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In general you want to give them sufficient Farad value so they do not rise and fall very much.
The input capacitor receives energy from the supply through a certain amount of resistance. so it can give it up quickly through very small resistance near to the converter. Ripple voltage maybe 5 or 10 percent should be okay.
The output capacitor discharges to the load during idle gaps in the output cycle. Suppose you want 3 percent ripple voltage. You can calculate what is a suitable C value by typical formulae. Example, the RC time constant, etc.
Keep the ripple to about 2% of voltage.
The ripple can be calculated by V = Q/C, where q is the charge that gets pumped into the cap every switching cycle.
Remember to account for the esr related ripple too, if that is significant.
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woops , i didnt realise Klaus had written that before i wrote mine.
Also, ensure the caps are'nt over-ripple-currented.
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If you do an v=ir calculation for the esr related ripple , then the overall voltage ripple is SQRT(v1^2 + v2^2)
Where v1 = peak current into cap * esr
V2 = Q/C as above
I got this equation from an application note for output capacitor design
c=I*D/(f*V)
C= minimum output capacitor
I= maximum output current
D= Maximum duty cycle
f= frequency
V= voltage ripple
is this okey???
As long as you get under your ripple spec, and dont over ripple current it, then it doesnt matter what is the value you use.
Here is a boost converter sim for you to experiment with, and check your calculations. it is in free ltspice
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