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INA240: Please give me some advice when using INA240 for DC brushless motor circuit

Naoki.N
Intellectual 515 points

Part Number: INA240
Other Parts Discussed in Thread: DRV8312

Hi 

I have a question about INA240A1.

I am using INA240A1 in DC brushless motor circuit  to measure motor current value.

When measuring the current on the high side of the motor, the output of the INA240A1 is not stable and I am in trouble.

This phenomenon can be observed both by simulation with PSpice for TI and by measurement with an actual oscilloscope.

This phenomenon occurs when the PWM frequency is set to 100kHz or higher, and when the PWM frequency is set to 10kHz, the Hi-side side can also be measured correctly.

Please check the attached document for details.

Question

- Do you understand why the low side current can be measured stably and the high side current fluctuates?

- Are there are any limitation of PWM frequency value when using INA240 ?

-Are there any alternative ICs for measuring high-side current values ​in the attached file's condition?

INA240A1_circuit trouble.pdf

Best regards

Naoki

  • 0
    TI__Mastermind 36730 points

    Hi Naoki,

    100KHz PWM and 10% duty cycle is too fast and doesn’t allow enough time for INA240 to settle. Here is a blog post that goes into much more details. You’re welcome to take a look.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Intellectual 515 points

    Hi 

    Thank you for introducing the blog!

    I could not perfectly understand in that blog, so I want to ask you further questions.

    - According to the datasheet, settling time is 9.6us ,so this is the reason why 100kHz is fast, is that right?

    - In the data I measured, Motor's high side current could not taken accurately, but the low side current was taken correctly. Do you understand why this is?

    (This phenomenon also can be reproduced Pspice for TI)

    - For example , It seems that INA241's settling time is faster than INA240. Is INA241 can measure high frequency current than INA240?

    Best regards

    Nishimura

  • 0 in reply to Naoki.N
    TI__Mastermind 36730 points

    Hi,

    Here are answers to your questions:

    - Mostly yes, although the datasheet spec is for step response and settling to 0.5%. A triangular input should be easier. At 100KHz, there is only 5uS after PWM transition, assuming 50% duty. It gets worse if it is 10%.

    - Strictly speaking you have a low-side and inline configuration. In the inline (or high-side as you called it), the 40V PWM common mode voltage causes output disturbances that takes about 2uS to settle. Within this 2uS window, the output is distorted and invalid. In the low-side however, there is no common mode issue. The device operates in linear mode.

    - Switching frequency and duty cycle are important parameters. Settling time of the current sensing amplifier is important, as well as common mode settling. They need to be evaluated together. INA241 has much improved specs, but I’m not so sure you’ll see big difference within a 1uS time window after PWM transition (100KHz, 10%).

    Regards, Guang

  • 0 in reply to Guang Zhou
    Intellectual 515 points

    Hi 

    Thank you for answering my questions. I want to ask additional questions.

    　- You mentioned that  "the 40V PWM common mode voltage causes output disturbances that takes about 2uS to settle. "

          Is this constraints included in the data sheet?

    　  Can you tell me which content of the datasheet applies to this?

       -  For example, your company's product has a motor driver IC such as the DRV8312, which states that it can apply PWM frequencies up to 500kHz.

    　  If I want to acquire the motor current with such an IC, what kind of usage is appropriate?

         Should I look at the low side current so that it is not affected by the common mode voltage?

    　 Can you give us some advice on an accurate current measurement method for driving a motor with vector control?

    Best regards

    Naoki

  • 0 in reply to Naoki.N
    TI__Mastermind 36730 points

    Hi,

    - These typical curves illustrate output settling after common mode jumps:

    - Since the need is to essentially measure current under a PWM pulse that is 1uS wide, you’ll probably have to adopt the low-side configuration. Here is such a reference design. Hopefully it is helpful in getting you started.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Intellectual 515 points

    Hi 

    Thank you for answering my questions.

    This is the last question.

    Is it correct of my understanding that it is recommended to detect on the low side in order to control the PWM of the motor using a high frequency?

    Best regards

    Naoki

  • +1 in reply to Naoki.N
    TI__Mastermind 36730 points

    Hi Naoki,

    For other configurations (eg high-side or inline), the common mode voltage is as high as the motor supply voltage. High voltage and high speed present a difficult combination for current sense amplifiers.

    An opamp may be fast enough but can’t survive under such high common mode voltages. Then low-side configuration becomes the only viable option.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Intellectual 515 points

    Hi

    Thank you  for your advice!

    It is helpful of our design.

    Best regards

    Naoki