I am building a DC-DC buck converting circuit from first principle with no on chip solution. Output Voltage must be 5vdc with a max current @500mA. input is 20+-4vDc. The problem that I am having is that my output voltage is 7.5vdc once bread boarded, but a 5vdc is required. I can solve this by increasing pull up resistance to the TIP122G but there is still a problem of the 5vdc output not being constant and dropping when a load is added. I think this my be due to my feedback circuit but I'm not sure as to what the problem may be and what I should change in it. Saw tooth generator has a frequency of 50kHz.
The main problem is the 741 op-amp - you appear to be powering it from a 7.5 volt zener diode yet, if you read the 741 data sheet the implied minimum supply voltage is 10 volts. See table 7.2 in the uA741 data sheet. Try using a modern device and not this crappy dinosaur. Think "rail-to-rail".
Also, I don't see any decouplers on the op-amp power rail.
- Q5's base is driven from 9.1V through high valued resistors, therefore it cannot saturate properly.
- Switching will also be extremely slow due to the 10k base drive. There is no way this schematic will work.
- 1N4007 is way too slow
- 741 should be replaced with something more modern
- There is no overcurrent protection
- C5 is way too low
You should scrap the entire schematic and use a proper, modern, MOSFET-based DC-DC solution (or an integrated, ready-made one, which you can purchase for a few bucks these days).