Determining input capacitor

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

  1. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
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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?
     
  3. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
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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
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  5. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
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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
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    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.
     
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