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Opto isolated flyback SMPS feedback loop

Discussion in 'Power Electronics' started by eem2am, Jun 5, 2021.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    For a FAN6300 based offline flyback with opto/TL431 feedback, what is the value of pullup resistor used in the feedback loop calculation? Ie the pullup resistor thats effecively connected to the FB pin of the FAN6300.

    Some background.....
    The FSDMxxxx series of flyback controllers, and the FSCQxxxx series of QR Flyback controllers, have an effective feedback loop primary side pullup resistor of value 2.8k.
    ..Considering the FAN6300 QR Flyback controller, would you agree that its “equivalent” pullup resistor value is 5k?
    The FAN6300 datasheet and App note (below) don’t say.

    FSCQxxxx datasheet:
    https://www.onsemi.com/pdf/datasheet/fscq1565rt-d.pdf

    FSDMxxxx datasheet:
    https://www.onsemi.com/pdf/datasheet/fsdm0465rb-d.pdf

    FAN6300 datasheet
    https://www.onsemi.com/pdf/datasheet/fan6300-d.pdf

    FSCQxxxx series App Note:
    https://www.onsemi.cn/pub/Collateral/AN-4146CN.pdf

    FSDMxxxx series App Note:
    http://educypedia.karadimov.info/library/AN-4137.pdf

    FAN6300 App Note:
    https://www.onsemi.com/pub/collateral/an-6300.pdf
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Circuit diagram? Can we assume you refer to this figure from the app note?
    upload_2021-6-6_18-41-28.png
    And here to Rb? If so, the conditions for calculating Rb are given in the paragraph preceding this figure.
     
  3. eem2am

    eem2am

    419
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    Aug 3, 2009
    Thanks but i refer to the pullup resistor on the primary side. Which isnt even present in the new controllers which simply have an inbuilt current source in the FB pin...so we need to calculate the "virtual" pullup resistor value.....so that we can dump this in the feedback loop expression.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Show us the relevant circuit diagram.
     
  5. eem2am

    eem2am

    419
    0
    Aug 3, 2009
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,417
    2,619
    Nov 17, 2011
    Rpullup = 20 kΩ in the examples used in these slides.
     
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