The present invention relates to a self-recovery current limiting fuse which establishes a conducting state through chaining of conductive particles in a liquid matrix and can reliably perform a cutoff operation upon occurrence of overcurrent.
In recent years, electronic equipment, such as cellular phones and notebook computers, use devices whose resistance has a positive temperature coefficient, or PTC devices, as protective devices for secondary cells. Demand exists for such electronic equipment to implement high functionality, long-hour operability, and higher efficiency. Under the circumstances, secondary cells are required to implement large capacity and high voltage. In association with these requirements, PTC devices are required to implement high voltage. At present, PTC devices of about 8 V are in practical use. For implementation of higher voltage, insulation performance in a current limiting condition, which is an OFF state, must be enhanced; i.e., dielectric strength must be enhanced. Mainstream materials for matrices of conventional PTC devices are solid materials, such as ceramics and polymers. For example, polyethylene-based PTC devices and barium-titanate-based PTC devices are used (refer to Patent Documents 1 and 2).
At present, PTC devices of low dielectric strength are widely used as protective devices for lithium ion cells for use in cellular phones and computers. However, in association with implementation of large-capacity cells, PTC devices of high dielectric strength are required. For a structural reason, a solid matrix involves the generation of cracks and voids in principle when the solid matrix expands. Since gas is present in such cracks and voids surrounded by the solid matrix having high dielectric constant, an electric field concentrates in cracks and voids, so that discharge is apt to be generated in cracks and voids. For this reason, a PTC device using a solid matrix suffers material deterioration caused by gaseous discharge, resulting in impairment in recovering characteristics. Thus, under present circumstances, difficulty is encountered in fabricating a reliably usable PTC device of 8 V or higher, depending on a PTC device structure.
Under the above-mentioned technological circumstances, the inventors of the present invention filed an application for a self-recovery current limiting fuse using a liquid matrix, which can suppress the generation of cracks and voids as compared with a solid matrix (refer to Patent Document 3). The self-recovery current limiting fuse using a liquid matrix disclosed in Patent Document 3 enhances dielectric strength through suppression of generation of cracks and voids and implements self-restoration characteristics by means of dielectrophoretic force of solid conductive particles generated through application of voltage. Thus, by means of solid conductive particles being mixed in a liquid matrix; i.e., solid conductive particles being fluidly dispersed in a liquid matrix, contact electric-resistance, or ON resistance, can be lowered; through enhancement of dielectric strength, a secondary cell having high rated voltage is protected; the range of applications is expanded; efficiency is improved; charging time is shortened; and maintenance-free operation is attained.
According to Patent Document 3, fusion cutting of a fuse element by overcurrent is utilized for operational change from an ON state to an OFF state. Specifically, when overcurrent flows between electrodes in an ON state, in which solid conductive particles are chained in a liquid matrix for establishment of a conducting state, Joule heat is generated in the liquid matrix. As a result, the solid conductive particles evaporate and disperse, whereby a cutoff/current-liming operation is effected, thereby establishing a cutoff/current-limiting state. Because of utilization of evaporation of solid conductive particles, particularly in the case of use of a fuse element having high melting point, some difficulty is involved in transfer to an OFF state. Also, the self-recovery current limiting fuse of Patent Document 3 does not have an emergency trip function.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H6-215903
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2005-285999
An object of the present invention is to solve the above-mentioned problems for more reliably performing a cutoff operation upon occurrence of overcurrent in a self-recovery current limiting fuse which establishes a conducting state through chaining of conductive particles in a liquid matrix by use of dielectrophoretic force.
The present invention devises an arrangement of current flowing to a device, a magnetic field applied to the device, and electrodes and a fuse element (conductive substance) of a self-recovery current limiting fuse so as to perform a cutoff operation of the self-recovery current limiting fuse through operation of the fuse element by means of an interaction of the current and the magnetic field (electromagnetic force). Thus, particularly in the case of use of a fuse element having high melting point, the present invention provides effective, indispensable cutoff means.
Also, the present invention may be applied to an emergency trip function and contributes to functional (safety) improvement of a device.
A self-recovery current limiting fuse of the present invention is configured as follows. A liquid matrix of a nonmagnetic material is accommodated within an insulative container of a nonmagnetic material, and a pair of electrodes are disposed within the insulative container such that the electrodes face each other via the liquid matrix. Conductive particles are fluidly dispersed in the liquid matrix. A magnetic field generation section is provided externally of the insulative container and adapted to generate a magnetic field having a component in a direction orthogonal to a fuse element to be formed between the paired electrodes through chaining of the conductive particles.
In an ON state in which the conductive particles are chained between the paired electrodes, a dielectrophoretic force which acts on the conductive particles in the liquid matrix through application of voltage to the paired electrodes causes the conductive particles to be continuously connected to one another. Upon occurrence of overcurrent, an electromagnetic force generated through interaction between the magnetic field generated by the magnetic field generation section and current flowing to the fuse element cuts the fuse element or pushes out the fuse element from the electrodes, thereby establishing an OFF state. In this manner, the ON state and the OFF state are repeated.
Also, the self-recovery current limiting fuse of the present invention further comprises a magnetic-field-intensity-varying apparatus capable of varying magnetic field intensity of the magnetic field generation section. Upon reception of a signal indicative of detection of overcurrent from an overcurrent detection section provided in series with the self-recovery current limiting fuse, or an emergency trip signal or an OFF operation check signal from an emergency trip input section, the magnetic-field-intensity-varying apparatus greatly varies the magnetic field intensity for bringing the fuse element into an OFF state.
According to the present invention, a fuse element material having high melting point can be cut based on a new cutoff principle different from conventional fusion cutting of a fuse element. Also, the present invention contributes to improvement of safety by providing operation check and emergency trip function, thereby expanding the range of use and application of devices.
According to the present invention, 1) an OFF operation can be performed without need to melt particles (even when unfusible particles are used), and 2) a reset function for checking an OFF operation like a test button of an earth leakage breaker may be added, thereby ensuring safe usage.
The present invention will be described by way of example.
The magnetic field generation section is disposed such that an electromagnetic force generated through interaction between a magnetic field generated by the magnetic field generation section and current flowing to a fuse element (chain of solid particles) in association with overcurrent cuts the fuse element or pushes out the fuse element from the electrodes. In
In a steady ON state shown in (a) of
In a steady ON state shown in (a) of
Next, suppose that overcurrent flows to the self-recovery current limiting fuse as shown in (b) of
(c) of
In this manner, the solid particles in the liquid matrix are collected between the electrodes and restored to the form of a pearl chain between the electrodes, whereby an OFF state is changed to an ON state. Again, in an ON state, in which the solid particles are chained, when overcurrent flows to the self-recovery current limiting fuse, the ON state is changed to an OFF state. In this manner, the self-recovery current limiting fuse repeats changeover between the above-mentioned states, thereby carrying out a self-recovery function.
Thus, each of the electrodes is formed into such a shape as to form a non-uniform electric field, to allow easy contact of particles with the electrodes, and to avoid an increase in contact resistance; for example, into a sloped or stepped shape or the like, in which the height increases gradually, whereby, in a region where the ends of the paired electrodes face each other, a gap is formed between the insulative container and side surfaces of the electrodes.
The electrodes may be formed from a high-melting-point material or an alloy which contains the high-melting-point material, and the high-melting-point material and the alloy are resistant to arc and electrolytic corrosion. For example, each of the electrodes may be configured such that a thin film of one or more conductive metals selected from the group consisting of Al, Cu, Ag, Au, Ni, and Cr is formed on an oxide film formed on a glass substrate or a metal substrate. Also, the electrodes may be configured by use or addition of a high-melting-point material, such as W, Ti, or stainless steel, for enabling repeated use.
When the overcurrent detection section detects overcurrent, the magnetic-field-intensity-varying apparatus greatly varies magnetic field intensity. The magnetic-field-intensity-varying apparatus is configured to be able to carry out cutoff even when overcurrent does not flow, upon reception of an emergency trip signal or an OFF operation check signal from an emergency trip input section. The magnetic field intensity may be varied by means of varying the position of a permanent magnet, if used, or varying a coil position or coil current, if a coil is used. The electromagnetic force F (=IBL) which acts on the solid particles of the self-recovery current limiting fuse is also proportional to the magnetic field intensity B of the magnetic field generation section. Therefore, in an emergency, by means of greatly varying the magnetic field intensity B, the self-recovery current limiting fuse may be externally brought to an OFF state.
Also, the self-recovery current limiting fuse may be used as a protection device against mechanical shock. Specifically, upon subjection to mechanical shock or vibration in the event of, for example, earthquake or collision, a pearl chain of solid particles connected to one another is cut, thereby cutting off current. Thus, the self-recovery current limiting fuse may be utilized as an emergency device against disaster or as a protective device against shock. The restoration speed from an OFF state to an ON state of the self-recovery current limiting fuse may be adjusted for applications by means of selection of a liquid matrix from among those of different viscosities and setting of electric field intensity through determination of electrode shape and a gap between electrodes.
In the self-recovery current limiting fuse of the present invention, a magnetic field generated by the magnetic field generation section acts on solid particles. Thus, the liquid matrix must be of a nonmagnetic material. For example, the liquid matrix may be of one or more materials selected from the group consisting of deionized water, including pure water, insulative oil, insulative organic polymeric material, and insulative organic polymeric material gel. The ON resistance of the liquid matrix can be lowered by means of cooling particles and metals, such as electrodes, by use of cooling medium, such as liquid nitrogen.
A conceivable liquid matrix encompasses not only liquid, which has complete fluidity, but also a gel substance. A self-recovery current limiting fuse using a gel substance has an advantage in that distant dispersion of solid particles, which causes a drop in efficiency of collection of solid particles, can be prevented, and liquid leakage or a like problem can be avoided in actual use.
The solid particles which serve as filler must be of a conductive material for forming a current path in an ON state. Additionally, in order for a dielectrophoretic force to act on the solid particles for restoration from an OFF state to an ON state, the solid particles must be of a conductive material. For example, one or more types of particles selected from among tin (Sn) particles, zinc (Zn) particles, indium (In) particles, bismuth (Bi) particles, etc., and one or more types of particles selected from among carbon particles, copper (Cu) particles, aluminum (Al) particles, silver (Ag) particles, gold (Au) particles, etc. may be mixedly used as material for the solid particles. Also, for example, mercury (Hg) may be used as a liquid material.
Example values for the self-recovery current limiting fuse of the present invention are as follows. The fuse device measures 30 mm×16 mm, and steady-state current is several mA to several tens of A. Cutoff was confirmed with an overcurrent ranging from 0.5 A to 7 A. The gap between the electrodes was, for example, 30 μm in the case of a narrow gap, and 150 μm in the case of a wide gap.
Number | Date | Country | Kind |
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2007-291555 | Nov 2007 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2008/068942 | 10/20/2008 | WO | 00 | 5/5/2010 |