Applications of Magnetics to Problems in Switched-Mode Power Conversion

Abstract

Two topics are presented which advance the use of magnetic devices in switched-mode power converters. Part I discusses a novel method of detecting impending saturation of magnetic materials. The technique exploits the interaction of perpendicular magnetic fields to provide a simple, direct and continuous measure of the increasing nonlinearity of the material without any elaborate unconventional devices or complex electronic circuitry. Theoretical foundations and applications to ferrite cores in switching converters are given. Specific practical hardware examples are described including magnetics for a 4-kilowatt push-pull buck converter. Part II presents a discussion of the analysis of zero ripple integrated magnetic structures. These complex devices are of great interest in the field of switched mode power conversion because they have the ability to exclude ac currents from some of their windings when they are excited by ac voltages. A thorough analysis is carried out to establish which of the many characteristic parameters of these devices are responsible for the unusual zero ripple behavior. Practical methods of modelling and analysis are developed by which a designer can quickly determine the conditions required for zero ripple. Equivalent electric circuit models are given to aid in the electrical design of converters and to support the simple physical explanations of the observed phenomena including the effects of parasitic quantities. A number of experiments confirm the validity of the models.