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Original question:

LM5156: Ccomp, Rcomp

LM5156-Q1: Design of double Rcomp and Ccomp

Sebastian Heinze
Prodigy 20 points
Part Number: LM5156-Q1
Other Parts Discussed in Thread: TL431

Tool/software:

Hello,

I am picking up the question about the double usage of Rcomp and Ccomp (RC) from the liked thread again.

I have not seen in other application notes. How to design these?

Are the values of the RCs on IC-side always identical to the value of the RC on Optocoupler-Input-side? Independant of the real opcocoupler-pole? Or do any premises apply?

I am using snva866a for design-procedure:

Are the formulars in chapter 5.2 valid for RC´s on both sides?

Which is what?

  • +1
    TI__Genius 13035 points

    Hi Sebastian,

    Thanks for reaching out.

    The picture you refer to is a bit misleading. The primary side RCOMP and CCOMP have nothing to do with the components on the secondary side. 

    The "real compensation" is done on the secondary side at TL431. Here, only CCOMP would be enough as it is not introducing a pole, but a zero (combines with upper feedback resistor), however RCOMP still can be used for increasing the phase margin.

    The reason for using the primary side components is to reduce the centerband gain of the inner loop, so that the total phaseshift of the inner loop stays roughly constant (earlier crossover of the inner loop).

    This topic is quite complex, for a bit more explanations, i recommend to watch this video: https://www.ti.com/video/5426178629001

    Also we plan to release an appnote that is dealing with this topic soon.

    Sebastian Heinze said:
    Are the formulars in chapter 5.2 valid for RC´s on both sides?

    The formulas are for the compensation components on the secondary side.

    Best regards

    Moritz

  • +1 in reply to Moritz
    Prodigy 20 points

    Hello Moritz,

    thanks for your feedback, which makes all sense to me.

    Yes, naming both RC´s on primary and secondary side the same (and using also same values in the example), can lead to misunderstandings.

    I will watch the recommended video and wait for the app-note.

    Two questions before:

    As the formulas in the appnote do not consider compensation on the primray side, do you have formulas available, which consider them?

    I quess the XLSX, which polts the loop-gain does also not consider the primary side compensation, correct?

    I would like to plot the transfer-function of the plant and the compensator and compare them to practical measuremenets.

    Best regards, Sebastian

  • 0 in reply to Sebastian Heinze
    TI__Genius 13035 points

    Hi Sebastian,

    Unfortunately, i cannot give you formulas for the primary components now. This is also a reason for the appnote coming.
    In some cases it is not even necessary to have these components placed at all. (When the inner loop is surpressed by a zener for example)

    In the video you will find some formulas on when and how to use these components. However, there are no transfer functions given i think.

    You can maybe simulate the Bode plot for both options and then measure in the lab.

    Best regards

    Moritz