I've been there and done that with respect to linear power supplies with cinder block size transformers. I'd like to explore the area of taking a high potential source, chopped at dynamically changing duty cycles, to achieve a constant output DC voltage.
What I had in mind was to use a LM339 quad voltage comparator, zener diode, power mosfet, and humungous electrolytic capacitor.
I will assume that the input AC mains power is rectified and filtered the same way in a linear supply. I'll refer to this available, high voltage DC source as "mains".
Connect the mains to the negative side of the voltage comparator, through a 1Mohm resistor. At that same terminal of the comparator, connect a zener diode, of the desired output voltage, to ground. This will present a constant voltage to the comparator, as long as the mains is greater.
Connect the output of the comparator to the gate of the power mosfet. Connect the mains power to the Source of the MOSFET and the Drain of the MOSFET will go to the positive terminal of the electrolytic capacitor. The negative of the capacitor will connect to Ground. The positive of the capacitor will feed into the positive side of the comparator. Once powered up, the voltage comparator will allow the MOSFET to conduct when the capacitor voltage is less than the desired reference voltage, and it will turn the MOSFET off when the voltage is greater than the desired output. So if the desired voltage is 14.4 volts, then I want the comparator/MOSFET pair to keep it between 14.39 and 14.41.
The issues that I know of are:
1. Voltage comparator has to be able to fast enough to recognize changes in capacitor voltage as compared to reference voltage.
2. The MOSFET also has to switch fast as well as be able to handle the available mains voltage at its Source terminal.
3. Possible drifting of zener voltage with temperature, although I think there are two temperature dependent factors that supposedly balance each other.
I've had two electrical engineers doubt this is a good design, but no one can explain why.
I just want the ability to change out zener diodes and get different outputs as well as have high efficiency.
Any input is greatly appreciated.