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DRV8312 Complementary PWM quiescent current

Rob Maguire
Prodigy 20 points
Other Parts Discussed in Thread: DRV8312

Hi,

I've been using the DRV8312 for the past few months to drive motors with the standard PWM scheme with no problems...

However, I recently changed the control scheme to complementary PWM in order to use the CBC current limiting. This also works fine but I've noticed that the motors get hot when they are not moving.. It amounts to about 80mA of current being drawn when they are being driven with a 50% duty cycle.

There was no extra current or heat generated when using the standard PWM control scheme so it's definitely something to do with the complementary scheme - Is this to be expected with this method or am I doing something wrong?

I've attached a screenshot of the scope trace - this motor is a brushed dc motor so I'm only using 2 phases but we experience the same behaviour with 3phase bldc motors as well.

Any help would be greatly appreciated!

Thanks & Regards,

Rob Maguire

  • 0
    TI__Guru** 114905 points
    Hi Rob,

    We will investigate and get back to you.

    Do you have a scope capture of the standard PWM for comparison?
  • 0 in reply to Rick Duncan
    Prodigy 20 points

    Hi Rick,

    When the motor is stationary with the standard PWM scheme, both phases are low, since the duty cycle is 0.

    However, for the motor to be stationary using the complementary scheme requires that the duty cycle is 50%. 

    Here is a screenshot of the standard PWM when the motor is stationary - as you can see both phases are low...

    Here is the standard method with the motor moving: phase 1 has ~30% duty cycle and phase 2 is off:

    And here is the motor moving at the same speed but with the complementary method, in this case I think the duty cycle is around 50 + (30/2) = ~65

    Hope this helps.

    Regards,

    Rob

  • 0 in reply to Rob Maguire
    TI__Guru** 114905 points
    Hi Rob,

    Thanks for the scope captures. Since the outputs are continuously on during complementary mode, there is extra current versus the 0% duty cycle in standard mode.

    There are a few possible options. Both require code changes, and the latter an extra PWM output.

    First is to switch out of complementary mode once the motor has stopped. When ready to move the motor again, switch back into complementary mode.

    Use a second PWM on the second output to generate a low duty cycle pulse (~50ns) at the end of each PWM cycle. This pulse is required to reset the outputs if the CBC has been activated in one of the two half bridges.

    As a general observation: For brushed motors, complementary mode is less efficient due to higher current ripple.