O transformador é você quem decide, depende da tensão de saida.
Umas dicas (http://sound.westhos...m/project89.htm
Transformer Construction Details
This is the most critical part of the design, and you have two options, buying a commercial unit with the required power rating and turns ratio (hard to find, only a single supplier found at the time of writing), or wind your own.
If you choose to wind your own transformer (as if you have much choice), you have to decide which shape of core to use. The preferred material is ferrite, which has high permeability (ability to "conduct" magnetic flux) or iron powder, which has a lower permeability, but is less likely to saturate. Most commercial transformers use ferrite, and iron powder is generally the best material for filter chokes (inductors) that carry substantial DC.
For example, with a standard ETD39 core you could theoretically build a > 350W supply. Winding this type of cores is not very difficult, but you will have to follow some guidelines I provide below in order to have good results.
Another possibility is using a toroid. You can extract it from a BIG power inductor. As a guide, a 4cm diameter toroid with a section of about 1cm2 can be used for a > 250W SMPS. Winding is a little bit more complicated than with ETD cores but with a little practice is not too difficult either.
Toroid from ITL 100 inductor (Wilco Corp).
(Remove the thick wire before winding! :-)
These are a few general winding guidelines for all types of cores:
You MUST use enamelled copper wire for all the windings. Keep also in mind that when working with high frequencies, the effective section of the wire is much smaller than the physical one, due to the "skin" effect (the current concentrates only in the outer part of the wire). As high currents are involved here, the section of the wire is very important, (if you don't want the enamel to fuse due to the heating produced by the resistive losses of the wire and short all the windings). A good practice is to use several thinner wires in parallel rather than a single thick one. This also eases winding. For example, six 0.4mm diameter wires can form a suitable primary for a 300W supply. The same applies to the secondary, although the current is reduced so you can use less wires (3 or 4, for example). From now on, I will refer to each composite wire as "winding", and to each thin wire as "wire".
The wires must be tightly wound. You must wind the primary first, trying to cover all the surface of the core, and then the secondary over it in the opposite direction, to maximise inter-winding coupling.
A good starting point is using 4 turns for each primary (that is, 4 turns, centre tap and another 4 turns IN THE SAME DIRECTION). To calculate the number of turns of the secondary winding, multiply by the turns ratio. For example, if you want to build a +/-30V supply, the turns ratio is 30/13.8=2.2 approx, so wind 2.2 x 4 = 8.8 turns (better 9 turns, to overcome the diode losses) for each secondary (that is, again, 9 turns, centre tap and another 9 turns IN THE SAME DIRECTION).
To start winding, take the number of thin wires you have decided to use (6, for example) in the primary, all together. Leave about 3 or 4 cm out of the core to ease connection to the board and start winding. When you have wound 4 COMPLETE turns, go out the core and cut at 3 or 4 cm. Now you have the first primary. Then start again IN THE SAME DIRECTION winding the other 4 turns and at the end leave another 3 or 4 cm for connection. Twist together the thin wires of each winding at the ends, to ease soldering.
The varnish of the wire is intended to provide electrical isolation, so you have to remove it at the ends to make the connections to the board. Be sure to remove about 1cm to the end in ALL the wires you use. You can do that using a special solvent or with sandpaper and a lot of patience BEFORE winding.