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what is the unit of OL_MagVect plant transfer function? (SFRA)

Leonardo Callegaro
Expert 1605 points
Other Parts Discussed in Thread: SFRA

Hi there,

I am running the SFRA tool on a buck converter and trying to measure the plant transfer function (i.e. without controller). I do not understand what is the unit of the values saved inside the vector "OL_MagVect"?

My understanding is that this vector 'should' give the dB value of the plant gain. Is this correct?

Also I understand that H_MagVec is calculated by SFRA_F_COLLECT(&Duty_pu,&Vout1_Read) - from SPRUHZ5A-, where "Vout1_read" is a value from 0 to 1. So, is OL_MagVect[f] = 20*log(value from 0 to 1) ?

Thank you so much

Leo

  • 0
    TI__Mastermind 43730 points

    Leonardo, 

    If you are in open loop, i.e. no controller

    ol_MagVect has calculation that is not going to mean anything as the loop is not closed 

    5.2 Plant TF Extraction

    ...

    .35. The results of the SFRA sweep are now displayed on the window. The open loop graph result has no meaning as the plant is not in in closed loop operation. Therefore, select Plant in the drop down menu to the left. Once Plant is selected in the drop down menu, the GUI will loo

  • 0 in reply to Manish Bhardwaj
    Expert 1605 points

    Hi Manish, thanks for your answer. OK, I understand now. my doubt was because I could not see anything (i.e. I had all zeros) in the content of the vector vector "Plant_PhaseVect" when I initially tried. That is because I simply copied and pasted what I found on pag. 19 in the SPRUHZ5A. In particular I believe that there is a mistake in the guide:

    I believe there are a couple of mistakes:

    1. It is missing: a line saying SFRA1.H_PhaseVect = Plant_PhaseVect

    2. Also it sould be SFRA1.ISR_Freq=SFRA_ISR_FREQ; rather than SFRA1.SFRA_Freq=SFRA_ISR_FREQ;

    3. I still would like to know what is the unit of the Plant_MagVect. Is it something like 20*log(number from 0 to 1)? As in your example in the guide, what you feed to the function SFRA_F_COLLECT(&Duty_pu,&Vout1_Read); is Vout1_Read which is the ADC reading/Max ADC reading, where the maximum ADC reading is 4096.

    I will try again my script and let you know if I can measure the plant (i.e. without controller) transfer function phase and magnitude.

    Thanks

    Leo