Windings and Cores
A transformer is both an electrical winding system and a magnetic structure. The winding determines resistance, current capability, insulation, and much of the leakage. The core determines how easily magnetic flux can be established.
Magnet wire
Small transformers commonly use enamel-insulated copper magnet wire. Larger units may use rectangular copper, aluminum, foil, or parallel conductors. Wire size must satisfy current, temperature rise, voltage drop, winding-window space, and insulation requirements.
Insulation
Turn-to-turn enamel is only one layer of the insulation system. Layer insulation, bobbins, tapes, barriers, sleeves, varnish, and spacing may all be needed—especially between primary and secondary.
Core materials
| Material | Typical territory |
|---|---|
| Laminated silicon steel | 50/60 Hz power and audio. |
| Ferrite | Switch-mode power and RF. |
| Powdered iron / composite | Selected power and RF magnetics. |
| Amorphous / nanocrystalline | High-efficiency and specialized transformers. |
| Air core | RF coupling where core loss or saturation is undesirable. |
Why laminations?
A solid steel core would support large circulating eddy currents. Thin insulated laminations interrupt those current paths and reduce loss.
E-I and toroidal cores
E-I laminations are inexpensive and easy to wind on a bobbin. Toroids provide a closed magnetic path and can have low stray field, but the winding process is more difficult.
Leakage inductance
Not all flux from one winding links all turns of another. The unshared part appears as leakage inductance. Close coupling and interleaved winding sections reduce leakage but usually increase interwinding capacitance.
Temperature
Insulation systems have temperature classes. Transformer life depends strongly on hot-spot winding temperature, so copper loss, core loss, ventilation, ambient temperature, enclosure, and duty cycle all matter.