tl431 usage
I apprecaite this VVV, but i believe if our friend wei9321 is going to be able to close his loop, he is going to need the transfer function for this little lot.....including as you say that load resistor on the primary side.
and that , as i say, wei9321, will not find, because it cannot be found.
Even the great text by Basso only includes the transfer function for the version of this circuit that has a CLC filter on the output with the L separating r(upper) and r(led)..........and to be truthful......for a type 2 amplifier it is a hulking great huge big transfer function..........and so too it will be pretty big transfer function in wei9321 's case.
Feedback is the part of SMPS that the industry wants no-one to know.....apart from their own staff.
And put it this way, and engineer who can derive transfer functions for modulators and error amps etc is a rarity.....and a fair genius.
Most will dust off the gain/phase analyzer and go for trial and error......soldering iron burning well!!