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TPS23753: Calculate loop compensation

Sergei Koskin
Prodigy 50 points
Part Number: TPS23753
Other Parts Discussed in Thread: PMP8803, TL431, , TPS23751

Hi!
I am read document slva305c, section 2.9-2.10. Equation to calculate Cctl (38) on page 14. How to calculate the Gmo value that is used in this equation?

  • 0
    TI__Genius 9230 points
    Hi Sergei,

    Gmo is the Modulator/opto transfer function specified in the 2.10.2. For calculating CCTL in 2.10.3, you would need to set this transfer function equation to 0.75, then work backwards through section 2.10.2 to find the rest of the values used in the CCTL equation.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 50 points

    Hi Tom, 

    I understood. In  expression (38) is error , instead of GMO, |GMO(2*Pi*F0)|  should be written.

    Thanks!

  • 0 in reply to Sergei Koskin
    Prodigy 50 points

    Hi Tom,

    I have another question on the expression (38):

    Do some conversions...

    If we use the following values, we get the negative values of the expression below the square root.

    Error in expression 38? Can an error in calculating the value of Cctl?

    Please, help...

  • 0 in reply to Sergei Koskin
    TI__Genius 9230 points
    Hi Sergei,

    Are you trying to derive the MPS Plus Filter and Load (MPF) Transfer Function? I recommend going further back in this document to 2.9.1 & working forward from there.

    Are you having issues with re-compensating your design? Let me know your requirements & I can recommend proven reference design you can try.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 50 points

    Hi Tom, 

    No, I tried to calculate the Cctl value using the value Rctl, Rob and ect. as in the document slva305c.pdf. I did not get the result of 50.3nF.
    I decided to do the inverse transform to make sure the original values were correct. In the end, I got that the root expression is negative at the specified values.

    Yes, I have problems with re-compensating of my design. Schematic is attached. Input voltage: 36-57V, Output: 24V, 540mA
    When design my schematic, I used applications reference designs PMP8803, PMP5595 and document slva305c.pdf.

    Thanks,

    Sergei

    PoE_Edited.pdf

  • 0 in reply to Sergei Koskin
    TI__Genius 9230 points

    Hi Sergei,

    Take a look at the feedback values in the attached 23V reference design (PMP21655). Notice R29: it there to make sure the minimum cathode current is satisfied to allow for regulation (see Imin on TL431 datasheet for sizing that R20 resistor). I recommend adding this.

    Also, I recommend significantly reducing your output capacitance. Your output capacitor pole may be too strong. For starters, 22uF may be sufficient.

    Keep in mind that PMP21655 uses TPS23751, not TPS23753A & is a type 2 design.

    PMP21655A(001)_Sch.PDF

    PMP21655A(001)_TI-BOM.xls

  • 0 in reply to Thomas Amlee
    TI__Genius 9230 points
    Hi Sergei,

    Are you still working on your issue or have your found resolution? If so, could you click the green button on the post you found most helpful?

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 50 points

    Hello Tom,

    I am still working on issue and I have  question. What is the capacitor C15 installed for ( PMP21655A(001)_Sch.PDF)?

    Thanks, 

    Sergei

  • 0 in reply to Sergei Koskin
    TI__Genius 9230 points
    Hi Sergei,

    C15 is an additional common mode isolation cap like C25. Putting them on the high side of the transformer & the low side (C25) give an additional path for the common mode noise. Additionally, this may provide benefits to EMI performance.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer