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DRV8801: Reverse Current Reduction

Hao Wang14
Expert 4415 points
Part Number: DRV8801

Hi Team,

One of the customer has the reverse current issue, the reverse current can go to -1.76A which is a little high.

They are wondering:

1. What could be the reason of the high current? 

2. Is it possible to reduce the reverse current? If so, how to do it?

Could you please help to analysis and give some suggestions? Thank you!

Here is the waveform:

CH2: Current; CH3: OUT2 voltage; CH4: OUT1 voltage

The schematic is as follows: the brushed motor is powered by 4 cell battery and the PWM frequency is 16kHz

  • 0
    TI__Guru 56650 points
    Wang,

    Do they test on an EVM or their board?
    "the reverse current can go to -1.76A" Is it input current? Where do they measure the current?

    This is high di/dt (1.76A under 1us; up and down) event.
    1. The board highest voltage is 10V, if the current goes through a inductor, v=Ldi/dt; L could be lower than (10V/1.76) uH. So, it may not a load issue unless you have load short.
    2. di/dt up and down If the FETs are short, it should be only one way.

    To me, it seems you measure a capacitor's charge and discharge current or a layout issue. Please let me know your measurement point.
  • 0 in reply to Wang5577
    Prodigy 20 points

    Hi Wang,

    Ch4 is OUT1 voltage and Ch3 is OUT2 voltage, The current is OUT1's Current, the peak high current was detected with every pwm whether it's slow decay mode or fast decay mode.

    Drv8801 was used for the sweeping robot, we found that there was lots of Carbon  which gather around the Carbon Brush after running for 1~2 month(normally, the motor can run as long as 3 months). the Carbon covered the rotor and caught a badly start, sometimes it even cound't start.

    1) the reversed current was detected in the Brushed Motor, By changing the decay mode, the reversed current can be smaller but still there.

    2) the peak  current is very large and I'm afrain a greater current might cause a worse Carbon Brush wear out. is it true?

    My questions are,

    1) Why the Carbon Brush wear out so quickly? which reason can lead to such badly wear out? How to decrease the wear out of Carbon Brush and  make the Brused motor work longer?

    2)  Is it possible to reduce the peak current especially reverse current? If so, how to do it?  Is there a theoretical mathematical model which we can use to predict

    and we notice the DRV8801's datasheet, the waveform of Out's Voltage and current was as follows  which distinguish  from our's,  Why was it be different .

    Our motor's inductance is 75uH and DCR is 13ohm. How to matching it with DRV8801 or other driver? Other parameter as following:

  • 0 in reply to Wang5577
    Prodigy 20 points

    Slow decay waveform

    details waveform

    fast decay mode

  • 0 in reply to 11 1192
    TI__Guru 56650 points
    Hello,

    1. This link (www.mawdsleysber.co.uk/.../ ) gives "Common Causes Of Carbon Brush Wear In DC Motors"
    a. Brushes being installed incorrectly or the wrong sized brush being fitted
    b. Brushes experiencing damage due to electrical underloading or overloading
    c. Winding faults
    d. Voltage surges to the motor
    e. Issues with the commutator – excessive sparking caused by worn brushes
    f. Brushes not being shaped to fit arc of commutator

    "we found that there was lots of Carbon which gather around the Carbon Brush after running for 1~2 month(normally, the motor can run as long as 3 months" Do you mean the motor could run 3 months with other driver or other control method?

    According the above listed carbon brush worn out reasons, we can try to reduce voltage surges to the motor. That is related your 2nd question.

    2. Some brushed motors have a small build-in capacitor in paralleling with the motor winding. That can cause a high di/dt event during the winding terminal voltage changes. If we want to reduce this high di/dt, we may need to reduce voltage surges to the motor during the DRV8801 switching. Please add a small inductor or magnetic bead in series with the motor.
    Would you verify this idea by adding an inductor in series with the motor to see the current spike change?
  • 0 in reply to Wang5577
    TI__Expert 4415 points
    Wang, thanks!