Electroyliic capacitor ripple current ratings

[sabu31]

Hi

I am operating a dc-dc converter at 100khz. The input voltage is about 35V and current is 12A with ripple of about 8A peak to peak. As the ripple current is to be supplied by capacitor. I would like to know about capacitor sizing equation and also ripple current rating of electrolytic capacitor. Thanking you.

 

[FvM]

Ripple current ratings are listed in capacitor datasheet, at least for low ESR capacitors. Capacitors without a ripple current rating have usually high ESR and aren't suited for switch mode applications.

In a rough estimation, the 100 kHz RMS ripple current and the total impedance Zc gives the ripple voltage. Zc² = ESR² + Xc²

 

[sabu31]

Hi FvM

Thanks for the reply. I would like to know,in general what would the ripple current rating of a standard 100uf 63 V electrolytic capacitor(not low ESR types).Is it okay to use 10*10uf ,63V electrolytic capacitors in parallel to get 100uf,63V for say 8A ripple current at 100khz switching frequency.

 

[FvM]

A cheap standard 100µ/63V capacitor has e.g. a 290 mA@120 Hz specification, 10µ/63V 65 mA. (Jamicon SK series, RMS values). Ripple current ratings at 100 kHz are e.g. factor 1.3 to 1.4. larger.

Even with very good low ESR capacitors, you'll probably need to parallel several capacitors.

 

[sabu31]

Thanks FvM

Then what would be good and economical choice to give ripple current of 10A roughly?.I saw in one paper,they have used 10uF ,50V ceramic capacitors but it seems to be very expensive.

 

[ftsolutions]

That is a very large amount of ripple current, and will require either alot of capacitors or some quite expensive high power capacitors to withstand it without overheating. Perhaps you can modify your design so the ripple current demands are minimized.

 

[sabu31]

Hi ,

I have seen data sheet for smd ceramic capacitor(10uF ,25V) that can give ripple current of 2A at 100Khz. I have attached data sheet. Is it possible to use this as input capacitor(at appropriate voltage rating)

 

[ftsolutions]

This capacitor could be used in a power supply design, but 2A is not 8A

 

[FvM]

When considering ceramic capacitors, you should also look at their voltage dependent capacitance. As a rule of thumb, don't use high Er ceramic capacitors above 50% of theit rated voltage when capacitance matters. As you wrote in post #3, a total capacitance of 100 µF seems to be a reasonable number for the said application, less capacitance will results in two much ripple at 100 kHz. 10*10 µF ceramic caps will surely handle the ripple current. If it's appropriate for your design depends e.g. on your component sources.

 

[sabu31]

When considering ceramic capacitors, you should also look at their voltage dependent capacitance. As a rule of thumb, don't use high Er ceramic capacitors above 50% of theit rated voltage when capacitance matters. As you wrote in post #3, a total capacitance of 100 µF seems to be a reasonable number for the said application, less capacitance will results in two much ripple at 100 kHz. 10*10 µF ceramic caps will surely handle the ripple current. If it's appropriate for your design depends e.g. on your component sources.

Thanks fvm

My source voltage maximum could be around 42V. Is it fine to use 10*10uf ,50V ceramic capacitors along with a bulk electrolytic capacitor of 470uF,100V?Will the capacitance of ceramic capacitors decreased. I am also confused why a film capacitor of 22uf 100V has a much bigger size than ceramic smd and also seems costlier.

 

[FvM]

Read the datasheets. The capacitance of 50V cepacitors will typically drop to 30 to 35%.

Secondly, a considerably ripple current share loads the electrolytic capacitor.