Time variable transformers operating at a near-unity transfer ratio and some possible applications

Time variable transformer network elements are reviewed. Four special topologies are presented which are of benefit, especially in the vicinity of a unity overall transfer ratio. Analytical expressions are derived which indicate the resulting improvements in terms of energy conversion efficiency and systems power density (as well as expected cost). It is shown, that the use of the proposed topologies may, in some cases, improve these figures of merit by a factor of two or more. Additionally, when the time variable transformer realisation is achieved by means of switched mode converters, the dynamic characteristics, and consequently, the dynamic load regulation will improve significantly. Application of the resulting topology in an active ripple cancellation stage of a unity-power-factor rectifier clearly validates the proposed approach.