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Original question:

LM34966-Q1: How to design an Isolated Flyback using the LM34966

LM34966-Q1: Rcomp and Ccomp Calculation

Ajay R
Prodigy 60 points
Part Number: LM34966-Q1


Tool/software:

Dear Mortiz/Team,

I'm designing the PSR with an AUX flyback converter, as discussed in this thread I'm stuck with the calculation of Rcomp and Ccomp, So I read your conversation, but as per your suggestion Acs=1 for LM34966Q1 but I'm unable to find the ACS value in the datasheet so please can you brief it more, and also I didn't get Cout-total from the formula how they have considered 300uF

Document: How to Design an Isolated Flyback Converter Using LM5157x/LM5158x (Rev. A) (Page 7)

  • 0
    TI__Genius 17835 points

    Hi Ajay,

    Thanks for reaching out.

    In this device, there is no number given for Acs. This is because there is no amplifier between the CS pin and the error amplifier. So the signal is not changed, which means the gain is 1.

    Regarding the Cout, according to the document, 120uF are used for the output that is calculated. The minumum capacitance for achieving the loadstep bahavior was calculated to be 13uF.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 60 points

    Hi Mortiz, 

    Thanks for the information.

    I have the following questions:

    1. Can I use the 750311591-3A Transformer for this IC?
      (Doubt is the datasheet of Transformer is recommended to use LT3573/LT3574/LT3575/LT3748)

    2. I can’t calculate the Rcomp value, I have used the recommended formula attached earlier/below . (I’m getting around Rcomp=1Meg)

    3. As you mentioned regarding the Cout, the calculated value is 13uF but why have they considered 120uF?(Is this just because of ripple elimination or something different?)

      And in the formula screenshot considered 300uF is this a typo error or something else?



  • 0 in reply to Ajay R
    TI__Genius 17835 points

    Hi Ajay,

    The transformer does not really depend on the IC you use, but on the specs of your design.

    1.2)  Please try to use the power stage designer to get a better overview of the voltages and currents the transformer will have to handle: https://www.ti.com/tool/POWERSTAGE-DESIGNER

    With this tool it is also possible to calculate the compensation components.

    3.) 13uF was just the minimum calculated for the load step requirement. But this is still a quite low value so to achieve a better ripple and transient behavior there was added more.

    Ajay R said:
    And in the formula screenshot considered 300uF is this a typo error or something else?

    I think this is a typo, i will need to check.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 60 points

    Hi Mortiz,

    Thanks for the information regarding the transformer and typo.

    But Point 2 is not addressed, please can you suggest any other formulas?


    Thanks,
    Ajay

  • 0 in reply to Ajay R
    TI__Genius 17835 points

    Hi Ajay,

    The powerstage designer is a good way to easily calculate the values.

    If you are intersted in the theory behin and the formulas, please refer to the following document presented in the video:

    https://www.ti.com/video/5099090674001#tab-1

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 60 points

    Thanks, Mortiz. I will connect with you for further information.

    Best Regards,
    Ajay