Design considerations for high power high frequency transformers

Various design considerations for high-power (multikilowatt), high-frequency transformers for DC-DC power converters are investigated. Although Ferrite (PC40) is lossier than Permalloy80 (0.5 mil) at the frequencies of interest (25-50 kHz) the use of the former is justified on the basis of cost, weight, and wide ranges of shapes. The influence of the leakage flux distribution in the window region on the copper losses has been demonstrated for various conventional winding arrangements. However, it is seen that in conventionally wound transformers a considerable amount of leakage flux enters the core, resulting in localized core saturation and hot-spots. This becomes a very critical issue for high-power, high-frequency transformers. Coaxially wound transformers are seen to be a viable alternative, in that the leakage flux is contained with the interwinding space, with very little or none of it permeating the core. Such transformers can also realize multiple benefits of a low distributed and controllable leakage inductance, robust construction, low electromechanical forces, and low core and copper losses. Test results on two coaxially wound transformers with different tube geometries, designed for 50 kW, 50 kHz, primary voltage of 200 V, and secondary voltage of 1000 V (two windings), are presented. Finally the concept of coaxial winding technique for three-phase transformers is also presented.>