Losses in foil-wound secondaries in high-frequency transformers

The eddy-current losses in the foil or strip secondary windings depend on radial components of leakage flux and not on axial leakage flux, which is the component used in conventional calculations. It is shown that existing methods of calculating eddy-current losses, sometimes known as proximity losses, which are suitable for wire-wound transformers are inadequate for foil-wound conductors. The eddy-current losses do not change significantly if the transformer core is removed, provided that primary and secondary ampere-turns are maintained equal and opposite. This suggests that leakage fields are not unduly influenced by presence of the core. A special coupled circuit model of a transformer in which the core can be assumed absent is suitable for calculating losses in a foil winding and simplifies calculation of the self- and mutual inductances. Experimental and theoretical results for two specimens show that the special air-core model predicts quite well the secondary winding losses in real, ferrite-cored transformers over the frequency range 20-100 kHz