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DRV8462EVM: DRV8462 reverse EMF and protection

Kevin Godfrey
Prodigy 20 points
Part Number: DRV8462EVM
Other Parts Discussed in Thread: DRV8462,

Good day.  I have two questions regarding the DRV8462 device. I have been testing the DRV8462EVM with a NEMA23 2A stepper motor.

The DRV8462 works very well with the motor, I am really impressed with this driver.

When power is removed, when the stepper is turned, the D4 LED comes on.  Of course, this would be due to EMF created by the stepper motor turning.

Would this have a negative impact on the driver or any other external electronics on the PCB being designed? ie if the user were to turn the motor in the system it's being used, while power is removed,

could this cause any driver or external component damage?  If so, is there any recommended protection?

Also, is external reverse EMF diodes or TVS or similar necessary as protection on the motors for reverse EMF during normal option (ie when power supplied and motors are running?) Or does the DRV8462 have sufficient

protection for reverse EMF up to the max current rating of the driver? Looking forward to you feedback. Thank you very much!

  • 0
    TI__Mastermind 20755 points

    Hi Kevin,

    Thank you for the post and thank you for evaluating the DRV8462. We're happy to hear it works very well with your stepper motor.

    You are correct. When the VM power supply is removed and the motor manually spun it will generate back EMF (BEMF). The stepper motor will behave like an alternator and generate a sinewave output AC voltage while it is mechanically driven. The body diodes of the output FETs will rectify this AC and the output DC will be presented on VM and GND. The polarity of the rectified voltage will be the same regardless of the direction with which the motor will be spun. Because the BEMF appears across VM and GND any other devices that may be powered by the same rail will also receive this voltage. So this must be kept in mind.

    As long as the BEMF voltage across VM and GND does not exceed the absolute maximum specification for the DRV8462 no harm will be done by this voltage. Usually in stepper motors BEMF amplitude of the voltage generated is less than the VM supply voltage as the speed of steppers are typically not as fast as brushed or brushless DC motors. The absolute maximum rating of the DRV8462 is 70V which is significantly higher than most commonly available full integrated stepper drivers. So in real world situation chances of BEMF amplitude exceeding 65V is not common.

    Back emf amplitude can either be measured by spinning the motor using another motor coupled to it and running it at the target velocity equal to the estimated mechanical back drive spinning velocity or it can be estimated using this formula. approximate_back_emf_amplitude = (sqrt(2) x pi x rated_holding_torque Nm x revs_per_second )/ rated_current. This formula requires the holding torque value is specified with both phases energized with the rated current which would be the typical specification. If the holding torque value is specified with only one phase energized explicitly, then multiply the result from the aforementioned formula by 2.

    Stepper motor drivers including the DRV8462 are designed to operate with the BEMF during normal drive operation. Matter of fact integrated stepper drivers leverage this BEMF for  several sensorless integrated features such as smart tune current decay, stall detection and auto torque. 

    In summary, no external protection is necessary to circumvent the BEMF on the VM and GND rail when the stepper is back driven as far as the stepper driver is concerned. If such a situation, the stepper would continuously get back driven mechanically and suspected it could generate higher amplitude than the absolute maximum voltage rating of the driver a TVS across VM and GND to keep the maximum voltage less than this rating may be used. Obviously the TVS should be rated higher than the maximum VM supply voltage applied. Most commonly this protection won't be necessary. The properly sized bulk capacitor would smooth out the BEMF effectively which would be good enough.

    Regards, Murugavel