Long life alternatives for Al Electrolytic bulk caps - Page 1
Forum section: electronics / projects

author: LabSpokane
author_rank: Super Contributor
timestamp: September 18, 2016, 04:34:52 pm
content: I may be moving into a project where we have a product that requires a long service life (24x7) in the great outdoors, but has been designed with copious amounts of electrolytic caps and even a super cap. This device has never been fielded widely yet, but what it's replacing has a service life of many decades.The Al electrolytics pretty limit the service life to ten years, and in fact that's exactly what the data for a similar device shows. I would like to look at ditching the wet Al electrolytics altogether.I realize that a different solution will be more expensive, but what are your experiences with other solutions like MLCCs, tantalum polymer, polymer electrolytics, etc? I understand the issues with DC bias voltage and ESR.I'd like to see a service life of at least 20 years.And what are folks experiences with Al electrolytics that are rated for long service lives, such as 20,000 hours and up?Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: September 18, 2016, 04:52:07 pm
content: You want a long service life use wet tantalum with glass frit seals. Lifetime is essentially limited by the case corroding through from the outside, or you running them past the specs. Just do not ask the price for parts over 22uF 35V.Otherwise really derated large format ceramic capacitors, run very conservatively, along with film capacitors where you need the stability, with the best quality high voltage electrolytics where you absolutely need the capacity. So a 24V rail might use 100V rated Nichicons, being around 3 times the volume, but they will last decades in service. Also, look for Telecoms rated components, they tend to be a better rated component, which sould do a good number of decades service.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 18, 2016, 04:54:16 pm
content: If desiged well and derated well, there's no reason why a al cap won't go the distance. A 20k cap rated for 105c will last 20 years @65C.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2016, 05:00:48 pm
content: The early Al electrolytic caps used in tube radios often lasted more than 50 years. However they where large and relatively expensive. They also had concerns about a limited lifetime when they first used them.The smaller one can be a problem, as the liquid to surface ratio is not that good. But here film or MLCC might be a good alternative.

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 18, 2016, 05:34:16 pm
content: You can de-rate what you want, the weak point stays the ageing of the (rubber) sealing. You can have luck with that and you can have not. No guarantee in advance. It's why the hermetically glass sealed tants last almost forever if properly used.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2016, 07:14:31 pm
content: Solid tantalum will be fine, as long as: they're a genuine decent brand, the voltage is suffiently de-rated and the surge current is limited to a safe level.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 18, 2016, 08:00:43 pm
content: There is no long-life alternative to a supercapacitor and worse is that supercaps tend to fail early and often even when treated with the proverbial kid gloves.For aluminum elkos used for bulk ripple filtering in switchmode converters an excellent - indeed, often superior - alternative is polypropylene film, and don't take up nearly as much volume as you might think because of their high ripple current rating. If you have volume to spare they can also be used for 50/60/300/360Hz ripple filtering as well, but in this application you do tend to need a lot more capacitance to meet ripple voltage specs.For general bypass/decoupling applications pretty much anything will do BUT be aware that the high ESR (relatively speaking) of al. elkos can actually be an advantage here because it "de-Q's" any parasitic LC resonant networks formed by trace/wiring inductance and the bypass capacitance. The - seemingly nuts - solution is to insert a bit of resistance in series with an otherwise Hi-Q bypass capacitor. Usually an ohm or two is sufficient.

author: LabSpokane
author_rank: Super Contributor
timestamp: September 18, 2016, 08:51:55 pm
content: Quote from: MagicSmoker on September 18, 2016, 08:00:43 pmThe super capacitor is the one that bugs me most, for like you state, there is no good alternative save for a permanent battery. In this instance, they did what the supercap companies typically recommend one *doesn't* do and bent the leads and glued it down, ensuring that the leads would be under continual stress.As for the resistor on the low ESR caps, I used to think that's weird, but now I see it as an advantage it at least one gets to pick the damping on the system and the resistance should stay consistent over time.

author: Kjelt
author_rank: Super Contributor
timestamp: September 18, 2016, 09:07:10 pm
content: If 24x7 outdoor is your target then you are going to pot the electronics and you better do a very good thermal analysis and testing job first and keep those caps cool.Esp. in the summer if it is in full sun. Then there is the other problem in the winter when it freezes -30oC make sure you keep it decently warm inside in those times.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 18, 2016, 09:33:38 pm
content: Quote from: LabSpokane on September 18, 2016, 08:51:55 pmOh boy... bending over the leads isn't going to end well...If I had to choose between a supercap or a long-life lithium coin cell I'd take the latter every time, but about the best you can expect from a good quality "10 year" Varta or Panasonic cell is a service life of 15 years.Sometimes you can reconfigure your circuit to not need a backup battery or supercap - if you are just trying to keep a RTC running you might instead consider getting the current time from WWV, GSM, GPS or the like at each power up.Quote from: LabSpokane on September 18, 2016, 08:51:55 pmYep! I've learned this lesson the hard way, too. Oh, one other thing I should've mentioned is to beware of a decrease in capacitance over time in high-k ceramic capacitors (including the better-performing X7R, X7S, etc. types, though it is much worse in the Y5V, Z5U types). See this article at Johanson:http://www.johansondielectrics.com/ceramic-capacitor-aging-made-simple

author: Vgkid
author_rank: Super Contributor
timestamp: September 18, 2016, 09:45:59 pm
content: Though it is an Al Elco. What about the PEG series from Kemet.http://www.kemet.com/FEBG_electrolytic_axialI almost ordered 1 to put in a power supply.

author: Kjelt
author_rank: Super Contributor
timestamp: September 18, 2016, 09:59:38 pm
content: There goes your BOM

author: splin
author_rank: Frequent Contributor
timestamp: September 19, 2016, 12:44:44 am
content: Quote from: MagicSmoker on September 18, 2016, 09:33:38 pmWhy not a lithium thionyl chloride battery? Tadiran, for example, claim a 25 year operating life or more. These have been around for a long time and are probably a reasonably safe bet. Other companies make similar cells but may not have been around long enough to have a proven track record on lifetimes.http://www.tadiranbat.com/index.php/product-overviewOr if you need a rechargeable cell, their TLI series with a claimed 5000 cycles and 20 year operating life:http://www.tadiranbat.com/index.php/tli-series-rechargeableOf course what their marketing department claim, compared to what they are prepared to guarantee, maybe another matter - I doubt these have been around very long and the datasheet and guidelines document don't seem to have anything to say about operating life. I don't suppose they'll be cheap either.[EDIT - $17 for 100+ 330mAh TLI1550A from Future electronics.]These cells do have one remarkable feature though as they claim they can be charged at temperatures down to -40C - most, if not all, other lithium rechargeables that I have seen must not be recharged at all below 0C. This also applies to (most/all?) Ni-mh; some Ni-cd can be recharged below 0C but at very restricted rates. Again what (the likely) effect charging at extreme temperatures has on their operating life is something you'd have to discuss with the manufacturer.

author: splin
author_rank: Frequent Contributor
timestamp: September 19, 2016, 05:56:30 am
content: Quote from: Pjotr on September 18, 2016, 05:34:16 pmHermetically sealed tantalums are very expensive and aren't available above 125V (from Digikey at least). There is an awful lot of electronic equipment out there that is more than 20 years old and still functioning perfectly well. It may be cheaper, or even much cheaper, to use electrolytics from two or more manufacturers in parallel to increase the probability that at least one of them will last the course. Of course this assumes that the most likely failure mode of dried out/aged electrolytics is increased ESR/reduced capacitance rather than short circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: September 19, 2016, 08:23:05 am
content: This probably ought to be a separate topic, but out of curiosity what other reliability aspects are being considered.One one aspect which would needle me (ho ho) is metal whiskers. Notice the source, and be shocked (or not) by some of the pictures inhttp://nepp.nasa.gov/whisker/reference/tech_papers/2012-Panashchenko-IPC-Art-of-Metal-Whisker-Appreciation.pdf

author: Pjotr
author_rank: Frequent Contributor
timestamp: September 19, 2016, 11:02:09 am
content: Quote from: splin on September 19, 2016, 05:56:30 amIndeed very expensive, if available at all. It belongs to the high-rel area where cost is no object The suggestion was merely intended to show there is not much in between this and long life wet alu electrolytes. Although the newer polymer hybrid ones look promising

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 19, 2016, 11:39:54 am
content: Quote from: splin on September 19, 2016, 12:44:44 amHuh... I was only thinking of rechargeable cells, but even then I was still under the impression that 10-15 year life was the best you could hope for and totally forgot about Tadiran which is, indeed, a top-tier small cell manufacturer.

author: Kjelt
author_rank: Super Contributor
timestamp: September 19, 2016, 01:12:28 pm
content: Quote from: Pjotr on September 19, 2016, 11:02:09 amDo I see that correctly >€100 for one capacitor and then an ESR > 1ohm ?http://nl.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=31&storeId=10168&categoryName=Alle%20categorie%C3%ABn&selectedCategoryId=&gs=true&st=vishay%20134D

author: tggzzz
author_rank: Super Contributor
timestamp: September 19, 2016, 01:55:15 pm
content: Quote from: Kjelt on September 19, 2016, 01:12:28 pmYou should always look at more than one supplier, and check the stock availaibility and volume discount.http://www.findchips.com/search/134D157X9125K6https://octopart.com/search?q=134D157X9125K6so you can see that you can get them at half that price -iffyou buy 1000

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 19, 2016, 02:03:08 pm
content: Don't know if this works for your application, but in some cases easy service can be substituted for reliability. Can you configure your device so that the components that demand very high capacitance are easily accessed? Perhaps even made into a plug in module.

author: jitter
author_rank: Frequent Contributor
timestamp: September 19, 2016, 05:18:33 pm
content: Quote from: LabSpokane on September 18, 2016, 04:34:52 pmThen the first thing I would have a look at is the design of the device that's being replaced. That is, assuming it did perform as desired.QuoteWell, if I look at the datasheet of (e.g.) the Vishay BC106 PED-ST, I'm looking at a very respectable 20,000 hours at 85 °C which, if used conservatively could be multiplied by as much as 40. Yikes: 800,000 hours (~91 years)... So I guess with the right choice of capacitor, 20 years usage won't be much (if any) of a problem. But these are figures I can only assume to be extrapolated from accelerated lifetime tests.QuoteHere's one, sort of. The cap I linked to is of the same brand that makes the somewhat more conservatively specced056/057 PSM-SI. We have been using lots of these for at least a decade, probably longer. AFAIK, no failures yet. These are used in the PSU of an instrument that could be run 24/7 by researchers, and is probably expected to last at least 20 years.

author: LabSpokane
author_rank: Super Contributor
timestamp: September 19, 2016, 08:01:29 pm
content: Well, the project didn't happen for me, but I'm still very interested in continuing the discussion, as I'm learning more from you all about design reliability.I probably should not have pointed out what I saw as design deficiencies so soon, but I am also happy not to be owning them.I didn't get a very close look at the product, but what I saw was enough reason to be concerned: bent leads on the supercap plus a ton of capacitors that purchasing would try to reduce-spec on the AVL for "cost reduction" or worse that they would buy outside of disty.Jitter, what you posted is exactly what I look at for in an Al electrolytic.

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2016, 08:18:19 pm
content: I was looking at an old Telecoms board today, made around 1993. Capacitors on the board are all Phillips or Siemens parts, so you probably will want to look at the new owners of those plants, and buy from there. Kemet does do a few large value Tantalums, and there are some rated at 350V, though they are modules made from stacked smaller capacitors with balancing resistors, and they have ones up to 15000uF, but the price on those is that of a mid range car, plus the few months lead time.

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2016, 09:45:10 pm
content: 10 to 20 years and more using aluminum electrolytic capacitors is certainly possible with proper derating. Beyond that, things become uncertain because of other failure modes like oxidation of the internal connections. Small capacitors do not scale as well as large ones with time because they have a smaller volume of electrolyte. Take advantage aluminum electrolytic capacitors rated for high temperature operation and voltage derate them so the loss of electrolyte over time is not as significant. For small aluminum electrolytic capacitors, use something else.If temperature cycling is extreme, then epoxy cased tantalum capacitors may develop stress fractures. Both solid and wet axial leaded tantalum capacitors avoid this problem and the solid ones are not that expensive. The economical alternative is to voltage derate the epoxy cased tantalum capacitors which will prevent failures do to thermal runaway.