OpAmp-Driven Variable-Output Voltage Supply

I am trying to create a circuit where I force a variable voltage (that is set by voltage source below) on a variable load for a power supply. The output voltage is "sensed" and corrected by the OPAMP, and is delivered by the main power supply (20V fixed DC source) to the load (1kOhm to the right) via power transistors (connected in Darlington pair). By doing this, I shunt all the current requirement and heating on the transistors instead of the OPAMP. My problem however is that I am wasting much power on the transistors when I force lower voltages on low impedance loads. Is there a clever way of avoiding this without using buck regulators? Perhaps there is a workaround with this that by decreasing the main voltage source to also decrease the voltage drop across the transistors. My main issue on this is with efficiency on converting to a lower voltage, as much as possible I want to avoid switching to different taps on the main transformer circuit of a powersupply so as to not deal with wear and tear. Also, the simulator can't seem to calculate initial conditions/biases so I added some pullups/pulldowns and an SPDT switch. Any input would be welcome!

published 5 years ago