"Fast Lane/Slow Lane" SMPS Feedback loop compensation with the TL431

[cupoftea]

Hi,
We all know to what this refers. Why do all Flyback SMPS application circuits show the divider being connected downstream of an output filter inductor?
That output filter inductor does not have to be there.
It is not inputted into the feedback loop calculations.
The "Slow Lane" is so called because at a certain frequency, the Cap from TL431 cathode to its ref becomes effectively short or low Z.

So why do they all tell us we must have that inductor in there?

 

[Rahul_d]

Flyback converter sometimes need post LC filter to reduce output voltage ripple derived by tighter specifications,
If you add this extra LC filter & close your feedback loop after the LC filter, stabilizing the feedback loop poses additional challenges, fast loop & slow loop concept will always stay irrespective of post LC filter present or not due to the opto-coupler LED current is involved in closing the loop.
please refer to Christophe Basso papers for more details.

Link to his & other informative pages.

SPICE

https://www.onsemi.com/pub/Collateral/TND381-D.PDF

https://www.power.com/sites/default/files/forum/files/Design_with_TL431.pdf

 

[cupoftea]

Thanks, also, just so its known what is meant by " fast lane - slow lane"...its on page 12 of this...

https://www.onsemi.com/pub/Collateral/TND381-D.PDF

 

[cupoftea]

In the following TND381 document, on page 19, they show the Type 2 error amp transfer function with TL431.
However, they fail to multiply it by the attenuation factor that gets applied to V(FB). In all onsemi flyback chips, the feedback pin voltage is divided down for input into the pwm comparator...why have they omitted this attenuation factor here? (and everywhere else).
The transfer function is wrong without this, since it is always there, in the real circuit.

TND381

https://www.onsemi.com/pub/Collateral/TND381-D.PDF