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DRV8870: Question about external freewheeling diode

BP
Prodigy 140 points
Part Number: DRV8870


Hello TI experts

Is it required to add external freewheeling diodes to protect DRV8870 from back emf of windings when motor is stopped ?

In which cases it is required generally ?

What is the maximum reverse voltage that internal body diodes of FET's can sustain without damage ?

  • 0
    Genius 5040 points
    BP said:
    Is it required to add external freewheeling diodes to protect DRV8870 from back emf of windings when motor is stopped ?

    How do you stop the motor --  coasting, brake, or reverse direction? If motor is stopped then there is zero BEMF, so I think you want to know what happens during the motor slowing down.

    BP said:
    In which cases it is required generally ?

    I couldn't think a case that it needs external addition free-wheeling diodes.

    BP said:
    What is the maximum reverse voltage that internal body diodes of FET's can sustain without damage ?

    In this case, the VM max is 45v, so at least the reverse voltage is 45 + 1 = 46 minimum.

    Brian

  • 0 in reply to Brian Dang
    TI__Mastermind 23441 points

    Hey BP,

    It can depend on how quickly you want the motor to stop, how much inertia the motor+load has, the size of the motor, and some other things.  

    The internal body diodes of our FETs are rated for the same voltage as the FET itself - in this case 45V recommended, 50V absolute max.   Because of this, it is uncommon to need an external additional diodes when the bulk capacitance is sufficiently large.  See this thread

    Check out this document - The art of stopping a motor – VM pumping. At the end of it there are some examples of where you might need good braking methods:

    • E-bike during quick deceleration or downhill.
    • Fan in windy conditions.
    • Quick release/stop the power tools.
    • Beat simulation in a massage chair with motor running back and forth all the time.
    • Quick valve on/off action driven by motor.

    You might also find this doc interesting:  Current Recirculation and Decay Modes

    Regards, 

    Jacob