1. Field of the Invention
The present invention relates to a coil for wireless power transmission and a wireless power transmission apparatus.
2. Description of the Related Art
In recent years, in order to transmit power without mechanical contact by a cable or the like, a wireless power transmission technique that uses an electromagnetic induction effect between primary (power feeding) coils and secondary (power receiving) coils that face one another, has been of interest.
In the case where a wireless power transmission technique is applied to a power feeding apparatus to an electric vehicle or the like, it is assumed that a positional relationship between primary coils that are fixedly arranged on the ground or the like, and secondary coils that are installed in an electric vehicle or the like, is not necessarily constant. Accordingly, there has been a rise in the demand for the development of wireless power transmission technique in which power transmission is also possible in a case in which positional shift occurs between the primary coils and the secondary coils.
In response to such demands, for example, in Japanese Unexamined Patent Application Publication No. 2009-164293, a non-contact power transmission apparatus has been suggested in which a primary side is configured by a plurality of planar coils and a secondary side is configured by one or more planar coils, and an external diameter of the secondary side coils is smaller than an external diameter of the primary side coils.
However, in the technique that is disclosed in Japanese Unexamined Patent Application Publication No. 2009-164293, the primary side coils are configured by a plurality of planar coils, and therefore magnetic leakage flux that flows to adjacent coils in the primary side is increased. As a result, a closed loop is formed between adjacent coils in the primary side to cause a problem in that coupling deteriorates in a case in which a distance is large between the primary side coils and the secondary side coils, where power transmission is intended.