Modeling Litz-wire winding losses in high-frequency power inductors

Abstract

The parasitic effects in stranded, twisted, and Litz wire windings operating at high frequencies are studied. The skin and proximity effects that cause the winding parasitic resistance of an inductor t o increase with the operating frequency are considered. An expression for the ac resistance as a function of the operating frequency i s given. The measured and calculated values of the inductor ac resistance and quality factor are plotted versus frequency and compared. The theoretical results were in good agreement with those experimentally measured.

Design and Optimization of Inductive Power Link for Biomedical Applications

“…Second, given the thickness of coating, cross section and inductance, reducing the size of a single strand inevitably decreases area efficiency of an individual strand, thus increasing the DC power dissipation. Third, experimental results show that an increase of the number of strands increases strand-level displaced currents hence reduces the self-resonant frequency of a coil [Bartoli et al (1996)]. Fourth, small wire is more expensive.…”

Analytical Computation of Ac Resistance of Single-Layer Air-Core Helmholtz Coils

“…Besides, litz-wire is also twisted so that every strand can cover all the positions in the wire cross section along the longitudinal axis, and this property will make sure that current in each strand is the same. Therefore, not only the skin effect but also proximity effect is reduced [6]. In the following analysis, it is assumed that current density distribution is absolutely uniform in every strand.…”

“…Many approaches to analytical computation of AC resistance of multi-strand and litz-wire windings have been presented [6][7][8][9][10][11]. However, most of the analytical modelsare developed to calculate AC resistance of windings of inductors and transformers.…”

Optimal choice for number of strands in a litz-wire transformer winding

“…The loss in the litz winding will be the same as in the equivalent single-strand winding as long as the currents flowing in all the strands are equal [6], [21]. Other methods of calculating loss in litz wire also assume equal current in all strands [17], [19], [22]. This assumption is equivalent to assuming that the bundle-level construction has been chosen properly to control bundle-level proximity and skin effects.…”

“…The simpler model is used because it is accurate for the small strand diameters that are found to be optimal, and because its simplicity facilitates finding those optimal diameters. Other models (such as [19] and the similar analysis in [22]) also model large strand diameters and circular bundle configurations accurately, but they fully calculate only internal (not external) proximity effect, and so are not useful for the present purposes.…”