LNK632DG circuit questions

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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 :)

Best regards

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LNK632.PNG90.36 KB

Hi dev,

LNK632 internally has a 6V linear regulator connected between the drain (D) pin and bypass (BP) pin. This is where the IC gets its power at startup. However, this results in very poor no load input power because of the large voltage difference across the linear regulator. An auxiliary winding is recommended because the rectified auxiliary voltage (across C4) can be made much closer to 6V than the Drain pin - resulting in much lower input power (<30mW at high line).
During the flyback off time interval, the voltage reflected across pin 2 - pin 5 of T1 will be proportional to the output voltage (by a scale determined by the secondary and bias turns ratio). The IC samples the voltage divided by R3 and R4 during the off time interval. R3 and R4 are selected so that FB pin is equal to V-FBth. This is explained in page 3 of the datasheet.

Hi Jango,

Can I inqury some question for above attached picture of LNK632 due to having interest by
looking your conversation。I have a doubt why FB sample winding isn't directly connect with
BP supply winding and using two winding in auxiliary is what is purpose。what is it have advantage? Thanks!