Here is a schematic i had found that is a 12vdc 2500W inverter. The 12vdc side is just what you need. The output is for 220vac, you would just erase that part and design the transformer for 48vdc output.
It may look simple, but because of the low voltages and the high currents this is a very difficult project. 2000W / 12vdc = 167A, and you are doing this on a PCB. This is almost a research project in itself. This would be very difficult even for people that have made smps before.