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UCC28780: TL431 Bias resistor affecting the regulation

HAMZA RHOUDDA
Prodigy 60 points
Part Number: UCC28780
Other Parts Discussed in Thread: TL431, ATL431

Hello,

I'm working on a Flyback active clamp using the UCC28780 controller, TL431 and FODFODM8801AV Optocoupler.

My output voltage should be 22V. The calculation excel file provided by Texas recommand to use 14 kohms in  Rbias resistor (R34 in the schematic)  but with this value i have 19V at the output. When i reduce the bias resistor to 1 kohms i get my 22V but at startup i have a lack of regulation and i have to consume 0.2W so that the voltage become stable. i don't see why changing the Rbias change the output voltage ? do you have any idea of what i'm doing wrong ?

Thank you for your help !

  • 0
    TI__Mastermind 40120 points

    Hello Hamza,

    The shunt regulator used in the excel file is ATL431, which requires much less bias current to operate than the TL431 regulator that you are using.
    That is why you needed to drop R34 to 1K.  

    The instability may be resulting from R35 = 62K.  This high value blocks the effectiveness of C33, so the loop has very little phase margin.
    I suggest to reduce R35 to 10K and then test your system.  Vary the value of R35 empirically to find the best loop response.

    Regards,
    Ulrich