A switched mode power supply is always dc-dc.
If you have AC it's normally better to use a transformer to give you the correct voltage and then rectify that. You could convert the AC to DC and then use a switched supply to give you the voltage you want but that's just adding an extra stage in front of what is basically a dc-dc converter. Make your life easier, just start with DC on the input.
First you need to decide if you want to build a step up or a step down supply. Roughly what voltages do you want in and out.
Then all you need are some FETs, an inductor, a diode, a few caps and a voltage controlled oscillator.
And I wouldn't try to model this in matlab, pspice is a far batter tool for modeling circuits.
Finally when you actually build the thing be very careful, switching power supplies are very sensitive to poor circuit layout. Modeling tools will only get you so far, if you lay the circuit out badly (e.g. long ground loops etc.) then don't expect the real world to match the theory very well.