Multi megahertz transformers

This paper reviews the electromagnetic considerations that determine the design of high frequency transformers, especially with regard to eddy current losses in the windings. It shows that the “E” core transformer cannot be evolved into dimensions which make it satisfactory at MHz frequencies. Similarly it is demonstrated that planar transformers have worse performance than “E” core versions, partly as a result of inherently 2-dimensional leakage fields and partly because their volume and losses are greater. An alternative winding arrangement is proposed which virtually eliminates proximity effect. It relies on interleaved primary and secondary turns on the same winding layer for this. It is best produced in toroidal form and requires wire bonding technologies for the production of the windings. Results from prototypes are present to show that proximity effect is eliminated and that pure skin effect is the main cause of residual eddy current loss. It is shown that noninteger turns ratios are feasible, that multiple secondaries can be produced conveniently using multilayer substrates, and that “sub-optimal” interleaving can be employed to simplify structure and design