TL431/OPTO Feedback with flyback with/without CLC filter

[eem2am]

tl431 circuit

Hello,

Basso’s book, page 290 gives the transfer function of the TL431/OPTO feedback network in a flyback converter as per the transfer function in the below diagram.

*****Diagram and transfer function of OPTO / TL431 Feedback network.

My question is, supposing that I don’t use the CLC filter on the flyback output as shown above. That is, supposing I omit the output inductor between the C1 and C2……….

….Then does this above given transfer function still hold?



*****UC3845 Datasheet
https://focus.ti.com/lit/ds/symlink/uc3842.pdf

 

[FvM]

tl431 feedback

Obviously, it's a simplified transfer function, only valid when certain assumptions come true.One assumption is written on the schematic. It also answers your question regarding L3. It's simply ignored in the feedback transfer function, otherwise it must play a role within.

The other simplification is by ignoring your beloved opto-coupler pole.

 

[eem2am]

flyback feedback

FvM:

"Obviously, it's a simplified transfer function, only valid when certain assumptions come true.One assumption is written on the schematic. It also answers your question regarding L3. It's simply ignored in the feedback transfer function, otherwise it must play a role within."
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.....-Indeed, the inductor L3 does not appear in the transfer function in the Original Post.

However, the derivation of the above transfer function is dependent on the assumption that V(out) is steady DC, but that the voltage at the secondary diode’s cathode is more dynamically changing.

In other words, as explained on page 288 & 289 of Basso’s book, this transfer function assumes that R(LED) is in the so-called “fast-lane” and R(upper) is in the so-called “slow-lane”.

-And the reason that the “fast-lane” is fast and the “slow-lane” is slow is because of the inductor L3.

So , the question is still, does the removal of L3 (ie shorting out L3) result in a different transfer function than the above?……and if so, what will the new transfer function be?

Unfortunately, there is not a book on the face of this planet which tells this.

 

[FvM]

flyback tl431

In other words, as explained on page 288 & 289 of Basso’s book, this transfer function assumes that R(LED) is in the so-called “fast-lane” and R(upper) is in the so-called “slow-lane”.

No. I don't have the book, but this assumption is neither according to the transfer function nor the text written at the schematic.

As said in the previous thread, the schematic comment regarding L3/C2 basically tells, that L3 is ignored in the transfer function. That's the whole story.

You may want to derive the transfer function of the TL431 circuit part, taking e.g. the voltage across Rbias as output. If you ignore the limited TL431 gain (and L3/C2) you simply get 1/R4C3s + 1 respectively (R4C3s +1)/R4C3s. So you see, that the said transfer function is a completely idealized function, without any slow or fast lanes.