Charles R. Sullivan

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

The number and diameter of strands to minimize loss in a litz-wire transformer winding is determined. With fine stranding, the AC resistance factor can be decreased, but DC resistance increases as a result of the space occupied by insulation. A power law to model insulation thickness is combined with standard analysis of proximity-effect losses to find the optimal stranding. Suboptimal choices under other constraints are also determined.

Accurate prediction of ferrite core loss with nonsinusoidal waveforms using only Steinmetz parameters

An improved calculation of ferrite core loss for nonsinusoidal waveforms separates a flux trajectory into major and minor loops via a new recursive algorithm. It is highly accurate and outperforms two previous methods for our measured data. The only characteristics of the material required are the standard Steinmetz-equation parameters.

Opportunities and Challenges in Very High Frequency Power Conversion

THIS paper explores opportunities and challenges in power conversion in the VHF frequency range of 30-300 MHz. The scaling of magnetic component size with frequency is investigated, and it is shown that substantial miniaturization is possible with increased frequencies even considering material and heat transfer limitations. Likewise, dramatic frequency increases are possible with existing and emerging semiconductor devices, but necessitate circuit designs that either compensate for or utilize device parasitics. We outline the characteristics of topologies and control methods that can meet the requirements of VHF power conversion, and present supporting examples from power converters operating at frequencies of up to 110 MHz.

Computationally efficient winding loss calculation with multiple windings, arbitrary waveforms, and two-dimensional or three-dimensional field geometry

The squared-field-derivative method for calculating eddy-current (proximity-effect) losses in round-wire or litz-wire transformer and inductor windings is derived. The method is capable of analyzing losses due to two-dimensional and three-dimensional field effects in multiple windings with arbitrary waveforms in each winding. It uses a simple set of numerical magnetostatic field calculations, which require orders of magnitude less computation time than numerical eddy-current solutions, to derive a frequency-independent matrix describing the transformer or inductor. This is combined with a second, independently calculated matrix, based on derivatives of winding currents, to compute total AC loss. Experiments confirm the accuracy of the method.

A high-efficiency maximum power point tracker for photovoltaic arrays in a solar-powered race vehicle

A maximum power point tracker for photovoltaic arrays is presented. Components are optimized for weight/power-loss tradeoff in a solar-powered vehicle, resulting in over 97% efficiency. The control circuit uses a robust auto-oscillation method. Measurement and multiplication of array voltage and current is shown to be unnecessary, and the control is based only on output current measurement. Multiple local maxima arising from partial shading of the solar array are discussed.<<ETX>>

Limits of localized heating by electromagnetically excited nanoparticles

Based on an analysis of the diffusive heat flow equation, we determine limits on the localization of heating of soft materials and biological tissues by electromagnetically excited nanoparticles. For heating by rf magnetic fields or heating by typical continuous wave lasers, the local temperature rise adjacent to magnetic or metallic nanoparticles is negligible. However, heat dissipation for a large number of nanoparticles dispersed in a macroscopic region of a material or tissue produces a global temperature rise that is orders of magnitude larger than the temperature rise adjacent to a single nanoparticle. One approach for producing a significant local temperature rise on nanometer length scales is heating by high-power pulsed or modulated lasers with low duty cycle.

An improved calculation of proximity-effect loss in high-frequency windings of round conductors

The two best-known methods for calculating high-frequency winding loss in round-wire windings-the Dowell method and the Ferreira method-give significantly different results at high frequency. We apply 2-D finite-element method (FEM) simulations to evaluate the accuracy of each method for predicting proximity-effect losses. We find that both methods can have substantial errors, exceeding 60%. The Ferreira method, which is based on the exact Bessel-function solution for the eddy current in an isolated conducting cylinder subjected to a time-varying magnetic field, is found to be most accurate for loosely packed windings, whereas the Dowell method, which approximates winding layers comprising multiple turns of round wire with a rectangular conducting sheet, is most accurate for closely-packed windings. To achieve higher accuracy than is possible with either method alone, we introduce a new formula, based on modifying the Dowell method. Parameters in the new formula are chosen based on fitting our FEM simulation data. By expressing the results in terms of normalized parameters, we construct a model that can be used to determine proximity-effect loss for any round-wire winding with error under 2%.

Design of microfabricated transformers and inductors for high-frequency power conversion

Transformers and inductors fabricated with micron-scale magnetic-alloy and copper thin films are designed for high-frequency power conversion applications. Fine patterning produced by photolithography reduces eddy current losses, thus enabling very high power densities. Calculated design graphs and design examples for 10 MHz transformers are presented.

Cost-constrained selection of strand diameter and number in a litz-wire transformer winding

The design of litz-wire windings subject to cost constraints is analyzed. An approximation of normalized cost is combined with analysis of proximity effect losses to find combinations of strand number and diameter that optimally trade off cost and loss. The relationship between wire size, normalized cost and normalized loss is shown to have a general form that applies to a wide range of designs. A practical design procedure is provided, and applied to an example design, for which it leads to less than half the original loss at lower than the original cost, or under one fifth the original cost with the same loss as the original design.

Automotive application of multi-phase coupled-inductor DC-DC converter

A set of four coupled inductors is applied to a four-phase interleaved 1 kW bi-directional 14 V to 42 V DC/DC converter for automotive applications. The coupled-inductor structure is optimized, and the performance is examined through simulations and experimental measurements. Although coupled inductors offer bigger advantages in applications that require fast transient response, they also have significant advantages in this type of application.