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Capacitor Ripple current rating

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zuq

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Hi guys

I have calculated the amount of capacitance I need at the input of a buck converter and I know how much RMS current my capacitors will need to carry.
Calculations say I need about 50mF and RMS current rating should be about 100A. Working at 24V

When I try finding a capacitor of 50mF rated at about 50V, it can only handle about 10A. To get 100A out of these caps I ll need like 100-200mF capacitance. Is that how it normally works? You find how much you need and then work out if it can handle the current ?

If I get a cap that can handle 10A at 400V doesn't mean it can do that at 24V right? Q=CV ??
 

A single cap is not likely to be able to absorb and release 100A at the speeds you require.

This is an ESR (equivalent series resistance) issue even though a cap is brand new and faultless.

Notice that even if we assume a very optimistic 1/100 ohm as the ESR, sending 100 A in and out of it will generate 100W of heat. (I x I x R). (Or less depending on duty cycle.) This must be considered even though safe watt rating is not a spec we usually see listed for capacitors.

This looks like a job for several capacitors. Each should be tested for lowest ESR possible.
 

Correct try to use 2 or more capacitors in parallel, but rating must be more than 24V.
 

If I get a cap that can handle 10A at 400V doesn't mean it can do that at 24V right? Q=CV ??
Click to expand...
Current rating is voltage independent. As explained. it's a matter of ESR rather than charge. It' however frequency dependent to some extent.

Your question seems to suggest a high power and high frequency switching converter. You're probably better with foil or ceramic capacitors to carry the high frequent ripple current. Electrolytic caps may be added as energy storage, if necessary.
 

Hi

Thanks for response guys,
It is a 5kHz DC motor drive, Data sheets normally give current ratings at 100Hz or 120Hz. I can find the total RMS current in capacitors but how do I find current at a particular frequency?
 

You're going to need a large quantity of mid-size PCB mounted electrolytics, or a handful of large screw terminal ones. Even large can capacitors generally have ripple current rated up to about 30Arms. I would opt for getting a small number of large capacitors.

The RMS ripple current ratings for capacitors will be somewhat frequency dependent, but it doesn't change a great deal. Maybe -25% max from 120Hz to 10KHz. Most datasheets will give ratings at a low frequency (100/120Hz) and a higher frequency (1KHz/10KHz/100KHz).

Make sure you properly derate the ripple current ratings for ambient temperature, as well.
 

With output currents in a 100 A range, it's often suggested to go for a multi-phase converter to reduce the input current ripple.
 

zuq said:
It is a 5kHz DC motor drive, Data sheets normally give current ratings at 100Hz or 120Hz. I can find the total RMS current in capacitors but how do I find current at a particular frequency?
Click to expand...
This cap, you are targeting for transient load or at supply to the regulator? I am not much clear in that.
Other than RMS , capacitor does not consume dc current, so dont worry about that. Anyway, Your capacitor is not about ripple current, its about ripple voltage.
 

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