Information for Magnetics Designers

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Information for Magnetics Designers

Most of the information on this site assumes you are already fairly proficient with magnetics design. If not, see below for suggested beginners' references.

High-Frequecy Winding Loss

It is well known that the resistance of wire goes up with frequency because of skin effect. Less well-known is that in windings, the high-frequency loss effects are much worse than would be predicted just based on skin effect. Generally, this effect of bunching conductors together is termed proximity effect. If this concept is new to you, see the references references below, particularly the notes by Lloyd Dixon. If it is not new to you, you may be familiar with the common 1-D approximation often called the Dowell method. However, Dowell's analysis can have errors as large as 60% for simple layer-wound windings, and much higher error when air gaps or 2-D winding arrangements change the field configuration. So better methods are often needed to design or optimize high-frequency windings. Here are some of the tools and information available here on that topic. See the publications page for a full list of papers, and the software page for more information on the software tools we have available.

Accurate modeling of proximity effect loss (i.e. without the up-to-60% error in the Dowell method, or more with the Bessel function method) is explained in the paper "Simplified High-Accuracy Calculation of Eddy-Current Loss in Round Wire Windings."
If you having different non-sinusoidal currents in different windings, the SFD method, described in the paper, "Computationally Efficient Winding Loss Calculation with Multiple Windings, Arbitrary Waveforms, and Two- or Three-Dimensional Field Geometry," can simplify correctly calcuating the loss in a wire winding that uses strands that are small compared to a skin depth.
One good way to reduce high-frequency losses is to use litz wire. "Optimal Choice for Number of Strands in a Litz-Wire Transformer Winding" gives general background and explains how to choose a strand number and diameter for the absolute minimum loss. Unfortunately, this minimum loss design is usually very expensive. Thus, "Cost-Constrained Selection of Strand Wire and Number in a Litz-Wire Transformer Winding." becomes very important. A general purpose version of this optimization technique has been implemented in a free CAD tool that is detailed in "Easy-To-Use CAD Tools for Litz-Wire Winding Optimization," and is available from the software page for download or use online.
In inductors, the gap has a strong effect on the field shape and thus on winding losses. One way to reduce the problems casued by the distored field shape is to use multiple small gaps instead of a single larger gap. This approach is often used without correct analysis of how many gaps to use. Find a simple design guideline in "The Quasi-Distributed Gap Technique for Planar Inductors--Design Guidelines".
In a wire winding, it is possible to do better than a distributed or quasi-distributed gap design by optimizing the winding shape. Several papers explain this. If you want to try it, see the ShapeOpt software page, to use the method online or download the software.
A low-cost alternative to litz wire is to use simple stranded wire without individually insulated strands. Although the loss will always be higher than with true litz wire, in some cases this option provides the lowest loss at a given cost. Plug your winding parameters into a simple equation in "Optimization of stranded-wire windings and comparison with litz wire on the basis of cost and loss" to find out whether your design is a good candidate for this approach.

Core Loss

Manufacturers' core loss data is based on sinusoidal waveforms. But actual power electronics waveforms are rarely sinusoidal. The "Generalized Steinmetz Equation," explained in the paper "Accurate Prediction of Ferrite Core Loss with Nonsinusoidal Waveforms Using Only Steinmetz Parameters" and implemented in software available on the software page. The new method overcomes problems with the "Modified Steinmetz Equation" and requires no data other than that provided by the manufacturer.

Measurements

High-performance components can be difficult to measure. See "Impedance-Analyzer Measurements of High-Frequency Power Passives: Techniques for High Power and Low Impedance" for evaluation of performance of some instruments and some special techniques.

Suggested references for beginners:

These two textbooks have good introductions to magnetics design in the context of power electronics:
Erickson, R.W. and Maksimovic, D., 2001, Fundamentals of Power Electronics 2nd Edition, Kluwer Academic Publishers, ISBN: 0792372700.
Krein, P.T., 1998, Elements of Power Electronics, Oxford University Press, ISBN 0195117018.

Some of the best explanations of high-frequency winding effects, as well as magnetics design in general, are in notes written by Lloyd Dixon for seminars held by Unitrode, now part of Texas Instruments. TI has made these notes available online (see the "archived unitrode design seminars"), and even has a five-module tutorial avaiable for streaming on-line, presented by Lloyd Dixon himself. The notes are also collected as part of the materials you can download when you enter the tutorial.

Consultant Ed Bloom sells books from his web site, and has a page of books on magnetics design that he sells, including the classic, out-of-print Applications of Magnetism
by J. K. Watson