You can also use a gate drive transformer, in conjucntion with a current sense transformer to drive a high side NFET.
This doc tells you Gate drive methods
http://www.radio-sensors.se/download/gate-driver2.pdfRemember to use a pnp turn-off circuit if you use a GDT.
The essential theory of GDTs is to ensure that your primary doesnt saturate with the V.us. -Its the magnetising current that saturates a GDT.
With a CST, its important to remember in the secondary, that your volt.seconds OFF must at least be equal to the volt.seconds ON, in order to reset the CST, and stop it saturating.
Give me a shout here if you want further info on this.
If you want a high side nfet and not a GDT, then you can use a bootstrap high side drive IC.....but watch if you go into total no load or burst mode then your bootstrap cap may discharge which may effect the return time from no load to full load.