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ISO224: Inverter Phase Voltage Measurement

alikaan
Expert 2265 points
Part Number: ISO224
Other Parts Discussed in Thread: AMC3330, AMC1311, OPA365

Hello,

I would like to measure the phase voltage value of inverter. Could you please check the schematic for ISO224 side ? 

DCLINK is 28V nominal.

Thanks.

  • 0
    TI__Guru 52752 points

    Hello,

    What is the expected maximum phase voltage that the ISO224B must measure?

    I would not recommend connecting a diode (D3) to the input of the device for two reasons: leakage through the diode can leak to inaccuracies and the ISO224B has a linear response for a +/-12V input voltage, therefor connecting a 5V1 diode would results in the input voltage not using the full-scale input range. The device is capable of withstanding +/-15V on the inputs before risk of damage as stated in the absolute maximum ratings table.  

    The specified output resistive load for OUTP and OUTN is 10kohm as stated in the electrical characteristics table of the datasheet. You may want to consider reducing the size of R76 and R77. 

  • 0 in reply to Alexander Smith
    Expert 2265 points

    Hello,

    Maximum phase voltage is 28V. I'll remove the D3 and reduce the value of R76&R77.

    Lastly, PH_B_RTN is power ground actually. Do you think it is OK for measurement ? In some application, PH_B_RTN path is connecting to switching node. PH_B path is connecting to DC bus. If you have, would you please send me some application note or reference design for isolated phase voltage measurement ?

    Thanks.

  • 0 in reply to alikaan
    TI__Guru 52752 points

    With a maximum phase voltage of 28V, I calculate that the maximum input range to the ISO224 will only be 2.14V of the possible +/-12V input range. You may want to increase the size of R20 as well. 

    It is best to refer the measurement node, high side device ground (GND1), and the high side voltage supply source (VDD1) all to the same node to prevent issues with common-mode input range and/or damage to the device. I recommend connecting PGND_F to PH_B_RTN.

    If this is not possible, you may want to consider a device with an integrated DC/DC converter such as the AMC3330. 

    I've provided a link to our device specific collateral below:

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1164993/faq-isolated-data-converter-collateral-overview

  • 0 in reply to Alexander Smith
    Expert 2265 points

    Hi,

    I modified the schematics with AMC1311, measurement range is same, 28V. Please review the schematic. U32 is OPA365

    Thanks.

  • 0 in reply to alikaan
    TI__Guru 52752 points

    Looks good! 

    The input voltage with 28V applied will be 2.14V, the linear input voltage range is 2V, so the accuracy will be linear, but not specified from 2V-2.14V, you may want to reduce the size of the sensing resistor slightly if highest accuracy is required at maximum input voltage of 28V. 

    Depending on the rail-rail output capabilities of your selected operational amplifier for the differential to single-ended conversion, you may want to adjust the Rf/Rg ratio so ensure the output does not clip as discussed in #6 design steps of this document: https://www.ti.com/lit/an/sbaa347/sbaa347.pdf

    As designed, 2.14V * (7.5/4.7) = 3.415V, beyond the supply rail of the amplifier - the output will be clipped.