UCC28056EVM-296: question about the meaning of external components

Hello Tsuyoshi-san, 

For L1, it is intended as a filter against differential-mode (DM) conducted noise currents.  Since the DM noise is expected to be a loop current through both sides of the line, a single inductor on one side should eb exactly as effective as two 1/2-sized inductors one both sides.  However, 1 inductor is less expensive than 2, so I think that is why most designs use 1 inductor on 1 side of the line. 

For D2, its purpose is to bypass the inrush current which charges Cout (C16+C17) when AC voltage is first applied.  D2 is supposed to keep the peak inrush current out of the boost inductor L3 to keep it from saturating in case the MOSFET Q1 starts switching before inrush is complete.   It also bypasses any peak current if there is an input line surge. 

Yes, RT1 is an NTC resistor which has higher resistance when it is cold to limit the peak amplitude of the inrush current to a level that does not cause problems, such as blowing the fuse or tripping a circuit breaker. 

Regards,
Ulrich