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LM25185-Q1: Design compensation components

Carlos Eduardo Possamai
Prodigy 20 points
Part Number: LM25185-Q1
Other Parts Discussed in Thread: LM25185

Tool/software:

Hi there,

I'm design a flyback converter but I'm getting some trouble during PSpice simulation related to compensasion network.

The converter specs are presented below.

Vin: 20~36V, Vout:16V, Pout:42W, VD:0.7, NPS: 1:1, Lm=10uH, Cout:200u, D_Nom = 0.3736, R_Cs = 10mΩ

Compensation design results:

      AM = 1.39

         ACS = 10

        WzRPH=640151.46

            WzESR=1000000

            Wp=2253.57

Calculating a Gain in cross-over frequency (F_Co = 1kHz)

Calculating RCOMP

    RCOMP = 261kΩ

 CCOMP = 0.85nF

CHF = 5.7pF

Simulating this circuit in PSPICE I got these currents in Lprimary (green) LSec (Red), it seams my compensation network is not working very well.

Am I doing something wrong during the compensation design?

Thank you

  • 0
    TI__Guru* 76525 points

    Hi Carlos,

    Thank you designing with the LM25185.  Why not use our design calculator which can help you do design the loop compensation?  We put efforts to create the calculator to help facilitate designs for customers.   You can find the calculator at: https://www.ti.com/tool/download/LM25185-DESIGN-CALC.  By checking your compensation network, the calculator shows that you do not have adequate Phase Margin, which can explain your unstable operation.

    I used the equations in MathCAD, and found the following results in your calculations are not correct.  

    • Wp.  It should be 1.13kHz
    • Rcomp.  It should be ~11 kOhm
    • Comp.  It should be 39nF
    • CHF.  It should be 260pF.

    If you use the calculator, it will suggest Rcomp=12.7k, Ccomp=34nF, and CHF=226pF. 

    Please double check your calculations.

    Hope this clarifies.

    Regards,

    Youhao

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hello Youhao,

    I thought it might be something related to this.

    I'm using Google Colab to perform the calculations, but I wasn't sure if I created the equations correctly.

    At first, I tried to use the Design Calculator but had some trouble opening it. Later, I discovered that it was an issue with the decimal separator, as we use a comma instead of a dot.

    Thank you for your support! I will adjust the decimal setting and check the Design Tool.

    Best regards,

    Carlos Eduardo

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao, 

    I would like to know if there's another way to change the turn ratio of the spreadsheet (LM25185-DESIGN-CALC) without using the dropdown list.

    Thank you

  • +1 in reply to Carlos Eduardo Possamai
    TI__Guru* 76525 points

    Hi Carlos,

    Very sorry that the spreadsheet was created that way and only a limited number of turns ratios were included. If your turns ratio is not on the pulldown menu list, use the spreadsheet with two adjacent turns-ratios, one smaller and one bigger, to check your design on both sides.  The key thing for you to decide is Rfb resistor value correctly with your actual turns ratio (please apply the Rfb equation in the datasheet). Other parameters can apply interpolation as a sanity check.

    Hope this clarifies.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao,

    I've realized my mistakes:

    1. I used a 100uF capacitor instead of 200uF for Cout.
    2. I misinterpreted the expression. It’s actually an exponential function, not a subtraction.

    Sorry for the confusion, and thank you so much for your help!