Design of a variable-phase contactless energy transfer platform using air-cored planar inductor technology

Contactless Energy Transfer (CET) describes the process in which electrical energy is transferred among two or more galvanically isolated electrical circuits or devices by means of magnetic induction (magnetic energy). The potential applications can range from the transfer of energy between low power home and office devices to high power industrial systems. Medical, marine, and other applications where physical electrical contact might be dangerous, impossible or at the very least problematic, are all prospective candidates for using CET. The work in this thesis mainly concentrates on fundamental concepts of a CET system that consists of multiple PCB inductors arranged to form a platform surface. When a mobile electronic device, embedded with a similar "power receiving" inductor, is placed on the platform surface, power is transferred from the platform to the mobile device through magnetic induction. Small low profile air-cored PCB inductors are used, since they can easily and cheaply be produced, and due to the small size and low profile easily be embedded in different devices without adding much weight or volume to the existing designs. As such, the CET platform attempts to remove the different cables, plugs and adaptors used to power and recharge various mobile electronic devices. Firstly, a library of analytical and numerical methods and procedures for modeling planar inductors constructed as copper tracks on printed circuit boards are presented. Here methods for calculating the DC- and AC-resistances, mutual inductances, selfinductances, magnetic field intensity, and parasitic capacitances are developed. Using these models, the maximum excitation frequency of the inductors are estimated in respect to their self-resonating frequencies. The purpose of this "library of techniques" is not only to model the inductors used in this work, but to act as a single repository for information on modeling similar planar PCB inductors for various future CET applications. The developed models are verified through measurements and FEM simulations, and they all show good agreement. Secondly, the development of the theory and the synthesis of a variable-phase CET system is performed. The platform is designed, based on user requirements, to transfer approximately 8 W of power from a cluster of three primary windings to a load connected to the secondary winding, over a 5 mm air gap. In this way about 5 W is available to load devices connected to the secondary winding and circuit after rectification and down conversion. The primary inductor platform consists of a matrix of hexagon spiral windings with radii of 12.5 mm, while the secondary windings have radii of 20 mm. The system operates at a frequency of 2.777 MHz, and is designed to induce a minimum voltage of 10 V (RMS) in the secondary winding, in order to ensure that at least 5 V - 6 V DC is available to the load devices. The CET platform is implemented in a prototype system. This high-frequency power electronic system is modularly designed and consists of several subsystems including a FPGA controller, quadratic buck converters, high-frequency half-bridge MOSFET drivers, half-bridge inverters, voltage and current estimators, as well as winding commutation circuits. Thirdly, in order for the CET system to locate the valid devices placed on the platform, an innovative load detection scheme is developed. The load detection scheme "scans" the primary windings while estimating their equivalent impedances through measurements. By comparing and detecting diffrences between the measurements and previously calibrated values, the system can find and locate the valid devices. Furthermore, this process has the added advantage of being able to distinguish between valid CET devices, metallic materials, as well as soft-magnetic materials placed on the platform surface. Objects like, soft-drink cans, lighters, pens, coins, and certain softmagnet containing devices, which might have been accidently placed on the platform, can thus be detected, and the activation of the windings closest to it, avoided. Fourthly, the CET platform investigates the effciency of a novel stray magnetic field shielding methodology using destructive wave interference. In contrast to the more conventional methods of shielding using magnetic- and conductive materials, this method uses the amplitudes and phases of the currents in the primary windings, to alter the distribution of the magnetic field above the windings, resulting in a reduction of the stray magnetic field. The system is designed to operate in one of three modes: single-phase, three-phase and variable-phase mode. During single-phase mode, all activated primary windings are excited with in-phase currents. Here, no additional reduction of the magnetic field is expected. During three-phase mode, the primary windings are excited with currents of equal amplitude but with 120 degrees phase shifts. Here, the magnetic fields of the three primary windings undergo destructive wave interference, attenuating the stray magnetic field. During power transfer, however, the secondary winding current still contributes to the field in a constructive manner. The variable-phase operational mode attempts to further reduce the stray field by taking into account the current in the secondary winding during power transfer. Here, the windings inside the activated winding cluster are excited with precalculated currents of different amplitudes and phases, which will give the minimum stray magnetic field values for given secondary winding placements and power transfer levels. The simulation results show that both the threephase and variable-phase modes are able to reduce the stray magnetic fields (for the same amount of power transferred to the load resistor) compared to the single-phase mode. Attenuation results of up to 12 dB are calculated. Finally, the power transfer capability, the operation of the load detection scheme, and the stray magnetic field shielding through destructive wave interferences are all verified through measurements performed on the prototype system. Furthermore, the prototype system is able to transfer 5 W of power (DC) to a load connected to the secondary circuit, in all three operational modes, and all measurement results show good agreement with calculated values.