Hello,
In the circuit below, what's the function of D18 and C37 at the junction of R24 and R27? Do R24 and R27 look like they are in series with R26?
The IC does some really clever things with the CS pin.
http://www.ti.com/lit/ds/symlink/ucc28711.pdfI still don't understand how it knows where to sample the correct part of the AUX waveform that represents secondary voltage. Anyways the reference is 4.05V which needs the top divider resistance to be close to 90K to get 16V AUX.
So R24 is really the odd value of 205Ω, not possibly 2MΩ.
Although the CS pin is supposed to reliably block the leakage inductance pulse, the designer of this PSU has decided to RC filter it.
D18 is likely there to quickly discharge C37 during downward swings so that the valleys can be properly detected after demagnetisation.