An Overview of Saturable Inductors: Applications to Power Supplies

This article presents an overview of saturable inductors that are intentionally saturated by the load current and their applications to power supplies. After introducing the fundamentals of magnetization and nonlinear inductance, three types of saturable inductors are differentiated. They are based on the way nonlinearity of the core material is used. The partial saturation, the saturation of swinging inductors and the full saturation. Subsequently, this article focuses on the application to power supplies: it identifies the main reasons for using saturable inductors, like improving the efficiency or reducing the size of the magnetic component by using stepped air-gaps or partial saturation and gives an overview on this topic and its main publications. Afterwards some concepts and circuits taking advantage of saturable inductors are presented in more detail, especially improving the light load efficiency of a point of load buck converter, ensuring zero voltage switching over a wide load range in a phase-shifted full bridge circuit, a nonlinear commutation circuit, and a passive and active power factor correction.

I. Introduction

Magnetic components are an integral part of almost every power electronic circuit [1, p. 25]. They fulfill various tasks such as storing energy temporarily (e.g., a choke in a switch mode power supply (SMPS) or a flyback transformer), transforming voltages (e.g., 50/60 Hz transformers or high frequency transformer in SMPSs) or operating as a filter choke due to electromagnetic compatibility (EMC) reasons. A material of high permeability (e.g., ferrite or electrical steel) is required as a core to increase the inductance or to improve the coupling between two or more windings. An external magnetic field strength magnetizes this material, which results in an increased flux density. However, the magnetization has limits, since it is a rotation of dipoles in the material [2, p. 5]. If all dipoles are aligned almost in the same direction, the material saturates. The first theories in physics were developed in the early 20th century to describe this process in magnetic material [3]. Today numerous models including the well-known of Jiles and Atherton [4], Hodgdon [5], or Preisach [6] exist to describe the nonlinear, hysteresis afflicted behavior.