Transformer for multimegahertz power applications

It is established that the eddy current losses in transformer windings at frequencies of 1 MHz or more, even when primary and secondary turns are interleaved in alternate layers, are unacceptably high. An improved winding arrangement is proposed in which primary and secondary turns are interleaved on the same winding layer. This almost eliminates leakage flux, to the extent that AC winding losses are due to skin effects only. Measurements on a prototype device confirm this. An improved transformer structure is described which makes interleaved turns practicable. A combination of substrate metallisation and aluminium wire bonding over a toroidal core is used to construct the transformer. Diodes are positioned in the centre of this structure to produce a transformer/rectifier assembly, and measurements on prototype units confirm that they operate efficiently. A hybrid method of computing winding losses is referred to and described briefly. It uses a three-dimensional finite element package to compute filament inductances, and tensor solution of a circuit description to calculate current distributions and winding losses. The effect of variable loadings in multiple secondaries on AC losses is also investigated. It is demonstrated that fractional turns ratios can be achieved with these transformers by suitable series and parallel connection of turns, similarly, a `suboptimal´ technique of interleaving turns is also introduced, which can reduce winding complexity at the expense of increased losses. A formal method of undertaking these designs is proposed