I am wondering about the BP and FP pins of the LNK632DG. In the application note DER207 we have a circuit as shown in the attached figure. What is the reason for connecting the BP pin to an auxiliary winding? In the datasheet the BP pin is described as, "It is the connection point for an external bypass capacitor for the internally generated 6 V supply" and "It is a mode selection for the cable drop compensation". Is it possible to have the BP pin only connected to a capacitor, and not connected to an auxiliary winding?
And for the FB pin, how are R3 and R4 determined? In the schematic is seems FB is connected to the auxiliary winding but without rectification and capacitor. I have poor understanding of how flyback transformers work, so I would be thankful if anyone could explain how this works :)