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-Frequency 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 nonsinusoidal 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 calculating 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 caused by the distorted 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
- 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
- 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.
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.
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.
These two textbooks have good introductions to magnetics design in the
context of power electronics:
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 available 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
- Erickson, R.W. and Maksimovic, D., 2001, Fundamentals
of Power Electronics 2nd Edition, Kluwer Academic Publishers, ISBN:
- Krein, P.T., 1998, Elements of Power Electronics, Oxford
University Press, ISBN 0195117018.
The Power Source Manufacturers
Association (PSMA) offers a number of interesting titles on their
current publications page, including:
- J. K. Watson, c.1980, Applications of Magnetism (reprinted 2008). A classic text
that explains the physics of magnetics from an electrical engineering
approach and discusses applications extensively.
- Rueben Lee, Leo Wilson, and Charles E. Carter,
1988, Electronic Transformers and Circuits, 3d ed. (reprinted
2007). A practical treatment of transformer and inductor design.
- Eric Snelling, 1988, Soft Ferrites: Properties and Applications, 2d ed.
(reprinted 2006). Another classic, which, in addition to discussing
ferrite materials, contains excellent discussions of power transformer
and inductor design and winding design considering high-frequency