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DRV8313: Phase current measurement

Jenny Cao
Prodigy 110 points
Part Number: DRV8313

Hi,

We have a project to measure the brushless motor torque through phase current.

We tried two methods:

1)Measure current through the low side resistor:

See DRV8313 datasheet on page12 Figure 12 shows connections to use the comparator to sense current for implementing a current limit. Current from all three low-side FETs is sensed using a single low-side sense resistor.

We measure the current through the Rsense, and then calculate the torque. But we found that under a same torque at different speed the motor has a different current value, which is incorrect.

1)Measure the current through a high side resistor:

Connect a small resistor (0.1 ohm) in-line with a motor phase(between a Vout and a motor phase)  and measure the average current through the resistor, we did have the current proportional to speed under same torque, it seems correct but the current is quite unstable, fluctuate about 2%(we need to achieve 0.1% or better).

And the current through the three phases added together is not equal to or proportional to the current in the Rsense. I would think all the three phases' current should added together and eventually went to ground through Rsense.

Could anyone please explain this to me? Any suggestions please?

Thanks and regards,

  • 0
    TI__Guru** 114905 points
    Hi Jenny,

    Can you provide a little more information?

    Have you verified the current with a current probe?
    At what point in time do you measure the current?
    What is your circuit to measure the current?
  • 0 in reply to Rick Duncan
    Prodigy 110 points

    Hi Rick,

    We apply a constant torque on the motor shaft, set different speed to the motor,  and then measured the average voltage across the resistor(low side and high side resistor).

    1)Use multimeter.

    2)Use our circuit:

    I don't know why I couldn't upload the circuit.

    Basically the voltage across the resistor was fed into the input of an amplifier, and then connected the output of an amplifier's output to a adc(ad7190)'s input.

    The firmware read the the adc value every 20ms and average it over 4 seconds(200 data)

    Do you have a email, so I can send the circuit to you?

    Thanks and best regards,

  • 0 in reply to Jenny Cao
    TI__Prodigy 325 points

    Hi Jenny,

    I believe if you will read current at the center of PWM pulse (1st method) then it will give correct value. You can verify this by observing the current waveform as rick suggested.

    When PWM is off then current don't flow through Rsense it will recirculate within inverter loop that's why current is not proportional to Rsense current.

    Thanks

    Abhishek

  • 0 in reply to Prashant Abhishek
    Prodigy 110 points

    Hi Abhishek and Rick,

    Thank you for your help.

    Jenny