low noise regulator

[user99]

As part of a learning project I'm designing a bench power supply.  My goal is for it to build it using parts in the bin and digitally controlled, low noise, reasonably safe and reliable. 

I tried out a few pre-regulator designs in spice and the simplest one ended up being a mosfet switched with a comparator.   

One of my concerns is transients generated when the mosfet turns off under a heavy load.   There doesn't seem to be much visible on the scope when it switches but I'm limiting the breadboard circuit to 20v and about 500mv load.

A diagram is attached.