This is a simple but efficient voltage regulator / buck-converter circuit.
High Side Mosfet driver for elegence and a 393 to power the charge pump and for the comparator circuit.
The IRF540 will power to 5A but the circuit will function at higher powers by changing the MOSFET.
The Zener Reference is 7.1v which the trim pot then allows you to select the output voltage (which must be lower). If a higher voltage is required change this reference diode to one higher than required
The circuit will function up to 30V input (the limit is the LM393), above that will require a driver transitor to isolate the charge pump and a voltage dropper for the LM393.