Bootstrap instead of any driver for mosfet

Discussion in 'The Projects Forum' started by rabiuls, Aug 7, 2012.

  1. rabiuls

    Thread Starter New Member

    Jan 18, 2012
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    0
    Hello,
    I want to make a buck converter for lead acid battery charging. I want to use IRF540 N Channel mosfet. And I don't want to use any driver IC to drive the mosfet gate. I shall send pwm signal from 5v microcontroller. Can I use the following bootstrap circuit (found on net) instead of using any driver IC for my purpose. Or is there any better method to drive the mosfet without a mosfet driver IC? PAD1 of the circuit would be connected to microcontroller's PWM pin.

    [​IMG]

    Thanks in advance.

    Best regards.
     
    Last edited: Aug 7, 2012
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    The answer depends on the frequency of operation. Which is?

    At high frequency, supplying and removing the gate charge adds up to a significant current. At "low" frequency, it's too small to matter.
     
  3. rabiuls

    Thread Starter New Member

    Jan 18, 2012
    7
    0
    The frequency can be 100kHz to 500kHz. Which frequency will work best for the circuit?

    Thanks.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    This is out of my range, but I believe the capacitance of the IRF540 is 2nF, or 20nC at 10V. Moving that charge in and out at 500kHz is a current of 10mC/s = 10mA (or is it double that, the sum of in+out?).

    That doesn't sound so bad. Maybe inductance increases the problem.

    Anyway, we'd better see if someone with more experience can help you here.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    It's a little worse than that. ;)

    Per the data sheet the IRF540 has a total gate switching charge of 71nC. At 500kHz the switch period is 2μs. Assuming you want a switch rise and fall time of no more than 10% of that or 0.2μs gives a switching current of 71nC/0.2μs = 355mA. That certainly requires a much lower resistor value than the 1kΩ pull-up shown in the schematic. A push-pull or totem-pole type driver would likely be required.
     
    #12 likes this.
  6. wayneh

    Expert

    Sep 9, 2010
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    I knew 500kHz wouldn't be that easy. Thanks for setting us right.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    50kHz is about as fast as you'd want to run that. You might could go somewhat faster depending on the value of the inductor that you would use with it.

    You're showing +12v for your supply, which won't be adequate for charging a 12v lead-acid battery. You would need ~13.7v just to float-maintain it (standby mode). If you're charging it after being partially drained, you should charge it at a constant current until you hit ~14v to ~14.5v before dropping back to the "float" value.
     
  8. rabiuls

    Thread Starter New Member

    Jan 18, 2012
    7
    0
    Hello all,
    By reading the replies I am not confident with the circuit. Can I use a NPN transistor like TIP-122 instead of mosfet? Since it does not have special gate voltage requirement like N Channel mosfet.
     
  9. Tim Woolaver

    New Member

    May 15, 2015
    2
    1
    Just though you would like to know

    I built this and it works and I am using it in a product design for a 30A PWM controlled voltage source.

    Values are different by it is the same schematically.

    And I am a real live electrical engineer who designs circuits for a living.

    Thanks
     
    wayneh likes this.
  10. ebeowulf17

    Active Member

    Aug 12, 2014
    678
    79
    At what frequency?
     
  11. Tim Woolaver

    New Member

    May 15, 2015
    2
    1
    100KHz pwm driven from a microcontroller
     
  12. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    So what are these circuit "different values"?
     
    Last edited: May 15, 2015
  13. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Why such a high frequency?

    You cannot use that circuit to charge a battery without some limiting impedance in series with the battery (resistance or inductance).
     
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