Flyback Transformer Design and Calculator
When the switching transistor is turn on in a fly-back converter, the primary winding of the transformer is energized, and no energy is transferred to the secondary windings. When the transistor is turned off the field collapses and the energy is transferred to the secondary windings. This differs from a forward converter topology, where energy is transferred to the secondary windings when the switching transistor is turned on. You can tell the difference between the two topologies, by looking at the orientation of the dots on the secondary compared to the primary. For the fly-back converter, the dot's are reversed, and for the forward converter the dots are aligned.
The calculator below calculates the number of turns, the inductance, and the wire gauge for the various windings of a discontinuous mode fly-back converter.