What is the fault with my hbdridge design in the below circuit using pchanel and n chanel mosfets?
MOSFETS have a significant gate capacitance. 

That means their turn on time is heavily dependent on the resistors in the gate drive line. 

With low voltages these resistors can be small, but with large voltages, with this kind of voltage divider circuit, the resistors need to be large so as not to waste too much power. That means the MOSFET turn on time will quickly become large and make this circuit unusable.

Driving high voltage bridges requires a much different technique. Fortunately, integrated drivers are available for this.

An alternative, since you are using opto-couplers anyway, is to add a -12V supply up at the 300V rail and use that to power the top-side drivers.







simulate this circuit &ndash; Schematic created using CircuitLab

BTW: If you are smart you will up those 12V supplies to like 10% less than whatever Vgs_Max is for your selected MOSFETS for maximum efficiency.