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DRV8353: DRV8353 1xPWM mode Current Sensing problem at %100 duty cycle

Ticola Nesla
Prodigy 40 points
Part Number: DRV8353


Hello

We use DRV8353S smart gate driver for BLDC motor control application.

We need to measure each phase current and also rms current.

ADC start sampling on center of the Low Side PWM. Outputs are correct between %15-%85 PWM Duty cycle.

But at %100 PWM Duty cycle, positive sides of phase currents do not exist. When I was monitoring real phase currents on oscilloscope, Positive and Negative sides of phase currents were seen.

Therefore RMS current that i measured reduces dramatically.

I can share oscilloscope screenshoot next days. 

How can I measure correctly phase current at %100 PWM Duty cycle?

  • 0
    TI__Mastermind 26046 points

    Hello,

    ADC sampling is done usually triggered from a PWM interrupt edge. But once you are running the motor at 100% duty cycle, you may need to generate a timer at your PWM frequency or interrupt from a PWM timer overflow (rather than the PWM transition itself) to trigger the interrupts for sampling the ADC. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 40 points

    Hi Aaron

    I can trigger the ADC at %100 PWM duty cycle. An analog signal does not occur in ADC input. Because when duty cycle is set as %100, Low side PWM duty cycle is %0. Due to no current flowing, There is no signal in ADC input.

  • 0 in reply to Ticola Nesla
    TI__Mastermind 27606 points

    Hi Ticola,

    The only way to measure current through a particular phase is to measure it when the low side MOSFET is on. As you pointed out, at 100% duty cycles the high side MOSFET is continually on, so there is no way to directly measure the current flowing through that phase. However, even with a 100% duty cycle, you should still have the low side MOSFET of one of the phases on to allow current to flow from VM, through one phase of the motor, through another phase of the motor, and back to ground. At 100% duty cycle you can only measure the current flowing through the phase where the low side MOSFET is on, however this should give you information on the total current that is flowing through the motor, since all current in the other phases would be flowing back to ground through the low side MOSFET that is on. In other words, if the low side MOSFET of phase B is on, then the current measured flowing into phase B would be equal to the sum of the current flowing out of phase A and C. 

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 40 points

    Thank you Anthony

     I  use DRV8353 as 1x PWM mode. The reference PWM signal was generated using center align mode. I also generated another PWM signal that was same frequency as the reference PWM to trigger ADC at center point of LOW side PWM. 

    How can we  implement an algortim that automaticly works.?

  • 0 in reply to Ticola Nesla
    TI__Mastermind 27606 points

    Hi Ticola,

    Do you have the hall sensor inputs going to the INLA, INHB, and INLB pins? What are you using the CSA current information for? to monitor the torque being applied to the motor and to correspondingly adjust the duty cycle of the PWM? Or are the CSAs only used for protection against motor overcurrent?

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 40 points

    Hello

    I read Hall sensor output with microprocessor and I implement them to DRV directly by using interrupt.

    I do not calculate torque. CSA current information is used for RMS current calculation and for monitoring by user. It is also used for overcurrent protection.

  • 0 in reply to Ticola Nesla
    TI__Mastermind 27606 points

    Hi Ticola,

    Thanks for the background. You mentioned that the CSA current information is used for RMS current calculation. Is this RMS current used in any way to control the motor? You can still detect the total current flowing through the motor while using 100% duty cycle by measuring the current through the low side MOSFET of the phase that has the low side MOSFET on. As a result, you can still have overcurrent protection on the phase that has the low side MOSFET on and you can also determine the total current flowing in the motor.

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 40 points

    Hello Anthony

    RMS current is not used for motor control yet. I will work on the  current control algorithm in the coming  days. 

    Should I read the phase current only when the phase low side Mosfet is on? I sample and read each phase every center align pwm update event. I guess that was wrong.

  • +1 in reply to Ticola Nesla
    TI__Mastermind 27606 points

    Hi Ticola,

    Sounds good.

    The reason you want to sample and read the CSA output when the low side MOSFET is on for a particular phase is because that is the time where you will have current flowing through that low side MOSFET, through the shunt resistor, and to ground. The CSA output will not be valid in being able to accurately measure the current through the phase if there is no current flowing through the shunt resistor and the shunt resistor is floating due to the low side MOSFET being off. The region to accurately measure the current is when the low side MOSFET is on.

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 40 points

    Thank you Anthony. I will try to read current when lowside Mosfet is on. Are there any application notes which includes software example? 

    Could you share  with me please?