1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Charge Boot Straps Motor driving

Discussion in 'Microcontrollers' started by elecLear78, Jul 30, 2017.

  1. elecLear78

    elecLear78 Member

    Joined:
    Jul 16, 2013
    Messages:
    226
    Likes:
    1
    Location:
    india
    I have seen the term charge bootstrap in motor driving. I don't know much about it. Can someone please explain me at the high level the schematic of a 3 phase driver circuit and where this capacitors will be present. What is the power supply we have to apply to the driver circuit? How much voltage and current does the driver IC provide? Does the motor require voltage or current? Is current important or Voltage? I am poor in hardware.
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,592
    Likes:
    477
    Location:
    L.A., USA Zulu -8
    Sorry, but I've never heard of charge bootstrapping as related to motor driving.
    Where did you see that term?
     
  3. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,668
    Likes:
    64
    Bootstrapping to provide a gate supply to the high-side NMOS in a half-bridge.

    I'll only briefly describe it to get you started. Read more on the links below.

    Bootstraps provide a floating voltage supply to drive the gate of N-channel devices. This requirement arises whenever you use N-channel devices like NMOS on the high-side of an half-bridge (a H-bridge is two half bridges and a 3-phase/BLDC motor driver is three half-bridges).

    The source terminal on these high-side N-channel devices is floating (not at a fixed potential). You can see that this a problem since it is the gate-to-source voltage that turns the FET on and off.

    Bootstrap is one method. A boostrap diode and boostrap capacitor are connected in series between the gate-drive voltage supply and the source terminal of the high-side FET (the output node of the half-bridge that connects the motor, the point where the high and low-side transistors connect). Look in the links provided to find a diagram.

    Whenever the low-side transistor closes, the diode lets the bootstrap capacitor charge up to the gate-drive supply. Then whenever the low-side FET opens, this boostrap capacitor floats on whatever voltage is at the high-side FET source terminal. The diode is there to prevent the boostrap capacitor from discharging back into the gate-drive supply when the low-side transistor opens and the capacitor floats up above the gate-drive supply. The boostrap capacitor then powers the circuits driving the high-side NMOS gate.

    You can't achieve 100% duty cycle with this method since the capacitor will eventually discharge. The boostrap capacitor recharges whenever the low-side transistor closes (which happens frequently in most motor drive algorithms. If it doesn't happen frequently, you need to make it happen..like synchronous switching in your half-bridge or something like that).

    The boostrap capacitor needs to charge up to a voltage high enough to fully turn on the MOSFET (which should happen if the bootstrap is charged by the gate-drive supply since this supply drives the low-side transistors directly.). The capacitor needs to be large enough to keep the transistor on.

    MOSFET gates are capacitors. Once they are charged to the voltage to turn them on, they draw almost zero current. But to turn them on fast for high frequency switching you need to provide a big current burst to charge up the gate capacitance quickly. This current is NOT the same as the motor current. I am only talking about the gate signal used to turn the MOSFET on and off.

    https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d462584d1d4a01585242c11947b1

    https://www.infineon.com/dgdl/an-978.pdf?fileId=5546d462533600a40153559f7cf21200

    https://www.infineon.com/dgdl/an-1123.pdf?fileId=5546d462533600a40153559692661096
     
    Last edited: Jul 30, 2017
    • Like Like x 1
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,592
    Likes:
    477
    Location:
    L.A., USA Zulu -8

    I am familiar with bootstrapping in MOSFET gate drivers.
    I just didn't understand his reference to it in 3-phase motor circuits.
     
  6. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,668
    Likes:
    64
    Not sure where the confusion is coming from but it seemed clear to me he's asking about about gate drivers in brushless motor drivers.
     
    Last edited: Jul 30, 2017

Share This Page