Aditya,
Your specifications suggest a straight forward flyback converter design, 2:1 input voltage range, probably operating in DCM over the entire line/load range. Here is a TI/Unitrode application note that will walk you through the design calculations…
Joey,
From the LM5021-1 datasheet Fig. 7 & eqn. 1, with Rt= 13k7, Fsw should be ~ 484 kHz - which tallies with the waveform you show above for 300-mA load.
At 600 mA, the freq appears to be doubled, which does not make sense.
In between these…
