Multi-turn high frequency co-axial winding power transformers

Three methods of attaining multiturn outer windings in coaxial winding power transformers were proposed and evaluated. Two of the methods utilize solid tube windings; the first employs concentric, series connected tubes, and the second uses a single tube split into uniformly spaced arc segments. The concentric tube winding allows use of additional winding material and, if the tube thicknesses are chosen close to the optimal values, results in the lowest AC resistance increases and ultimately exceeds that of the split tube winding. The split tube winding with n turns results in an AC resistance which is n² times the resistance of the original unsplit tube. Unlike the concentric tube winding, the frequency dependence of the resistance is identical to the single tube case which can be advantageous when the current has a large harmonic content. The third technique utilizes litz wire for the outer as well as the inner winding. By employing a number of wires arranged in a cylindrical pattern, a reasonable approximation to a coaxial outer winding can be produced. A prototype transformer having a 10-turn outer winding was constructed and tested