As methods of charging secondary batteries mounted in electronic equipment such as portable terminals and video cameras, there are two types of charging methods, i.e., a contact type charging method and a non-contact type charging method. The contact type charging method carries out a charging operation by making an electrode of a power reception device in direct contact with an electrode of a power feeding device.
The contact type charging method is commonly used in a wide range of applications, since a structure of a device implementing the contact type charging method is simple. However, in association with miniaturization and weight reduction of electronic equipment, various electronic devices become light in the weight thereof, and accordingly a low contact pressure between electrodes of the power reception device and the power feeding device may cause problems such as charge failure charge error. Further, secondary batteries are weak at heat, which needs to prevent the temperature rise of the batteries, and to pay attention to a circuit design so as not to cause overcharge and overdischarge. To cope with these problems, a non-contact type charging method is being considered in recent years.
The non-contact type charging method is a charging method using an electromagnetic induction principle in which coils are mounted at both sides of the power reception device and the power feeding device.
A non-contact type charger can be miniaturized by putting a ferrite core to be in a magnetic core and winding coils around the ferrite core. Furthermore, for miniaturization and reduction in thickness, a technique of forming a resin substrate by mixing ferrite powder and amorphous powder and mounting a coil and the like on the resin substrate, has been proposed. However, in the case that a ferrite sheet is processed thinly, the thinly processed ferrite sheet may be easily broken and weak in impact resistance. As a result, there have been problems that defects have occurred in the power reception device due to a fall or collision of the non-contact type charger.
Further, in order to reduce thickness of a power reception portion of an electronic device in response to reduction in the thickness of the electronic device, a planar coil that is formed by printing a metal powder paste as a coil have been employed. A structure of strengthening a coupling of a planar coil and a magnetic sheet has been proposed. In the proposed structure, a magnetic body or a magnetic sheet is used as a core material to strengthen the coupling between primary and secondary coils.
Meanwhile, if a power transmission speed increases, defects between adjacent transformers, as well as defects caused by heat from the surrounding components, may be likely to occur. That is, in the case that the planar coils are used, the magnetic flux passing through the planar coils is connected to a substrate or the like inside an electronic device, an internal portion of the electronic device may be heated due to eddy currents caused by electromagnetic induction. As a result, large power cannot be transmitted and thus a time-consuming problem may be caused for charging the electronic device.
To cope with this problem, a magnetic body or a magnetic sheet was used as a shielding member on the back of the substrate. In order to obtain a sufficient shielding effect, as the magnetic body or the magnetic sheet may have the larger magnetic permeability, and the larger area and thickness, a more effective shielding effect can be obtained.
In general, a magnetic body such as an amorphous ribbon, a ferrite sheet, or a polymer sheet containing magnetic powder is used as the magnetic field shield sheet. An effect of focusing a magnetic field for improving magnetic field shielding performance and additional features may be good in the order of amorphous ribbons, a ferrite sheet, and a polymer sheet containing magnetic powder, with high magnetic permeability.
In the case of a power reception device of a conventional non-contact type charging system, a magnetic body or a magnetic sheet with high magnetic permeability and large volume is disposed on the opposite surface to a primary coil side, i.e., on the surface of a secondary coil, for reinforcement of a coupling for improving transmission efficiency, and for improving a shielding performance for suppression of heat generation. According to this arrangement, fluctuations in the inductance of the primary coil become large, and an operation condition of a resonant circuit is shifted from a resonance condition at which a sufficient effect can be exhibited according to a relative positional relationship between the magnetic body and the primary coil.
Korean Patent Application Publication No. 10-2010-31139 provides a power reception device for improving a resonance performance and also suppressing heat generation to solve the aforementioned problems, and proposes a technique of enabling large transmission power and shortening charge time, through an the electronic device and a power reception system using the power reception device.
In other words, according to Korean Patent Application Publication No. 10-2010-31139, a composite magnetic body including a plurality of magnetic sheets magnetic ribbons are arranged at at least one location between a spiral coil a power reception-side spiral coil: a secondary coil and a secondary battery, and between a rectifier and the spiral coil, to thereby prevent a magnetic flux generated from the power reception-side spiral coil from interlinking a circuit board and a secondary battery, and to thereby suppress noise and heat generation caused by an induced electromotive force electromagnetic induction, and the amount of fluctuation of inductance in the primary coil is controlled due to presence or absence of the secondary coil to thus enhance a resonance performance of a resonant circuit constituted by the primary coil and to thereby effectively control oscillation.
The composite magnetic body is set so that first magnetoresistance of a first magnetic sheet adjacent to the spiral coil is less than or equal to 60, second magnetoresistance of a second magnetic sheet laminated on the first magnetic sheet is greater than or equal to 100, and a value of the second magnetoresistance divided by the first magnetoresistance is equal to or greater than 1.0.
The first magnetic sheet is prepared by bonding polycarbonate resins on both surfaces of a first amorphous ribbon by using adhesive layers, respectively, and the second magnetic sheet is prepared by bonding polycarbonate resins on both surfaces of a second amorphous ribbon with large relative permeability by using adhesive layers, respectively. Then, the first magnetic sheet and the second magnetic sheet are integrally bonded via an adhesive layer.
Meanwhile, the ferrite sheet or a polymer sheet containing magnetic powder has the magnetic permeability a little lower than the amorphous ribbon, and thus in order to improve the performance of such low magnetic permeability, thickness of the ferrite sheet or a polymer sheet becomes large compared to the thin amorphous ribbon of several tens μm. Therefore, it is difficult to respond to a thinning tendency of terminals.
Further, in the case of amorphous ribbon with high magnetic permeability, the ribbon itself is a metal thin plate, and thus there is no burden on thickness of the amorphous ribbon. However, when an alternating-current magnetic field according to frequency of 100 kHz used for power transmission is applied to the amorphous ribbon, functionality of applications may be reduced due to an influence of eddy currents of the ribbon surface, or problems of reducing wireless charging efficiency and causing heat generation may occur.
Co-based or Fe-based amorphous ribbons can increase surface resistance slightly, through heat treatment. However, in the case that a processing treatment such as a flake treatment process of reducing a surface area of the ribbon is performed in order to further reduce the eddy current effects, the magnetic permeability is significantly degraded and the function as the shield sheet is greatly degraded.
Also, most of wireless chargers employ a structure of adopting permanent magnets that assist an alignment with a power receiver in a power transmitter for power transmission, in order to increase the power transfer efficiency of the chargers to the maximum. A magnetization or saturation phenomenon occurs in a thin shield sheet due to a direct-current magnetic field of the permanent magnets, to thereby decrease the performance of the chargers or sharply decreasing the power transmission efficiency.
Accordingly, in the case of the conventional chargers, the thickness of the shield sheet must be quite thick in the order of 0.5 T or higher, in order to indicate shielding features without being affected by the permanent magnets, and to maintain high power transmission efficiency, which may cause a major obstacle on slimming of portable terminals.
A voltage induced in a secondary coil of a wireless charger is determined by the Faraday's law and the Lenz's law, and thus it is more advantageous to have the greater amount of magnetic flux linked with the secondary coil in order to obtain a high voltage signal. The amount of the magnetic flux becomes large as the amount of a soft magnetic material contained in the secondary coil becomes large and the magnetic permeability of the material becomes high. In particular, since the wireless chargers essentially employ a non-contact power transmission system, a magnetic field shield sheet in which the secondary coil is mounted is needed to be made of a magnetic material with high permeability, in order to focus wireless electromagnetic waves made from the primary coil of a power transmission device, on the secondary coil of a power reception device.
Conventional magnetic field shield sheets for wireless chargers do not present solutions for attaining the thin film but solving the heat generation problem due to shields and improving the wireless charging efficiency. Thus, the present inventors recognized that inductance (magnetic permeability) is less reduced and magnetoresistance is greatly reduced, although an amorphous ribbon undergoes flakes in the case of the amorphous ribbon, and thus a quality factor (Q) of the secondary coil is increased, to thereby reach the present invention.