1. Field of the Invention
The present invention generally relates to a wireless charging of a mobile terminal, and more particularly to a system and a method for wireless charging using electromagnetic induction.
2. Description of the Related Art
A mobile terminal, such as a cellular phone or PDA (Personal Digital Assistant), is driven by a rechargeable battery. In order to charge the battery, electric energy is supplied to the battery of the mobile terminal by using a separate charging apparatus. Typically, the charging apparatus and the battery have separate contact terminals on their exteriors, respectively, so that by contacting the contact terminals with each other, the charging apparatus is electrically connected with the battery.
However, since the contact terminal protrudes from the external side of the charging apparatus and the battery, the contact-type charging method creates a problem in that the contact terminal can be easily contaminated by foreign materials, thereby failing to appropriately charge the battery. Further, when the contact terminal is exposed to a moist or humid environment, it is nearly impossible to charge the battery.
In order to solve these problems, wireless charging technology or contactless charging technology has recently developed, and has been applied to many electronic devices. This technology is representatively applied to electronic devices, such as an electric toothbrush and an electric shaver, and has been recently adapted to a battery of a hybrid car and a mobile terminal.
The wireless charging technology is classified into three schemes, i.e. an inductive coupling scheme, an RF (Radio Frequency) wave radiation scheme, and an evanescent wave resonance scheme. Among them, the inductive coupling scheme has been most efficiently and widely used until now. The basic principle of the inductive coupling scheme is the same as that of a transformer. In the wireless charging system, a coil of the transformer is divided into a first coil and a second coil, and the first coil is mounted in the charging apparatus and the second coil is mounted in the mobile terminal.
The coil for use can be manufactured in various shapes, and in particular, a planar spiral coil is widely used in small devices, such as a mobile terminal. In this case, the power efficiency of the charging can be represented as a ratio of power applied to the first coil to power induced in the second coil. In order to increase the charging efficiency, the arrangement of the first coil and the second coil is very important. When an arrangement offset is generated, induced electromotive force is not sufficiently induced to the second coil, so that the charging efficiency deteriorates in comparison with a wired charging system. That is, when the coils are not appropriately arranged, the charging efficiency remarkably deteriorates, and thus the time for charging also increases.
A method for arranging the first coil and the second coil includes an arrangement method using a permanent magnet, an arrangement method using mechanical assembling of the charging apparatus and the mobile terminal, and an arrangement method using a change of an arrangement pattern of the first coil, among others. However, those arrangement methods have the following problems.
The arrangement method using the permanent magnet has a problem in that a static magnetic field created by the permanent magnet influences on a time-varying magnetic field, so as to deteriorate the assembling efficiency, as well as a design problem due to the mechanical size and weight of the permanent magnet. Further, when the permanent magnet is mounted on the mobile terminal or the charging apparatus, various metal things can attach to the mobile terminal or the charging apparatus, thereby incurring the inconvenience to the user.
In the arrangement method using mechanical assembling of the charging apparatus and the mobile terminal, the charging apparatus and the mobile terminal should be compatibly designed on a one-to-one basis, so that one charging apparatus cannot be used for a different mobile terminal, thereby limiting the compatibility with other mobile terminals.
In the arrangement method using the changing of the arrangement pattern of the first coil, it is possible to guarantee a relatively high charging efficiency, regardless of the location and direction of the second coil. However, energy waste is excessive in comparison with a case where a single coil is used.