# Determining input capacitor

Discussion in 'General Electronics Chat' started by pradeeba, Feb 28, 2008.

Feb 27, 2008
104
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What is the Formula for calculating input Capacitor in a SMPS?

2. ### rherber1 Member

Jan 6, 2008
15
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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?

Feb 27, 2008
104
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for eg: my input is 20-60 VAC,50 Hz, & my output power is 17W.

4. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12

Feb 27, 2008
104
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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.

6. ### rherber1 Member

Jan 6, 2008
15
0
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.