Free circuit schematics - wiparat.com

The 78S40 is a very old, but still useful power conversion chip. It contains a 1.25V reference, comparator, gating, output transistor, flyback diode and an uncommitted operational amplifier, which is not used here, and draws very little power. As the internal transistor and diode are good only for 1A maximum, I had to use external [...]

The design is a simple saturation-limited push-pull converter. There is no special reason to use PNP transistors; I used them simply because I had a box full of them around. You may well turn over the design to use NPN transistors.
The 2SC945 is a bias switch for startup. When applying 12V power, this transistor applies [...]