hi.
a flyback topology is not suitable for
high power over 300W.
the stress on the transistor and the output dioder is very high
especialy at low input voltage.
i suggest you to use mc34262 for the
pfc for the first stage with output
voltage at 400V.
for the second stage use a forward
current mode topology, use mc3842.
infineon (simenes) have one chip for
this work.
look also at
https://onsemi.com
sorry, i dont have now the time to upload.
if you want to upload pdf files on switching
power supplies including pfc and other topologies.
i will post you my login and password at freedrive.
regards - haymeron