PROTECTION ELEMENT

To provide a protection element evens the amount of flux on a fusible conductor and improves variations in fusing characteristics. A protection element is provided with: an insulated substrate; a heating body; an insulating member; two electrodes; a heating body internal electrode; a fusible conductor which is stacked over the two electrodes from the heating body internal electrode and fuses a current path between the two electrodes by heating; a flux coated on the fusible conductor so as to superimpose on the heating body; and a cover member attached to the insulated substrate covering at least the fusible conductor. The cover member has a cylindrical protrusion formed on an inner surface of the cover member so as to contact the flux, and a communication hole communicating with the inner surface side of the cover member from the protrusion is opened on a side wall surface of the cover member.

FIELD OF THE INVENTION

The present invention relates to a protection element protecting a circuit connected onto a current path by fusing the current path.

This application claims priority to Japanese Patent Application No. 2012-069658 filed on Mar. 26, 2012, the entire contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The most of rechargeable and iteratively usable secondary batteries are fabricated in the form of a battery pack to provide a user. Preferably, in a lithium ion secondary battery having high weight energy density, several protection circuits, such as an overcharging protection circuit or an over-discharging protection circuit or the like are typically built in the battery pack, in order to ensure the safety of a user and electronic equipment. The lithium ion secondary battery has a capability of shutting down an output from the battery pack upon occurrence of predetermined emergencies.

An overcharging protection operation or an over-discharging protection operation is performed to the battery pack by turning ON/OFF an output from the battery pack by using a FET switch built in the battery pack. The battery pack and the electronic equipment must be protected from unexpected sudden accidents, such as firing or the like, even when the FET switch is short-circuited and damaged for some sort of causes, when lightning surge or the like is applied and large current is instantaneously flown, when an output voltage is unusually dropped due to the dead of the battery pack, or conversely when an abnormal over voltage is output. To this end, it has been used so far a protection element consisting of fuse elements having a capability of shutting down a current path, in response to a signal from the outside, in order for an output from the battery pack to securely shut down, even in any predictable abnormal state, such as those described above.

As a protection element of the protection circuit for such lithium ion secondary battery or the like, it is generally known a structure in which a heating body is provided within the protection element, by which a fusible conductor provided on a current path is fused, as disclosed in Patent Document 1.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

In the protection element disclosed in Patent Document 1, a flux is coated onto a surface of a fusible conductor (fuse) made of a low-fusing metal, for the sake of antioxidation, fusing acceleration, and improvement of fusing characteristics. Further, a cover member is provided so as to cover a substrate consisting the protection element for quality assurance of the protection element. Uniformly coating the flux on the fusible conductor allows for a uniform heat generation distribution of the fusible conductor, which reduces variations in fusing characteristics of the fusible conductor. For this purpose, the cover member has a cylindrical protrusion in an inner surface of the cover member so as to enclose the center on the fusible conductor, for the purpose of maintaining the coated flux for making the amount of flux coated on the fusible conductor uniform.

However, even when the cylindrical protrusion is provided on the fusible conductor, a problem emerges that if void (air bubble) exists in the flux, the amount of flux undergoes a change depending on where the void is generated, occurring variations in the fusing characteristics. To deal with this problem, Patent Document 1 discloses a protection element in which a notch is formed in the cylindrical protrusion for discharging the void. However, such a measure causes another problem that not only the void but also the flux flows out from the notch formed in the cylindrical protrusion, resulting in occurring variations in the amount of flux coated on the fusible conductor.

An object of the present invention is to implement a protection element in which the amount of flux is uniformly distributed to thereby improving the variations in the fusing characteristics, even if void is generated in the flux coated onto the fusible conductor.

Namely, as a means for resolving the foregoing problem, the protection element according to the present invention comprises: an insulated substrate; a heating body stacked on the insulated substrate; an insulating member stacked on the insulated substrate so as to cover at least the heating body; first and second electrodes stacked on the insulated substrate on which the insulating member is stacked; a heating body internal electrode which is stacked on the insulating member so as to superimpose on the heating body, and is electrically connected on a current path between the first and second electrodes and to the heating body; a fusible conductor which is stacked from the heating body internal electrode to the first and second electrodes, and fuses a current path between the first and second electrodes by heating; a flux coated on the fusible conductor so as to superimpose on the heating body; and a cover member attached to the insulated substrate covering at least fusible conductor, wherein the cover member has a circular protrusion formed so as to contact the flux on an inner surface of the cover member opposingly to the heating body, and a communication hole communicating with an inner surface side of the cover member from the inside of the protrusion is opened on a wall surface of the circular protrusion.

Advantageous effect of the Invention

Since the present invention is configured such that the cover member has the circular protrusion which is formed so as to contact the flux opposingly to the heating body, and the communication hole communicating with an inner surface side of the cover member from the inside of the protrusion is opened to a wall surface of the circular protrusion, the invention allows void generated in the flux to be discharged through the communication hole. The discharge of the void is instrumental in forming a uniform flux coated onto the fusible conductor, which creates a uniform heat generation distribution of the fusible conductor, thereby reducing the variations in the fusing characteristics.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is needless to say that the present invention is not necessarily intended to be limited the present invention to the following embodiments, and thus various modifications may be made within the scope without departing from the gist of the present invention.

[Structure of Protection Element]

As shown inFIG. 1A,1B,2A and2B, a cover member1has in its inner surface1aa protrusion2whose head is arranged at a position opposite to a position of a heating body14. The protrusion2takes the inner surface1aof the cover member1as a bottom surface and presents cylindrical geometry whose head is opened. The cylindrical protrusion2is arranged at a position opposite to a rectangular heating body14via a fusible conductor13when the cover member1is attached by putting it over the body of the protection element10. The height of the cylindrical protrusion2is defined so that a head of the cylindrical protrusion2comes to a position where the head contacts a surface of the flux17coated on a surface of the fusible conductor13. An inner surface2aof the cylindrical protrusion2is formed into a space where the flux17is drawn by dint of surface tension to which the head is contacted, and filled with the flux17. A wall surface of the inner surface2amay be smooth or may be finished in a pearskin texture to impart roughness thereon. On a wall surface of the protrusion2, a communication hole3communicating with an inner surface2aof the protrusion2and an inner surface side la of the cover member1is opened. The cover member1is used for protecting the inside of the protection element10and is made of insulating materials. For example, it may use insulating materials having specified heat resisting properties, such as glass epoxy and ceramics or the like. It is common to integrally form the cover member1including the protrusion2by utilizing the injection molding technique. Alternatively the protrusion2may be formed, as a separate component, at a proper position of the inner surface1aof the cover member1by gluing or inlaying or the like.

As shown inFIG. 2A, the body of the protection element10comprises an insulated substrate; a heating body14stacked on the insulated substrate11and covered by an insulating member15; an electrode12(A1) and12(A2) provided at the both ends of the insulated substrate11; a heating body internal electrode16stacked on the insulated substrate15so as to superimpose on the heating body14; and a fusible conductor13whose both ends are connected to electrodes12(A1) and12(A2), whose center is connected to the heating body internal electrode16. On the fusible conductor13, a flux17is coated. As shown inFIG. 2B, the protection element10according to the present invention is formed as a whole by putting the cover member1shown inFIG. 1over the insulated substrate11of the body of the protection element10. When the cover member1is put over the insulated substrate11, the cylindrical protrusion2contacts a surface of the flux17, and the inside of the cylindrical protrusion2is filled with the flux17by dint of surface tension of the flux17.

The rectangular insulated substrate11is made, for example, of member having insulation properties, such as alumina, glass ceramics, mullite, and zirconia. Other than the above, materials used for a print circuit board, such as a glass epoxy substrate and a phenol substrate or the like may be used, but it needs to take notice of temperature on fuse fusing.

The heating body14is a member which has a relatively high resistance value and generates heat upon energization. For example, the heating body14is made of W, Mo, and Ru or the like. The heating body14is fabricated through the following processes. Powder body of these alloy, composition, and compound are mingled with a resin binder or the like to produce a paste-like material. Then, the paste-like material is formed, in a pattern form, on the insulated substrate11by the screen printing technology, followed by firing or the like.

The insulating member15is disposed so as to cover the heating body14, and the heating body internal electrode16is arranged so as to be opposite to the heating body14via the insulating member15.

One end of the heating body internal electrode16is connected to one heating body electrode18(P1). Further, one end of the heating body14is connected to the other heating body electrode18(P2).

The fusible conductor13may be made of a conductive material which is melted and fused by predetermined electric power and heat. For example, BiSn alloy, BiPb alloy, BiSn alloy, SnPb alloy, PbIn alloy, ZnAl alloy, InSn alloy, and PbAgSn alloy or the like may be available therefor.

The flux17has low viscosity, and when the flux17is coated on the fusible conductor13, the flux17is almost uniformly spread and distributed thereover, at the time of manufacture of the protection element10. With the lapse of time, a solvent contained in the flux17is volatilized and viscosity thereof will increase accordingly.

[Use of Protection Element]

As shown inFIG. 3, the aforesaid protection element10is used for a circuitry within a battery pack of a lithium ion secondary battery.

For example, the protection element10is used by being embedded into a battery pack20having a battery stack25composed of totally4lithium ion secondary batteries of battery cells21to24.

The battery pack20includes a battery stack25; a discharging and charging control circuit30controlling discharge and charge of the battery pack25; a protection element10which protects the battery stack25and the discharging and charging control circuit30, and to which the present invention is applied; a detection circuit26detecting a voltage of each battery cell21to24; and a current control element27controlling a behavior of the protection circuit10depending on detection results obtained by the detection circuit26.

The battery stack25is that in which the battery cells21to24are serially connected to one another and which is required to control for protection of over-discharging and overcharging states. The battery stack25is connected to a charging device35via a positive electrode terminal20aand a negative electrode terminal20bof the battery pack20, and charge voltage is applied from the charging device35. Connecting the battery pack20charged by the charging device35to the electronics which is activated by the positive electrode terminal20aand the negative electrode terminal20benables activation of the electronics.

The discharging and charging control circuit30includes two electric current control elements31and32serially connected to a current path through which an electric current flows to the charging device35from the battery stack25, and a controller33controlling behavior of the electric current control elements31and32. The electric current control elements31and32are configured, for example, by a Field-Effect Transistor (hereinafter, referred to as a “FET” for brevity's sake). Conduction and disconnection of the current path from the battery stack25are controlled by controlling a gate voltage with the controller33. The controller33is activated upon reception of power supply from the charging device35. The controller33controls behavior of the current control elements31and32so as to shut down the current path, when the battery stack25is in an over-discharging or an overcharging state, depending on the detection results obtained by the detection circuit26.

The protection element10is, for example, connected on a discharge and charge current path between the battery stack25and the discharging and charging control circuit30, and a behavior thereof is controlled by the current control element27.

The detection circuit26is connected to each battery cell21to24to detect a voltage value of the each battery cell21to24, and the each voltage value is supplied to the controller33of the discharging and charging control circuit30. Moreover, the detection circuit26outputs a control signal controlling the current control element27when at least one of the battery cells21to24falls into an overcharge voltage or an over-discharge voltage.

The current control element27activates the protection element10when a voltage value of the each battery cell21to24runs up to a voltage exceeding a predetermined over-discharging state or an overcharging state, in response to a detection signal output from the detection circuit26. The current control element27controls to shut down an over-discharging and charging current path of the battery stack25without relying upon a switching operation of the current control elements31and32.

A specific description will be made to a structure of the protection element10in the battery pack20having such a configuration as above.

The protection element10to which the present invention is applied has a circuit configuration as shown, for example, inFIG. 4. That is, the protection element10has a circuit configuration including a fusible conductor13serially connected via a heating body internal electrode16, and a heating body14fusing the fusible conductor13by energizing through a junction point of the fusible conductor13for heating. Further, in the protection element10, the fusible conductor13is, for example, serially connected on a discharging and charging current path and the heating body14is connected to the current control element27. Out of two electrodes12(A1) and12(A2) of the protection element10, one is connected to A1and the other is connected to A2. Moreover, the heating body internal electrode16and a heating body electrode18connected thereto are connected to P1, and the other heating body electrode18is connected P2.

The protection element10having such a circuit configuration realizes the height reduction and at the same time can ensure fusing of the fusible conductor13on a current path.

[Capability of Cover Member]

Prior to making reference to a capability of the cover member1used for the protection element10according to the present invention, a description will be made to a capability of the cover member of the conventional protection element.

As shown inFIGS. 5A and 5B, a cover member1of the conventional protection element has a protrusion42arranged to an inner surface1aof the cover member1, at a position opposite to a position where the heating body14is disposed. The protrusion42is formed in cylindrical geometry. The protrusion42whose bottom surface is formed of an inner surface1aof the cover member1has an inner surface42a,and its head is opened. An area of the head of the protrusion42is set to cover approximately 80% of an area of the rectangular heating body14opposite to the protrusion42. In some cases, void (air bubble)44is occasionally generated in the flux17when the flux17is drawn by dint of surface tension so as to fill a space formed of the inner surface42aof the protrusion42, or when the flux17is coated on the fusible conductor13. In the event the void44stays in the protrusion42, the flux17unevenly distributes in the protrusion42, and impairs uniformity of the flux17on the fusible conductor13. Consequently, an ununiform heat generation distribution will occur in the fusible conductor13due to the heating body14, and causes variations in the fusing characteristics of the fusible conductor13.

As shown inFIG. 5B, the cover member1has a notch43formed towards the inner surface1aof the cover member1that is a bottom surface of the protrusion42from a head of the protrusion42.

As shown inFIG. 5C, void44at a position represented by a broken line, which is generated in the flux in the protrusion42is discharged to the inner surface1aside of the cover member1through the notch43with the aid of buoyancy. Nonetheless, the flux17flows out on the fusible conductor13through the notch43, as at a position where the notch43is formed, surface tension does not act between the head of the protrusion2and the flux17. On this account, the amount of flux17held at the protrusion42decreases, leading to arising a difference in a coating amount of the flux17on the fusible conductor13. As a result, an uneven heat generation distribution occurs at a position corresponding to the heating body14on the fusible conductor13. It will cause variations in the fusing characteristics of the fusible conductor13.

As shown inFIG. 6A, since the protection element10according to the present invention, the protrusion2is provided so that the flux17contacts a whole circumference of the head of the protrusion2of the cover member1opposingly to the heating body14, the provision enables a sufficient amount of flux17to be drawn, over a large area, enough to fully cover a surface area of the heating body14. Moreover, as shown inFIG. 6B, since the communication hole3communicating with the inner surface la side of the cover member1from the inner surface2aof the protrusion2is opened to the cylindrical protrusion2, void4generated in the flux17is discharged toward the inner surface1aof the cover member1from the inside of the protrusion2through the communication hole3with the aid of buoyancy. Accordingly, only the void4is discharged through the communication hole and the flux17stays at a position corresponding to the heating body14on the fusible conductor13with a uniform amount.

Herein, as shown inFIG. 7A, the communication hole3formed on a wall surface of the protrusion2is surroundingly formed by a surface3aof a head side of the protrusion2of communication hole3, a surface3bof the cover member1side, and a side (not shown). In this instance, it is preferable for the surface3bof the cover member1side to be on the same surface as the inner surface1aof the cover member1. As shown inFIG. 7B, in a case where the surface3bof the cover member1side of the communication hole3is formed to have a step from the inner surface1aof the cover member1, an attention should be paid to a point that it becomes hard to discharge the void4, as the step will act as a barrier to the void4migrated with the aid of buoyancy to the inner surface1aside of the cover member1. Besides, if a thickness of a portion corresponding to a bottom surface in a cylindrical protrusion46out of the inner surface1aof the cover member1is set so as to gradually increase with an inclination from the communication hole3, the generated void is guided towards the communication hole3, thus facilitating discharge of the void.

In the above configuration, it goes without saying that geometry of the communication hole is not necessarily limited to rectangular, but may be elliptic or any geometry.

A communication hole47opened to a wall surface of the protrusion46may be plural, not necessarily limited to one. As shown inFIGS. 8A and 8B, 4 communication holes may be opened being separated from one another at equal intervals on a circumference of the protrusion46. The void in the flux17of the protrusion46migrates with the aid of buoyancy within an inner surface46aside of the protrusion46, and thus an increase in the number of portion where the communication hole47opens enables effective discharge of the void stayed in the protrusion46.

An increase in an opening area of the communication hole allows for more effective discharge of the void stayed in the flux17of the protrusion. As shown inFIGS. 9A and 9B, an end of the cover member1is either opened and the cover member1has the cylindrical protrusion2having an inner surface2aand a columnar supporting member48whose one end is connected to a portion of the surface2cof the inner surface1aside of the cover member1of the protrusion2. Then, the other end of the columnar supporting member48is connected to the inner surface1aof the cover member1. In the cover member1of the modification, the protrusion2is supported by the columnar supporting member48, and a communication hole3is formed in a slit shape between a surface2cat the side which does not contact the flux17of the protrusion2and the inner surface1aof the cover member1. The formation of the protrusion2allows the void4in the flux17generated in the inner surface2aside of the protrusion2to be discharged to the inner surface1aside of the cover member1in almost all the directions.

Meanwhile, a circumferential surface at the side contacting the flux17of the protrusion2can contact the flux17over the entire surface. Hence, it makes it possible to evenly draw the flux17on the fusible conductor13without taking the trouble of forming an outflow path for the flux17.

The protrusion2and the columnar supporting member48connected to the protrusion2, as shown inFIGS. 9A and 9B, can be integrally formed with the cover member1utilizing the injection molding technique.

Alternatively, as shown inFIGS. 10A to 10C, it may also be feasible to fabricate the cover member1by integrally molding individually a protrusion member composed of the protrusion2and a columnar supporting portion48, and then by connecting the protrusion member to a cover member with no separately integrally molded protrusion.

In the above configuration, the descriptions were made in view of mathematical common sense that the figure having the maximum area relative to the same peripheral length is a circle, the protrusion assumed the circular shape as an annular protrusion. However, it is needless to say that the protrusion may be an elliptic cylinder, a triangular cylinder, and another polygonal cylinder, not necessarily limited to cylindrical.

REFERENCE SIGN LIST