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Opto isolated flyback SMPS feedback loop

eem2am

Aug 3, 2009
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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
 

Harald Kapp

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Nov 17, 2011
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For a FAN6300 based offline flyback with opto/TL431 feedback, what is the value of pullup resistor used in the feedback loop calculation?
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.
 

eem2am

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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.
 

Harald Kapp

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Rpullup = 20 kΩ in the examples used in these slides.
 
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