Increase life of electrolytic capacitors? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2021, 08:41:12 am
content: Hi,We are doing an offline 180-265VAC Flyback (15W) with Mornsun LS15-13B12 AC/DC power module. We must add a primary side HV DC bus capacitor….Which of the following capacitance choices would dry out quicker?...1…One 33uF , 450V EKXF451ELL330MM20S Electrolytic capacitorOR2..Two 68uF , 250V UCY2E680MHD3TN Electrolytic capacitors in series.Please also answer with relation to…A)…Flyback on 1W standby load all the timeB)…Flyback on Full power all the time…approx. (200mA of AC rms ripple current at 100Hz)EKXF451ELL330MM20S Electrolytic capacitor datasheet (33uF, 450V)https://www.chemi-con.co.jp/products/relatedfiles/capacitor/catalog/KXFLL-e.PDFUCY2E680MHD3TN Electrolytic capacitor datasheet (68uF, 250V)https://www.nichicon.co.jp/english/products/pdfs/e-ucy.pdf

author: bob91343
author_rank: Super Contributor
timestamp: July 09, 2021, 05:56:09 pm
content: Two capacitors will be less reliable than one, if you need the capacitance. If one fails, you are out of luck, and since you have two, you double your liability.None of this deals with drying out. That is more a function of quality of seal and purity of materials. The higher temperature rating parts are less likely to fail in this way.

author: wraper
author_rank: Unknown
timestamp: July 09, 2021, 06:16:05 pm
content: Quote from: bob91343 on July 09, 2021, 05:56:09 pmExcept it's not. They are in series and either of them failing obstructs operation of second cap. Not to say connecting capacitors in series generally is not a good idea to begin with, and comes with it's own issues.

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2021, 08:02:38 pm
content: Thanks, the thing is that as you know, the 68uF,250V caps have twice the ripple current rating as the single 33uF, 450V capacitor. As such, would you think that they would run cooler and thus be les likely overall to dry out?...or may thus take longer to dry out.Also, on the same note, is a longer capacitor less likely to dry out then a shorter, squatter capacitor, because more of the dielectric is distant from the seal? And on the same note still........what about squirting some silicone rubber round the capacitor base so that the bottom seal is kind of more "blocked up" and less able to vent dielectric?...will that mean it taking longer to dry out?I once took apart a Philips DALI radio receiver, which dims streetlights, and they had two 250V electrolytic capacitors in series in there. (As the offline flyback primary DC bus capacitor.)

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2021, 08:56:32 pm
content: Quote from: wraper on July 09, 2021, 06:16:05 pmYou misread him, I think. He said 'liability' not 'reliability'. The idea being that if you have two parts and need them both you now have two ways to have a critical failure. But this is applying a more sophisticated analysis than this idea deserves...

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2021, 09:02:15 pm
content: Quote from: Faringdon on July 09, 2021, 08:41:12 amActual iife and reliability results rarely line up with simple assumptions based on a few datasheet parameters. The first capacitor has very long specified life, you've derated the voltage a decent amount and the ripple current is only about 25% of its rating at full power, thus it seems a good fit. Nobody can predict exactly how long it will take to 'dry up', but at a reasonable operating temperature, probably a very long time. High quality caps often outlast the other components. Filter caps in series is not a commonly used method and I'm pretty sure that solution will be less reliable. You would need to balance them as well.

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2021, 11:17:53 pm
content: Thanks, balancing is interesting here....peak of high mains is 373V.....so if we imaging one capacitor has 250V on it, then the other would have 123V on it....but this level of imbalance is extremely unlikely.......and even that imbalance is OK since the caps are 250V rated.

author: bob91343
author_rank: Super Contributor
timestamp: July 10, 2021, 01:14:07 am
content: Blocking the seal with rubber or anything else may not be wise, as it may be used as a vent. A vent is important in case the internal pressure increases. Better to make a mess and lose capacitance than have an explosion.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2021, 06:58:00 am
content: Just say no to series capacitors because you don't have to go there, plenty of 450V rated parts are available. If you have shape constraints and a single part doesn't meet yourenergystorage requirement and can't fit a larger single case, use two 450V parts inparallelinstead.Series capacitors, even with some high-value balancing resistors, require a bit of extra voltage derating so 2*250V is not actually 500V, a single 450V rated part is better.Key to very long lifetime is,1) using well known brands,2) parts rated to 105 degC or more, and rated for some 5000 hours or more,3) derating voltage and ripple current rating, maybe to 80% and 50%, respectively, if you can do that, that is,4) derating temperature rating, i.e., make sure the local ambient doesn't go nowhere near the rated 105 degC.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 10, 2021, 12:12:51 pm
content: "drying out" is also not the same as service life - do you have a description of what drying out means for the cap and the product?Are you trying to prepare a design life or mtbf to quote in product docs? Have you done a monte carlo on parameter changes that could lead to an out of spec product? Are you providing maintenance data on what cap parameter degradation would indicate cap replacement?

author: jonpaul
author_rank: Super Contributor
timestamp: July 10, 2021, 03:50:28 pm
content: Bonjour, the rating you need is the temperature85/105/135 dec C.It gives the average part 2000 hours to decrease C 20% or increase ESR 50%.Most cheap commercial parts are 85 C.105 are better.We used 135C caps from Rubycon, Nichcon etc that were for electronic ballst use in 1980s..1990s.Ever 10 deg change in temp = 2x change in chemical activity.Thus the 135 C caps are many time longer life.Cap internal temp = ambient temp plus ESR x RMS ripple current exp 2 (P=I exp 2 R)Check any detailed lytic cap catalog for all the info and calculations.Using two in series is seldom needed and increases cost and PCB footprint.Bon ChanceJon

author: Faringdon
author_rank: Super Contributor
timestamp: July 10, 2021, 08:57:19 pm
content: The thing is, the 68uF , 250V capacitors have twice the ripple current rating as the single 33uF,450V capacitor.As such, they will run cooler, and last longer.(i admit it means more PCB space, but we accept that in return for longer lasting capacitance.)In case anyone was wondering, we cant use two 33uF , 450V caps in parallel as it would give too much capacitance, and too much inrush current.

author: bdunham7
author_rank: Super Contributor
timestamp: July 10, 2021, 09:18:21 pm
content: Is there any reason you aren't looking at the EKXF451ELL330ML25S which would have a ripple current rating of 980mA @ 100kHz vs 1300mA for the doubled-up 68uF setup? You'd have 5X derating vs 6.5X, but a 20k-hour specified life vs 12k. Given those numbers, I don't think you can conclude that one or the other would last longer.

author: Faringdon
author_rank: Super Contributor
timestamp: July 10, 2021, 10:55:46 pm
content: Thanks yes, you are obviously correct concerning those particular capacitor part numbers.....but, and i believe you would agree, generally speaking it will be easier to find 68uF,250V caps with higher ripple current rating than 450V, 33uF capacitors. Also, i believe you would agree that it will also be possible to find 68uF,250V caps with just as much hours life rating as for 33uF,450V caps.The only worrying thing about two electrolytics in series is if they get voltage unbalanced......but with two 250V caps in series across the mains rectified DC Bus, the difference in voltage could never get bad enough to overvoltage one of the 250V series'd caps.There wouldnt even be any need for balancing resistors.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 03:48:45 am
content: Quote from: Faringdon on July 10, 2021, 10:55:46 pmWhat is the basis (evidence or theory) for this assertion?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2021, 08:28:40 am
content: Quote from: Faringdon on July 10, 2021, 08:57:19 pmLook further, I'm sure you'll find a single part (or two in parallel) that satisfies your size constraints, ripple current and maximum capacitance requirements without series connection.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 09:30:33 am
content: QuoteThanks, because its a capacitance voltage divider, and the capacitors are no more than 20% tolerance. The max bus voltage across the pair would be 373V. So with two 250V caps in series, neither is ever going to get overvoltagedQuoteThanks, Maybe there is some combination somewhere with parallel caps thats better...but it will be rarer than with series caps, and so we would get caught out with nil-stocks and production would get held up.There is a Philips offline DALI light dimming module (for use with streetlights) that uses two 250V caps in series for the primary DC bus (i took it apart, ~3 yrs ago)....however, i cant remember if they used a resistor balancing network.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2021, 10:01:05 am
content: Quote from: Faringdon on July 11, 2021, 09:30:33 amCapacitive voltage divider, by definition, works for AC signals. If the tolerances are good match, then this means they initially power up properly balanced thanks to being capacitive voltage divider indeed.But after that power-on surge, they see DC over it. Now sustained balance is only defined by the leakage currents, and good luck finding leakage current tolerance specified!Now in practice, elcaps leak more current near and slightly over the rated voltage so this mechnanism usually/likely keeps them balanced: one cap going slightly overvoltage "shunts" the charge, charging the other cap. And often the capacitors are not seriously damaged by this limited-current, limited time overvoltage. Hopefully.But it's taking a risk as there are assumptions and uncertainties in this process, so avoid if you can. If you can't avoid it, do your due diligence.Balancing resistors add a known "leakage" current which is highly accurate (because 5% resistors are cheap as dirt) and also follows a predictable I = U/R curve. Zeners/TVS would have much steeper curve.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 11, 2021, 11:26:55 am
content: Unless there is a reference for a study about disadvantages of series capacitors, I don't necessarily see a problem in this configuration other than what was mentioned before about balancing.The ripple current will be equally applied to both capacitors (as they are in series) and obviously the weakest link would go down first (just like in an imbalanced set of batteries, where it is usual for one discharge faster than the others). With the calculated ripple and keeping the temperature low, the set will last for quite a while.On the other hand, a case could also be made for a pair of 15 or 22μF in parallel, which will distribute the ripple current among them and take about the same space as two 68μF in series. So, it might simply be a matter of choice, really.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 11:44:02 am
content: Thanks, actually you are all correct.The series capacitor thing is a gamble that we wont get widely disparate leakage current capacitors.....and i now dont think its a gamble worth taking.The trouble with two 15uF, 450V in parallel is that (generally) they will have worse overall ripple than one 33uF.

author: coppice
author_rank: Super Contributor
timestamp: July 11, 2021, 12:00:54 pm
content: The caps that will last longest and the ones that are kept coolest. If two smaller caps can best be worked into a cool location, that will be best. If the single cap can best be worked into a cool location, that will be best.

author: SeanB
author_rank: Super Contributor
timestamp: July 11, 2021, 12:17:46 pm
content: True, but the ripple will be distributed over a much larger surface area, so the heating will be less. You can also add a parallel 100-220n 630VDC polyester film capacitor in parallel as well, which will reduce the high frequency ripple a lot, and which likely will fit between the 2 15uF capacitors as well. You want to use a capacitor with a long life there, ripple current rating greater than your 200mA current draw, and if you want use an inductor to isolate the 2 capacitors from each other at high frequency, it will make the film capacitor and the one handle the HF ripple heating, while they both have the mains frequency on charging. But here capacitor manufacturer and temperature rating, along with rated life, is the determining factor, you want a good name brand in a series with a long life at 105, 125 or 135C rating, and to choose among them, and run the rest cool enough to give decades of service life.

author: coppice
author_rank: Super Contributor
timestamp: July 11, 2021, 12:46:41 pm
content: Quote from: SeanB on July 11, 2021, 12:17:46 pmIn most designs failing regularly through dried out electrolytics, the proximity of a heat sink has far more influence than any self heating. Bad designers love nothing more than to bunch their electrolytics around the equipment's hot spots.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2021, 01:08:18 pm
content: Quote from: coppice on July 11, 2021, 12:46:41 pmSometimes you can't help that because it's conflict of interests, the capacitors are there in the first place to offer low-inductance (i.e., close by) supply rail for parts that require the current (i.e., heat up). They can't be far away.Of course, you can often find a better compromise, and in some cases, i.e. "card edge damping bulk caps" not seeing much ripple, you could have placed them further away no problem.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 01:54:40 pm
content: Quote from: coppice on July 11, 2021, 12:00:54 pmTrue, but everything works together. If this is an enclosed device, then you would also want the lowest total self-heating for any combination of capacitors. Sometimes there is no cool location and you just need to improve the efficiency of the whole device.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 02:08:11 pm
content: Quote from: Faringdon on July 11, 2021, 11:44:02 amSometimes the key is to realize when you have solved the problem. I misstated the ripple current derating above (I used 100kHz, not 100Hz), but according to your calculated parameters, I'd still say your first selection solves the problem--although the 16x25 model from the same line would be even better if it fits. It has very long life at its max ratings and in your application you have derated it some, so I think the only thing left is to make sure it doesn't get cooked.It might be more comforting so see 3X or 4X derating on ripple, but unless you have specific data showing what the life extension would be for derating various parameters, you don't really know if any changes you make will offer any actual improvement. Also, I'm curious--how do you get 200mA ripple with a 15W PSU?

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 02:26:10 pm
content: Hi,Its the thing with the capacitor only getting change current for a small time due to the poor power factor operation.Ill append the LTspice if you wish. (its attached now)Its 269mA AC RMS ripple at 220vac in and 15w out.Attached is the ripple current scope, and the FFT of it....it shows the majority is at 50-800Hz (unless the Blackman-Harris FFT function got it wrong)

author: fcb
author_rank: Super Contributor
timestamp: July 11, 2021, 02:44:26 pm
content: Heat is the primary life-shortener (as I'm sure everyone else has mentioned).Ripple current causes heating. Derate on ripple if you care about life, by how much is anyone's guess.If I was concerned, I'd build two prototypes and measure just how hot the caps get on each, you should be able to estimate the caps life from the mfr datasheets.And 270mA RMS ripple is mad for a small supply, you sure?

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 02:55:10 pm
content: Thansk, yes Ltspice confirms it as in post above but one.Worse still, most of this ripple is at 50-800Hz, and not at the 65Khz of the switching frequency.As you understand, we dont want to derate ripple by having more capacitance, as it means more inrush.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 03:05:54 pm
content: Quote from: Faringdon on July 11, 2021, 02:26:10 pmCan you post the assumed schematic? Also, are you using the recommended circuit design for your PSU, including the NTC and EMI chokes?I think there is actually a fair amount of energy at higher frequencies, although I'm glad to see it is down -320dB @ 100MHz.Try replacing the PSU circuitry with a resistor that draws exactly 15 watts and see what the calculated RMS is then. I'm thinking that the various parasitics in the circuit will blunt the peaks enough that your ripple to load ratio will be less than an incomplete LTSpice simulation shows. I would at least try to experimentally measure it in an actual circuit before you go into production. Just based on experience and observation, 33uF is not a small primary filter cap for a 15W PSU, and even lower quality third-tier caps are usually not the failure point in these types of supplies.In any case, I think your (or my) selected capacitor will likely outlast this economy PSU module. If you really want to worry about something, start looking at the secondary capacitors.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 03:11:45 pm
content: Thanks, yes, the attached is it..

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 04:07:42 pm
content: OK, there's a few things I don't see.How does LTSpice model the PSU unit itself?Where is the problem with inrush current if you use a larger capacitor? (not that I think you need to)Can you try running the simulation with the capacitor changed to 15 and then 68uF? I think the main reason you have such a large ratio of ripple to load is that the capacitor is more than large enough.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 04:20:35 pm
content: Thanks, yes you're right the capacitor is a wee bit too big. Thing is, as such, it will offer a decent bit of small_mains_transient protection without activating the MOV. And as you know, MOVs die a little bit after every transient that they quench. So the too big Elco helps the mov live a little longer, or so the story goes.Even though the vishay AC-05 can handle the inrush, its datasheet says nothing as to how much it will be slightly damaged after every inrush event....and says nothing about whether such damages are cumulative or not, and may result in the inrush resistor dying at some point.QuoteLTspice (in the sim a few posts back) is modelling a DCM flyback at 65KHZ, which is whats on the Mornsun AC/DC