Other Parts Discussed in Thread: UCC28950
The application note SLUA287 describes a simplified method to drive the synchronous rectifier mosfets from signals A and B.
However it seems to me as if with this method avoids soft swithing in half the transitions.
If you look at t2, when QA turns off, the same does Q2. The voltage in the drain of QB starts to decrease, so the secondary voltage starts to increase, but with the negative polarity in the dot side. This seems to mean that Q1 diode will conduct. The current of L2 will go through the secondary but in the wrong direction, that is cancelling the primary current, with the result of a hard swithing.
The same seems to occur in t4 transition.
On the contrary, in the transitions t1 and t3, the current reflected to the primary works in the right direction and helps the auxiliary inductor.
So it seems to me as if this system cannot fulfill the soft switching requirements. Also that with a current doubler you have to use the E and F outputs of the UCC28950 and adjust the delay TAF to the complete transition time, and then set TAB to a value slightly longer. TCD could be very short thanks to the help of the reflected output current and can be adaptative.
