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DRV8840: When the motor is commutating, high pulsed current is present before entering the chopping current platform

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8840

Hi team,

Here's an issue from the customer may need your help:

In slow decay mode, high and low (no PWM) motor commutation occurs when the motor current is pulsed high before entering the chopping current platform, but no high pulsed current occurs in fast decay mode. Motor VM 24V, 0.5A running current. The chopping current value is set at 4 A. 

The following figure shows the slow decay mode current waveform with high pulsed current. Chopping does not start, but start until after a period of time. Why is this? 

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Guru 55565 points

    Cherry,

    It is hard for me to understand the question exactly.  In the waveform attached, what are the two scope channels?  One is current and other is ???  

    What are the inputs to the device and outputs?  Schematic?  I am not sure I even understand the question fully.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Mastermind 22235 points

    Hi Ryan,

    Thanks for your help.

    Please let me clarify, the issue here is why the spike in current is not limited by chopping when commutating?

    Ryan Kehr said:
    In the waveform attached, what are the two scope channels?  One is current and other is ??? 

    One is 100 mV/a and the other is meaningless which can be ignored.

    Ryan Kehr said:
    What are the inputs to the device and outputs?

    The inputs are controlled by high and low levels. 

    Ryan Kehr said:
    Schematic?

    And in the figure below, blue is the current waveform, and yellow is the out waveform:

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Guru 55565 points

    Cherry,

    Can I see what the other output is doing please?  I assume they are using a current probe to measure the current and it is around the motor wires?

    Regards,

    Ryan