Transformers for switching power supply
The transformers realized on the ferrite cores often with air gap by reason of increasing of the possible drawn current. The transformer consist of primary winding (usually divided in two halves), of BIAS winding and one or more secondary windings (usually realized by litz wire cable).
As far as the shapes of the cores, there is a noticeable trend in the last time: from the cores EF to the cores ETD.
Power supply transformers for the PCB´s
Encapsulated power supply transformers use for small sources on the PCB´s. The transformer is usually realized on the two chambers bobbin (separated primary and secondary winding) and it can contain a thermal fuse. It usually use trafo sheet at intervals EI30-EI48.