Aluminum electrolytic capacitor failure modes and ripple current ratings

I am trying to get a handle on ripple current ratings for capacitors, start ing with the ChemiCon KZN series. The data sheet has specifications for ra ted ripple current and maximum temperature. It seems I can reliably operat e well beyond the ripple current rating, but I don't have a good understand ing of what the real limitations are.

How do capacitor manufacturers come up with the ripple current rating for t heir parts? I had thought this was mostly a function of the capacitor's in ternal temperature, and the effect on the electrolyte. If it gets too hot the electrolyte might boil, or in a less extreme case the electrolyte perme ates through the plug faster effectively reducing the life of the capacitor . Does this sound correct? From the little bit of testing I have been doi ng, it seems more complicated than that. I would like to come up with a go od way to measure this or otherwise put numbers on it.

Does anyone know of a good reference for the design of capacitors? I would like to get a better understanding of the chemistry. From what little I k now, the chemistry in the electrolyte is pretty sophisticated. This is a m ature technology.

Thanks,

Ethan

Reply to
Ethan Petersen
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I suspect that they rate them conservatively as having them fail catastrophically in service can be pretty horrible and messy.

This is one such estimate of MTBF vs operating conditions:

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The hotter it runs the shorter the lifetime. +10C roughly halving it.

Mature technology with a twist and critical hidden trade secrets in the electrolyte formulations that will catch out the unwary.

ISTR a decade or two back someone stole the electrolyte formula from a well known Japanese manufacturer and made cheaper clones. The resulting capacitors failed spectacularly after about 3 years in use but not before they were installed on a heck of a lot of PC motherboards. eg.

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I had one motherboard of mine that failed this way. Fortunately I got to it before the thing actually blew up. It showed symptoms of unreliable ram when the ripple started to get a bit too high. The worst failing capacitor had already bulged enough to be at a very rakish angle.

--
Regards, 
Martin Brown
Reply to
Martin Brown

Cooling, namely air flow, makes a big difference to temp rise from ESR dissipation. Or being near warm parts.

ESR increases with temperature, sort of a runaway effect.

Several small caps in parallel is better thermally than one big one.

Reply to
jlarkin

I found the Illinois Capacitor lifetime calculators helpful:

piglet

Reply to
piglet

All the manufacturers' application notes I have looked at claim that Al electrolytic capacitor ESR decreases as temperature increases, so no thermal runaway.

John

Reply to
jrwalliker

** Nonsense.

The exact opposite is the case, ESR typically FALLS by a factor of 5 between room temp and max rated temp.

Anyone with a hot air gun ( ie hair dryer) and a basic ESR meter can verify this.

.... Phil

Reply to
Phil Allison

You do NOT want to operate beyond the ripple rating, in fact you should stay at least 2x below the rating. And 3x or 4x is even better.

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 Thanks, 
    - Win
Reply to
Winfield Hill

The limiting factor is core temperature. Which they conveniently don't show you how to measure, of course*.

*Very rarely, you find a datasheet with RthCA (core to ambient) so you can figure this out without having to special-order be-thermocouple'd capacitors to measure it directly (which they can actually supply, if you ask nicely enough).

Presumably, you can operate at higher ripple, at significantly lower ambient, for the same lifetime. So, maybe a 2khr 105C part handles 1A at

105C ambient, say 1.4A at 85C, 2A at 65C, and so on.

It's not a lot of improvement for such a big cost in terms of lifetime, and one much better served by buying the right part in the first place (say a

5khr 105C part that does 2A to begin with), or enough in parallel for the same effect.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

rting with the ChemiCon KZN series. The data sheet has specifications for rated ripple current and maximum temperature. It seems I can reliably oper ate well beyond the ripple current rating, but I don't have a good understa nding of what the real limitations are.

their parts? I had thought this was mostly a function of the capacitor's internal temperature, and the effect on the electrolyte. If it gets too ho t the electrolyte might boil, or in a less extreme case the electrolyte per meates through the plug faster effectively reducing the life of the capacit or. Does this sound correct? From the little bit of testing I have been d oing, it seems more complicated than that. I would like to come up with a good way to measure this or otherwise put numbers on it.

ld like to get a better understanding of the chemistry. From what little I know, the chemistry in the electrolyte is pretty sophisticated. This is a mature technology.

It seems to be a heat problem, but you won't notice caps getting hot in ser vice, even when their service life is much reduced by the ripple current. Y ou don't have to go anywhere near BP for problems to happen.

As others have said, stay well away from max ripple rating unless you don't want it to last. MTTF is the point at which 50% of your caps have failed, normally you want way more reliability than that.

NT

Reply to
tabbypurr

tarting with the ChemiCon KZN series. The data sheet has specifications fo r rated ripple current and maximum temperature. It seems I can reliably op erate well beyond the ripple current rating, but I don't have a good unders tanding of what the real limitations are.

or their parts? I had thought this was mostly a function of the capacitor' s internal temperature, and the effect on the electrolyte. If it gets too hot the electrolyte might boil, or in a less extreme case the electrolyte p ermeates through the plug faster effectively reducing the life of the capac itor. Does this sound correct? From the little bit of testing I have been doing, it seems more complicated than that. I would like to come up with a good way to measure this or otherwise put numbers on it.

ould like to get a better understanding of the chemistry. From what little I know, the chemistry in the electrolyte is pretty sophisticated. This is a mature technology.

ervice, even when their service life is much reduced by the ripple current. You don't have to go anywhere near BP for problems to happen.

't want it to last. MTTF is the point at which 50% of your caps have failed , normally you want way more reliability than that.

Years ago when vacuum tubes still freely roamed the Earth, half wave voltag e doublers were common in low end consumer products. a 160uF, 250 electroly tic was used as the input capacitor. These would get hot enough to melt the tar inside the aluminum can, and blow its guts out of the can. Those capac itors were rated for ripple current, rather than 'Power Factor' or ESR. Som e capacitor OEMs would sell crap that was supposed to be high current, but they were unmarked. The real techs would laugh at the penny pinchers who bo ught crap that wouldn'y last 30 days of heavy use.

Reply to
Michael Terrell

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Like I mentioned earlier, I still don't have a good understanding of the fa ilure mechanisms, or the criteria the manufacturer uses to determine the ri pple current rating. What I am hearing here contradicts experience from th e field. Millions of devices shipped, over many years, and no field return s for capacitors, despite routinely operating at ripple current well over t he rating. Competitors products are even more aggressive with ripple curre nt ratings.

The Illinois Capacitor Lifetime Calculator looks useful. It seems to line up with other academic literature I have been reading. Thanks piglet.

There doesn't seem to be much published information on the chemistry in the capacitors. I guess everything is proprietary and held close. Some artic les are useful such as the ones recommended by Klaus Bahner, thanks.

Since the failure mode seems to be excessive temperature, I have been tryin g to get a capacitor to fail due to excessive temperature and ripple curren t. Heating the capacitor to 160C for 30 minutes did not cause it to blow u p, or change capacitance and ESR. I was monitoring this while running the test. Capacitance increased, and ESR decreased as temperature increased, t hen returned to normal when brought back to room temp. I was hoping to see a catastrophic failure such as the electrolyte boiling and the cap explodi ng, but no luck. That doesn't seem to be the failure mechanism.

So I tried again, except with 15 A rms ripple current, also at 160C. The c apacitor is rated for 2.5 A rms ripple, so I am operating at 6x the rating. I had one capacitor fail because the leads fused. I re-soldered the lead s much closer to the body of the cap, and it continued to run. Again no fa ilures.

I suspect the ultimate failure mechanism is something more subtle, such as corrosion or some other electro-chemical reaction degrading the internal st ructure. This is probably going to require a big long life test, but it wo uld be good to have a better understanding of the part before starting some thing like that.

The adventure continues,

Ethan

Reply to
Ethan Petersen

----------------------

** Alluding to some mysterious example nobody is allowed to know about is F UCKING STUPID !!

.
** Really ?

** More mysterious allusions.

Some of their 500V electros are garbage.

** Should be "a" failure mode, not "the".

e and ripple current. Heating the capacitor to 160C for 30 minutes did not cause it to blow up, or change capacitance and ESR.

** Another mystery example that goes against common experience. 160C is enough to make any electrolyte boil and an explosion is very much o n the cards - or more accurately a sudden venting of lots of white smoke.

ESR decreased as temperature increased, then returned to normal when broug ht back to room temp. I was hoping to see a catastrophic failure such as t he electrolyte boiling and the cap exploding, but no luck. That doesn't se em to be the failure mechanism.

** I have *witnessed* electro caps venting violently from either excessive ambient ( inside and tube amplifier ) or from overvoltage causing excessive leakage current.

capacitor is rated for 2.5 A rms ripple, so I am operating at 6x the ratin g. I had one capacitor fail because the leads fused. I re-soldered the le ads much closer to the body of the cap, and it continued to run. Again no failures.

** Bizarre.

s corrosion or some other electro-chemical reaction degrading the internal structure.

** Internal corrosion does kill electro caps by eating away the aluminium s trips that connect to the terminals. This is more likely however with caps that have had little or no use.

Far and away the *main* failure mode is the cap going high ESR due to gradu al loss of electrolyte over years. Poor sealing at the ends and or a high a mbient are the main causes.

You are one colossal fool wasting your own time chasing non existent rabbit s down holes.

Please go ahead, make my day.....

..... Phil

Reply to
Phil Allison

----------------------

** Alluding to some mysterious example nobody is allowed to know about is F UCKING STUPID !!

.
** Really ?

** More mysterious allusions.

Some of their 500V electros are garbage.

** Should be "a" failure mode, not "the".

e and ripple current. Heating the capacitor to 160C for 30 minutes did not cause it to blow up, or change capacitance and ESR.

** Another mystery example that goes against common experience. 160C is enough to make any electrolyte boil and an explosion is very much o n the cards - or more accurately a sudden venting of lots of white smoke.

ESR decreased as temperature increased, then returned to normal when broug ht back to room temp. I was hoping to see a catastrophic failure such as t he electrolyte boiling and the cap exploding, but no luck. That doesn't se em to be the failure mechanism.

** I have *witnessed* electro caps venting violently from either excessive ambient ( inside and tube amplifier ) or from overvoltage causing excessive leakage current.

capacitor is rated for 2.5 A rms ripple, so I am operating at 6x the ratin g. I had one capacitor fail because the leads fused. I re-soldered the le ads much closer to the body of the cap, and it continued to run. Again no failures.

** Bizarre.

s corrosion or some other electro-chemical reaction degrading the internal structure.

** Internal corrosion does kill electro caps by eating away the aluminium s trips that connect to the terminals. This is more likely however with caps that have had little or no use.

Far and away the *main* failure mode is the cap going high ESR due to gradu al loss of electrolyte over years. Poor sealing at the ends and or a high a mbient are the main causes.

You are one colossal fool wasting your own time chasing non existent rabbit s down holes.

Please go ahead, make my day.....

..... Phil

Reply to
Phil Allison

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failure mechanisms, or the criteria the manufacturer uses to determine the ripple current rating. What I am hearing here contradicts experience from the field. Millions of devices shipped, over many years, and no field retu rns for capacitors, despite routinely operating at ripple current well over the rating. Competitors products are even more aggressive with ripple cur rent ratings.

e up with other academic literature I have been reading. Thanks piglet.

he capacitors. I guess everything is proprietary and held close. Some art icles are useful such as the ones recommended by Klaus Bahner, thanks.

ing to get a capacitor to fail due to excessive temperature and ripple curr ent. Heating the capacitor to 160C for 30 minutes did not cause it to blow up, or change capacitance and ESR. I was monitoring this while running th e test. Capacitance increased, and ESR decreased as temperature increased, then returned to normal when brought back to room temp. I was hoping to s ee a catastrophic failure such as the electrolyte boiling and the cap explo ding, but no luck. That doesn't seem to be the failure mechanism.

Well 30 minutes is not much time. If you put numbers into the Illinois Capacitor Lifetime Calculator, what sort of time do you get? Have you tried contacting the cap manufacturer?

George h.

capacitor is rated for 2.5 A rms ripple, so I am operating at 6x the ratin g. I had one capacitor fail because the leads fused. I re-soldered the le ads much closer to the body of the cap, and it continued to run. Again no failures.

s corrosion or some other electro-chemical reaction degrading the internal structure. This is probably going to require a big long life test, but it would be good to have a better understanding of the part before starting so mething like that.

Reply to
George Herold

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