CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
Inductive power transmission has become a more and more popular method to deliver power to mobile electronic devices and small appliances with a power consumption of up to 100W [1]. In its large meaning, inductive power transfer can take the form of electricity, light, heat or even sound and wind. The inductive power transfer involves a starting point (source) and an ending point (receiver) without intermediate material that would be intentionally added to achieve some purpose. In the scope of this thesis, the starting and ending points are both characterized by electric energy, with in between no additional wires or material.
Light energy transfer can be obtained by photons travel from a source such as the sun or a laser, to a receiver that is generally a photovoltaic cell. The advantage of the laser as a source is that it provides a very directional illumination thanks to its high degree of temporal and spatial coherence. However, the efficiency of light energy transfer is very low, because the losses at each step of the transfer processes are important, namely in the source, in the air and in the receiver. Furthermore, the economical aspects are not attractive [45]. For these reasons, light energy is interesting only for applications that require a transfer over a very large distance, for example in the space.
The microwaves can also transfer energy over a relatively large distance that can be greater than the emitter and receiver sizes. They are electromagnetic waves ranging from 1 GHz to 300 GHz and are used for far-field energy transfer [83]. The most common source for domestic applications is the magnetron thanks to its unexpensive cost, but high frequency waves can be also generated by antennas. The receiver is a rectenna which is actually an antenna with a rectifier used to convert microwaves into DC electricity. The main drawbacks of this technology are the health risks and the relatively low efficiency that can be obtained.
The most used way to transfer contactless energy is the magnetic induction whose principle is shown in Fig. 1.1. It uses near-field electromagnetic waves in the range of frequencies between 10 kHz and a few MHz. The characteristic distance of transfer is the same order as the size of the receiver (secondary coil) and the emitter (primary coil). Such inductive power transfer systems are based on the coreless transformers theory. The working principle consists in applying a high frequency current in a primary coil. In the surrounding air, it generates a magnetic field that induces a voltage to a secondary coil if placed in proximity. It is necessary to operate at high frequency because coreless transformers suffer from weak mutual coupling. To enhance the coupling between primary and secondary, both coils must be well aligned and the air gap must be as small as possible. With this description, a distinction can be made between inductive inductive power transfer and RFID systems.
Indeed, the former involves a transfer of power through a relatively short distance as defined above, while the latter is used to transfer information (with power generally lower than 100mW) through a large distance.