full bridge dc dc converter

If you don't need input to output ground isolation, go for a simple buck converter, this will avoid complexity and save time. If ground isolation is required, you would have to go for a transformer coupled topology, and for 2A I recommend either flyback or forward topologies rather than full bridge because they use just a single primary side switch and one or two secondary side diodes.
razunitem said:
full bridge dc dc converter 30v/12v and Iout=2A

how can design it this circuit?

If you don't know how to design it (or even if you do), there are two pretty-good ways to get started:

1) Go to http://www.national.com/appinfo/power/webench.html .

2) Download and install http://ltspice.linear.com/software/swcadiii.exe . Then run it and select File-->Switch Selector Guide.

Both 1 and 2, above, can automatically design several different suitable SMPSs for you, using the SMPS controller ICs of either National Semiconductor (1) or Linear Technology Corp (2). I would try both of them.