What is the Formula for calculating input Capacitor in a SMPS?

 

What is the Formula for calculating input Capacitor in a SMPS?

A very open-ended and not too specific question...

You need to provide a lot more detail. For example;

Input requirement - is it DC input or off-line?
Output requirement - what output power?

 

for eg: my input is 20-60 VAC,50 Hz, & my output power is 17W.

 

Although a little bit on the expensive side, I highly recommend a flux capacitor for highest efficiency:

http://kalecoauto.com/index.php?main_page=product_info&products_id=28

 

I think all are aware of the thumb rule for determinig input caapcitor.
I/P Voltage Cin
115V 2uF/W
230V 1uF/W
85-270V 2....3uF/W

But my input voltage range is 20-60 VAC.
Then how to determine input capacitor. am looking for a formula so that i can calulate the value.

 

I think all are aware of the thumb rule for determinig input caapcitor.
I/P Voltage Cin
115V 2uF/W
230V 1uF/W
85-270V 2....3uF/W

But my input voltage range is 20-60 VAC.
Then how to determine input capacitor. am looking for a formula so that i can calulate the value.

There is nothing critical at all to worry about so there is no need for a specific or accurate calculation. Ball park on the high side is good enough.

You can see from the figures you quote that for a low end input voltage of 85V, a value of 3uF/W is nominal. Therefore, the lower input voltage you use the larger the capacitance/W needs to be. for 20 - 60V guesstimate 5uF/W which gives you around 85uF for 17W. Now you won't be able to buy a cap with this value so you go to a readily available value larger than this, eg. 100uF.

Of course the major factor governing your final decision will be "how much space is available for your caps"? Will a 100uF cap fit in? If you have more than enough space you may be better off going for a 150uF cap because in this case "bigger is better" in regard to filtering and input regulation.