The development of portable telecommunications equipment is remarkable in these years, and especially cellular phones are being made smaller and lighter quickly. In addition to the cellular phones, electronic apparatuses such as video cameras (handy cameras etc.), cordless telephones, laptop personal computers (notebook personal computers) and the like are also being made smaller and lighter. They have a secondary battery provided within an electronic apparatus main body so as to be usable without connection to a wall outlet. Thus, their portability and usability are enhanced. At present, the capacity of the secondary battery is limited, and it is required to recharge once per several days to several weeks.
Recharging methods include a contact charging method and a non-contact charging method. The contact charging method performs charging with an electrode of a power receiver directly contacted with an electrode of a power feeder. The contact charging method is generally used because its apparatus has a simple structure. But, the weight of the electronic apparatus has been reduced with the downsizing and weight reduction of the electronic apparatus in these years, and the contact pressure between the electrode of the power receiver and the electrode of the power feeder becomes insufficient, causing a problem of defective charging. In addition, since the secondary battery is weak against heat, it is necessary to design a circuit so that the secondary battery is not overdischarged or overcharged in order to prevent the battery from increasing its temperature. Under the circumstances described above, the application of the non-contact charging method is being studied.
The non-contact charging method provides a coil to both the power receiver and the power feeder and performs charging by electromagnetic induction. The non-contact charging method does not need to consider a contact pressure between the electrodes. Therefore, a charging voltage can be fed stably without being influenced by the contact state of the electrodes. As a coil for the non-contact charger, a structure that a coil is wound around a ferrite core is known (see Patent References 1, 2). A structure that a coil is mounted on a resin substrate having a ferrite powder and a amorphous powder mixed is also known (see Patent Reference 3). But, the ferrite becomes brittle and has poor impact resistance when it is fabricated into a thin form, and the power receiver tends to have a malfunction if equipment is dropped.
Besides, to provide a thin power receiving portion in correspondence with the thinning of the equipment, it is considered to adopt a planar coil that is formed with a metal powder paste printed in a spiral form on a substrate. But, magnetic flux passing through the planar coil is interlinked with the substrate and the like within the equipment, so that there is a problem that heat is produced within the device because of an eddy current generated by the electromagnetic induction. Therefore, high electric power cannot be transmitted, and a charging time becomes long as a result. Specifically, when a contact charger is used to recharge a cellular phone, it takes about 90 minutes, while the non-contact charger takes about 120 minutes.
As described above, the power receiver applying the conventional non-contact charging method fails to take sufficient measures against the eddy current generated by the electromagnetic induction. Especially, since the power receiver is provided with the secondary battery, it is required to suppress the heat generation as much as possible. Besides, since the power receiver is attached to the electronic apparatus main body, the heat generation has an adverse effect on the circuit parts and the like. Therefore, high electric power cannot be transmitted when charging, resulting in a problem of having a long charging time. The generation of the eddy current also induces the generation of noise, becoming a factor for degrading the charging efficiency.
[Patent Reference 1] JP-A 11-265814(KOKAI)
[Patent Reference 2] JP-A 2000-23393(KOKAI)
[Patent Reference 3] JP-A 9-190938(KOKAI)