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INA239: Low-Side Power sensing application

Douglas Lee
Prodigy 40 points
Part Number: INA239
Other Parts Discussed in Thread: ISO7763, ISO7741, INA238

Hi! 

I need help with using a low side sensing application with the INA239.

So far I have it working with a high-side sensing application but it does not work for the low side sensing application. Also I would like to add that it only works when I share the common ground of the two PSU's (power supplies). Please refer to my wiring diagram that illustrates the + and - wires of my system.

I tested the low-side sensing by moving the shunt resistor from the positive leg to the negative leg and adjusted the two wires that goes to the INA239 accordingly but I got no readings. I then checked the voltage on the negative side and realized there is no voltage on the negative leg therefore I get no readings.

May someone assist me with the wiring diagram of for the low-side sensing? 

Thank you

  • 0
    TI__Expert 5880 points

    Hello,

    Could you also post the low-side circuit diagram you are using?

    Douglas Lee said:
    I would like to add that it only works when I share the common ground of the two PSU's (power supplies).

    Do you mean that the high-side circuit only works when you share the ground or that the low-side starts to work?

    Also, what do you consider "working"? is the chip not responding to the MCU, or is it reading values that you don't expect? If so, what values are being read?

    Thanks,

    Levi DeVries

  • 0 in reply to Levi DV
    Prodigy 40 points

    Hi Levi!

    Below is the wiring diagram of the power wires in the low-side sensing circuit I tried. There is no voltage on the low-side from the 750Watt PSU and Load so it makes sense that I do not get a reading from the INA239. Which leads me to question how does the low-side sensing applications work for the INA239?

    For the high-side sensing diagram (first diagram) it only works when I connect the negative (-) wire shown at the mcu to the negative (-) wire of the load. When I connect those negatives (-), I get voltage and current readings that match my multimeter. If I do not connect those two negatives, I get random values sporadically. From searching on the web, some suggest that it is unsafe to share those negatives. But so far it looks like that is the only way I can get the INA239 to work properly. 

  • 0 in reply to Douglas Lee
    TI__Expert 5880 points

    Hello,

    Douglas Lee said:
    how does the low-side sensing applications work for the INA239?

    The INA239 Senses the voltage drop across the small shunt resistor as the current flows back from the load to the PSU. What current range are you trying to sense and what is the value of your shunt resistor for your circuit? If there is current flowing from the load the voltage on the negative side of the load should be greater than zero because of the voltage drop across the resistor.

    Douglas Lee said:
    it only works when I connect the negative (-) wire shown at the mcu to the negative (-) wire of the load.

    Although the inputs of the INA239 are differential inputs, the voltage still needs to be kept within a certain range referenced to the device's ground pin, -0.3-85V for the INA239. You should connect the ground references together, or keep the voltages within range another way, to avoid the risk of leaving the voltages floating, potentially causing damage to the device. 

    It's hard for me to comment much on the safety of connecting the ground levels together because I don't know much about where the power is coming from.

    Levi DeVries

  • 0 in reply to Levi DV
    Prodigy 40 points

    Hi Levi,

    I retraced my low-side circuit application and got it working. Again, the ground of the mcu needs to reference the V- of the load, after the shunt resistor. Now in low-side it reads the current pretty much the same as when I had it on the high-side of the load.

    But now I am reading the small voltage before the shunt resistor on the low-side. Can I just connect the Vbus pin to the high side to read voltage and keep the current sensing resistor on the low-side of the load?

    Another question I had was, is there a safety advantage of sensing current on the low-side? Most references online say its just more cost sensitive since there is more readily available devices with a smaller common-mode range.

    My current range is roughly 0-30 amps, and my dc voltage has the potential to go from 0-60Vdc depending on the change of the current I want the load to pull. 

    My shunt resistor is about 1.333 mOhms. specs of 75A 100mV 

    Essentially I have a 24Vdc 600W power supply supplying power to my electronics (there are some dc-dc converters after that to step down the 24 to other dc values). And the 750W power supply is for the load to use. So the MCU and INA239 is being powered from a different power supply than the 750W PSU.

    I plan on replacing the 750W PSU with a rectifier that converts 3-phase AC to DC. So that is where my safety question evolved from. I am not sure if it would be safe to connect the ground of the 24Vdc PSU with the V- of the rectifier. But I have to if I want the INA239 to read the proper values for voltage and current.

    Thank you,

    DL

  • 0 in reply to Douglas Lee
    TI__Expert 5880 points

    Douglas,

    Douglas Lee said:
    Can I just connect the Vbus pin to the high side to read voltage and keep the current sensing resistor on the low-side of the load?

    Yes, actually that is how the Vbus pin is intended to be used for most of our digital power monitors. The pinout descriptions of the INA239 may be helpful to you here:

    Douglas Lee said:
    I am not sure if it would be safe to connect the ground of the 24Vdc PSU with the V- of the rectifier.

    If you would like to isolate your power supplies we have a few options you can consider:

    If you would like to keep using the INA239, I would recommend you use a digital isolator IC such as the ISO7741 (or ISO7763 if you would like an extra digital output for the alert pin) to keep the INA239 referenced to the 750W PSU's voltage levels. We also have this reference design that describes a similar setup using I2C if you would like to use the INA238 (the INA238 is a similar chip to the INA239 that uses I2C).

    We also have magnetic current sensors that would allow you to remove the shunt resistor, but they are limited to 20A max. Our magnetic devices are analog devices, so you would also need to digitize the output with an ADC.

    We also have Isolation Current Amplifiers that will operate similar to analog current sense amplifiers, but use isolation to operate at two different voltages. This will also require an ADC for your application, and may require you step down the voltage from the 750W PSU.

    Let me know if you have any follow-up questions regarding any of these solutions.

    Levi DeVries