Lifetime of electrolytic capacitors vs film capacitors

[treez]

Hello,
The MKP1848 film capacitor has a life quoted as 100000 hrs at 70 degC.
The life of a Rubycon BXC series wet electrolytic capacitor is quoted as 12000 hrs at 105degC.
..By way of the “10 degree rule”, we can say that the lifetime of the Rubycon electrolytic capacitor at 70 degC would be 96000 hrs.
This puts the lifetime of the wet electrolytic on a par with the film capacitor.
This doesn’t sound right, please advise if they are talking about a different kind of “end of life” failure mode in each case?

MKP1848 polypropylene fim capacitor
https://www.digikey.com/product-detail/en/vishay-bc-components/MKP1848S62010JY5F/BC2777-ND/3053110

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

- - - Updated - - -

..sorry, better links here

MKP1848 polypropylene fim capacitor
https://www.vishay.com/doc?28164

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

 

[barry]

The life of an electrolytic is generally considered to be 10 years; that's about 88000 hours.

 

[Easy peasy]

What you are missing is that the film/foil cap can take a lot more current for a lesser temp rise than the electro - very good quality electro's can last for 15 years - as long as the current is suitably de-rated to keep the Trise in bounds. What ultimately gets electro's is seal failure allowing the inside electrolyte out - temp cycling to extremes makes this worse (lower life). For film foil it is the terminations (zinc sputtering) from the connecting wires to the ends of the wound cap - that give out over time - high currents seem to affect this as well as temp cycling - some of these caps could easily last 100 years if the currents are reasonable and the temp cycling is minimal - however knowing which have the best process for end termination is very problematic ....

 

[treez]

Thanks, can we extend the life of an electrolytic capacitor by sealing it over in potting compound? This will stop it from drying out when it heats up?

 

[KlausST]

Hi,

Electrolytic capacitors usually are boxed in a aluminum cup.
The aluminum should prevent the elecrolytic from drying out, so only the top and the bottom - where no aluminum is - of the capacitor will be responsible for drying out.

Electrolytic capacitors have a safety vent. I don't think it's a good idea to modify this safety feature by potting the capacitor.

With an automotive application I've heard that even an air bubble in the glue underneath the capacitor (for fixing it) caused some problems. It had to do with some enclosed humidity within the bubble. I can't remember exactly what happened. If I remember right the prevented the air bubble by using vaccuum..

All in all I don't think that potting will reliably extend lifetime of an electrolytic capacitor.
You have to consider negative side effects.

Klaus

 

[c_mitra]

12000 hrs = 500 working days (about 1.5 years)

You will be lucky if the cap survives 1 year at 105C; the ratings must have been calculated for some low duty cycle. The heating is due to losses and causes the water to boil out.

The 10C rule is only approximate but good as a rough guide; but successive applications of the rule will not work. Electrolytic capacitors also have a shelf life (the corrosion problem).

 

[brushhead]

It is also very much affected by the ripple current and the frequency of operation of the ripple. When I have designed drives in the past it is important to gauge through simulation the harmonics of the ripple current. Manufacturers can normally give you a lifetime on an ambient operating temperature based on the current harmonics present. As a rule it is not a good idea to operate them at high ripple (at a low frequency) whilst at the same time hammering the ambient temperature.