Lifetime calculations for electrolythic caps

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Lars Clausen said:


Bruno: Are you saying that the caps used on your modules have this spec'ed lifespan?

Please read the explanation again. I'll try to clarify. Lifetime of an electrolytic capacitor is a function of ambient temperature and ripple current. The lifetime spec given in a data sheet is one single point from this function. Because the function further evolves identically for all capacitors, it is not plotted. Like in a data sheet for resistors there is no graph for current vs voltage.

Thus, extrapolating from this one data point it can be shown that the caps I'm using (which are specified as 5000hrs at 85 degrees) have over 50k hours in them in the conditions at which I use them.
 
Lars Clausen said:
About EMI / RFI we made huge reductions 2 years ago when we introduced ver 2.1. Of course improving step by step since then. And we are at comparable levels with the other Class D amplifiers on the market, not only from Bruno and Jan-Peter, but in general.

Our modules are around 20dB better than the legal limits. This margin is necessary to insure users have equally good radio reception with our modules compared to a linear amplifier, even if the antenna runs parallel to the speaker wires (not unusual). This level is unparallelled by any competitor.
 
Thus, extrapolating from this one data point it can be shown that the caps I'm using (which are specified as 5000hrs at 85 degrees) have over 50k hours in them in the conditions at which I use them.
Can you show us how you made this calculation? And also give a brand and type of capacitors you are using? 50.000 Hrs that's very impressing ;)

Our modules are around 20dB better than the legal limits.

Sounds very impressing. What legal limit are you using for this? :)
 
Bruno Putzeys said:
Lifetime of an electrolytic capacitor is a function of ambient temperature and ripple current. The lifetime spec given in a data sheet is one single point from this function....
Lifetime is really difficult to calculate if you don't already know the results.

At work we had a low price product and suddenly the lifetime was less than 12 months, very irritating becauase of the product garantee. At that time I started to investigate the lifetime and the conclusion was that the caps should last 6-10 months but in real life it was 3-5 years.

So our problem was that suddenly our caps had become worse but within specs. After that we changed brand to Rubycon and we got much better results. Later we also made the design run cooler, less than 60 deg C.

At the moment we have no dried up caps at all.

So, keep the caps cool and you will have no problems with lifetime.
 
Sorry to butt in, I have a question regarding the life time calculations you refer to:

Do you measure the frequency content in the ripple current, or do you “just” use the fundamental switching frequency as ripple frequency for the calculation. I ask because I’m working (at work :() with life time estimations for electrolytic caps in switching applications (SMPS).

I ask because I have used ceramics exclusively until now.

Thanks

\Jens
 
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