Patent Publication Number: US-6661633-B1

Title: Protective element

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a protective element for protecting a circuit to be protected from an overcurrent or an overvoltage by tripping of a PTC element. 
     2. Description of the Related Art 
     Conventionally, current fuses that are made of a low-melting-point metal, such as lead, tin or antimony that are heated and melted off by overcurrents, are known as protective elements for interrupting overcurrents to circuits to be protected. 
     Moreover, PTC (Positive Temperature Coefficient) elements are known as elements that, when overcurrent flows, generate heat, thereby increasing their resistance and limiting the current flowing through the circuit to be protected. 
     Furthermore, protective devices have been proposed, in which such a protective element is combined with a voltage sensing means, so that the circuit to be protected is protected not only from overcurrent but also from overvoltages. 
     FIG. 9 is a circuit diagram of such a protective device (see Japanese Patent Application Laid-Open No. 8-236305). In this circuit, the terminals A 1  and A 2  are connected to the electrode terminals of the device to be protected, such as for example a lithium-ion battery, and the terminals B 1  and B 2  are connected to the electrode terminals of, for example, a charging device. 
     In the protective element  20 X used for the protective device in FIG. 9, a PTC element  1  and a low-melting-point metal member  2  are connected in series between a first terminal “a” and a second terminal “b” of the protective element  20 X, and a heat-generating member  3  is connected between a third terminal “c” and an electrode  5   d . Moreover, this protective device is provided with a Zehner diode and a transistor as a voltage sensing means and a switching means. 
     With this protective device, under normal conditions, current does not flow through the heat-generating member  3 , and the PTC element  1  and the low-melting-point metal member  2  form a conduction path between the terminals of the device to be protected, such as a lithium-ion battery, and the electrode terminals of the charging device or the like. However, but when a reverse voltage that is larger than a predetermined breakdown voltage is applied to the Zehner diode, an abrupt base current i b  flows, which causes a large collector current i c  to flow through the heat-generating member  3  and heat the heat-generating member  3 , the low-melting-point metal member  2  that is at a position near the heat-generating member  3  melts off, and the continuation of an overcharge in the device to be protected, such as a lithium-ion battery connected to the terminals A 1  and A 2 , can be prevented. 
     On the other hand, when an overcurrent exceeding a certain value flows between the terminals A 1  and B 1 , first, the PTC element  1  limits the current, then the low-melting-point metal member  2  melts off and interrupts the current. 
     In the protective element  20 X used for the protective device as shown in FIG. 9, electrodes  5   a ,  5   b ,  5   c  and  5   d  are formed on a substrate  4 , as shown in a top view by FIG. 10A and a cross-sectional view by FIG.  10 B. The heat-generating member  3  is formed between the electrode  5   c  and the electrode  5   d , and is covered with an insulating layer  6 . The PTC element  1  is layered on the electrode  5   a , the low-melting-point metal member  2  is formed extending over the PTC element  1 , the electrode  5   b  and the electrode  5   d  on the substrate  4 , and these parts are covered by a protective cap  7 . 
     Thus, there is the problem that there is a limit to how much the height h of the protective element  20 X can be reduced, so that it is not possible to make the device thinner. Moreover, the number of parts is great, so that there is the problem that it is difficult to reduce the manufacturing costs. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to overcome the problems of the related art, and to provide a protective element using a PTC element, which can be manufactured easily, with fewer components, and at lower costs, and which can be made thinner. 
     The inventors of the present invention have found out that when the protective element is made of (i) a first PTC element and (ii) a low-melting-point metal member, a heat-generating member or a second PTC element, then the substrate, which hitherto used to be necessary for mounting the protective element on, becomes unnecessary, and the entire protective element can be made thinner, if a PTC material constituting (i) the first PTC element serves as a substrate for the (ii) low-melting-point metal member, the heat-generating member or the second PTC element, thus conceiving the protective element of the present invention. 
     In other words, the present invention provides a protective element comprising: a first PTC element; and a low-melting-point metal member, a heat-generating member or a second PTC element, where a PTC material constituting the first PTC element serves as a substrate for a low-melting-point metal member, a heat-generating member or a second PTC element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a top view of a protective element in accordance with the present invention, and FIG. 1B is a cross-section thereof. 
     FIG. 2 is a circuit diagram of a protective device using a protective element of the present invention. 
     FIG. 3 is a circuit diagram of a protective device using a protective element of the present invention. 
     FIGS.  4 A 1  to  4 E 1  and  4 A 2  to  4 E 2  illustrate the steps for manufacturing a protective element in accordance with the present invention. 
     FIG. 5 is a circuit diagram of a protective element in accordance with the present invention. 
     FIGS.  6 A 1  to  6 E 1  and  6 A 2  to  6 E 2  illustrate the steps for manufacturing a protective element in accordance with the present invention. 
     FIG. 7 is a circuit diagram of a protective device using a protective element in accordance with the present invention. 
     FIGS.  8 A 1  to  8 D 1  and  8 A 2  to  8 D 2  illustrate the steps for manufacturing a protective element in accordance with the present invention. 
     FIG. 9 is a circuit diagram of a conventional protective device. 
     FIG. 10A is a top view and FIG. 10B is a cross-section of a conventional protective element. 
     FIG. 11 is a circuit diagram of a conventional protective device. 
     FIG. 12 is a circuit diagram of a conventional protective device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a detailed explanation of the present invention, with reference to the accompanying drawings. In the drawings, like symbols denote like or similar elements. 
     FIG. 1A is a top view of a protective element  20 A in accordance with the present invention, and FIG. 1B is a cross-section thereof. FIG. 2 is a circuit diagram of a protective device using this protective element  20 A. 
     As shown in FIG. 2, the protective element  20 A is made of a PTC element  1  and a heat-generating member  3 . To be more specific, an electrode  5   a  and an electrode  5   b  are provided on an upper and a lower surface of a rectangular PTC material  1 ′ constituting the PTC element  1 , as shown in FIG.  1 . An insulating layer  6  is formed on the electrode  5   b , electrodes  5   b  and  5   c  are formed on side surfaces, and the protective element  20 A is provided with a heat-generating member  3 . The letters “a”, “b”, and “c” denote the terminals of the protective element  20 A. 
     There is no particular limitation on the PTC material itself that can be used for the PTC material  1 ′, and it is possible to use for example so-called polymer PTC in which conductive particles have been dispersed into a crystalline polymer (for example, a polyolefin-based resin), barium-titanate-based PTC, or christobalite-based PTC (see JP H10-261505A). 
     Moreover, there is no particular limitation on the material and method for forming the electrodes  5   a ,  5   b , and  5   c , and it is possible to form them, for example, by printing and baking a silver. The electrodes  5   a  and  5   b  and the PTC material  1 ′ sandwiched therebetween functions as the PTC element  1 . 
     The insulating layer  6  can be made, for example by printing, with an inorganic insulator such as glass, or with various an organic resins such as an epoxy, acrylic or polyester resin. 
     The heat-generating member  3  can be made for example by printing with a carbon paste, a ruthenium oxide paste or the like. 
     If necessary, it is also possible to print a cover glass onto the heat-generating member  3 , or cover it with a molding material, so as to protect the protection element  20 A and prevent damaging during its handling. 
     In this protective element  20 A, the PTC material  1 ′ constituting the PTC element  1  also serves as a substrate for mounting the heat-generating member  3 , so that the substrate that conventionally had to be provided separately from the PTC element becomes unnecessary, and the material costs can be reduced. Furthermore, the electrodes  5   a ,  5   b , and  5   c , the heat-generating member  3 , and the insulating layer  6  are formed or layered flatly on the PTC material  1 ′. Consequently, the protective element  20 A can be made thinner. 
     Moreover, since the element is integrated into one element, its heat efficiency is good, and the PTC element  1  can be heated quickly with the heat generated by the heat-generating member  3 . 
     For the PTC element  1 , a conventional element tripping at about 100° C. can be used. Consequently, with the protective element  20 A, a protective element can be obtained that trips at temperatures that are lower than the melting point of the solder used for the mounting. 
     Moreover, in the protective element  20 A, all the terminals a, b, and c of the protective element are formed on one surface of the PTC material  1 ′, so that it is suitable for surface mounting. 
     There is no particular limitation concerning the circuit using the protective element  20 A. For example, in the circuit shown in FIG. 2, the terminals A 1  and A 2  are connected to the electrode terminals of a device to be protected, such as a lithium-ion battery, and the terminals B 1  and B 2  are connected to the electrode terminals of a charging device or the like. Under normal conditions, almost no current flows through the heat-generating member  3 , and a conduction path with the PTC element  1  in series is formed between the terminals A 1  and B 1 . When an overcurrent flows between the terminals A 1  and B 1 , the PTC element  1  trips, thereby limiting the overcurrent. On the other hand, when the IC senses an overvoltage exceeding a certain voltage between the terminals A 1  and A 2 , the IC changes the gate potential of the FET, and allows a large source current to flow through the FET. Therefore, the heat-generating member  3  generates heat, so that the PTC element  1  trips, and the circuit to be protected can be protected from excessive charging. 
     FIG. 3 is a circuit diagram of a protective device using another protective element  20 B in accordance with the present invention. This protective element  20 B is provided with a low-melting-point metal member  2  between its terminals a and b, and a heat-generating member  3  and a PTC element  1  are connected in series between the terminals a and c. Like the conventional protective device shown in FIG. 9, the protective device in FIG. 3 is provided with a Zehner diode and a transistor as a voltage sensing means and a switching means. 
     Like the conventional protective device in FIG. 9, this protective device causes a collector current i c  to flow. abruptly at an overvoltage, heating the heat-generating member  3 , and melting off the low-melting-point metal member  2 . After the low-melting-point metal member  2  has melted off, the continuation of current through the heat-generating member  3  is limited by the PTC element  1  by virtue of the fact that this current does not heat up the heat-generating member  3  too much. Thus, this protective device provides for enhanced safety. 
     Conventionally, as protective elements used in protective devices for preventing overvoltages and provided with a low-melting-point metal member on a heat-generating member, elements are known that are provided with an intermediate electrode  5   d  between two low-melting-point metal members  2 , like the protective element  20 Y shown in FIG. 11, one side of the heat-generating member  3  being connected to the intermediate electrode  5   d . Also in the protective device using this protective element  20 Y, a Zehner diode and a transistor are used as a voltage sensing means and a switching means. The reason why an intermediate electrode  5   d  has to be provided in the protective element  20 Y in FIG. 11 is that when the low-melting-point metal members  2  on both sides of the intermediate electrode  5   d  melt off at an overvoltage, then the current to the heat-generating member  3  is completely interrupted. That is to say, when no intermediate electrode is provided, and a protective element  20 Z is used that is provided with only one low-melting-point metal member  2  on the heat-generating member  3  as shown in FIG. 12, then a current may continue to flow through the heat-generating member  3 , although the low-melting-point metal member  2  has melted off at an overvoltage, and there is the danger that the heat-generating member  3  continues to generate heat. 
     On the other hand, the afore-mentioned protective element  20 B of the present invention as shown in FIG. 3 is not provided with an intermediate electrode  5   b  as in FIG. 9, but excessive heat generation by the heat-generating member  3  after the low-melting-point metal member  2  has melted off can be prevented because the PTC element  1  is connected in series with the heat-generating member  3 . 
     The protective element  20 B is manufactured as shown in FIGS.  4 A 1  to  4 E 1  and  4 A 2  to  4 E 2 : First, as shown by the top view of FIG.  4 A 1  and the cross-sectional view of FIG.  4 A 2 , the electrodes  5   a ,  5   b ,  5   c , and  5   e  are formed on the two sides of the PTC material  1 ′. 
     Then, the heat-generating member  3  is formed, extending over the electrodes  5   a  and  5   e  (see top view in FIG.  4 B 1  and cross-sectional view in FIG.  4 B 2 ). The heat-generating member  3  is covered by the insulating layer  6  (see top view in FIG.  4 C 1  and cross-sectional view in FIG.  4 C 2 ). On top of the insulating layer  6 , an electrode  5   a ′ is formed so that is in connection with the electrode  5   a  (see top view in FIG.  4 D 1  and cross-sectional view in FIG.  4 D 2 ). Then, the low-melting-point metal member  2  is formed, extending over the electrode  5   b  and the electrode  5   a ′ (see top view in FIG.  4 E 1  and cross-sectional view in FIG.  4 E 2 ). Thus, it is possible to obtain the protective element  20 B of the present invention. 
     The PTC material  1 ′, the electrodes  5   a ,  5   a ′,  5   b ,  5   c , and  5   e , the heat-generating member  3 , and the insulating layer  6  constituting the protective element  20 B can be the same as in the afore-mentioned protective element  20 A. For the low-melting-point metal member  2 , it is possible to use one of the conventional materials used for current fuses or the like. 
     On the other hand, it is also possible to provide the protective element of the present invention with an intermediate electrode. An example of this is the protective element  20 C, which has an intermediate electrode  5   d  as shown in FIG.  5 . With this protective element  20 C, safety can be improved even further due to the current-limiting function of the PTC element  1  and the melting of the low-melting-point metal member  2 . 
     The protective element  20 C can be manufactured as shown in FIGS.  6 A 1  to  6 E 1  and  6 A 2  to  6 E 2 , following the manufacturing method for the protective element  20 B described above. 
     FIG. 7 is a circuit diagram of a protective device using yet a different protective element  20 D in accordance with the present invention. In this protective element  20 D a first PTC element  1 — 1  and a second PTC element  1 - 2  are provided on a heat-generating member  3 , and an intermediate electrode  5   d  is provided between these two PTC elements  1 — 1  and  1 - 2 . One end of the heat-generating member  3  is connected to the intermediate electrode  5   d , and the other end of the heat-generating member  3  is connected to a third PTC element  1 - 3 . Also this protective device has a Zehner diode and a transistor as a voltage detection means and a switching means, so that a collector current i c  flows abruptly at an overvoltage, so that the heat-generating member  3  generates heat, and the device to be protected can be protected from an overcurrent by an increase in the resistance of the PTC element  1 — 1  or the PTC element  1 - 2 . Consequently, if the terminals A 1  and A 2  are connected to the electrode terminals of a device to be protected, for example a lithium-ion battery, and the terminals B 1  and B 2  are connected to the electrode terminals of, for example, a charging device, the device to be protected, such as the lithium-ion battery, can be protected from overcharging. Even if there is an overvoltage after the PTC element  1 — 1  has failed and led to a short, the resistance of the second PTC element  1 - 2  rises due to the heat generation by the heat-generating member  3 , and currents through the device to be protected can be limited. Furthermore, although there is the possibility that current continues to flow through the heat-generating member  3  even when the current through the device to be protected is limited at an overvoltage, the resistance of the third PTC-element  1 - 3 , too, increases due to the heat generation of the heat-generating member  3 , so that the current can be limited. Consequently, with this protective element  20 D, the safety can be improved even better than with the aforesaid protective element  20 C. 
     The protective element  20 D can be manufactured as shown in FIGS.  8 A 1  to  8 D 1  and  8 A 2  to  8 D 2 , following the manufacturing method for the protective element  20 B described above. 
     The protective element of the present invention has at least one PTC element. As long as a heat-generating member, a low-melting-point metal member, as well as a second and third PTC element are formed taking the PTC material constituting this PTC element as a substrate, there is no limitation concerning the above examples, and various variations thereof are conceivable. For example, the shape of the PTC material, the number of electrodes formed thereon, the shape of the electrodes etc. can be selected as appropriate. 
     EXAMPLES 
     The following illustrates the present invention specifically, with reference to examples. 
     Example 1 
     As Example 1, the protective element of FIG. 1 was manufactured as follows. 
     A PTC material  1 ′ was prepared with the following steps. First, a high-density polyethylene (HDPE: HI-ZEX 5000H by Mitsui Chemicals Corp.), which is a crystalline polymer, an ethylene-ethyl acrylate copolymer (EEA: NVC6170 by Nihon Unica Co., Ltd.), and conductive particles (MSB-10A by Nippon Carbon Co., Ltd.), which are small globular carbon particles plated with silver, were mixed at a ratio of 44:22:34 by weight, and after kneading at 190° C. with a pressure kneader, the mixture was pressed into a film of 300 μm thickness with a hot press (at 190° C., 5 kg/cm 2 , 20 sec). The film was cut into pieces of 7 mm×4 mm size and taken as the PCT material  1 ′ of the protective element. 
     The electrodes  5   a ,  5   b , and  5   c  were formed using silver paste (by Fujikura Kasei Co., Ltd.). 
     After the components listed in Table 1 were pre-mixed, they were dispersed by three rolls to prepare an epoxy-based insulating paste, which was applied to form the insulating member  6 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Epoxy-based insulating paste 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Epoxy resin (YDF-170 by Toto 
                 100 parts by 
               
               
                   
                 Kasei Co., Ltd.) 
                 weight 
               
               
                   
                 Alumina powder (A-42-6 by Showa 
                 200 parts by 
               
               
                   
                 Denko K.K.) 
                 weight 
               
               
                   
                 Dicyandiamide (by ICI Japan, 
                 7.4 parts by 
               
               
                   
                 Ltd.) 
                 weight 
               
               
                   
                   
               
            
           
         
       
     
     The heat-generating member  3  was formed by applying carbon paste (FC-403R by Fujikura Kasei Co., Ltd.). 
     The size of the resulting protective element  20 A was 7×4×0.35 mm (thickness), and was thus extremely thin. 
     Then, the protective element  20 A was built into the circuit shown in FIG. 2, the electrode terminals of a lithium-ion battery were connected to the terminals A 1  and A 2 , a stabilized power source was connected to the terminals B 1  and B 2 , and the operation of the protective element  20 A was examined. When the stabilized power source was set to  5 V and  1 A to charge the lithium-ion battery, a current started to flow through the heat-generating member  3  when the lithium-ion battery was near 4.3 V, and as a consequence, the resistance of the PTC element  1  increased to about 90 Ù, and the current decreased to 0.05 A. Thus, it could be verified that the protective element sufficiently limits the current at an overvoltage. 
     In a protective element using a PTC element of the present invention, a PTC material is used for the substrate on which a heat-generating member, a low-melting-point metal member, a second PTC element etc. are mounted, so that the protective element can be manufactured easily, with fewer components, and at lower costs. Furthermore, the protective element can be made thinner. 
     The entire disclosure of the specification, claims and drawings of Japanese Patent Application No. 11-221802 filed on Aug. 4, 1999 is hereby incorporated by reference.