Exclusive Technology Feature © 2011 How2Power. All rights reserved. Page 1 of 6 ISSE: July 2011 Why Use Aluminum Wire? by Dennis Feucht, Innovatia Laboratories, Cayo, Belize Aluminum wire is produ ced by major wire co mpanies yet it is not commonly found in the transformers and inductors (transductors of switching power converters. Aluminum (Al wire has an advantage over copper (Cu wire at high frequencies because its conductivity is lower than copper, causing its skin depth to be greater at the same frequency. Its conductive area is greater and thus its resistance for the same length and cross- sectional area of wire is lower. When is it advantageous to use it? This article explores this question. Skin Effect For copper and aluminum wire of the type used in transductors, the ratio of Al and Cu resistivities at a typical operating temperature of 80°C is 637 . 1 131 . 2 487 . 3 C) 80 ( C) 80 ( Cu Al q q U U . In other words, the same wire in aluminum is about 64% higher in resistance than it would be in copper. The density of time-varying cu rrent in a round wire is highest at the conduc tor surface because of the sk in effect. Current density exponentially decreases from the surface along a radial axis toward the center of the wire. The surface current is that of the static (dc current. One length constant, or skin depth, , defines an annulus or ring of current from the surface of the wire to a distance of inward toward the center. If the value of the current at the surface were uniformly distributed in this ring it would equal the total current of the exponential distribution. Consequently, for a given switching frequency, f s , and a given wire conduc tor of r adius, r c , the sk in eff ect of increased resistance is minimized when r c d . The skin depth for copper at 80°C is f Hz mm 73.5 Cu | G . For aluminum at the same temperature, f Hz mm 94 Al | G . At the same frequency, Al skin depth is greater than Cu though its static resistivity is higher. The lower resistance from the greater cross-sectional area of the greater of Al can overcome its greater static resistivity to make its total (frequency-depen dent resistance less than copper. To determine the conditions for lower Al resistance, frequency-dependent resistance equations are needed. Frequency-Dependent Resistance Ratio, F R The ratio of the total (ac resistance of a round wire at frequency, f , to its static (dc resistance, R 0 , is designated as 0 ) ( R f R F R .
