Hello,
I have the following design using a UCC28631.
Vnom: 270V
Vout: Multiple
Po:80W.
The device needs a minimum preload value from Eqn 64 in datasheet, PPRELOADmin = 0.5 × LPRI× (VCS:min / RCS)^2 × fSW:min , but for my design to work I have to set my outputs preloads to almost 10W!
My Lpri is 250uH (similar to the example equation) and my Rcs is 10 Ohms ( .2 Ohm going through 50:1 CT).