An Investigation into the Core Losses of Mn-Zn Ferrite Materials under DC Bias Conditions

Abstract

The range of products in modern society which are dependent on electronic power supplies is extensive. A non-exhaustive list of examples includes: computers, battery chargers, lighting systems, televisions, automobiles, DVD players, and mobile phones. As new technologies emerge and new products become available consumer demand for power driven equipment can be expected grow. Concurrently, environmental protection agencies are advocating that demand be constrained in order to limit power consumption and, thus, carbon emissions. The conflicting interests of consumers and environmental protection agencies can be at least partially reconciled through improving power supply efficiency levels and, consequently, research in this area has assumed great importance. A significant contribution to the total energy losses of power supplies are made by its magnetic components, which consist essentially of an electrically conductive coil wound around a soft magnetic core that is often excited under a dc bias condition, as both ac and dc magnetic fields are applied. Although the material commonly used to implement the core is Mn-Zn ferrite, its physical loss mechanisms under dc bias conditions are not well understood. Therefore, there is a need for an investigation in this area. This Thesis aims to fulfill this need by presenting the results of an investigation into the losses of Mn-Zn ferrite cores under dc bias conditions. With the use of a laser vibrometer, experimental results are presented showing that the amplitude of the magnetostrictive vibration of a Mn-Zn ferrite core increases with dc bias levels. This increase is shown to be general in the sense that it occurs at frequencies distant from, as well as close to, the natural resonant frequency of the core. Using an accurate core loss measurement circuit it is also shown that core losses increase significantly with dc bias, and it is proposed that a correlation between magnetostrictive vibration and core losses exists. In addition, an unusual phenomenon caused by the interactions between the mechanical and magnetic states of a Mn-Zn ferrite core is reported. During this phenomenon figure-eight shaped B-H loops can be induced, as a negative core loss occurs during a portion of a magnetic excitation cycle. The experimental evidence related to the figure-eight shaped B-H loops, as well as the correlation between core losses and magnetostictive vibration, is supported by theory proposed in this Thesis, and results generated by a model of the magnetization process under dc bias conditions.