High Voltage, MCU controled 300Volt power supply

[pedja089]

That is first example that I found to get theory of operation for phase shifted forward smps. You calculate your transformer for maximum voltage that you need on output, with lowest voltage on input.
If your phase is 180deg out of phase, you will have full voltage on output.
If your signals are just little bit off phase you will have very short impulses on transformer, and output filter(this is basically buck converter), will average that short impulse to low voltage. With relatively high ripple, so that is reason I suggested linear output stage. Output voltage of SMPS just need to be few volts higher than output voltage. So linear post regulator will have relatively easy job(not much disipation).
Any other architecture, as far as I know, cant generate very short impulses without causing major loses in mosfets.

For example if you calculate your flyback to give 300V with 80% duty then you need 0,5% of PWM for 10V on output(approximately). If your flyback work around 40KHz, your PWM period is 25uS. Your PWM pulse width for lowest voltage will be 0,125uS.
I wouldn't even try to generate that...
But if you have enough resolution for phase control, you can relatively easy generate that pulse on transformer. You have relatively small output current, so you can omit synchronous rectifying.