Bootstrap Capacitor Calculation Issue

Discussion in 'General Electronics Chat' started by DumbDummy77, Feb 1, 2010.

  1. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    Hi

    I am currently learning how to calculate the bootstrap capacitor for IR2110 Application. So far I have identified every parameters apart from one:

    Qgate 27E-06
    Frequency 21600
    Icbs(leak) 0
    Qls 5.00E-09
    Vcc 14
    Vf 0.78
    Vls 4.5
    Iqbs(max) 230E-06
    Vmin ??

    I use the forumla from Application Note AN-978 on page 6. At the moment I am not certain on how to obtain Vmin, I use a half bridge configuration and the voltage is about 100V and the IGBT is IRG4BC20UD.

    Please explain me how to obtain Vmin?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Vmin = the minimum voltage between Vb and Vs.
    Vb = the high side of Cboot.
    Vs = the junction of the high side MOSFET source and the low side MOSFET drain; the load connection point.
     
  3. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    Can you please give me an example in number?

    Regards

    MM
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It depends on the MOSFET you're using.

    If it's a standard level MOSFET, you'd want Vmin to be 10v.
    If it's a logic level MOSFET, you'd want Vmin to be 5v.
    If it's a very low threshold MOSFET, you could use a lower number for Vmin.

    Cboot is what supplies the current to charge the MOSFET gate to turn it on. In order for a standard level MOSFET to be considered fully turned on, you need to get the Vgs (gate relative to the source voltage) up to 10v.
     
  5. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    Right I get the picture, for my application, I use IGBT such as RG4BC20UD at the moment. According to the datasheet, the Vge is +/-20V so that mean I apply 20V in Vmin. Is it correct?

    I am struggling to understand it, to be honest with you. Can't you give me a design example? Hopefully i grasp the idea quickly

    Regards

    MM
     
    Last edited: Feb 1, 2010
  6. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    Nothing too complicated. Just make that voltage a suitable voltage to turn on the IGBT.
    Try 15v it's what most IGBT gate drive circuits i've seen use.
    It's somewhere inbetween VGE(th), which is the voltage where the device beings to conduct, and VGE(max), the maximum voltage you can put on the gate.
     
  7. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    What about MOSFET?
     
  8. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    0
    Vmin is the difference between Vb and Vs. Hence since Vb is connected to Vcc which make 12V (Vcc = 12V). I am not sure about Vs to be honest. If I am mistaken about this, please correct me asap
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You need to look at the Vce(on) specification in the datasheet.
    There you will find Vge=voltage - for that particular IGBT, it's 15v.

    If the design is simply handed to you, how much would you learn in the process?

    You say you already have all the other numbers - plug them into the formula, and you'll have your result.
     
  10. DumbDummy77

    Thread Starter Member

    Jan 21, 2010
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    Thank you SgtWookie and I have another question to ask you do I need to connect correct resistor from the IR2110 to the gate of IGBT. If yes, what forumla should I use?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Go back to that same Application Note, and start reading on page 9.

    Note the reference to DT97-3 on page 11.
     
  12. legaspito

    New Member

    Nov 21, 2011
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    i believe you should use a resistance value that does not make your voltage drops under the specified Vg you know what i'm saying
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    This topic has been asleep for 21 months, and the original poster has not returned since the end of May. It is not likely that they still require help on this.

    Please view the last reply date of the original poster before replying to a thread.
     
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