You are using a digital stimulation signal instead of an analog pulse generator to drive the MOSFET, apparently without understanding the specific behaviour of the mixed signal (PSpice AD) tools.
Two options:
- read the AD user manual to learn how to use the mixed signal features correctly
- use a regular VPULSE source
Use a PWM controller IC model, or more instructive, design a simple behavioral model of a PWM controller comprised of a reference voltage, an error amplifier with PI characteristic, a triangle voltage source and a comparator.
The load capacitor is also too large to put in an IC.
I suppose you plan to add the control circuit? The control circuit needs to be smart. It must manage two behaviors. It must maintain on-off action in the transistor. It cannot allow it to stay on.
The control circuit must also monitor output voltage. It must alter the duty cycle, so that it maintains your desired output voltage.