Do you need to simulate a full DC_DC convertor using only BJT (lots of BJT ) or just using a normal dedicated controller and some externals power BJT?
Like here: **broken link removed**
You can use a LINEAR DROPPER by using NPN BJT with a zener & a bias resistor at the base to clamp the base at 5.1 so emitter will float at 5.1V-0.7V = 5.3VDC aprox.