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Original question:

TIDM-1000: I don't understand why Errn is based off of Vref = vBusRefSlewed, which is only read once

TIDM-1000: Sensor Gain Feedbacks

stevenh
Guru 20045 points
Part Number: TIDM-1000


Hello Manish,

I decided to simulate the TIDM in PSIM.

For the simulation, I don't have to worry about feedback offsets because:

1. I assume all feedbacks are perfect (no op-amp offsets and etc.)

2. I don't have to worry about unipolar ADCs, e.g. I compute the voltage and current ADC input value as  (Actual Voltage or Current * feedback gain) and then digitize the value between -2048 and 2047.

Could you please verify the feedback gains (shown below) I am using in the simulation.

Thanks,
Stephen

Current Feedback gain:

Current sensor Gain = (4.5V-2.5V)/19.2A

Op-amp gain = 330k/499k

Total Current Feedback gain =  Current sensor Gain * Op-amp gain = 0.068887776

Input Voltage Feedback Gain:

ADC Input Voltage at 416.917V Input Voltage  = 1.65V

So, Input Voltage Feedback gain = 1.65/416.917 = 0.00395762226

DC Bus Voltage Feedback Gain:

ADC Input Voltage at 454V DC Bus Voltage = 1.67V

So, DC Bus Voltage Feedback gain = 1.67/454 = 0.00367841409

Is 454V the total DC Bus Voltage (Postiive + Negative DC Bus Voltage)?

  • 0
    TI__Mastermind 43730 points

    Kv_gain = (1/454), this is just for the positive bus, on the design VbusPM is in range of 0-1 , and VbusPN is in range of 0-2

    Ki_gain = (1/12)

    Please note for simulation purpose you can assume 454 as the gain for both Vac and Vbus because the software normalizes it to the bus voltage sense range by an multiplication which i had explained to you on an older post of yours. 

  • 0 in reply to Manish Bhardwaj
    Guru 20045 points

    Do you mean VbusMN instead of VbusPN?  According to the hardware and software, the feedback gain for both VbusPM and VbusMN should be the same.

    Does the y=mx+b equation located in readCurrVolADCSignals scale the DC Bus feedback from 0.0 to 1.0?  Where does the code/documentation mention scaling VbusMN differently from VbusPM?

    Is Ki_gain the total current feedback gain?  Where did you get (1/12)?

  • 0 in reply to Manish Bhardwaj
    Guru 20045 points

    Ok, I finally realized VbusPN is the total DC Bus Voltage, so it should range from 0 to 2.

    I am still wondering how you got 1/12 for the feedback current.

  • 0 in reply to stevenh
    TI__Mastermind 43730 points

    Please see the calculations.xlsx file , Isense has a range of 12Amps hence Ki_gain =1/12

  • 0 in reply to Manish Bhardwaj
    Guru 20045 points

    Ok, the equation (shown in the spreadsheet cell) doesn't have an offset value, but, for some reason, it still computes the correct value. 

    The equation in the cell is:

    Inominal_rms * 3.3 * 0.5/(Vnominal*Rf/Re) 

    I thought it should be:

    ((3.3/(Rf/Re))-2.5)/(0.625/3)

    For my simulation, I don't add the sensor offset, so it is not necessary for my simulation code to remove the offset as was done in the original code..  I just multiply the actual sensed current by (0.624/6) and then digitizing that value to a 12-bit value from -2047 to 2047. 

    I am going to change the DC Bus gain from what I previously had (1.67/454) to 3.3/631.279.  I had incorrectly thought I had to shift the voltage down like I did for voltage and current. That is not necessary since the DC bus is always positive. After multiplying by 3.3/631.279, the simulation digitizes the resulting value to a 12-bit value from 0 to 4096.  Then the code scales the DC Bus ADC value by (1/4096), but does compute the y=mx+b equation as was done by the original code because calibration is not necessary since there isn't any offset and 631.279 is already equivalent to 1.0.

    For the Input voltage, my simulation multiplies the actual input voltage by a gain of 1.65/416.917 and then digitizes that to a 12-bit value from -2047 to 2047.  There isn't any need to remove the offset because my simulation doesn't add any offset.

    Do you agree?

    Stephen

  • 0 in reply to stevenh
    TI__Mastermind 43730 points

    equation in the calculation sheet is correct for the current sensor. 

  • 0 in reply to Manish Bhardwaj
    Guru 20045 points

    The equation below is correct (as shown in the spreadsheet), but solving for Iinv doesn't give the equation in the spreadsheet cell B44.  If 3.3V in cell B1 is changed to 0 you should get -12 in cell B44, which is not the case.