Mathematica--A System for Doing Mathematics by Computer.

The next generation of implantable high-power neuroprosthetic devices such as visual prostheses and brain computer interfaces are going to be powered by transcutaneous inductive power links formed between a pair of printed spiral coils (PSC) that are batch-fabricated using micromachining technology. Optimizing the power efficiency of the wireless link is imperative to minimize the size of the external energy source, heating dissipation in the tissue, and interference with other devices. Previous design methodologies for coils made of 1-D filaments are not comprehensive and accurate enough to consider all geometrical aspects of PSCs with planar 3-D conductors as well as design constraints imposed by implantable device application and fabrication technology. We have outlined the theoretical foundation of optimal power transmission efficiency in an inductive link, and combined it with semi-empirical models to predict parasitic components in PSCs. We have used this foundation to devise an iterative PSC design methodology that starts with a set of realistic design constraints and ends with the optimal PSC pair geometries. We have executed this procedure on two design examples at 1 and 5 MHz achieving power transmission efficiencies of 41.2% and 85.8%, respectively, at 10-mm spacing. All results are verified with simulations using a commercial field solver (HFSS) as well as measurements using PSCs fabricated on printed circuit boards.

Design and Optimization of Printed Spiral Coils for Efficient Transcutaneous Inductive Power Transmission

The mutual inductance between two coils is a key parameter in the inductively coupled wireless power transmission. The spiral rectangular coils are easier to implement than the circular ones, but the study of the mutual inductance of the spiral rectangular coils is not enough. In this paper, a new analytical calculation of the coaxial spiral rectangular coils is presented, which is simpler than the greenhouse method. The impact of the track thickness and width is also analyzed. The calculation results are verified by the existing methods and the experimental results. Based on these results, some guidelines for optimizing the coupling coefficient are given, when the received coil's size is limited by space constraints.

A New Analytical Calculation of the Mutual Inductance of the Coaxial Spiral Rectangular Coils

Magnetic cores have been used in transformers for over a century. In this paper, the authors present a fundamental concept of using "coreless" printed-circuit-board (PCB) transformers. With the aid of a high-frequency equivalent circuit, the use and basic characteristics of coreless PCB transformers are described. Optimal operating conditions for minimum input power requirement and maximum efficiency operations are identified. Coreless PCB transformers have the advantages of low costs, very high power density, no limitation due to magnetic cores, no magnetic loss and ease of manufacturing. They have the potential to be developed in microcircuits. A printed planar PCB transformer with a diameter of about 1.0 cm and power capability of 19 W has been successfully tested. The power density of the PCB transformer demonstrated in this paper is 24 W/cm/sup 2/. The maximum efficiency can be greater than 90%. The analysis has been confirmed with experiments. Coreless printed transformers have great potential in applications in which stringent height and space requirements have to be met.

Coreless planar printed-circuit-board (PCB) transformers-a fundamental concept for signal and energy transfer

Fully integrated microinductors and microtransformers based on two different geometries and ferrite composite materials are designed, fabricated, tested, and compared. These devices are based on screen printed polymer/ferrite composites and electroplated copper coils, and are deposited at low temperature, making them compatible with organic electronic packaging substrates.

Ferrite-based Integrated Planar Inductors And Transformers Fabricated At Low Temperature

A method for calculating the reluctance of air gaps in magnetic circuits making use of a Schwarz-Christoffel transformation is described. The method is applied to the calculation of the inductance of example inductor and transformer configurations and its validity established through comparisons with calculations based on the finite-element method of numerical analysis. >

Air-gap reluctance and inductance calculations for magnetic circuits using a Schwarz-Christoffel transformation

A method for calculating the inductance of planar-spiral structures in the presence, and in the absence, of magnetic core material is presented. In this method, the current is assumed to flow in the exact center of the planar conductor. Simplified expressions for the self inductance of each turn comprising the inductor, and the mutual inductance between turns on the same layer of the inductor are developed. An expression for the leakage flux between turns on adjacent layers is presented. The effect of a magnetic core on the inductance is considered, and an equivalent magnetic circuit for the core is developed. At the measurement frequency of 100 kHz, the new expressions exhibit better agreement with measurements than do existing ones presented in the literature. >

The inductance of planar structures

Two methods are presented for calculating the inductance of planar rectangular spirals. The first method assumes current flows only in the exact center of the conductor. The second method is based on uniform current distribution across the conductor width, and results in significant improvements in the calculated value of the inductance. The effect of introducing a magnetic core is also studied. The calculated values of inductance from both methods are compared with laboratory measurements, and the accuracy of the methods is demonstrated. >

A. Balakrishnan

Papers

Air-gap reluctance and inductance calculations for magnetic circuits using a Schwarz-Christoffel transformation

The inductance of planar structures

Inductance of planar rectangular-spiral strip conductors for low-profile inductors