Effect of pulse time on electrolytic capacitor overvoltage damage? - Page 1
Forum section: electronics / projects

author: katzohki
author_rank: Frequent Contributor
timestamp: February 23, 2022, 05:59:40 pm
content: Hi all, trying to find some information or perhaps a scholarly paper discussing this. I'd like to understand what effect the pulse length of overvoltage on an electrolytic capacitor will have.I think it's predictable to say that 600V on a 550V rated cap for 100ms is probably worse than 100us, but I'm trying to find some resources that expound onwhyandhowthis is the case.Much thanks if you can provide any insight on this!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2022, 06:14:46 pm
content: Heck, depending on value, 100us might not even reach the dielectric as such.Afraid I have no such data, though. I would also be interested to know.Note that electrolytics generally fail by excessive temperature or venting, both of which are relevant in overvoltage conditions. If the overvoltage occurs slowly enough (hours?), the (hydrogen) gas generated can diffuse through the seal, while the temperature remains modest. After such reforming, the capacitor may well serve a normal working life, even under continuous operation at the elevated voltage. (But, excess voltage IS a lifetime factor, so it should be that this is a relatively rare occurrence, with only "lucky" individuals surviving such long lives.)I at least wouldn't think a <10% surge for such short times is very important.BTW, note that such high voltage ratings are only achieved by series connection (well, maybe 550 is still single, I'm not sure, but 630+ are two cells internally AFAIK?), so anything that's true of say 300V caps will be proportionally true for those.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2022, 06:32:30 pm
content: Quote from: katzohki on February 23, 2022, 05:59:40 pmI've no citations, but just some basic info. Electrolytic capacitors used to have 'working' and 'surge' voltage ratings, the surge being 10-15% higher than the working. The time of a 'surge' was more like your 100ms, something that it could withstand for a short while without excessive current leakage, temperature rise or outgassing. HV transients, on the other hand, will cause a temporary breakdown of the barrier and substantially increased leakage current, effectively limiting and absorbing the transient. This apparently causes little or no long term damage to liquid electrolytics and they will heal as soon as the HV transient is over, as long as total energy involved doesn't cause physical damage.

author: katzohki
author_rank: Frequent Contributor
timestamp: February 23, 2022, 06:39:21 pm
content: It does happen to be a 550V rated cap we're looking at in this case. Can't comment on the internal structure since the datasheet doesn't illustrate anything about the internal structureQuote from: bdunham7 on February 23, 2022, 06:32:30 pmI did see something about this self-healing effect (still reading up on what I could find this morning), but that sounds like it must be the kind of thing that would have to be causing damage if it's repeated a lot over the lifetime of the cap.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2022, 07:13:19 pm
content: Quote from: katzohki on February 23, 2022, 06:39:21 pmI don't know about the long term effects of repeated HV low energy transients. I don't think it works like a self-healing film cap where each event results in a film penetration and then the loss of a small area of the capacitor. But a related concern as to what you are doing if it isn't just academic--running HV electrolytics close to the edge of their limits will drastically shorten their lifespan and I have seen examples where this has resulted in a product with a propensity to explode quite spectacularly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2022, 07:14:39 pm
content: Actual dielectric breakdown of an electrolytic layer is... weird? I'm not sure quite how it works, if you can actually apply enough electric field to pierce it as such, or if that's an adequate description of the "self healing" process (with consequent electrolytic oxidation, hence healing), or if it's a more continuous process (like electroplating is), just, behind enough of a barrier that the voltage keeps going up. (The difference I suppose being, the nature of the current flow through the barrier? If it's a billion zaps, there should be substantial shot noise due to that process; if it's more like a tunneling current, or ionic current, the noise level should be closer to ideal shot noise.)(I do know, dry tantalum capacitors self-heal in this fashion: microcracks in the oxide layer pass current, causing local heating; the momentary short circuit partially discharges the capacitance into the site. The heat decomposes the MnO2 oxide, simultaneously oxidizing any exposed Ta and leaving a nonconductive Mn2O3 scab over the site. Downside: if this happens en masse, the whole fuckin' thing ignites as a thermite reaction (Ta + MnO2 --> Ta2O5 + Mn (unbal)). Which is a big hazard if the circuit can supply additional fault current into that short -- hence why tantalums are discouraged for power supply bypass applications. Also, because the process generates considerable noise (momentary shorts), it can be bad for high-reliability systems. Supposedly, some telecom equipment was affected in this way (random crashes), which they eventually solved by going to ceramic and electrolytic types.)In any case, it's just an extension of how the dielectric was originally prepared -- aluminum foil is textured and etched (increases surface area), then anodized to rated voltage. What you get in the capacitor, is basically hard-anodize foil, interleaved with a cathode, and soaked paper spacers. (And, probably they reform it again after rolling? Surely the anodize isn't flexible enough to wind up and ship? Dunno.) The electrolyte is not very different from what it was anodized in, in the first place; it's not as corrosive, but still works the same in operation.The oxide may degrade over time, as well; typically over several decades, in storage (unpowered), effectively the voltage rating falls. Such caps can be reconditioned by reforming (charging slowly, allowing the anodization reaction to proceed gradually). Put another way: the leakage current is massively increased at relatively low voltages; but the leakage goes away as charge is delivered, until eventually the rating is reached (or indeed exceeded), and then low-leakage operation is had while the part remains in service. (Respectively: if it's been in service the whole time, it will have "deformed" only to the nominal operating voltage, drawing low to modest (nA~uA?) leakage.)There's something about anodizing, that it doesn't work, or at least very well, beyond 450V or so; I don't know if there are process refinements / formulations that allow it to be pushed higher, or if there's only so much that can be done, whatever. For sure, there are fewer parts above 450V (granted, less need for such as well!), and I think none above 550 without being multi-section (and, yeah, I don't know that they'll tell you, either).Interesting; I realize I either haven't read about several of these topics in a long time, or perhaps at all (i.e. word of mouth only). (The tantalum thing at least can be readily found from KEMET, Vishay, etc.) All the more reason, sources would be welcome.Tim

author: katzohki
author_rank: Frequent Contributor
timestamp: February 23, 2022, 07:40:06 pm
content: Some more background, then: Yes, we are running these HV capacitors close to their limits, or actually ourvendoris. And this information I'm gathering is in an effort to discuss this as an issue with them.Besides that, they're switching pairs between parallel and series. One mode runs 1000V across a pair in series of these caps which are rated 550V each. I already pointed out that the variation in ESR could cause an imbalance of how the voltage is divided between them. And then there's heating, this thing is really packed and gets pretty hot...So yeah it's not just academic, but in an effort to say "hey guys we think this is seriously an issue here..." and we have had several blow already, which is what kicked off my investigation in the first place.Interesting stuff on the tantalums and other info as well. Here's a couple papers I've found so far that have some good information on the topic:https://www.nrel.gov/docs/fy19osti/71386.pdfhttps://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.693.1444&rep=rep1&type=pdf

author: uer166
author_rank: Super Contributor
timestamp: February 23, 2022, 08:19:41 pm
content: Do electrolytics have a wear-out mechanism caused by voltage assuming they run below the rating? I thought they did not. If they're paralleling 2 550V caps to make a 1000V one, without any ballast/balance resistors, then most likely you'll have some overvoltage on one of the caps, but it should be fairly self-limiting since the high-cap will have increased leakage, which will sort of self-correct the voltage. I would never put 2 caps in series without some ballast resistance though, seems like an oversight.Are you sure they're not just cooking the caps? If they're packed and run hot, you'd do well checking their temps in operation, that alone could explain why you've had several of them blow.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2022, 08:24:04 pm
content: Quote from: katzohki on February 23, 2022, 07:40:06 pmIs it a resonant converter? High ambient temp, high ripple current, running close to max voltage all lead to short lives. Running 2X in series with only a 10% margin sounds 'specially 'xplody. Balancing by leakage!

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2022, 08:28:45 pm
content: Quote from: uer166 on February 23, 2022, 08:19:41 pmI don't know why, but I've seen it stated by presumably reliable sources (that I can't recall at the moment) that the operating voltage has little effect on lifespan for capacitors rated <100V, but for higher voltage electrolytics there is a pronounced difference based on the applied voltage as well as temperature.

author: uer166
author_rank: Super Contributor
timestamp: February 23, 2022, 08:32:25 pm
content: Quote from: bdunham7 on February 23, 2022, 08:28:45 pmI've been re-broadcasting that as a fact and actually would be good to make sure that is correct. If you have any source that states that HV elcos' lifespan is limited due to only voltage, that would be interesting to see. What kind of mechanism would it even be? Some chemical reaction that eats away the electrodes, or maybe dries out the electrolyte?

author: inse
author_rank: Super Contributor
timestamp: February 23, 2022, 08:48:44 pm
content: Somehow, I have this piece of information in my mind:An electrolytic capacitor can be reformed by applying the desired working voltage with current limitation.So the aluminium oxide layer will grow accordingly whilst the capacity will shrink.Is this still correct? Although it is not correlated to surge voltage behaviour directly.

author: bdunham7
author_rank: Super Contributor
timestamp: February 23, 2022, 08:52:37 pm
content: Quote from: uer166 on February 23, 2022, 08:32:25 pmThere's no simple consensus AFAIK. I don't think I'd want to count on a life extension based on voltage derating to meet a design criteria unless it were spelled out in a datasheet. The electrolyte of <100V caps is different than the HV types, for one thing. If you look at this document, it shows statements purportedly from different manufacturers. Scroll down to Rubycon and they distinguish between miniature and large-can capacitors, with the latter having a strong lifespan correlation with voltage but the former none. Perhaps the differences simply reflect differing opinions or perhaps they are actual product differences.https://www.dfrsolutions.com/hubfs/Resources/services/Long-Term-Storage-of-Al-E-Capacitors.pdf

author: katzohki
author_rank: Frequent Contributor
timestamp: February 23, 2022, 09:11:43 pm
content: I wish it was Rubycon... that's another thing...Doesn't seem particularly tied to temperature exactly, since I've run the units at up to 80C all day long at full power without issues. But when we load drop the voltage spikes before the control loop can compensate / shut down and this is when capacitors are blowing. Or at least getting slammed with HV at / over their limit.Oh yeah, and the manufacturer told me none of their other customers are having this problemI believe one or both of the links I posted may have some of the details on the damage and repair mechanism you guys are talking about. I think I saw something, but coffee may not have soaked in yet. I'd pull a quote, but I got to get back to testing

author: uer166
author_rank: Super Contributor
timestamp: February 23, 2022, 09:32:20 pm
content: Quote from: katzohki on February 23, 2022, 09:11:43 pmAre you able to scope the voltage on those caps? If it's a load dump scenario with a massive overvoltage then that could certainly explain it. It's entirely possible the spikes are much higher than you think, and would damage stuff in a timely manner.. Is this some kind of crappily designed PFC?

author: uer166
author_rank: Super Contributor
timestamp: February 23, 2022, 09:45:43 pm
content: Quote from: bdunham7 on February 23, 2022, 08:52:37 pmIt is interesting that only Rubycon does that. Nichicon, Nippon Chemi-con, and Panasonic have no voltage terms in their lifetime calculations. According to the Rubycon equation, de-rating the voltage by 2X from rated would increase the lifetime by a factor of 5.6 (!). It seems like everyone has wildly different approach to this, and universal data is missing.

author: katzohki
author_rank: Frequent Contributor
timestamp: February 23, 2022, 11:58:54 pm
content: I am getting scope shots of the capacitor voltages right now and in fact yes we're seeing OV spikes on the caps. Not sure what you mean by PFC, but crappy design... yeah that's what I'm leaning towards.This are neither Rubycon, Nichicon nor Nippon-Chemicon. I bet you can guess what that means...

author: uer166
author_rank: Super Contributor
timestamp: February 24, 2022, 12:25:45 am
content: Quote from: katzohki on February 23, 2022, 11:58:54 pmIn my unprofessional opinion, you can totally use Lelon, Crapxon, relatively safely, if you use them according to their ratings. You might have lower MTBF or faster wear-out, but I think they have a bad rep because they're always associated with badly engineered product that push the rating beyond the max. If you use them within the prescribed ratings, good chance they'll work as well as any Tier-1 brand. Your case seems to be systematic failures due to a bad design which would happen on Nichicon caps just as easily.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2022, 12:42:42 am
content: Quote from: katzohki on February 23, 2022, 11:58:54 pmIf you can, you should scope the current through the capacitors. If those OV spikes correspond to a large surge in current, the capacitors are being severely abused.There are plenty of good brands other than the ones you listed, but it is also possible that your "550V" capacitor is no different than the usual 450V electrolytic, just with an optimistic rating attached. Do you have a make/model/part number/dimensions/photo? What is the device?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2022, 01:05:27 am
content: Again, applied voltage isn't necessarily indicative of internal voltage. An IEC 61000-4-5 mains surge into a power supply filter cap for example, drops most of the voltage across the ESR, for most values of capacitance. The surge generator itself has about 10uF inside it; over 100uF and you have significant leverage against such surges. ESR of a fraction of an ohm is enough to keep things safe (hence why many supplies got away without MOVs on the front end!).During such a surge, typically the applied voltage rises momentarily during the pulse -- due to the voltage drop across the ESR -- then returns to only slightly above nominal. Like an extra 20V versus the 100 or 200 extra during the pulse. The final level is indicative of what stress the actual dielectric experienced.Note that ESR is largely due to the electrolyte, so all this voltage is dropping across bulk goop, not the dielectric. Unless it's high enough to cause physical breakdown (like, it'll probably spark across at a kV or something), I don't see that this causes any actual problems for the capacitor. Just heating, if it's repetitive.For that matter, if the waveform happens to be nanoseconds, it's irrelevant, not even the capacitor itself, but voltage drop along the lead length itself -- or induced into the probes themselves. Until OP provides more data of the situation, we have no way to judge the scenario further.Tim

author: uer166
author_rank: Super Contributor
timestamp: February 24, 2022, 01:08:55 am
content: Quote from: T3sl4co1l on February 24, 2022, 01:05:27 amThat's true for surges, but in OP's case it seems like a badly compensated converter that overshoots when load is removed, which is a much longer time-frame event (probably well over >1ms). I suppose it's all guesswork until we see some traces from OP.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2022, 01:21:16 am
content: Quote from: T3sl4co1l on February 24, 2022, 01:05:27 amAn conversely, the observed voltage across the capacitor doesn't accurately indicate the energy of the spike, since the capacitor breakdown itself, if it occurs, will limit the voltage. A large disproportionate spike in current would tell you what has happened.

author: katzohki
author_rank: Frequent Contributor
timestamp: February 24, 2022, 02:12:46 am
content: Manufacturer is Jianghai Capacitor and their FA indicates electrical overstress.I wanted to measure the current on those caps, but boss-man wants to test this all in-situ in their enclosure. I can't really find a place to put any sensor in, but if anybody has advice on this I'll try it.Image below is a good example of what I'm seeing on 4 of the internal caps. Running at 819V was the highest I could pull because of my limited load configs. It tails off to 500V since my demand was 1000V (CC mode). Going to try building a custom load tomorrow out of some old equipment and run again at 1000V. We do actually run up at that level in application.

author: uer166
author_rank: Super Contributor
timestamp: February 24, 2022, 02:26:00 am
content: These guys actually give really well written app notes regarding reliability and lifetime estimation.http://jianghai-america.com/uploads/technology/JIANGHAI_Elcap_Lifetime_-_Estimation_AAL.pdfhttp://jianghai-america.com/uploads/technology/JIANGHAI_Life_Time_Estimation.pdfTheir equations seem comprehensive and include a voltage factor for stress calculation. Seems to be a step-up from the average Crapxon cap with no data.Your Vmax in trace is 527V, which seems well under the 550V max. If you scale that to 1000V/819V=1.22, then Vmax is 527*1.22=642V (some serious assumptions here).I think applying almost 100V over spec on a scale of multiple seconds would blow up pretty much any manufacturers' cap..

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2022, 02:56:29 am
content: That's 819V at 37 amps? Can you tell us the topology of this power supply or some other information? How many and what size capacitors are used on this 37 amp output? I assume the capacitors that are blowing up are the output filters? Is this 480VAC/3PH input?

author: katzohki
author_rank: Frequent Contributor
timestamp: February 24, 2022, 10:03:18 pm
content: Topology seems like a Vienna rectifier into a full bridge resonant converter. I'm not the expert on this one so I had to ask.Going to run at 1000V today and see what we get it peaking at. The capacitors are on the output and 6 in parallel or 2 series x 3 parallel (LV vs HV mode). 480V 3Ph input yep.According to others, we have seen it peak at 1050V on output during load drop condition. Hope to see something interesting.Been out / working on other stuff most of today so just getting back on to this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2022, 10:17:57 pm
content: Sounds like a control thing. Why hasn't the control been tested at smaller scale, without risk of damage to components? (That's also >1000V on the, whatever the semiconductors are; I'd have to guess---hope---this is 1700V SiC -- 1200V would be much too marginal?)Tim

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2022, 10:42:12 pm
content: If you recall I asked if it was a resonant converter....I'm not so interested in the long term sort of graphs you have of that surge. If the max it reaches is 1050 and it doesn't go over 550 per leg, then that's probably not the issue unless the capacitors are substandard. I'd be more interested in a look at a faster timebase, one on the order of the switching frequency of the resonant converter. Then you really need to get a picture of the current in those capacitors, as well as measure their temp in operation. If you can, try and do that with as close to a full load as possible at both 500V (parallel mode) and 1000V (series mode).Do you have a part number and datasheet for the capacitors themselves?

author: katzohki
author_rank: Frequent Contributor
timestamp: February 25, 2022, 02:32:46 am
content: Some "particular individual" absconded with all our higher performance differential probes. I sent out to have at least one back so I can get a closer look at the shorter time base impulse. Right now my bandwidth is 100kHz...Not sure on the current in the capacitors. Any recommendations? We talked about a rogowski coil since it was the smallest thing I could find, but it was pretty pricey so I didn't want to go for that unless I was sure it would work out. I really can't think of anything that will fit inside this case to measure the current while it's in operation. If I have to run this thing at high power while it's open like a clam...

author: bdunham7
author_rank: Super Contributor
timestamp: February 25, 2022, 02:57:17 am
content: Surely you don't mean that those are 150uF capacitors?? They should be physically huge for a power supply that size.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2022, 09:26:29 am
content: I assume there's a lot of them in parallel, ala:Tim

author: katzohki
author_rank: Frequent Contributor
timestamp: February 25, 2022, 09:56:19 pm
content: yeah the caps are 150uF. Six in parallel for 900uF. Or in high mode they put them in series pairs 3x2.