Ignacio Lope

Analysis of the Mutual Inductance of Planar-Lumped Inductive Power Transfer Systems

Mutual inductance is a key parameter of the inductively coupled circuits such as transformers and contactless energy transfer systems. This parameter is particularly required to design the power electronics associated with each application. However, its value is usually extracted from measurements performed in a previously built prototype, which is an expensive and time-consuming task. In this paper, an analysis of the mutual inductance between two planar circular windings is developed. The analysis takes into account the effect of the media that can be part of the complete system. As a result, an analytical calculation of the mutual inductance with respect to the main parameters of the system such as the number of turns of the coils, geometry, frequency of the currents, and the properties of the media can be carried out. The analysis can also be used to explore the tendencies of the mutual inductance for design purposes and, thus, it allows speeding up the design process. The analysis has been verified by means of experimental results.

AC Power Losses Model for Planar Windings With Rectangular Cross-Sectional Conductors

In this letter, a method to calculate the ac losses, including skin effect and proximity losses, in planar windings with rectangular cross-sectional conductors is proposed. The aim is proposing proper ac losses expressions similar to the formulas available for round cross-sectional wires, to be used for the calculation of the ac losses and the optimization of planar magnetic windings implemented in the printed circuit board. The proposed model is based on the decomposition into conduction and proximity losses. Conduction losses only depend on the properties of the conductor, whereas proximity losses are calculated by using the orthogonal decomposition of the magnetic fields in which the conductors are immersed. Functions including the frequency and geometrical dependences of the both types of losses are extracted by means of finite element method simulation. Finally, several prototypes are used to verify the proposed expressions and some design considerations are also outlined.

Analysis and Optimization of the Efficiency of Induction Heating Applications With Litz-Wire Planar and Solenoidal Coils

Optimization of the efficiency of an induction heating application is essential in order to improve both reliability and performance. For this purpose, multistranded cables with litz structure are often used in induction heating applications. This paper presents an analysis and optimization of the efficiency of induction heating systems focusing on the optimal copper volume of the winding with respect to different constraints. The analysis is based on the concept of a one-strand one-turn coil, which captures the dissipative effects of an induction heating system and reduces the number of variables of the analysis. An expression for the efficiency of the induction heating system is derived. It is found that, with the geometry and the other parameters of the system fixed, efficiency depends on the copper volume of the windings. In order to use this result to optimize the efficiency of an application, volume restrictions, the packing factor and the window utilization factor are also considered. The optimum frequency for an induction heating system is also studied in this study. An experimental verification for both planar and solenoidal cases is also presented.

A Unified Approach to the Calculation of Self- and Mutual-Inductance for Coaxial Coils in Air

This paper extends a previous formula for the mutual inductance between single-turn coils to include all coils in air with rectangular cross sections, without any restrictions on the dimensions (including overlapping coils). The formula is compared with a wide spectrum of examples from the literature and agreement is excellent in every case. Experimental results are presented to validate the formula for both solenoid and disk coils. The formula is relevant to coreless transformers, inductive coupling, wireless power transfer, and leakage inductance in resonant converters.

Frequency-Dependent Resistance of Planar Coils in Printed Circuit Board With Litz Structure

Printed circuit board (PCB) windings are convenient for many applications given their ease of manufacture, high repeatability, and low profile. In many cases, the use of multistranded litz wires is appropriate due to the rated power, frequency range, and efficiency constraints. This paper proposes a manufacturing technique and a semianalytical loss model for PCB windings using planar litz structure to obtain a similar ac loss reduction to that of conventional windings of round wires with litz structure. Different coil prototypes have been tested in several configurations to validate the proposal.

Practical issues when calculating AC losses for magnetic devices in PCB implementations

In this paper, a method to calculate the ac losses (including skin effect and proximity losses) in planar magnetic devices having rectangular cross-section conductors is presented. A similar model to that employed in the losses calculation with round wires is developed for the mentioned conductors. The aim is to obtain a proper design procedure, especially for magnetic devices in printed circuit board (PCB) implementations. Basically, the losses model is based on the superposition of different losses in the wire (conduction and proximity losses) and also in an orthogonal decomposition of the field where the conductors are immersed. As a consequence, to provide us a computation time reduction, the number of functions, needed to calculate the proximity losses is reduced to one, as it occurs in round wires. This function and also the function that describes the conduction losses, both include the size and frequency dependencies, are calculated and tabulated by means of Finite Element (FEM) calculations. Thus, the ac losses are obtained with a computation time reduction employing the losses model developed with FEM field calculations. Finally, several prototypes are used to verify the proposed method and some optimization results are also extracted.

Design and Implementation of PCB Inductors With Litz-Wire Structure for Conventional-Size Large-Signal Domestic Induction Heating Applications

Printed circuit board (PCB) implementations provide high repeatability and ease of manufacturing as well as potential cost savings for many applications. An optimization procedure is proposed for designing PCB inductors for use in domestic induction cooktops. Cables for classical inductors are built of multistranded round litz wire for efficiency reasons. A planar litz structure in a PCB has been applied to achieve a similar performance to that of traditional constructions (in terms of maximum output power, inductive efficiency, and thermal behavior). The inductor performance has been optimized by a finite-element-analysis-tool-based method taking into account the power losses in rectangular cross-sectional tracks of the cable as well as the geometrical constraints of PCB implementations and the requirements of the associated electronics. Finally, the proposed method has been validated by means of experimental measurements under large-signal conditions on a PCB inductor prototype.

FEA-Based Model of Elliptic Coils of Rectangular Cross Section

We present a 3-D finite element analysis (FEA)-based model of elliptic coils. The type of elliptic coil addressed in this paper is the constant-turn density coil, which corresponds to practical implementations of flat or solenoid-type coils wound on an ellipse-shaped bobbin. We assume a coil model consisting of constant current density perpendicular to the cross section along the current trajectory. This model covers the dc operation and the ac applications with multistranded litz wires. The perpendicular current condition of the elliptic coil is implemented in the FEA tool by the gradient of the winding function and the vector trajectory of the currents. In addition, a frequency-dependent calculation of the resistance of the litz wire is included. Hence, electric parameters as the self-inductance or resistance of the elliptic coil can be extracted from the FEA tool as a function of the geometrical parameters. The model is applied to obtain the mutual inductance between two elliptic coils for different misalignments. Finally, a comparison between the self-inductance and winding resistance measurements obtained from a flat coil prototype in several scenarios and the simulation results is presented.

Printed circuit board implementation of small inductors for domestic induction heating applications using a planar litz wire structure

In this paper, the design and implementation of small inductors in printed circuit board (PCB) for domestic induction heating applications is presented. With this purpose, we have developed both a manufacturing technique and an electromagnetic model of the system based on finite-element method (FEM) simulations. The inductor arrangement consists of a stack of printed circuit boards in which a planar litz wire structure is implemented. The developed PCB litz wire structure minimizes the losses in a similar way to the conventional multi-stranded litz wires; whereas the stack of PCBs allows increasing the power transferred to the pot. Different prototypes of the proposed PCB inductor have been measured at low signal levels. Finally, a PCB inductor has been integrated in an electronic stage to test at high signal levels, i.e. in the similar working conditions to the commercial application.

Minimization of vias in PCB implementations of planar coils with litz-wire structure

PCB implementation of planar windings replicates the structure of conventional litz wires in order to reduce the ac losses. This structure consists of a set of fine trace electrically connected in parallel which are transposed by means of changes of both trajectory and layer. An appropriate transposition strategy is essential to reduce the winding ac losses, however PCBs with high number of vias could result expensive and not reliable. Taking advantage of the field profile of planar windings, in this work is studied the minimization of the number of vias without significant reduction of performance. The work is carried out by means of a combination of finite-element analysis (FEA) simulations and experimental verification on ultra-thin (0.3 mm) PCB prototypes of planar inductors oriented to a domestic induction heating application.