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DRV8872: startup must with over 50% PWM duty

Part Number: DRV8872

Hi,

My customer let FAN startup must with over 50% PWM duty when using drv8872.

But it just needs 15%  PWM duty when using P+N mosfet.

Any reason to explain this situation?

Is it  a reason that the DRV8872's RDSon is bigger than P+N mosfet?

thanks,

best regards,

Simen

  • Hi Simen,

    There is no specific reason that >50% duty cycle is required when using the DRV8872.

    Can you provide a little more information to help narrow down what the possible reason?

    Are you using the DRV8872EVM?
    Can you provide scope captures of the inputs and outputs for both the DRV8872 and P+N mosfet?
  • Hi Simon,

    We have not heard from you and are assuming your issue has been resolved. If it has not, please reply to this thread. If the thread is locked, please select "Ask a related question" at the top of the thread.
  • Hi Rick,

    Customer's FAN is single-phase brush-less DC FAN.

    It used a hall sensor to change phase control.

    I have done some experiments recently.

    I found that fan startup and uses  >50% duty cycle with drv8872  coast control(see datasheet 7.3.1 Bridge Control ).

    But fan can startup  and just uses  <10% duty cycle with drv8872  brake control (slow decay).

    They just modify PWM active high ( coast control) and active low ( brake control) different.

    These results are the same with the fan experiments of two customers.

     

    Do you have ideas why coast control and brake control make startup should use different PWM duty?

    thanks,

     

    best regards,

    Simen

     

     

     

     

     

  • Hi Simen,

    Please ask the customer to examine the current when using coast and brake.

    In coast mode, the average current will be less at lower duty cycles. The lower current will not provide enough torque to move the fan.

    In brake mode, the current recirculates and should be higher at lower duty cycles. This will provide enough torque to move the motor.

    In most applications, it is recommended to use brake mode rather than coast for two reasons:
    1) Higher torque as described above
    2) Current flowing through the FETs uses less power than current flowing through the body diodes of the FETs
  • Hi Rick,


    Thanks for your detail answers.

    best regards,
    Simen