Applications of half-turn on E-core in switching power supplies

A turn around an outside leg of an E-core is called a half-turn because it encloses only one-half of the cross-sectional area of a turn around the center leg. It is well known that a half-turn in a secondary winding of a power transformer greatly increases the leakage inductance between windings, thus causing an adverse effect on cross-regulation. However, the increased leakage inductance of a half-turn can be very beneficial in tapped inductors for boost circuits and in coupled output chokes. This paper explains some of these little-known applications of a half-turn. The theory and formulae for prediction of leakage inductance added by such a half-turn are presented.