Capacitor lifetime over varying temperature.

[dbvanhorn]

I have an application where I need to use aluminum electrolytics in a
fairly uninteresting environment electrically, but the temperature
will vary widely on a daily and annual basis.

Can anyone point me at an authoritative source for a method of
predicting lifetime in this application? Worst case is ridiculous,
and I suspect that averaging will be rather optimistic, but I don't
have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

Overall temp range, while operating, can be anywhere from 0-ish to 90-
ish C, varying slowly through the day, quickly while operating, and on
a day to day basis.


Thanks

 

[Oliver Betz]

dbvanhorn wrote:

(aluminum electrolytics lifetime prediction)

have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

why exactly do you think it's not good enough?

Oliver
--
Oliver Betz, Munich
despammed.com is broken, use Reply-To:

 

[Lostgallifreyan]

dbvanhorn <microbrix@gmail.com> wrote in news:454d8ae7-1a08-4ce4-8786-
1afc300cc466@s9g2000yqd.googlegroups.com:

I have an application where I need to use aluminum electrolytics in a
fairly uninteresting environment electrically, but the temperature
will vary widely on a daily and annual basis.

Can anyone point me at an authoritative source for a method of
predicting lifetime in this application? Worst case is ridiculous,
and I suspect that averaging will be rather optimistic, but I don't
have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

Overall temp range, while operating, can be anywhere from 0-ish to 90-
ish C, varying slowly through the day, quickly while operating, and on
a day to day basis.

With temperature swings that great and frequent I think you'll have more to
think about than lifetime of electrolytics alone. If the capacitance isn't
more than a few hundreds of ľF it might be better to pay for ceramics in X7R
or Y5V dielectric, each one having 10, 22 or 47 ľF and choosing your best
compromise on cost, ease of ganging them into arraye, etc.. Easiest is if
they have leads and epoxy coatings, they survive stresses better, but the
costs can be horrible, and bags of SMT 10ľF caps are cheap enough, but
assembly time might not be. Some firms sell pre-made assemblies to help with
that. This also allows a small board, which is cheaper to isolate against the
faster temperature changes. The smaller it is, the easier and cheaper it is
to protect it. Electrolytics are low density parts so I wouldn't even
consider them if there was any way to avoid them in this sort of situation.

 

[ehsjr]

dbvanhorn wrote:

I have an application where I need to use aluminum electrolytics in a
fairly uninteresting environment electrically, but the temperature
will vary widely on a daily and annual basis.

Can anyone point me at an authoritative source for a method of
predicting lifetime in this application? Worst case is ridiculous,
and I suspect that averaging will be rather optimistic, but I don't
have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

To get you started:
http://www.cde.com/calc/

Click on tools, then select Life & Temperature calculators.

If that's insufficient, define precisely what "good enough here"
means to you, then discuss that with the cap manufacturer(s).

My guess is that you won't get the kind of prediction you want,
but at least you'll have the opportunity to explore your options
more fully.

Meanwhile, design your gizmo for maximum cap life, since that
seems to be your goal.

Ed

Overall temp range, while operating, can be anywhere from 0-ish to 90-
ish C, varying slowly through the day, quickly while operating, and on
a day to day basis.


Thanks

 

dbvanhorn schrieb:

I have an application where I need to use aluminum electrolytics in a
fairly uninteresting environment electrically, but the temperature
will vary widely on a daily and annual basis.

Can anyone point me at an authoritative source for a method of
predicting lifetime in this application? Worst case is ridiculous,
and I suspect that averaging will be rather optimistic, but I don't
have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

Overall temp range, while operating, can be anywhere from 0-ish to 90-
ish C, varying slowly through the day, quickly while operating, and on
a day to day basis.

Let's do an example calculation for a Sikorel

<http://tw.ic-on-line.cn/IOL/datasheet/b41570e9688q000_1215949.pdf>

10000uF, 100V capacitor (case dimensions 64.3 mm x 80.7 mm). Let's also
say that ambient temperatures at the capacitor location are distributed
as follows over the span of one year: 25 entire days at 100°C, 50 entire
days at 90°C, 110 entire days at 80°C, and the remainder at 70°C or
below. We use the bottom diagram on page 98 to find expected lifetimes
(for a ripple current of 16A at 100Hz) of roughly 30000, 65000, and
135000 hours for the first three temperature bins, corresponding roughly
to 1250, 2500, and 5500 days of operation. Your yearly operating days at
these temperatures therefore eat up about 2.0%, 2.0% and 2.0% of the
capacitor lifetime budget. In other words, with this Sikorel capacitor,
your gizmo can be expected to last about 17 years. Good enough?

Taking lower temperature bins into account will decrease the expected
lifetime somewhat, albeit not dramatically so (one would first have to
extrapolate the diagram on page 98 to lower temperatures for this, which
is fairly easy to do).

Martin.

 

[Oliver Betz]

dbvanhorn wrote:

(aluminum electrolytics lifetime prediction)

have a feel for how much. I'm aware of the usual "half the life for
every +10C" but that isn't good enough here.

why exactly do you think it's not good enough?

First off, it assumes that the temperature is constant.

no, it doesn't if you rephrase it "twice the aging speed every 10K".
See below.

[...]

Assume that I could have a sine-wave-ish temperature curve over a 24
hour period. This is probably wrong, but it will do.
To keep things simple, assume a 40C swing with 25% of the time at -20,
25% at -10, 25% at +10, and 25% at +20 around some nominal
temperature.
Does the cycling itself take away lifetime? I would assume that it

I don't think this slow cycle causes a problem for the aluminum
electrolytics, but to be sure, you could ask the manufacturer.

It could also be that mechanical stress causes other failures.

does, but I've never seen anything to tell me how much.
Do I calculate with the average temperature, or try to bin it, and
assume that the effects are relatively linear?

So you have to integrate the aging over time, IOW average 2^(T/10).

Oliver
--
Oliver Betz, Munich
despammed.com is broken, use Reply-To:

 

[dbvanhorn]

On Jul 22, 1:14 am, Oliver Betz <ob...@despammed.com> wrote:

why exactly do you think it's not good enough?

First off, it assumes that the temperature is constant.

I'm fighting a couple of issues here. We don't have a good handle on
operating temperatures at the moment, but I know that the variation is
large.

Given the application, it's reasonable to assume that the temperature
won't be below 10C or above 40C in the room, but the high end is not
so well defined, since these are located in windows where the
insolation could bring the temps up substantially.

Assume that I could have a sine-wave-ish temperature curve over a 24
hour period. This is probably wrong, but it will do.
To keep things simple, assume a 40C swing with 25% of the time at -20,
25% at -10, 25% at +10, and 25% at +20 around some nominal
temperature.
Does the cycling itself take away lifetime? I would assume that it
does, but I've never seen anything to tell me how much.
Do I calculate with the average temperature, or try to bin it, and
assume that the effects are relatively linear?

These are caps in the 2000uF range, so I don't have the option of
going to ceramics.

 

[dbvanhorn]

10000uF, 100V capacitor (case dimensions 64.3 mm x 80.7 mm). Let's also
say that ambient temperatures at the capacitor location are distributed
as follows over the span of one year: 25 entire days at 100°C, 50 entire
days at 90°C, 110 entire days at 80°C, and the remainder at 70°C or
below.

This is closer to what I'm looking for, but my temperature variations
are fairly large, and will occur on a daily cycle.

I may just have to assume some derating number and wing it, it doesn't
look like anyone has data for this sort of applicaiton.
Maybe automotive would but their temperature swings would be larger
than what I'm dealing with, so they would probably be judging the
parts too harshly.

The good news is that my ripple currents are relatively low, about 1/4
of rated, so I have that working for me.

 

[Lostgallifreyan]

dbvanhorn <microbrix@gmail.com> wrote in news:38482363-828b-4efa-a082-
bc866b2c4952@v23g2000vbi.googlegroups.com:

Do I calculate with the average temperature, or try to bin it, and
assume that the effects are relatively linear?

I'd bin it. Completely. As in, throw away the calculations outright. Most
industries derive stats from observations, then optimise for longer life,
better performance, whatever... If you're trying to model that kind of
complexity in advance, forget it. As Sam Goldwasser said to me a couple of
times when discussing SPICE, 'a simulated circuit gets a simulated degree'.
(Was a professor's remark to him, badly transcribed my me).

You can do a very detailed set of calculations but I doubt they'll be worth
the bytespace they occupy unless you start building and testing.

These are caps in the 2000uF range, so I don't have the option of
going to ceramics.

Could you use tantalum with a high voltage rating? If the answer is 'no
because of cost', then I doubt the project is a money-is-no-object exercise
in perfection so you might as well just get the best electrolytics you can
find and be prepared to replace them, and note how frequently you end up
having to do it. Choose some with higher voltage and temperature ratings than
you need, that's an easy cheap way to extend safety margins generally. You
can always try to economise later if tests show that you can risk it.

And even in a sunlit window you likely have a way to mount the caps in the
space least likely to get solar heating, and to provide a grille for
convection. There really ARE too many imponderables for pretty calculations
here, I think you will just have to build one and start basing your
assessments on observations.

 

[Lostgallifreyan]

dbvanhorn <microbrix@gmail.com> wrote in news:a92abc93-eb79-4061-92a5-
8facd424e0be@u11g2000vbd.googlegroups.com:

I may just have to assume some derating number and wing it, it doesn't
look like anyone has data for this sort of applicaiton.
Maybe automotive would but their temperature swings would be larger
than what I'm dealing with, so they would probably be judging the
parts too harshly.

In many cases they probably do, but that would be their trade secret, earned
from a lot of testing. They won't easily be persuaded to sacrifice that to
save others from doing the work required. Look at how laser diodes are
spec'd, and you'll soon find out how much of the exalted specs are derived
from mass longterm testing, and how few actual specs there are for new
devices. None of these people calculate in advance when the time is better
spent in setting up tests and gathering data.