value of cap in middle of half bridge.

Discussion in 'The Projects Forum' started by iamhere, Jul 12, 2009.

  1. iamhere

    Thread Starter Active Member

    May 1, 2009
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    Hello all,

    We can see in many half bridge circuits , there is a capacitor connected to the middle of half bridge(between 'Source' of High mosfet,and 'Drain' of Low mosfet) to power the driver IC , now the question is :

    if I want to get 20ma current, what will the value of capacitor connected to the middle of half bridge when voltage =400v , freq=30khz ? I know the way to calculate capacitance and current in AC circuit, but here in the middle of half bridge, voltage is DC near square wave.

    Thanks a lot.

    attached: example schematic
    V=400v
    Frequency: 30khz
    Question: what is the value of CSNUB if I want to get 20ma current?

    [​IMG]
     
    Last edited: Jul 12, 2009
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic and a link to the datasheet of the driver?

    Greetings,
    Bertus
     
  3. iamhere

    Thread Starter Active Member

    May 1, 2009
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    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think the capacitor CSNUB is used at startup of the cfl tube.
    At startup the tube uses a much higher voltage than it needs to burn.
    The capacitor CSNUB makes a booster with the diodes DCP1 and DCP2.

    [​IMG]

    In the datasheet they do not mention the value.
    I think it will be someting between 1 and 10 nF.

    Others may correct me if I am wrong.

    Greetings,
    Bertus
     
  5. iamhere

    Thread Starter Active Member

    May 1, 2009
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    Thanks Bertus,
    undoutly , CSNUB provide power source to IC when half bridge works .
    now I need to know the capacitance needed for 10ma , 20ma etc.
    DCP1 is a zener.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here is an application note with some practical values.

    Greetings,
    Bertus
     
  7. iamhere

    Thread Starter Active Member

    May 1, 2009
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    Thanks Bertus again, unfortunately I will not use IR2320( and I can not find the the current data of Ir2320 from sheet),

    BTW: I like your cute cat very much.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Wich chip will you be using?
    Here is the datasheet of the IR2520.

    Greetings,
    Bertus
     
  9. iamhere

    Thread Starter Active Member

    May 1, 2009
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    I will not use IR2520. I need about 20ma current to power a mcu.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    So you need 5 Volts 20 mA for a microcontroller.
    Do you want to power it right from the mains (240 Volts)?

    Greetings,
    Bertus
     
  11. millwood

    Guest

    I would suggest that you look for a topology with isolation. This circuit, as posted, will expose your entire mcu and you in the process of debugging it to mains voltage.

    a $10 cell phone charger would be better and safer than this.
     
  12. iamhere

    Thread Starter Active Member

    May 1, 2009
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    I do not want to consider other way to power the IC, capacitor from middle of half bridge is the most economical way.

    Vbus= 400 v
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    So you want to calculate the capacitor so it will drop (400 - 18) = 382 Volts at 20 mA.
    The impedance will be 382 / 0.02 = 19100 Ohms.

    Use the formula:

    [​IMG]

    C=1/(2 X π X 30000 X 19100) = 277 pF.

    Greetings,
    Bertus
     
  14. iamhere

    Thread Starter Active Member

    May 1, 2009
    33
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    Thanks Bertus again, this formula is for AC case, but voltage here is DC square wave ( about 200v in average) I do not know if this formula is still proper.
     
  15. iamhere

    Thread Starter Active Member

    May 1, 2009
    33
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    in datasheet of IR2159 , the Icc about 10ma only, but the CSNUB is 1.5nF . if the formula is proper, current will be about 100ma when 50khz.
     
  16. iamhere

    Thread Starter Active Member

    May 1, 2009
    33
    0
    in my application, 1.5n can run the IC well , but in testing , current only about 23ma.
     
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