Switchmode dc-dc converter, input capacitor questions.

[cages]

I am designing a high current DC-DC convert and from the equations on the data sheet.
I have calculated input capacitor of about 200uF

and the RMS current that will flow through the input capacitors will be 5A.

without going into too much information
if I choose to use 15uF ceramic that has a ripplle current rating of about 6A and get the rest of the capacitance from a aluminum electrolitic capacitor with ripple current rating of 0.5A will most of the current flow through the ceramic capacitors.

Are there any problems with this solution.

Thanks Cages.

 

[max0412]

Simply put no. Heres why.

i(t) = C dv/dt v(t)

I dont know what your rail voltage is or frequency is but plug your variables into that equation and you'll find out how much current goes into your Caps.

 

[dfullmer]

Hi,

max0412 is correct don't do it. Current will be distributed based on the capacitance network and the energy stored in each cap.


regards

dfullmer

 

[tillu]

the ceramic capacitor may help you only for a short transient peroid at the instant of switching, but it will not help you any further.max0412 is right.

 

[cages]

Thanks for the answers to my questions.
I have come back to this problem after taking a break over the holiday's

The DC-DC buck converter operates at 200khz
Input voltage is 40V output Voltage is 5V supplly max 10A.

As I stated in the original question I need a total 200uF min capacitace with rms current 5A as the input capcitor.
I am not able to find any suitable capacitors without either using a lot of capacitors or a really big and expensive capacitor.
both solutions are costly and will take up space on my design.
If any one could sugest suitable capacitors.
Or am I missing something in the data sheet that will make the selection a lot more straight forward.

thanks,
Cages.