I am designing a powered USB HUB (UFP) which can provide power to the host through USB PD. This means I need a PD controller which supports DR_Swap and it seems that STUSB4700/STUSB4710 cannot be used and something like TPS65987D is overkill.
However, the STUSB4761 is designed for AC-DC converters and I am unsure how to re-purpose it for something like a DC-DC buck converter.
Is it possible to take this design and replace the AC-DC circuit with a buck converter circuit?
This is already a DC-DC converter, but it is an isolated flyback, not a buck. You will have to wind the primary with proportionally fewer turns if you have a lower input voltage than the rectified voltage you would get from the rectified AC, and the switching MOSFET will have to handle proportionally larger current. Otherwise, replace the bridge rectifier output with your DC input voltage and everything should work just fine as shown in the detail from your figure below.
You definitely want isolation between primary and secondary, which is why this approach was used, to keep high voltage from getting into your low voltage circuit if something fails on the primary side. (Not a good application for a buck converter.)
