Can we build the switching dc power supply circuit with 2 transistor? This may be a good choice for you.
Here is a 12V 3A Power Supply Switching Regulator circuit, or Step down DC Converter voltage. Use IC LM2576-12 SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator. Input DC Voltage 40Vmax and 16Vmin 3A.