PWM Control with MOSFET switch
If I'm reading this correctly you are using the MOSFET (switch) to charge a capacitor to a certain voltage. The voltage across the capacitor is being drained by the (fixed) load connected across it. The turn ON/OFF times are to be controlled by a 555 used as a PWM. Something like this:



Essentially it is an open loop system. The ripple voltage across the capacitor will be a function of the chop rate and the M/S ratio of the PWM.
My first reaction would be to make it a closed loop system by monitoring and comparing the voltage across the capacitor (too high/too low) and using this information to control the PWM. Instead of altering the PWM 'manually' you would simply change the reference voltage (variable resistor) and the circuit would self adjust.
I would also think an inductor  between the  storage capacitor (C) and load (and possibly a second capacitor across the load - pi filter)  would be helpful to smooth the ripples.
You may have to add a resistor in series with the MOSFET to limit the charging current and a fuse (from 300V supply) would be helpful in case of a short circuit caused by a faulty capacitor or an accidental short circuit output.