through” up to 900V, so we needed the extra bit of transient protection circuitry that you see here….(so as to hopefully limit the voltage across the LED driver to <450V)QuoteThe problem is that the TVS voltage climbs up above 450V as its current goes up…..and we want the voltage across the LED driver to always stay below 450V, so we put the M3 based shunt there aswell.I hope we're not1.5SMC400A TVS datasheethttp://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_1_5smc_datasheet.pdf.pdf

author: CopperCone
author_rank: Super Contributor
timestamp: January 07, 2018, 02:08:51 am
content: wtf is the point of all that shit. You want a fuse first of all, then a MOV, and you probably want a thermal fuse on the MOV so it does not catch on fire.The fuse should be the first thing in the circuit. All the other protection components and related PCB trace is at risk of fire.Can you give more details on the application?Thats the basics. Then a GDT if you got cash. Semiconductors wont sink shit if there is no fuse to blow.I think I would put the money in putting a bunch of MOV in parallel, like you see in a surge protector. Then you can afford to clamp at a lower voltage for longer.A PCB sparkgap might help too. Always an option

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 02:24:02 am
content: Thanks yes, there is a littlefuse LSP05 MOV module at the mains input, but it "lets through" up to 900V, so we need the circuitry shown. We also have a fuse on the AC side of the rectifier, after the MOV.We actually hit the circuit with the attached Mains Transient spike generator in order to test it, and it survived.(We put an isolated 10V supply across the 3n3 capacitor first, because that capacitor would be charged up to 10V in normal running).The spike generator turns the back-to-back IGBTs on at mains peak, and thus a voltage spike goes into the circuit due to the LC circuit...By "survived", I mean the LED driver didn’t die and the M3 and M1 FETs did not die. The MOV module was not used for this spike generator test. Admittedly the spike only went up to some 600V. We found that the voltage across the LED driver stayed below 450V……….but when we removed the TVS, it does not always stay below 450V for some reason. In other words, the M3 obviously doesnt do a great job of clamping the rail to 413V.However, I do share your concerns.

author: wraper
author_rank: Unknown
timestamp: January 07, 2018, 02:25:08 am
content: Quote from: treez on January 07, 2018, 02:01:24 amBy the time when M1 shuts off, M3 will already blow up. These mosfets are not supposed to work in linear range. If they do not remain fully open or closed, their SOA will be exceeded very fast. Also this "transient protection circuit" is not protected from transients itself.

author: wraper
author_rank: Unknown
timestamp: January 07, 2018, 02:38:40 am
content: Quote from: treez on January 07, 2018, 02:24:02 amFirst of all, most likely you don't have proper setup to measure actual spikes and may see insane values which actually are not there. Secondly this is a joke, not a spike generator.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2018, 02:47:49 am
content: I tried this years ago with a zener based transistor shunt regulator, which is much simpler than your circuit, as a substitute for a high power TVS or zener. I will not say that it cannot work when combined with a current limit as you have done but I am not convinced that it is the best way. Incidently, Tektronix implemented a MOSFET based line side current limiter on their early 2213/2215 oscilloscopes which might be worth checking out.Better in my opinion is to implement foldback current limiting with a high voltage cascode transistor or several after higher voltage shunt protection.

author: hermit
author_rank: Frequent Contributor
timestamp: January 07, 2018, 04:00:35 am
content: Quote from: treez on January 07, 2018, 12:45:40 amQuote from: treez on January 07, 2018, 01:51:26 amIs this like, "Doc, I have a 'friend' with this problem....."?

author: Unknown
author_rank: Unknown
timestamp: January 07, 2018, 06:08:33 am
content: I'm only a hobbyist, but it does appear that the Littlefuse website has several application notes regarding transient protection. Look inHome> Technical Resources> Datasheets and Downloads, select the application notes check box, and search for MOV.http://www.littelfuse.com/technical-resources/datasheets-and-downloads.aspx#q=mov&f:@ftechnicalresourcecategory30330=%5BApplication%20Notes%5D-Steve

author: Unknown
author_rank: Unknown
timestamp: January 07, 2018, 06:17:00 am
content: Also found this under the "Application Guides" on the littlefuse site: (Led Design Guide)http://www.littelfuse.com/~/media/electronics/design_guides/led_protectors/littelfuse_led_lighting_design_guide.pdf.pdf-Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2018, 06:58:58 am
content: Quote from: treez on January 07, 2018, 02:24:02 amYes, your toy; easily defeated with a small capacitor or TVS.The real world is much harsher than this.Tim

author: jbb
author_rank: Super Contributor
timestamp: January 07, 2018, 07:13:37 am
content: Add me to the ‘fuse and MOV camp.’ Especially if you want to pass any safety standards. The MOV should provide enough clamping to protect M1 from overvoltage (if M1 suitably chosen).I would also say that the sheer number of components is a bit worrying - in terms of 1) response time, 2) reliability and 3) “all those bits cost money, and they don’t do anything, so let’s leave them out.”I think David Hess is right that some kind of cascode trick with a small TVS to fire a power MOS might be better.Or maybe you could add another NPN next to Q1 (I.e. M1 gate pull down), and drive its base with a TVS diode to set the trigger level. When a spike comes, the MOV takes the top off, and M1 is rapidly gated off. No need for a second divider chain, DC rail, xx431 etc.It often turns out that the main DC link cap is important for absorbing the last of a surge.

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2018, 07:16:02 am
content: nay a capacitor to be found in that design. Bin it.

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 01:44:26 pm
content: Well the current clamp is not automatically a horrible idea, with the proviso that whatever device you use there must be able to deal with whatever voltage the MOV lets thru (And I would have the current clamp be a hard switch for over voltage not a current limiter, but whatever).But the rest of the thing?40W led array? So maybe something like 200mA nominal running current?First thing, series resistor, 10 ohms or so, surge rated for high voltage, before the MOV, your problem just got much less nasty (And the MOV will now last better). Probably be something like a classic 2W carbon composition jobbie, they have really good surge behaviour (About the only nice thing you can say about carbon composition resistors!), Pd will be ~0.5W nominal.Now, that current clamp, add a transistor (or better schmitt trigger (2 transistors)) to yank the gate down if the voltage across the bridge rectifier becomes excessive, add some gate series resistance and a cap to make it take a few ms to recover.I would still want some bus capacitance, just to give the thing a reasonable chance of reacting sufficiently quickly. Measure the reaction time then do some sums with the series resistor and whatever the MOV lets thru to see how much cap you need to hold the bus down until the mosfet cuts off. Do it right and use an appropriate bridge and you even get protection from some numb nuts installer wiring the thing across two phases...I might also consider that inherent in that surge stopper being where it is, is that the LED array will, when bolted to the chassis form a capacitive shunt across the thing, probably worth some measurement and modelling both for conducted interference and what happens to a tall pole when lightning happens nearby.Finally, test the thing, with proper gear for testing EFT and surge, not that joke you have lashed up, your local test house probably has the kit.I am tempted to put him out of our misery and just design a sodding street light as an open hardware project....Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 02:55:51 pm
content: QuoteThanks, sorry, the attached is the more complete version of the circuit which shows the 22n DC Bus capacitor thats there aswell. It also shows the MOV.QuoteThanks i couldnt agree more, the problem for us is that we are restricted to this small PCB, and the PCB space needed for the overvoltage switch-out circuit is too much...the current clamp just needs the SOT323 BJT and DPAK FET.QuoteThanks, its a nice idea, though if it blows open, then the whole product doesnt work any more.Sorry to sound lousy, but our investors will say this. Also, theyll moan about the cost of the resistor, and the fact theres no room on the all-surface-mount PCB for it.....and that it will have to be mounted in the incoming live wire, -more assembly expense, etc etcQuoteThanks, we have plenty of better designs "on paper", but we cannot get component price-downs that the big manufacturers do, so we are priced out of the market...instead we have to go for the very cheap end of the market. And much of our profit comes from direct-from-China stuff. There isnt much money to develop products ourselves because the company doesnt really need its own product base, since the Chinese stuff is our main support. If we are to get money to re-do this circuit, then we need absolute proof to our investors that its a total no-goer.The problem is, that many units have been out in the field in pub gardens for 2-3 years and not failed, so the investors dont think its a total sham like the posts here suggest.QuoteThanks, the problem is that there's absolutely no more room on the PCB for those mods which would need a bit more PCB space which we just don't have.The investors dont want to sanction a new bigger PCB unless we can absolutely proove the shown circuit is a total sack of spuds.If we mod it, then the mod will have to be very small.************ Moisture on the 8MEG2 resistors *************Thanks all, Do you think that the 8MEG2 resistors could have their resistance affected by atmospheric moisture settling on them? If so, then this could result in the circuit destroying itself….The lamp enclosure is actually open to the elements, -it is not *totally* sealed. Well, there are gaskets around most of the mechanical interfaces…however, the wire entrance hole for live and neutral to get to the PCB, comes up through a hole in the heatsink on which the PCB rests……this hole is not sealed as it would be expensive to seal it. So moist external air can first come in through the gap between the lamp and the vertical mounting pole which is dug into the ground….this cannot be totally sealed since the pole belongs to the customer and is obvioulsy a separate part to the lamp itself.So what I am saying is, moist air can make its insidious way in to and around the PCB, and could settle on the 8.2Meg resistors, and thereby reduce their resistance, and thereby make the ZR431 false trip…..which would mean M3 shunting the high voltage DC rail when it shouldn’t, and thereby blowing up M3 and indeed R4. The PCB hangs upside down, so water can't easily collect over the PCB, but a film of moisture from the atmosphere could still settle on the 8MEG2 resistors and reduce their resistance?*****************************

author: CopperCone
author_rank: Super Contributor
timestamp: January 07, 2018, 04:24:40 pm
content: this is worse then apollo 13, you can't do anything because of investors. it passed the safety standards needed to get it into beer halls without a fuse?is using an adhesive caulk gun really that expensive? You gotta think about insects too, they like warmth.

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 04:46:54 pm
content: If you cannot compete on BOM then the low end is the one place you should not be trying to play!Competing on selling price when you do not have the volume to get the COG down is insane, no wonder you guys are skint.Meanwell do an entirely usable 30W flyback power supply as a little potted board mount thing for less then a tenner, and IME the things are absolutely reliable, I have a few hundred in the field no failures. They do this by doing **massive** volumes, leaning on suppliers, scheduling deliveries a year out, and probably squeezing the margins til they scream, if they do a few million units a year at less then 50% margin that is still reasonable profit, You cannot afford to be doing less then 50% margins on low end because you are not making the volumes.If you instead play in the high end then the conversation becomes one about TCO and unique selling points, not unit cost, and the BOM really ceases to be a major consideration. I have never once worried about the cost of a resistor, never mind such things getting past the department head and the board to become an INVESTOR concern!You can get a dual NPN in a SC-70/SOT323, I use them routinely, cheap as anything, and with two NPNs you can trivially form a schmitt trigger to yank that gate down.If a 10 Ohm 2 or 3W fusible resistor blows open on a 200mA rail then something else in the circuit has already shit the bed big time, the resistor will not have been the weakest link. You can get such things in ZL lead form that pick and place on an SMT board just fine, so no issues there, and it might be worth seeing if such a part could substitute for the fuse?And yes, condensation can be an issue, particularly in seaside towns where it is more that salt spray gets into **everything**, the stuff is near impossible to really keep out. Conformal coat (Or better, full up potting) and more reasonable values...regards, Dan.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 07, 2018, 04:56:18 pm
content: Quote from: treez on January 07, 2018, 02:55:51 pmYes, that's a risk indeed. Conformal coating might help you here.Among what the others said here, shunting the transient (effectively trying to reflect the energy) is a bad idea using the components you did. The current limiter MOSFET will easily be driven to destruction by either a non-standard conforming transient (I've never seen a real surge / transient that conforms to the standards that you apply while testing and designing the circuit, I believe it's because they don't bother to read the standards and behave what they like to) or some other malfunction like the condensation issue.If the max voltage level of the incoming transient is limited by a upstream protection devices, it would be a way much better way to open your circuit using a high enough voltage rated MOSFET that is normally on. Just turn it off while the input voltage is too high. That's a common way for semiconductor based transient / surge protection devices.Any kind of protection device for a harsh environment is a multi-stage device. Consisting of a coarse high energy absorbing stage (e.g. spark gaps), a medium energy stage at a lower voltage level (e.g. MOV or TVS), and if you need fine protection, a stage like yours but designed to go open circuit in case of overvoltage.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 05:27:03 pm
content: QuoteThanks, in the lab, the M3 NFET did successfully clamp the rail to an extent, as the attached scope shot shows. –Though this was from a slow-rising transient from the ringing of 1mH and 1uF when switched at mains peak (with the 3n3 capacitor pre-charged to 10v).Also, the ltspice simulation attached shows the circuit working as intended. The ZR431 .asy file is also attached in case anyone wants to run the simulation

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 05:32:49 pm
content: QuoteWe are playing down at the low end because the big manufacturers are not playing down here…so we don’t have that much competition down at this cheap end……….we only have competition from China, but after the Trade Tariff and shipping costs of these somewhat heavy outdoor lights, the Chinese can’t compete with us…….plus a Chinese company can’t really give a 10 year warrantee because nobody can hold a Chinese company to that from Europe……you can’t sell these lights without giving a 10 year warrantee, and guarantee to replace any failed units……..you just can’t do that from China to Europe.QuoteThanks, but i bet it wouldnt fit in our solution size......ours is very low profile because we dont use transformers etc and everything is surface mount. (apart from the LSP05 MOV module)Also, i doubt that Flyback is dimmable. Also, we need 40W.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 05:45:17 pm
content: QuoteThanks, due to our circuit situation, I don’t believe we can do that. Our LED driver is basically several successively switched linear regulators, and we need to limit the current through the leds when a transient comes, so we do really need the current clamp.I mean, we could put the turn-OFF fet in there aswell as the current clamp, but there simply is not room on the current PCB size.......so we have to have the current clamp and just make do.

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 05:48:44 pm
content: I was not suggesting you USE the thing, I was giving it as an example of why you should not play in the low end, because down there volume and buying power really matters.Seems to me that you thought you could play in the low end at an acceptable margin and are now finding that meeting both COG and product reliability targets is way harder then you thought.The way to look at warranties on things like garden lights is to realise that your retail chain can just refund the sale as long as the failure rate is low enough, which takes the fact that the OEM has vanished after 5 years rather out of the picture (Not that many will still be able to find the purchase paperwork for something like that after 5 years anyway, I know I never can).

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 05:51:44 pm
content: Quote from: treez on January 07, 2018, 05:45:17 pmJust switch off the current clamp mosfet completely when the voltage gets too high, not hard.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 06:59:30 pm
content: QuoteThanks, however, that can’t exactly be applied to our particular unusual type of offline LED driver….we use PFC’d successively switched linear regulators, each with their own LED string……when the mains is at the low part of the cycle, only the first led bank is switched in, …..at this point a serious overvoltage would be a DC bus voltage of > 70V……as the mains cycle goes higher, the value of an “overvoltage” gets higher, up to the mains peak, where the mains would need to peak above 400V to represent an “overvoltage”.I understand this is a very unusual way we are doing it, The chips we use are little known about.But yes, there is simply not an single “overvoltage level” that we can use as the “overvoltage threshold”….it depends on which particular linear regulator is on at the time, and what part of the mains cycle it is at. This is why we absolutely need the current clamp and cannot use an overvoltage triggered FET switch off.....Uuuurrr, but Having said that, you are right in that an overvoltage switch off, set at say 413V or so, would be better than not having one. And it would have the advantage of protecting the Vdd of the LED driver which is 450V max. So i will look into it, but will struggle to find the room for it.QuoteIf im honest, ive no idea if our own built product is profitable or not.......maybe we just survive because of the direct-from-china stuff which comes straight from china to the customer, middle-manned through us.

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 07:19:04 pm
content: So just have the bus exceeding 400V or so cause a transistor to pull the current clamps gate voltage, job done.Then you can probably remove that unreliable shunt thing and use the space saved for an input usable resistor.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 07:37:22 pm
content: Thanks very much DMills. Much appreciated.The attached shows what i believe you meant. There will be space for this.(schem in pdf and ltspice attached, plus ZR431 .asy file if needed.)

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 10:06:21 pm
content: Why not just replace Q1 with the ZR431 directly, then you can eliminate R1,R2,R25,R24,R28,D20,C2, Q4 & Q1?You have ~200uA down the R20 chain, and the ZR431 is good down to 50uA so that should work?I would be a little concerned about the open loop gain, particularly in the 8us rise time case, it is rather unimpressive by the time you hit ~100kHz, could possibly improve that with a small cap across R16 or something? I cannot be bothered to work the numbers now. Possibly put Q1 back in but make it PNP and do the obvious. If going there I comment to your attentions such parts as the BC847BVN and related things.I still say you want that 10 ohm fusable resistor in there ahead of the MOV well worth the cost.Who do I make the invoice out to?Regards, Dan.

author: CopperCone
author_rank: Super Contributor
timestamp: January 07, 2018, 11:01:12 pm
content: i think the 'board of investors' is going to send a hit man after you if you mail anyone an invoice from the sound of things

author: dmills
author_rank: Super Contributor
timestamp: January 07, 2018, 11:13:22 pm
content: Seems likely.Well that is around half the parts from that input section gone, let me sleep on it and I will try to get us down the the fuse, MOV, Class X and pair of class Y plus CM choke that we all know is what the universe intended for a reliable mains input section.Weird company setup, having investor types sticking fingers in Engineerings turf.Regards, Dan.

author: CopperCone
author_rank: Super Contributor
timestamp: January 07, 2018, 11:34:34 pm
content: yes, most people don't realize that transients of high frequency are attenuated by filter components, I see them as 'taking the edge off' for further components down stream.Protecting capacitors with MOV or protecting MOV with capacitors though, which is better? I would at least put it behind the magnetic though, but the fuse in front of everything. I guess capacitors don't degrade like a MOV does?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2018, 12:24:57 am
content: Mind that filters can saturate, depending on materials used.CMCs saturate in common mode, but diff mode is air cored, so that helps.Y caps are Y5P ceramic and saturate, but at fairly high voltages (low kV).If both of these combine, you can end up with significantly higher peak common mode voltage than you were expecting. This can be a good reason to add spark gaps or GDTs around the CMC, or to ground, as is sometimes seen.X type film caps don't saturate, but do self-heal. Which actually helps, since it's a current sink acting in parallel with the MOV -- yay, at least until the cap is self-healed in half, which does happen over time!There are a number of reasons the IEC 61000-4-5 surge shape was chosen. The mains network itself has a filtering effect, due to stray inductances (power lines are typically a higher characteristic impedance than the power they transfer, thus appear inductive), filter capacitances (typically for PFC), lightning arrestors (stacks of MOVs), transformers (a ball of wire, having parasitic capacitance and leakage inductance), and other loads (what with SMPSs being the norm, it's not uncommon for a few thousand uF to be clamping a household's mains line under transient conditions). Lightning itself is not terrifically fast; it may be a spark discharge, but due to its length, the 100kA+ is delivered at a fairly relaxed rate (many microseconds).Overall, especially to differential mode -- a typical mains input DC rectifier circuit has good filtering effect, which greatly helps the MOV. The increased source impedance (mainly due to the CMC's LL and DCR) absorbs a lot of surge voltage, when the MOV is placed after it.Tim

author: ocset
author_rank: Super Contributor
timestamp: January 08, 2018, 06:02:36 am
content: Thanks yes i should be paying up for this.I've just realised what will be said about this.The original use of a DPAK NFET (M3) to shunt the HV DC Bus was said (not in this thread) to be beneficial because it conducts the transient and takes the energy out of it, whereas now, since the rail is just being switched off when the transient comes, ...well this means that we are no longer de-energising the transient, and therefore it will not be de-energised, and therefore it will likely overvoltage the current clamp FET which gets switched off.For this reason, i know they are not going to like me getting rid of the shunt NFET M3.The space for the current clamp FET means it has to be DPAK, and the highest voltage we can find in low enough RDSon is 900V....as such, i think we will need the shunt FET (M3) back in to de-energise the transient...Yes, the let-thru voltage of the LSP05 MOV is 1100v, and so we need to de-energise this let-thru.LSP05 MOVhttp://m.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_lsp05_datasheet.pdf.pdf(...I confess that some of the units dont even have the MOV fitted...to save cost.)QuoteThanks, do you think a power BJT with a zener fed base would make a better active-TVS than the M3 based circuit of the first post?..ie would it be more robust because of using a BJT?

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2018, 09:03:51 pm
content: Quote from: treez on January 08, 2018, 06:02:36 amIt amounted to the same thing but was simpler and it could be implemented with a power MOSFET just as easily. Using a 431 shunt regulator strikes me as unnecessarily complicated.The only virtue to the circuit was that we had bipolar power transistors which were much larger than available zener diodes. But failure was still possible if the power transistor's power ratings were exceeded and with a bipolar part, secondary breakdown becomes a major limitation. The only reason it worked was because we could throw a large area of silicon at the problem.

author: hermit
author_rank: Frequent Contributor
timestamp: January 08, 2018, 10:42:57 pm
content: I can understand the reluctance to share a proprietary design but has the OP ever reasonably demonstrated they are even chasing the right problem? I know this stuff is spread over two or three (or more?) threads so the information gets needlessly diffuse. Seriously though how propriety can an LED driver on the low end market be? I think the OP could get some excellent help here if they were more open.

author: ocset
author_rank: Super Contributor
timestamp: January 09, 2018, 06:01:44 am
content: QuoteThanks yes i appreciate that.IGBTs are known for their robustness, -certainly more so than mosfets, and i wonder if an IGBT with a zener fed gate woudl be the most robust way to make an active TVS.I also remember that MOSFETs, whilst not suffering from secondary breakdown, do have paraisitic BJTs etc which suffer things similar to secondary breakdown.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2018, 06:45:10 am
content: Quote from: treez on January 09, 2018, 06:01:44 amIf by "more" you mean "more important to protect". IGBTs have, uh, ballpark 5 times or so higher current density, and have never been optimized for linear operation as far as I know.Tim

author: dmills
author_rank: Super Contributor
timestamp: January 09, 2018, 11:26:53 am
content: Also a 10kHz IGBT is considered to be really fast.Any sort of active thing is really going to be fighting against the miller effect, so you will need stiff gate drive.I still say a few ohms or so of series R is what you want in there, and that that shunt clamp cannot do much for the energy in a transient because it is in series with a current clamp which limits the shunts ability to reduce the line impedance seen by a transient (In fact once the current clamp actually clamps the line impedance becomes very high which makes the over voltage actually worse at the input to the circuit).Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: January 09, 2018, 09:19:38 pm
content: Thanks,We are wondering of the attached Mains Transient absorber would be effective?(its to go ahead of the product, in the mains line)Its just basically a 1500V, 1.8uF film capacitor in the Mains line leading up to the product.185PSB152K6R film capacitor, 1500V, 1.8uFhttp://www.illinoiscapacitor.com/pdf/seriesDocuments/PSB_RSB%20series.pdfWe were thinking that if we used this, then it gives some absorbtion of a transient, and then we can do DMills kindly given idea of switching off the current clamp FET when the voltage goes above aprox 430V.

author: splin
author_rank: Frequent Contributor
timestamp: January 09, 2018, 11:57:14 pm
content: If you don't have enough space for a discrete resistor, how about a PCB trace resistor? 5 ohms would require a .127mm (5mil) trace on 1oz copper and 18mm x 18mm board area and should be good for 500mA. Not sure what it would do for EMI though.You could make it into an 13uH inductor to slow things down a little too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2018, 01:53:56 am
content: Surge is tested with a 2 ohm source impedance. What's the 4.7 ohm resistor for? It will have a small effect, but between the series resistance and the small capacitor, it will be maybe 10%, an insignificant reduction.Consider that the surge tester (the real thing, not your toy circuit) uses 20uF charged to 1.5kV or more. To get that under 800V, you need more like 50uF or more.Commercial surge protectors of this sort use electrolytic capacitors around 470uF, and a large series inductor (100uH or so, air core).Tim