# Gain of TL431 needs to be found for feedback loop calculation

1. ## Gain of TL431 needs to be found for feedback loop calculation

Equation 23, page 14 , of AN57 by power integrations (below) shows a parameter called
“K TL31” which is the gain of the TL431.

From the attached TL431 datasheet , does this gain equal [Change in V(KA)]/[Change in V(ref)]?….which equals 1/0.0014 = 714

TL431 datasheet:

AN57 by Power Integrations
https://ac-dc.power.com/sites/defaul...-docs/an57.pdf

2. ## Re: Gain of TL431 needs to be found for feedback loop calculation

From the datasheet, simplified diagram, I see that V(KA) is same as V(ref). It should be something like d(Vref)/d(IkA).

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3. ## Re: Gain of TL431 needs to be found for feedback loop calculation

thanks, the connection method of the TL431 , to which the gain is required to be known for, is as in fig 5 , page 4 of AN57, (AN57 is linked in the top post)

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4. ## Re: Gain of TL431 needs to be found for feedback loop calculation

From the datasheet TL431, Fig 21, I see that dV(KA)/d(Vref)=1+(R1/R2); they are using an op-amp model for the gain. This equation is given in the Fig page 18.

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5. ## Re: Gain of TL431 needs to be found for feedback loop calculation

Originally Posted by c_mitra
From the datasheet TL431, Fig 21, I see that dV(KA)/d(Vref)=1+(R1/R2); they are using an op-amp model for the gain. This equation is given in the Fig page 18.
This would be the closed loop gain in a zener reference application but the application note in question specifically refers to the TL431 open loop gain.

Googling "TL431 open loop gain" hits some sources. ONSemi includes a chart in their TL431 spec on page 6:

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6. ## Re: Gain of TL431 needs to be found for feedback loop calculation

Strange that the AN tries to model the TL431 as an op amp with unitless gain. The TL431 has an open collector output, and thus should be characterized by transconductance. The open loop voltage gain will surely depend on the cathode impedance.

That being said, if the open loop gain of the TL431 is found to have a significant impact on overall control loop, then I would suggest revising the design, since that open loop gain will likely not be well-controlled.

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7. ## Re: Gain of TL431 needs to be found for feedback loop calculation

Originally Posted by asdf44
This would be the closed loop gain in a zener reference application but the application note in question specifically refers to the TL431 open loop gain.

Googling "TL431 open loop gain" hits some sources. ONSemi includes a chart in their TL431 spec on page 6:
The reference is detailed and the model for calculating the gain is given on pages 11 and 12. It will take some time for me to digest.

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8. ## Re: Gain of TL431 needs to be found for feedback loop calculation

Hi,

usually: Gain = Vout/Vin, or Gain = dVout / dVin

Especially when you see it as an OPAMP.
So I´d say gain = -0.0014.

large input span, small output span, negative.

Klaus

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9. ## Re: Gain of TL431 needs to be found for feedback loop calculation

Thanks,
Ive just realised that the top (right) of page 13 of the AN57 app note (given in the top post) states that the TL431 open loop gain is 55-60dB. This indicates a gain of about 700.
It does indeed seem strange that the gain of the TL431 is being used in the controller small signal transfer function. Pages 286-299 of the book “Switch Mode power supplies” by Christophe Basso derives expressions for TL431/Opto controller transfer functions and Basso certainly never includes the open loop gain of the TL431 in any of his expressions.

Equation (3-58) of page 290 of the Basso book (attached), gives the transfer function for the TL431/opto based controller transfer function and nowhere is the TL431 open loop gain.

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