Leakage Inductance at High Frequencies

Provided that a transformer is operated at frequencies below resonance, the leakage inductance measured at low frequencies governs response at high frequencies. Leakage inductance in concentric windings is lowest if the windings are symmetrically spaced in the traverse direction, as in Fig. 168(a).

Fig. 168. (a) Symmetrical and (b) asymmetrical spacing of concentric windings.

For a given number of turns, the leakage flux is least in Fig. 166, somewhat greater in Fig. 168(a), and much greater in Fig. 168 (b). The increase in leakage flux is a function of core dimensions, winding-to-winding spacing, and margin inequality. Figure 169 shows typical increase of leakage inductance when one secondary margin is increased with respect to the other, as in Fig. 168(b).

Fig. 169. Leakage inductance of asymmetrical windings.

Leakage inductance is very low in toroids with windings which cover the whole magnetic path. Toroids are wound on special machines which thread wire in and out of the core. Carefully wound toroidal transformers function at very high frequencies.⁽¹⁾ If part of the core is not covered by the windings, as indicated by dimension G in Fig. 170, leakage flux sprays out of the ends of the coils and reduces the frequency range.

Fig. 170. Toroidal core and coil.

(1)	See "Very-Wide Band Radio-Frequency Transformers," by D. Maurice and R. H. Minns, Wireless Engr., 24, 168 (June, 1947).