Field of the Invention
The present invention relates to an antenna for transferring power.
Description of the Related Art
With wireless power transfer, in order to achieve an improvement in transfer efficiency or an extension of the transfer distance, the idea of reducing the resistance of an antenna while increasing the inductance value of the antenna has been proposed. The inductance value of the antenna can be increased by increasing the size of the antenna, or increasing the number of turns of its coil. However, the antenna size is limited by a wireless communication apparatus on which the antenna is mounted. There is also a limit to increasing the number of turns of the coil due to the size of the wireless communication apparatus. If the number of turns is increased in a small space, conductors constituting the antenna are closely located to each other, which may increase the resistance due to the stray capacitance or proximity effect generated between the conductors.
Meanwhile, the idea of forming a coil on a multi-layer substrate in which a plurality of substrates are stacked has been proposed. Japanese Patent Laid-Open No. 10-145267 discloses a technique for increasing the inductance value by constituting an RFID antenna with a multi-layer structure. However, according to Japanese Patent Laid-Open No. 10-145267, a spiral conductor is arranged on a front surface and a back surface of a substrate such that the conductor formed on the front surface and the conductor formed on the back surface face each other. Accordingly, the resistance component increases due to the stray capacitance or proximity effect generated between the conductor formed on the front surface and the conductor formed on the back surface.
Here, Q factor of the coil in the antenna is given by the following equation:Q=2πfL/r  (1),
where L represents the inductance of the antenna, and r represents the internal resistance of the antenna. Japanese Patent Laid-Open No. 10-145267 relates to an antenna designed for RFID. With RFID antennas, because Q is set to a low value, it is important to increase the internal resistance according to an increase in the inductance. With wireless power transfer, on the other hand, the internal resistance affects the transfer efficiency, and it is therefore important to minimize the internal resistance while increasing the inductance value.
Meanwhile, Japanese Patent Laid-Open Nos. 2012-120410 and 2011-086890 disclose a multi-layer structure in which a conductor pattern on the front surface and a conductor pattern on the back surface are arranged such that they are offset from each other. According to the techniques disclosed in Japanese Patent Laid-Open Nos. 2012-120410 and 2011-086890, in order to suppress the stray capacitance generated when the front surface pattern and the back surface pattern are formed to be aligned, the conductor-to-conductor gap is set to be greater than the conductor width so as to reduce the stray capacitance.
The methods of Japanese Patent Laid-Open Nos. 2012-120410 and 2011-086890, however, are problematic in that there is a limit to miniaturization of the antennas because the conductor arranged on a first layer and the conductor arranged on a second layer need to be spaced apart from each other by an appropriate distance in order to reduce the stray capacitance between a plurality of layers.
The present invention has been conceived to address the problems described above, and provides a miniaturized antenna whose resistance is reduced while its inductance value is increased.