Minimize Winding Losses in High-Frequency Inductors

Jul 1, 2008 12:00 PM
By Weyman Lundquist, President and Engineering Manager, West Coast Magnetics, Stockton, Calif.

Power electronics are rapidly expanding into new applications as high-power semiconductor devices increase their rated operating current and frequencies to higher and higher levels. As a basic building block of virtually all power electronics equipment, inductors have a unique potential for improvement. That's because inductors are usually the largest, most expensive and volumetrically inefficient items in a power system.

Consequently, improvements in inductor design can have a great impact on the size and cost of the inductor, which can have a significant impact on the rest of the power electronics design. One way to help shrink a power inductor is to reduce its losses at high frequencies, so that a lower value of inductance may be specified and, therefore, a smaller inductor may be used in the system.

To lower an inductor's power losses at high frequencies, designers must understand the role of winding losses and the options available for reducing those losses. Those options include a new foil-winding technique that achieves low losses for inductors operating at high ripple current and high power levels. For designers looking to optimize inductor designs using Litz wire, there's also a new tool to aid the design process. This tool takes the form of a freeware program that allows the user to optimize the stranding and positioning of the winding inside of the available winding window.

Understanding Winding Losses

There are two principle mechanisms for loss in inductors, core losses and winding losses. Core losses involve the magnetic properties of the core material, which exhibits power losses in the form of hysteresis and eddy currents within the core itself. Winding losses come from the resistance in the winding, typically copper.

Inductors used for switch-mode power applications are subject to high-frequency current ripple, which can make the effective winding resistance and the associated copper losses very high. The winding resistance of power inductors includes both the dc resistance and an ac component of resistance that is a result of both skin effects and proximity effects.

A time-dependent current induces a flux, which in turn induces small currents within the wire. Since very little current passes through the center of the winding, the effective cross-sectional area is reduced and the resistance is increased. These losses increase in magnitude as the frequency and current increase.

At switch-mode frequencies, the ac component of resistance can be very high, often greatly exceeding dc resistance and resulting in high copper losses. With gapped-power inductors, the field near the gap produces a strong local proximity effect and can produce very high ac copper resistance and losses, even leading to the failure of the inductor.

Power loss in any magnetic device is the sum of these effects, and the design process is made more difficult by their relationship to one another. For instance, common methods of reducing ac resistance, such as the use of Litz wire, greatly reduce the cross-sectional area of the conductor and drastically increase dc resistance. Foil inductors are often used to minimize winding losses in an application of high dc current, because of their efficient use of the winding window. However, even a small amount of ac current can cause significant losses in these coils.

Such sacrifices are unacceptable in many of today's applications. Many dc-dc converters require an inductor that can carry a large dc current with an ac ripple. Even when the ac component is small in comparison to the dc current, the ac resistance can be orders of magnitude larger than the dc resistance. The problem is more acute as current level and frequency of operation increase in modern designs.

Fortunately, there are solutions to the problem of ac copper losses. Keeping the windings to a single layer substantially mitigates ac copper losses. Using a powdered core with no gap will substantially reduce proximity effects and the resulting ac copper losses.

However, powdered cores typically have significantly higher core losses than ferrite cores, and for high-ripple applications, a gapped core is sometimes preferred due to lower core losses. Or, it may also be desirable to use a relatively high-permeability powdered core with a gap, to take advantage of the higher B_SAT available with this type of core material. In these cases, the gap-fringing field must be dealt with or copper losses can be very high.

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