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TIDA-00365: how to selcet zenar diode for motor line

LIN SUN
Intellectual 620 points
Part Number: TIDA-00365
Other Parts Discussed in Thread: SM72295,

Dear expert:

Can you kindly find the design guideline please? Can you teach me how to select D4 of Figure6?

Is D4 also used to protect against motor Induced electromotive force？

Thanks!

  • 0
    TI__Prodigy 640 points

    Hi,

    Thanks for the question. D3 and D4 are used to protect the SM72295 gate driver pins HSA and HSB from overshoot and undershoot at switch node during switching events.  HSA and HSB pins have a max. rating of (-5 to 100)V and a recommended operating range of (-1 to 100)V. So, allowing a safety margin of 5V, 95V is the maximum allowable voltage at HSA/HSB for safe operation. Typically zener diodes have a 5% variation in clamping voltage and therefore a 91V zener(max clamp voltage of 95.5V at 5% variation) is chosen for protection. R6 and R7 are current limiting resistors.

    Hope this helps.

    Regards
    Siva

  • 0 in reply to Siva Mohan
    Intellectual 620 points

    Dear expert:

    Thank you. Can you teach me how to protect against motor Induced electromotive force during suddenly power off? Should one schottky diode be added?

  • 0 in reply to LIN SUN
    TI__Prodigy 640 points

    Hi,

    I assume you are referring to operation in the regenerative region when the motor speed increases beyond rated speed due to regenerative loads. In these cases using a brake resistor controlled by a MOSFET(not included in TIDA-00365) to dissipate the motor kinetic energy(seen as motor EMF) is necessary to protect against DC link exceeding safe values.

    Regards

    Siva

  • 0 in reply to Siva Mohan
    Intellectual 620 points

    Dear expert Siva,

    I don't know how to design "a brake resistor controlled by a MOSFET". Can you share me some valuable experiences please?

    And I'm not sure whether schottky diode can be used to protect against such motor EMF. Have you ever heared about this?

    Thanks a lot!

  • 0 in reply to LIN SUN
    TI__Prodigy 640 points

    Hi,

    Please refer to TIDA-00195 for an example implementation of brake resistor controlled by MOSFET(in the design schematic, IGBT is used instead of MOSFET but the implementation is similar). The brake resistor will be connected between MOSFET drain and DC bus positive and MOSFET will be controlled from MCU(and suitable gate driver) depending on DC bus voltage level.

    I do not understand the Schottky diode implementation. Can you please share a schematic or circuit diagram on how you plan to use the schottky diode for protection against back EMF?

    Regards

    Siva

  • 0 in reply to Siva Mohan
    Intellectual 620 points

    Dear expert:

    TI DRV8872QQARQ1 is used to drive my brushed-DC motor. My motor can forward or reverse rotated via current direction adjustment.

    And the redrawwn diode is intended to protect against EMF. Do you think it's necessary and capable?

    D35 is zenar diode.

    Thanks a lot!

  • 0 in reply to LIN SUN
    TI__Prodigy 640 points

    Hi Lin Sun,

    What is the nominal operating voltage of VM? D35 is a 27V zener and hence will start regulating(clamping) output voltages at 27V if VM > 27V. Please use a Zener that is rated at least 10% higher than nominal VM. Also, D35 has a continuous power rating of only 225mW and the motor braking power should not exceed this value for safe operation. Also overvoltage protection or braking mechanisms will be needed only if the motor back EMF will exceed the maximum VM rating, which is 50V - this will happen if the motor is driven above rated speed by the load(regenerative or active loads). Can such a situation happen in your application - if not, then D35 should be a sufficient solution to protect from transient over/undervoltages during switching events(do add a series resistor between BU+ and D35, BU- and D35 for current limiting). However, if the motor can operate in regenerative mode, then D35 may not be sufficient to protect from overvoltage and brake resistor with MOSFET may be needed depending on the regenerative power to be dissipated.

    Regards
    Siva

  • 0 in reply to Siva Mohan
    Intellectual 620 points

    Dear expert Siva,

    Many thanks for your professional work!

    Norminal operating voltage of VM is 12V, so 27V D35 should be okay for clamping. My motor typically works at 500mA,but I cannot understand your suggestions thouroughly. I'll learn it and feedback later. Thanks!