If you can get ferrite cores, I can recommend you the SG33524, easy design. The trouble is the transformer, but if you have the core, it's only matter of som probes.
You can get the datasheet of the SG3524 or LM3524 or UC3524: www.national.com www.ti.com www.unitrode.com www.fairclildsemi.com
I suggest You'd use some ready-made DC/DC converter module, for instanse, Aimtec or Franmar or some other (18...36)-12V output module.
It isn't an easy task to design your own power supply from scratch that would sutisfy all the specifications.
The problem you'd have to solve is how to get 48V from 12V DC - don't know a ready solution.
Is this supposed to be an isolated P/S?
If so, you could probably use an additional winding on the transformer, that would produce 48V, then after 5ms, switch to the normal 12V one.
It it is not isolated, then you can try using both a buck and a boost converter, with OR-ing diodes. After 5ms, you turn off the boost and let the buck produce 12V.