Recently, wireless power transmission technology to perform non-contact transmission of power wirelessly using a power transmission coil and a power reception coil is adopted to a wide range of devices such as an IC card and a portable phone. A power transmission characteristic significantly varies depending on a positional relationship between the power transmission coil and the power reception coil. Therefore, actually, the positional relationship between the coils is fixed by means of a cradle and the like and a distance between the coils is limited to 0 cm to approximately few cm.
In order to realize a transmission distance not shorter than few tens of cm without limiting the positional relationship between the coils, it is required to detect the transmission characteristics different according to the position of the coils to adjust a circuit parameter. Therefore, a method of improving power transmission efficiency by adjusting a tuning capacitor by deriving an electromagnetic coupling coefficient by a circuit sensor and a table provided in a power transmission device is suggested.
The transmission characteristic is detected by detecting only a state of a resonance circuit in the power transmission device by a method of adjusting the power described above, so that an operational state of a power reception device is not understood. Therefore, there is a problem that correct transmission characteristic cannot be detected and sufficient power transmission efficiency cannot be obtained when the transmission distance is extended and the electromagnetic coupling coefficient becomes smaller.