Feedback loop graph of flyback smps is wrong?

[treez]

Hello,
The "total loop small signal response" graph on page 13 of the following...
https://www.ti.com/lit/an/slva559/slva559.pdf

..cannot possibly be the total loop response, as they have neglected to also check the second feedback path through the opto resistor?

Putting an injection resistor where they have is not sufficient to test for the complete feedback loop? (schematic is on page 2 and also 9 shows the feedback components.

 

[mtwieg]

So long as the value of R3 is small and doesn't affect the loop response, it should be a valid measurement. They also refer to injecting at R4, and explain that this neglects the path through the optocoupler, and that this is therefore not a rigorous check on loop stability. But there is no parallel path around R3.

 

[treez]

**broken link removed**

..according to figure 8 on page 4 of the above, the way the ti.com article has done it in the top post is wrong.
I should have made it clear to yourself that in the ti.com artcle they are injecting across the 49 ohm resistor.

 

[mtwieg]

I should have made it clear to yourself that in the ti.com artcle they are injecting across the 49 ohm resistor.

You must be confusing R3 with R4. The TI article explicitly says they inject at R3, which agrees with the second article.