Design of Inductors With Significant AC Flux

A design methodology is introduced to select a minimum core volume for an inductor or coupled inductors experiencing appreciable core loss. The geometric constant (Kgac) has been found to be a power function of the core volume for commercial toroidal, ER, and PQ cores, permitting the total loss to be expressed as a direct function of the core volume. The inductor is designed to meet specific loss or thermal constraints. An iterative procedure is described in which 2-D or 3-D proximity effects are first neglected and then subsequently incorporated via finite-element simulation. The methodology is demonstrated for coupled inductors in a 5-MHz converter in critical conduction mode. The core loss is verified using rectangular excitation and the winding loss is inferred from thermal measurements.