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Bootstrap and DRV8301 (on DRV8301-69M-KIT)

Philipp L.
Intellectual 795 points
Other Parts Discussed in Thread: DRV8301, DRV8301-69M-KIT

Hello dear e2e-Community,

We are planning a motor control project, and for the first steps, we want to use the DRV8301-69M-KIT, where a DRV8301 is applied as motor driver. 

Now, one very important functionality is the direct control of the mosfets for tests and measurements. I need to be able to set a specific combination, for example:

Phase A, top -> high, or ON

Phase A, bot -> low, or OFF

Phase B, top -> low, or OFF

Phase B, bot -> high, or ON

Phase C, top -> low, or OFF

Phase C, bot -> high, or ON

Then, I want to perform some measurements with the motor over some seconds, or even minutes, and switch to another mosfet-combination, and so on. 

Am I right that this will be difficult with the DRV8301 driver because of the bootstrap circuit? Is it even possible?

And the next step would be the block commutation without PWM and without any current control, just switching the mosfets as the hall sensors dictate. At low speeds, this would be nearly the same as the above, and it should also be difficult with a normal bootstrap circuit.

And what is the maximum and minimum duty cycle with the DRV8301-driver?

What do you say?

Thank you for your help!

Best regards,

Philipp L.

  • 0
    TI__Mastermind 21320 points

    Phillip,

    An internal trickle charge pump allows the DRV8301 to support 100% duty cycle. The bootstrap cap just needs to be initially charged to support the turn ON of the MOSFET.

    Min duty cycle should just be related to the slew rate of the MOSFET.

    Max duty cycle depend on the slew rate of the MOSFET and the gate charge required by the MOSFET (how much charge must be replenished to the bootstrap capacitor). You should be able to achieve close to 1-2us periods.

  • 0 in reply to Nicholas Oborny
    Intellectual 795 points
    Hello Nick,
    thanks for your quick answer! So that means, I can leave the individual switches ON or OFF as long as I want, and then I am still able to switch afterwards? With a "normal" bootstrap, this would not be possible, but this trickle charge pump allows it?

    And when I neglect the slew rate of my mosfet, 100% and 0% duty cycle are possible?

    -Philipp
  • 0 in reply to Philipp L.
    TI__Mastermind 21320 points

    Ignoring slew rate you should be able to achieve essentially 0% duty cycle.

    Max duty cycle is a different story. If you are still switching (<100), there is max duty cycle limit. This limit is the period needed to recharge the bootstrap capacitor.

    So the example is that you can go 0 to 98%, after 98% you need to skip to 100%. Let me know if this makes sense.

  • 0 in reply to Nicholas Oborny
    Intellectual 795 points

    Hello Nick,

    I fear I am a little bit slow on this topic - power electronics is not my strongest field ;)

    So lets make a very simple example: Lets take the DRV8301, and just look at one half-brige (e.g. Motor phase A top and bottom switch).

    Lets say I let the capacitor charge up at the beginning, and then I want to do this procedure:

    top switch on, bottom switch off - wait 10 sec - top OFF, bot ON - wait 10 sec - top ON, bot OFF - wait 10 sec - ... and so on. (No PWM, just ON or OFF)

    Will this work reliably? 

    Thank you very much for your patience with me! ;)

    Philipp

  • 0 in reply to Philipp L.
    TI__Mastermind 21320 points

    This should be no issue.

    No problem, let me know if you have other questions.