Title page for ETD etd-543273119623370

Type of Document Dissertation
Author Skutt, Glenn R.
URN etd-543273119623370
Title High-Frequency Dimensional Effects in Ferrite-Core Magnetic Devices
Degree PhD
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Besieris, Ioannis M.
Boroyevich, Dushan
Chen, Dan Y.
Kohler, Werner E.
Lee, Fred C. Committee Chair
  • ferrites
  • dimensional resonance
  • eddy currents
  • transformers
  • inductors
Date of Defense 1996-10-04
Availability unrestricted
MnZn ferrites are widely used in power electronics applications where

the switching frequency is in the range of several tens of kilohertz

to a megahertz. In this range of frequencies the combination of

relatively high permeability and relatively low conductivity found in

MnZn ferrite helps to minimize the size of magnetic devices while

maintaining high efficiency. The continuing improvement in

semiconductor switches and circuit topologies has led to use of

high-frequency switching circuits at ever increasing power levels.

The magnetic devices for these high-power, high-frequency circuits

require magnetic CORES that are significantly larger than standard

ferrite-core devices used at lower power levels. Often such large

ferrite cores must be custom designed, and at present this custom

design is based on available material information without regard for

the physical size of the structure.

This thesis examines the issues encountered in the use of larger MnZn

ferrite cores for high-frequency, high-power applications. The two

main issues of concern are the increased power dissipation due to

induced currents in the structure and the change in inductance that

results as the flux within the core is redistributed at higher

frequencies. In order to model these problems using either numerical

or analytical methods requires a reliable and complete set of

material information. A significant portion of this work is devoted

to methods for acquiring such material information since such

information is not generally available from the manufacturers. Once

the material constants required for the analysis are determined, they

are used in both closed-form and numerical model to illustrate that

large ferrite cores suffer significant increases in loss and

significant decreases in inductance for frequencies as low as several

hundred kilohertz. The separate impacts of the electrical and

magnetic losses in the core are illustrated through the use of linear

finite element analyses of several example core structures. The

device impedances calculated using the FEA tools show fair agreement

with measurement. An analysis of gapped structures and segmented

cross-sections shows that these design options can reduce the severity

of the dimensional problems for some designs.

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