No announcement yet.

Question about gate drivers and bootstrapping

  • Filter
  • Time
  • Show
Clear All
new posts

  • Question about gate drivers and bootstrapping

    Hello everyone
    I want to design an h-bridge DC motor driver IC using discrete MOSFETs and a gate driver.
    Now, most motor drivers I have seen use a half-bridge gate driver like the IR2184, which utilizes a bootstrap circuit to use the NMOS as a high-side switch.
    As far as I understand, bootstrap operation requires you to continuously alternate between the 2 MOSFETs in order for the bootstrap capacitor to have time to charge.
    This is all good if, for example, I feed a 50% duty cycle PWM signal to the driver, in that case, the motors will keep running at 50% speed just fine. Now what if I run them at 100% (i.e. feed a fixed input voltage) for a long time? will the driver work for a little bit then the bootstrap capacitor gets discharged and the driver stops? and how can this be handled without limiting the duty cycle to, for example, a maximum of 90%?
    thank you

  • #2
    If you want to run a DC brush motor at 100% for an extended time, you need a separate bootstrap circuit.
    I have seen such circuits in 'complete' drivers (most of them include the FETs too), but not in (half bridge) gate drivers (at least not for higher voltages > 100V).

    From experience you can go up to 97% duty cycle.

    If you want to create one of your own, look at L6205 (from ST) for inspiration.
    Last edited by qdrives; 04-03-2023, 12:48 PM.


    • colhany
      colhany commented
      Editing a comment
      Hi qdrives, thank you for the reply.
      I want to make a driver for low voltages, maybe up to 35 volts only.
      I have found a document from Infineon suggesting using one of their half-bridge drivers (which uses bootstrap) with an additional charge pump circuit using the 555 timer, I will try it out.

      This is the document I'm talking about.
      You can also look at it and tell me what you think, if you have time for this.
      Thank you

  • #3
    Question is - when you are switching (so not 100%) will you damage the FET as you are feeding Vbus+12V and not Vs+12V?
    If your gate resistor has a high(er) resistance it may be ok.


    • #4
      Thank you for the hint
      I will try running a simulation to see what happens


      • #5
        You will need to add the parasitics too.