In recent years, a non-contact power transmission system supplying power by wireless, that is, without contact is actively developed. A magnetic field resonance method attracts attention as a method realizing non-contact power transmission. The magnetic field resonance method uses magnetic field coupling between a transmission-side coil and a reception-side coil to perform power transmission. The magnetic field resonance method has characteristics that magnetic fluxes shared between a power feed source and a power feed destination is reduced by actively using resonance phenomenon.
In a well-known electromagnetic induction method, the coupling degree of the power transmission side and the power reception side is distinctly high, and power feeding with high efficiency is possible. However, since a coupling factor need to be maintained high, power transmission efficiency between a power transmission side coil and a power reception side coil (hereinafter, referred to as “efficiency between coils”) is largely deteriorated when a distance between the power transmission side and the power reception side is increased or when positional deviation occurs. On the other hand, the magnetic field resonance method has characteristics that the efficiency between coils is not deteriorated, even if a coupling factor is small, if a quality factor is high. In other words, there is an advantage that an axial alignment between the power transmission side coil and the power reception side coil is not necessary and degree of freedom in position and distance between coils is high. The quality factor is an index indicating relationship between retention and loss of energy in a circuit having the power transmission side coil or the power reception side coil (indicating intensity of resonance of a resonance circuit).
One of the most important factors in the non-contact power transmission system is measures against heat generation of a foreign metal. When power feeding is performed without contact, if a metal exists between the power transmission side and the power reception side, an eddy current occurs and thus the metal may generate heat, irrespective of the electromagnetic induction method or the magnetic field resonance method. To suppress the heat generation, various methods of detecting a foreign metal are proposed. For example, a method using an optical sensor or a temperature sensor is known. However, a detection method using a sensor is high in cost when feeding range is wide like the magnetic field resonance method. In addition, for example, if the used sensor is a temperature sensor, output results of the temperature sensor depend on heat conductivity therearound so that devices on the transmission side and the reception side are limited in design.
Accordingly, a method of observing change in parameters (a current, a voltage, and the like) when a foreign metal exists between the power transmission side and the power reception side and determining presence of a foreign metal is proposed. In such a method, its cost is allowed to be suppressed without design limitation. For example, in Patent Literature 1, a method of detecting a foreign metal with use of modulation degree of parameters at communication between the power transmission side and the power reception side is proposed. In Patent Literature 2, a method of detecting a foreign metal with use of eddy current loss (detection of foreign substance by DC-DC efficiency) is proposed.