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DRV8308: DRV8308

Andrei Shamkovich
Prodigy 30 points
Part Number: DRV8308

Dear Expert,

We use DRV8308 for BLDC. It is 28VDC 4000W drive. We use 3 Hall effect sensors.

We need to provide certain torque in a wide range of RPM. We do not control torque, but must satisfy the load demand. We do it in nominal and high line. At low line, about 24VDC, the PWM reaches its limit and motor can't put out more power. The controller senses out-of-lock condition and switches from (1) Hall to (3) Hall operation. As a result there is an erratic operation, "hick-up" mode. We operate in close loop mode,  setting the speed as a control parameter.

Is it possible to detect imminent "out-of -lock" event ( running out of pulse width) ? If we do that, we can command "reduce speed operation", thereby reducing current, and maintaining below "saturation" operation.

Regards,

Andrei

  • 0
    TI__Guru 63710 points
    Andrei,

    The internal loop doesn't have a way to notify the outside world of an impending out of lock. What I would recommend is using an ADC or something to sense the PWM outputs and correlate that with the hall sensors to check for RPM change and PWM duty change.

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Prodigy 30 points
    Adam,

    Adam,

    Here is another question:

    Can DRV8308 recognize impending out-of-lock in the following fashion:

    1. Torque increases, leading to increased PWM
    2. PWM is approaching 100% Duty Cycle.
    3. Assume that we sense (1) Hall signal outside of the main loop.
    4. Now we compare that Hall signal and drive signal to the gate of the FET.
    5. If there is a phase lag, which increases with the heavier load, we can measure it.
    6. Once we measure the lug, we can throttle down the torque or speed.


    Can we delay (1) Hall to (3) Hall transition?



    Adam, see if this scenario makes sense.

    Regards,

    Andrei
  • 0 in reply to Andrei Shamkovich
    TI__Guru 63710 points
    Andrei,

    This scheme makes sense. Unfortunately there is no way to delay the hall 1 to hall 3 transition since that is controlled internal to the part.

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Prodigy 30 points

    Thanks Adam,

    Delay is not possible, understood. Is there a lag, or phase delay under torque? I.e. If torque increases,  is there a tendency for a rotor to pull out relating to control signal? If so, we can measure the command signal to the Hall signal phase lag and take some action before it goes out of lock completely.

    See if it makes sense.

    Regards

  • 0 in reply to Andrei Shamkovich
    TI__Guru 63710 points
    Andrei,

    Please check it on your system.

    Regards,

    -Adam