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Choosing ceramic capacitor - I RMS rating

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ln023

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My appilcation is a DC-DC converter design with V-out 3V, I out 5A.
I rms at output = 2.5A (assuming worst case Irms as Iout/2).

How to choose Output decoupling capacitors in DC-DC design to stand this rms current rating?
RMS current rating is not provided in most capacitor datasheets.

If i need to choose a 0.01uF cap , what should the Voltage rating of the capacitor be?
I think rather than choosing a cap with Voltage rating around 6V, it should be based on ESR value, and assumed I-rms, the voltage rating should be greater than ( I-rms x ESR ) Volts.
Please provide your suggestions and comments

Thanks
Ln
 

Some points are completely unclear in your post, respectively sound confused.

Its very unlikely that a 0.01uF decoupling capacitor will carry considerable ripple current in a 3V supply. What's the total output capacitance, switching frequency and expected ripple voltage? Current rating can be a problem with large capcitance values, you'll find specification and calculation tools at least from some manufacturers, e.g. Murata.

A reasonable calculation goes the other way around:
Maximum filter capacitor impedance √(ESR² + Xc²) = acceptable ripple voltage/ ripple current.
 
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    ln023

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Hi FvM,

Thanks for your reply.

In my application, total output decoupling capacitance used near 220uF.
Maximum allowed ripple is 60mVpp. Switching frequency is 500kHz.

How do I choose any capacitor's voltage rating for this application?

Regards
Ln
 

Dear LM,

This document should solve your problem. and about the missing values of ripple current, I match mfd,voltage,case size, and temp. with the good known mfg. like panasonic or others. then I pick the worst case available value of ESR or ripple current.

The document is made by TI.
 

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Ans for your post no #3

**broken link removed**

in the attached pdf read page 10.

In other words voltage stress is one factor which decides the life of capacitor in circuit.
 

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