1. Field of the Invention
The present invention relates to power feeding devices and wireless power feeding systems including the power feeding devices.
2. Description of the Related Art
A variety of electronic devices have spread, and a wide range of products are shipped to the market. In recent years, portable electronic devices such as mobile phones and digital video cameras have spread widely. Further, electric propulsion vehicles that are powered by electric power, such as electric cars, are coming on the market as products.
Mobile phones, digital video cameras, and electric propulsion vehicles include batteries which are energy storage means. At the moment, the batteries are charged in most cases by being directly connected to home AC sources which are power feeding means. Other devices that have no battery or do not use power charged in a battery are directly supplied with power from a home AC source through a wiring or the like.
On the other hand, methods by which batteries are charged wirelessly or power is transmitted to loads wirelessly have been researched and developed. Typical methods are an electromagnetic induction method (also referred to as an electromagnetic coupling method), a radio wave method (also referred to as a microwave method), and a resonance method. An electromagnetic induction method comes into wide use for some electronic device, such as small consumer electronics.
A resonant wireless power feeding system has attracted attention since it provides high transmission efficiency at middle and long distance. In the resonant wireless power feeding system, however, it is known that the power transmission efficiency changes enormously depending on the distance between a resonant coil included in a device that receives power (hereinafter “power reception device”) and a resonant coil included in a device that supplies power (hereinafter “power feeding device”). For that reason, structures for maintaining high transmission efficiency even when the distance between the power reception device and the power feeding device is changed have been actively researched and developed (e.g., see Patent Documents 1 and 2).