Why LC output filter always with TL431 in "Dual vout connection"?

[Faringdon]

Hi,
We always see LC output filter in SMPS being analysed with their connection as in fig 22, page 12 of the following..

Practical design considerations.....Dr Choi
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjtlfe7wq-AAxVCVkEAHYuMA94QFnoECBgQAQ&url=https%3A%2F%2Fwww.researchgate.net%2Fprofile%2FMohamed-Mourad-Lafifi%2Fpost%2FHello_it_is_a_power_system_stabilizer_How_to_calculate_the_gain_margin_in_the_bode_plot_if_the_phase_is_always_above_the_0_degree%2Fattachment%2F59d635f479197b80779936c8%2FAS%253A386285110874112%25401469108955496%2Fdownload%2FPractical-Feedback-Loop-Design-Considerations-for-Switched-Mode-Power-Supplies.pdf&usg=AOvVaw1nV06ZJ6OWyxExmMFKNo_H&opi=89978449

...ie, the TL431 feedback "straddles" the output filter inductor.

However, most applications do not have the typical TL431  feedback connection , ,and simply use an opamp error amp feeding off a bias voltage, and they only have one connection to the vout for feedback purposes.

So why dont we see that case investigated more....?

In fact, that case is seen here  on page 10 of the above link.....it simply means multiplying the initial power stage transfer function by the extra factor shown there that depicts the output LC filter.
___---___---___
The typical TL431 feedback connection involves  two connections to vout.....and this makes the feedback compensation more awkward to do.....the following TND381 on page 36 shows why the "fast lane/slow lane" approach of the typical TL431 connection is disavantageous. The fact is that the "dual vout conenction" means that you end up on a limit to the available gain settings...as page 36 says.

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

Pages 45 and 46 of TND381 show more woes of the "dual vout conenction"....here you see the problem that the RLED resistor unfortunately ends up playing a role both in fixing the gain , and fixing a zero position........page 46 shows this being solved by the ridding of the "dual vout connection".

So why are all texts obsessed with the "fast lane / slow lane" connection of the TL431?

As discussed, page 10 of the top link from  Prof Choi shows the factor that one simply needs to multiply with  whichever power stage  transfer function is being worked with.

Page 12 of the Choi document actually says QUOTE>>This configuration is very effective in  increasing the phase margin when the crossover  frequency is close to the double pole of post LC filter.<<UNQUOTE

However, no other author lists this as an advantage of the "fast lane/slow lane" approach...not even Basso in his books......maybe Basso feels that he said enough to imply it...but who knows.

[Faringdon]

Second stage LC filter sizing:
As an example, here are the bode plots for a flyback SMPS with xover at 600Hz.
...And also for the same flyback, but this time with a second stage LC filter with F(resonance) = 919Hz

As you can see, the Bode plots still show stability , even after the addition of the LC filter (10uH and 3mF).

This shows that the " rule of thumb" discussed, is not correct.

The factor seen in the equation on page 10 of Dr Hangseok Choi's doc "practical feedback loop...." was used to multiply by the original transfer function to get the Transfer function with the second stage LC.

This shows why only extreme cases of external LC filter cause instability.

In the shown plots, the phase margin is certainly reduced, but not giving total instability.....the phase is still above 180degrees at the crossover.