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Low AC And DC Resistance Inductor

Weyman Lundquist, President, West Coast Magnetics, Licensee

Professor Charles R. Sullivan, at the Thayer School of Engineering at Dartmouth College, has invented an important new inductor technology. This inductor technology is the first major advance in wire wound inductors since the invention of litz wire. It is likely to become the industry standard for a certain class of inductors. Professor Sullivan’s invention consists of a method to make an inductor which combines the very low DC resistance of a foil-wound inductor and low AC resistance close to that of a litz-wire inductor. The resulting inductor is far more efficient than current inductor designs and results in an inductor that is significantly smaller and more efficient than current technology.

I. Introduction

Electrical energy is rarely supplied in the form it is needed. The process of power conversion is used in an extremely large number of devices that perform a wide variety of functions. Vast size reductions and efficiency improvements have been achieved through the use of semiconductor technology controlling power conversion circuitry that operates at much higher frequencies (10 kHz to 500 kHz) than the 50/60 Hz at which the power is generated. Types of high-frequency power conversion equipment utilizing advanced control technology include DC/DC converters, AC/DC power supplies, inverters and electric motor controllers. The applications include household and consumer electronics, industrial electronics, medical equipment, all computers from notebooks to servers (including the computers that power the internet), power generation and distribution, wind energy, hybrid vehicles, automobile electronics, military electronics and other applications too numerous to list. The use of modern power electronics will continue to grow and spread rapidly as our society becomes more and more electrified.

One of the basic magnetic building blocks of power conversion equipment is the inductor. In fact it is very rare to see any power supply without at least one inductor, and most power conversion circuits require numerous inductors. Inductors used in power electronics have particular potential for improvement, as the inductors are typically the largest, most expensive, and "volumetrically inefficient" in a power circuit. Inductors are often a great source of power loss. The technology for other building blocks in power conversion circuits such as control chips has been advancing rapidly with smaller and smaller power conversion devices operating at higher and higher frequencies. In contrast, wire-wound inductor technology has advanced very little in the last 25 years. Inductors are large, expensive, and inefficient. These deficiencies will only become more acute as power supply technology continues to advance rapidly, keeping pace with the computer chip technology revolution, absent of technological advances in wire wound inductors.

The new inductor technology described in this report was tested in the laboratory at Dartmouth College in order to precisely measure the potential energy savings. An examination of the data in Figure 1 will serve to illustrate these savings. The hatched area represents the energy savings that are observed with the new foil winding technology vs. conventionally wound inductors. These reduced losses result in a more efficient inductor but they have another very important feature. These reduced losses also permit the design of a smaller, lighter and cheaper inductor that will perform precisely the same function as a heavier and more expensive inductor using conventional technology. For many of the potential markets this cost advantage is critical.

II. How It Works

There are two principle causes of loss in magnetic components: magnetic 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. Winding losses come from the resistance in the conductive windings. This loss has both DC and AC components. The DC component depends on the length and cross-sectional area of wire used. If the device is carrying a current with an AC component, there are also losses due to eddy currents and the proximity effect. A time-dependent current induces a flux, which in turn induces localized currents within the wire.

Total Power Loss vs. AC Ripple at 25 kHz

Fig. 1. In tests performed on a 90 µΗ, 40 amp inductor with a ripple current at 25 kHz, it was shown that the new Dartmouth technology substantially outperformed conventional wire wound and litz wire wound inductors using the same core geometry and number of turns. The total winding losses for this example were from 17% to 35% lower than both solid wire and litz wire over a ripple range from 0 to 30%.

The result is a current density profile concentrated on the surface of the conductor at a distance of a skin depth, δ defined as √ρ/𵃠where µ is the permeability, ρ the resistivity, and ƒ the frequency. Since very little current passes through the center of the winding, the effective cross-sectional area is reduced and the resistance is increased. In multiple-layer windings, eddy currents can also be induced in neighboring wires leading to further losses, a phenomenon known as the proximity effect. These losses increase in magnitude as the frequency and current increase. The power loss in any magnetic device is the sum of these effects. The design process is made more difficult by their relationship to one another. 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 and DC chokes require an inductor that can carry a large DC current with an AC ripple. Even when the AC ripple is small in comparison to the DC current, the AC resistance can be orders of magnitude larger than the DC resistance. Professor Charles Sullivan has invented a method of displacing the AC eddy currents to defined locations inside any copper foil winding which effectively neutralizes the loss-causing eddy current effects inside the windings. This effect is created by shaping the copper winding around the field created by a gap in the magnetic core path. (Gaps in the magnetic core are commonly used in this class of inductor.) When this technique is employed, the foil winding retains the very low DC resistance typical of this class of winding, and also has very low AC resistance. Typically, the total loss is lower that what is achievable with a litz-wire winding. The resulting combination of low AC and low DC copper losses results in an inductor that is far more efficient than its conventional cousins, solid-wire and litz-wire wound inductors.

III. Where Does It Apply?

Power converters are normally converting DC Voltage to DC Voltage or DC Voltage to AC Voltage. DC to DC converters will always require inductors. DC to AC converters will normally require inductors when they are converting to an AC Voltage requiring power factor correction (i.e. for connection to the utility grid). The most promising markets are green energy conversion applications including:

• Wind Energy
• Hybrid Vehicles
• Photovoltaics
• Fuel Cells
• Flexible AC Transmission Equipment