Radial leaded Electrolytic capacitor lifetime equations

[Flyback]

Thanks, the thing is , why is it that no Radial electolytic capacitor datasheet actually gives the internal temperature rise of the capacitor when it has rated ripple current flowing in it?......without this information, it is absolutely impossible to do a proper lifetime calculation....unless you are lucky enough to get one of those capacitors with a thermocouple built into it..or the capacitor datasheet gives you the Heat radiation factor of the capacitor, so that you can work out the internal temperature rise from the case temperature rise.....but again, that data is never in the datasheet.

Just imagine if a power fet datasheet didnt tell you the theta(jc).....it would be ridiculous.
Is this all about industrial secrecy or something i wonder?

 

[Diver300]

Surely the heating is spread around the capacitor, and the thermal resistance is within the capacitor as well as from the capacitor to the outside. Therefore the temperature rise will be different in different parts.

But you don't care about the temperature rise, it is the lifetime that matters, so as long as the formula allows the lifetime to be calculated from the ambient temperature, the ripple current and the average voltage, that is all you need.

 

[Flyback]

Thanks, the formula for electrolytic capacitor lifetime calculation is attached, and there is never enough data in the datasheet to satisfy this equation. Certainly not for radial electrolytics.
(BTW thankyou very much for correcting this equation earlier)

Rubycon BXC series...
https://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/e_bxc.pdf

The way i always do it (lifetime calculation) is to assume a value of alpha as in the exerpt already shown.
Alpha is the ratio of internal temperature rise to top surface temp rise.
You can measure top surface temp rise, and then with the assumed value of alpha, you calculate internal temp rise...then from that you can calculate the internal temp rise with rated ripple current.
But this assumes a value for alpha....which is usually said to be a value of about 1.2 for 18mm diameter radial electrolytics.

 

[Flyback]

Hello,
Please may i put this here as its related to the above....
Do you know where to get the lifetime equation for this panasonic film capacitor.....

ECWFD series film capacitor, 4u7,450V
https://uk.farnell.com/panasonic-el...film-capacitor-4-7uf-450v-5-radial/dp/2418957

The Panasonic website gives no lifetime equation or data for it....
https://industrial.panasonic.com/ww/products/capacitors/film-capacitors/film-cap-electroequip

 

[Diver300]

Film capacitors don't have a lifetime limit like electrolytic ones do. There is no electrolyte to dry out in film capacitors.

 

[Flyback]

Thanks, thats what i thought, until i read post #19 above...repeated here...

https://www.illinoiscapacitor.com/tech-center/life-calculators.aspx

However, when you read it, you see that no film capcaitor datasheet gives the information required for you to use the equation for film capacitor lifetime given there...eg the "thermal constant , B".........no film cap datasheet gives it.

 

[Diver300]

Yes, I stand corrected.

However, something like this https://www.illinoiscapacitor.com/pdf/seriesDocuments/PHC series.pdf has a life with DC of 100,000 hours. Also the increase in life as the voltage goes down is very large as well. According to the link in post #19, it is the 7th power of the voltage, so a 400 V capacitor running at 300 V lasts 7.5 times as long, which puts it at 750,000 hours or 94 years.

Strangely, other film capacitors, like this one:- https://www.farnell.com/datasheets/1598281.pdf?_ga=1.264073547.699924908.1484343272 only start with a life of 2000 hours, but that is at 125% of rated voltage. I suppose it is something to be aware of.

 

[Flyback]

Thanks, i wish the PHC series film cap datasheet gave lifetime calculation parameters