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DRV8332 - current limiting too early.

Ian Hopper
Prodigy 30 points
Other Parts Discussed in Thread: DRV8332

Hi,

I'm using the DRV8332 on a motor driver board in development and am seeing current limiting taking place at too low a current. I suspect it might be pcb layout related but before I jump into redesign could you give some insight?

I'm driving a 3ph BLDC at very low / stalled conditions, max continuous current draw is expected about 4.5 Amps. Powered by 22V, with 50% duty cycle on all phases for zero output torque, with variation above / below in SVM modulation to generate torque. 2.23 mH motor inductance.

CBC current limiting, 30kHz PWM, 22k OC_Adj resistor so hoping to see current limiting well above the expected draw. However current limiting occurs consistently about 3.5 Amps. Similar behaviour apparent in 2nd board and 2nd motor, so appears to be circuit / component related.

PCB shown below. Note the board is intended for hand soldering with through-hole components mainly, so I've not been able to follow the PCB layout recommendations exactly. The though hole caps are a bit further from the chip so as not to clash with the heatsink.

Unfortunately I'm not able to measure the phase currents separately, but here's the typical Phase 1 and 2 voltages in operation..


10V/div vertical . This is with additional R-C snubbers fitted to outputs as per another thread to limit the voltage overshoots on the sharp edges.


Overall the drive works well - very smooth motor drive at low speed, OT latches operate as expected, good motor torque control by varying output. But the current limiting appears to set in much too early- when the card draws about 3.5 A. I can also progressively drop the current at which this happens by varying the OC_Adj resistor, so I don't think it is a boot strap issue etc.

Voltages on the Gate Drive supply pins appears steady.

Could the PCB layout be the issue? Any suggestions as to how I can debug to be sure would be appreciated...

Thanks

  • 0
    TI__Prodigy 735 points

    Hi Ian,

    It seems like you are measuring the current going from your power supply to your PCB, is this correct? Without measuring the output phase current, it is difficult to know how much current is actually travelling through your motor. 

    Is there any way you could measure a phase current of your BLDC using a current probe and capture a scope shot for us? Often the current draw shown by the power supply will be much lower than the actual current that your motor is drawing.

    Thanks,

    Fabio

  • 0 in reply to F2
    Prodigy 30 points
    Hi Fabio,

    Thanks for your quick response.

    I'm afraid I don't have the equipment to do that measurement.

    Could you expand on your comment. Is this because the DRV8332 measures phase current flowing only during the ON period of the bridge, whereas the external current at the power supply is the "smoothed" always-on current which will naturally be lower as it is more of an average?

    I can see why the two would be different. Perhaps the chip is just a bit too small for the application...
  • 0 in reply to Ian Hopper
    TI__Prodigy 735 points

    Hi Ian,

    Like you stated, reading current from the power supply is more of an average current. The actual peak current that the device is tripping at could be significantly higher, especially since some of the current through your motor is being supplied by the bulk capacitors. 

    If you do not have access to a current probe, you could connect a sense resistor from GND_x to GND and capture the voltage using an oscilloscope. Please make sure that the chosen resistor is of low inductance and rated for high enough power. This might require for you to scratch off some of your GND_x trace in order to connect the resistor, but it will give you a better understanding of how much current your motor is actually drawing.

    The DRV8332 is good for 8 A continuous using a proper top side heat sink and board layout, so as long as you stay well within that range you will be fine. 

    Please let me know what you find.

    Regards,

    Fabio

  • 0 in reply to F2
    Prodigy 30 points

    Hi Fabio,

    Thanks again for your reply.

    I will let you know if I am able to make the measurement.

    Ian