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How to achieve motor phase current inline sensing with detailed waveform variations in hundreds of ns?

Other Parts Discussed in Thread: INA240, OPA320, INA301, OPA365, THS4551

I want to observe the influence of the mos switching on phase current. It happens in hundreds of ns. So the amplifier should have at least a 10ns resolution. Does it indicate the bandwidth of the amplifier should be around 100M? 

The rated phase current is 3.68A and phase resistance is 200mΩ. The voltage across the 5mΩ shunt resistor is around 15 mV, at most 50mV. But the DC voltage is 24V. So the CMRR of the amplifier should be above 10000:1.

At first I want to use a current probe to observe the current directly. I scanned the current probes in tek.com and found that my requirements are satisfied except a new emerged problem. There is input impedance reflected into the phase when using the current probe. And the least value is 36mΩ at 1MHz. I’m not sure if it’s a disturbance comparing the 200mΩ phase resistance since it is not constant, it varies along the frequency and the value can’t be neglected comparing 200mΩ. Is it a disturbance? By the way, the phase to phase inductance is 0.065mH.

Then I turned to differential probe. But the CMRR is 1000:1 at just 1MHz. Obviously, it isn’t enough.

Now I plan to amplify the differential signal first and observe the output of the amplifier. Is there an amplifier meeting all the requirements above? I’ve noticed INA240, but the bandwidth is just 400KHz.

Any other amplifiers or any other solutions?

Best regards.

  • Hello user4793821,

    Thanks for coming the TI forum!

    So are you saying the mosfet switches from on/off in 10ns or are you saying the on-time for a phase current is 10ns?

    If you want to measure the current before and after the mos switching you could do this with an INA240, but if the on-time is only 10ns to 100ns before the mos is switched off, then INA240 can’t do this since the BW will limit is output rise time to 0.35/BW = 875ns for small-signal analysis. This does not even include slew-induced distortion or the settling time of INA240 output so total time from mos switching on until INA240 VOUT settles can be up to 1-2µs. Basically if you need the output voltage to settle within 100ns to a voltage that indicates true phase current, then this won’t be possible with INA240.

    In order to optimize the INA240’s response time, you would want reference the device to it middle-supply voltage (VS/2), if you are doing bi-directional current sensing which I assume you are. Referencing the device to mid-supply will ensure the output of INA240 will remain in its linear range and this will utilize the most out of the device BW.

    Hope this helps,
    Peter Iliya
  • Thank you for your reply!

    But what do you mean by referencing the device to mid-supply voltage? Is it the first method "Common Mode Voltage Divider Using Resistors" in Extending Beyond the Max Common-Mode Range of Discrete Current-Sense Amplifiers or the second method "Extending Common Mode Range for Current Output Amplifiers"? Besides, even though the device BW is fully used through the mentioned method, it's still 400KHz.This BW still can't reflect the true current variations that happens in hundreds of ns, does it? Or are you suggesting me using this method to decrease the common-mode voltage and choosing another amplifier to meet the new easy-to-achieve requirements? Sorry, I'm really puzzled. 

    What will TIers do if you want to analyze the influence of the mos switching on phase current?

    Best regards.

  • No problem user4793821,

    What I mean by referencing the device to mid-supply voltage is driving REF pin to a voltage that is half of the device's supply voltage. So for the INA240 on a 5-V supply (VS) this can be accomplished by connecting REF1 and REF2 pins together and driving it with a 2.5V source or by connecting REF1 to GND and REF2 to VS as shown in Figures 28 and 29 of datasheet. When you reference a current sense amplifier like this then VOUT = VREF + Vsense*Gain, so the output is always centered at the mid-scale of your output range which is limited by your supply voltage. So to answer your question further, I am not referencing the "Extending Beyond the Max Common-Mode Range of Discrete Current-Sense Amplifiers” document at all.

    Well if I wanted to analyze influence of MOS switching on phase current I would first establish what switching frequencies and duty cycles I want to explore. If I know these values, then I know what settling times are required for my amplifiers when trying to measure peak phase current. I would also probably read up on the documents below to understand more about this topic.

     

    http://www.ti.com/lit/an/sboa174a/sboa174a.pdf

    http://www.ti.com/lit/an/sboa161b/sboa161b.pdf

    http://www.ti.com/lit/an/sboa171d/sboa171d.pdf

     

    I would then develop a testing board where I could insert three shunt resistors and INA240’s to measure in-line current. I would then add another set of three shunt resistors and INA301’s to measure low-side current. Since you seem to be pushing for very low duty cycles (short on-times), I would also add in three more low-side current-sensing circuits using very fast operational or differential amplifiers. OPA320 (20MHz UGBW) and OPA365 (50MHz UGBW) are good fast, precision rail-rail input/output amplifiers, but the THS4551 (150MHz) is also a precision fully differential amplifier that can provide even faster settling. I would read the outputs of these amplifiers with an oscilloscope with a 50% duty cycle of reasonable frequency 20kHz and then begin reducing the duty cycle until I observe the output voltages are not tracking the current anymore.

     

    Hope this helps.

    Sincerely,

    Peter Iliya

  • Thanks a lot! I'll have a try.