Capacitor life calculation

[dharithothi]

Hello Guys,

I was checking for capacitor life... How long it can run...

I found calculator for measuring life. In general we are mostly using electrolyte and ceramic caps usually in routine Basic Electronic circuits.

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

I found one parameter ripple current is affecting for electrolyte capacitor life. How ever it is not considered in calculating in ceramic capacitor life... Why so??? What is the reason behind it???

 

[KlausST]

Hi,

ripple current in combination with ESR causes heat.

ESR is relatively high for electrolytic capacitor, so the produced heat is much more than with ceramic capacitors.
Heat causes drying of the electrolyte of electrolytic capacitors, so the influence of heat causes more damage with electrolytic capacitors than with ceramic capacitors.

Klaus

 

[schmitt trigger]

Complementing Klaus comments:

Electrolytic capacitors have a liquid electrolyte. That liquid slowly but surely escapes thru the bottom seal.
This can take many, many years if the capacitor is not being used.

However....temperature causes liquids to expand, even to boil. The increased pressure will significantly increase the electrolyte loss.
High ripple currents cause internal self heating. If additionally you have a high ambient temeprature, the electrolyte will be lost very quickly.

Since you are posting in the power electronics forum, I assume that you have an interest in building a power supply.
If so, I'm going to give you the best advice: Avoid the 85-C rated capacitors. Always use 105-C or 125-C rated capacitors. They cost a little more, but will last significantly longer.

 

[dharithothi]

ok...

So basically ESR is there behind increasing temperature... So what should be recommended ESR if we are using Ele Cap for power supply design??? Can anyone suggest me capacitor selection guidelines...

 

[Warpspeed]

Its really all about temperature rise as mentioned above.

There are two main factors, the ambient temperature the capacitor has to work in, and internal capacitor temperature rise due to ripple current.
You first need to work out your maximum internal ambient air temperature due to all the other components in the power supply.
Obviously a cooling fan helps significantly.

Then any additional temperature rise due to internal capacitor heating added onto that.
The capacitor manufacturer should then be able to supply an estimated lifetime curve.

Using multiple capacitors in parallel will work better than one large capacitor if the ripple current is unusually high, simply because the surface area to volume ratio gives much better cooling.

Often its only the first capacitor straight after the rectifier that has a very high ripple current, a second stage filter rarely suffers quite as much.

The first stage capacitor may require an expensive Low ESR electrolytic, or a ceramic capacitor.
Second stage capacitor can often be a low cost standard aluminium type.

The first thing you really need to do is run the supply and determine the maximum internal ambient temperature.
Next thing is to measure the RMS ripple current.
With that information you can then start to approach the problem from a known starting point.

 

[KlausST]

Hi,

Can anyone suggest me capacitor selection guidelines...

In many smps IC datasheets there are guidelines on how to select the capacitors.


Klaus