Thanks, yes granted the LTC4440 is only rated to 80V....though my apologies as its the 2ED2182 (650v) that we will end up using, as its says on the schem above.
So it seems that a bootstrap driver cannot be used here.....though i suspect that we may use a digital isolator chip instead to level shift the gate drive signal...then shovel that into a hi side gate driver.
Would you agree?
Also, AYK, We cannot use a pulse transformer as we sometimes will need the FET to be held on for several hours.
I am amazed theres nothing offtheshelf for this, as driving a buck fet from such a high "Underside" is common, eg with pfet buck drivers, which need converting to nfet drive.....the hi "underside" being needed as the pfet driver needs to connect to a hi side sense resistor.