Abstract:
A protective element includes at least three electrodes provided on one PTC material. The PTC material operates as at least two PTC elements. This protective element can be manufactured, with fewer components, in an easy manner, and at lower cost, and can cope with overcurrent and overvoltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a protective element and a protective circuit 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, which 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 devices, which are heated by overcurrents, whereby their resistance is increased, and the current flowing through the circuit to be protected is curbed. 
     Furthermore, protective devices have been proposed, in which a protective element is combined with a voltage sensing means, and the circuit to be protected is protected not only from overcurrent but also from overvoltages. 
     FIG. 6 is a circuit diagram of such a protective device, and FIG. 7A is a plane view and FIG. 7B is a cross-sectional view of the protective element  20 X used in this circuit (see Japanese Patent Application Laid-Open No.8-236305). In the circuit in FIG. 6, the terminals A 1  and A 2  are connected to the electrode terminals of the 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. 
     In this circuit, 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, a heat-generating member  3  is connected between their connection point (electrode  5   d ) and a third terminal c, and the low-melting-point metal member  2  and the heat-generating member  3  are arranged in proximity to each other. Thus, the PTC element  1 , the low-melting-point metal member  2  and the heat-generating member  3  constitute one protective element  20 X. 
     In the protective element  20 X, electrodes  5   a,    5   b,    5   c  and  5   d  are formed on a substrate  4 , as shown in FIGS. 7A and 7B, the heat-generating member  3  is formed between the electrode  5   c  and the electrode  5   d,  the heat-generating member  3  is covered with an insulating layer  6 , the PTC element  1  is formed on the electrode  5   a,  the low-melting-point metal member  2  is formed so as to bridge the PTC element  1 , the electrode  5   b  and the electrode  5   d  on the substrate  4 , and these are covered by a protective cap  7 . 
     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, when current is conducted to the heat-generating member  3  under regular operation, the PTC element  1  and the low-melting-point metal member  2  form a conduction line between the electrode 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, 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 progression of an overcurrent in the device to be protected, such as a lithium-ion battery, which is connected to the terminals A 1  and A 2  can be prevented. 
     On the other hand, when an overcurrent exceeding a certain value flows through the terminals A 1  and B 1 , first, the PTC element  1  curbs the current, then the low-melting-point metal member  2  melts off and interrupts the current. 
     FIG. 8 is a circuit diagram of still another protective device, and FIG. 9A is a front view and FIG. 9B is a side view of the protective element  20 Y used in this circuit (see Japanese Patent Application Laid-open No. 10-98829). In this circuit, a first PTC element  1   a  and a second PTC element  1   b  are arranged on opposite sides of a stainless-steel plate  8  of the protective element  20 Y. The first and the second PTC element  1   a  and  1   b  are elastically clamped by springs  9   a  and  9   b,  and assembled into a polymer protective casing  7 . Moreover, as in the circuit in FIG. 6 above, a Zehner diode and a transistor are provided as a voltage sensing means and a switching means. 
     Consequently, with the protective circuit of FIG. 8, only the first PTC element  1   a  is conducted during regular operation, and during an overvoltage, current flows abruptly through the second PTC element  1   b  to heat the second PTC element, this heat is quickly conducted to the first PTC element, and the first PTC element trips, whereby the current into the circuit to be protected is curbed. 
     However, in a protective element using the above-described PTC element, to operate a PTC element  1  or  1   a  (FIGS. 6 and 8) that curbs the current in a conductive line in case of abnormal stances, a heat-generating member  3  (FIG. 6) or a second PTC element  1   b  (FIG. 8) have to be used separately. 
     Furthermore, to configure the protective elements  20 X and  20 Y using the PTC elements  1 ,  1   a,  and  1   b  or the heat-generating member  3 , a substrate  4  or a casing  7  becomes necessary. 
     Therefore, there are the problems that the number of components for conventional protective elements is large and miniaturization is difficult, and manufacturing costs are high. 
     SUMMARY OF THE INVENTION 
     With regard to these problems of the prior art, it is an object of the present invention to provide a protective element that can be manufactured with fewer components, in an easy manner, and at lower cost, and can cope with overcurrent and overvoltage. 
     The inventors have discovered that when one PTC material is provided with three or more electrodes, the PTC material that is sandwiched by two of these operates as one separate PTC element, and thus the inventors perfected the protective element of the present invention. Moreover, they have discovered that when a circuit is configured such that two PTC elements are connected in parallel with a conduction line to a circuit to be protected, current flows through both PTC elements during regular operation, and current flows only through one of them during abnormality, then even if the current during the abnormality is comparatively smaller than during regular operation, it still trips the PTC element, and thus the protective circuit of the present invention was perfected. 
     In other words, the present invention provides a protective element which comprises one PTC material and at least three electrodes provided thereon, and the PTC material operates as at least two PTC elements. 
     Moreover, the present invention provides a protective circuit, comprising a first PTC element, a switching element and a second PTC element, wherein the first PTC element is connected in parallel with the switching element and the second PTC element connected in series; the switching element enables the conduction of current to the second PTC element under regular operation and interrupts the conduction of current to the second PTC element when a terminal voltage of the circuit to be protected is higher than a predetermined voltage; and the first PTC element trips due to the conducted current. Other objects and features of the present invention will be described or become apparent from the following disclosure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plane view and FIG. 1B is a cross-sectional view of a protective element of the present invention. 
     FIG. 2 is a circuit diagram showing a protective circuit of the present invention 
     FIG. 3A is a plane view and FIG. 3B is a cross-sectional view of a protective element of the present invention. 
     FIG. 4A is a plane view and FIG. 4B is a cross-sectional view of a protective element of the present invention. 
     FIG. 5A is a plane view and FIG. 5B is a cross-sectional view of a protective element of the present invention. 
     FIG. 6 is a circuit diagram of a conventional protective device. 
     FIG. 7A is a plane view and FIG. 7B is a cross-sectional view of a conventional protective element. 
     FIG. 8 is a circuit diagram of a conventional protective device. 
     FIG. 9A is a front view and FIG. 9B is a side view of a conventional protective element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following is a detailed description of the present invention with reference to the drawings. It should be noted that in the drawings, equal numerals denote equal or equivalent structural elements. 
     FIG. 1A is a plane view and FIG. 1B is a cross-sectional view of a protective element  20 A of the present invention. 
     In this protective element  20 A, a first electrode  11   a,  a second electrode  11   b,  and a third electrode  11   c  are provided in stripes on the surface of one side of a rectangular PTC material  1 ′. There is no particular limitation to the PTC material itself that can be used for the PTC material  1 ′, and it is possible to use 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 Japanese Patent Application Laid-Open No. 10-261505). 
     Moreover, there is not particular limitation to the materials and methods for forming the electrodes  11   a,    11   b,  and  11   c,  and it is possible to form the electrodes  11   a,    11   b,  and  11   c  by sputtering or the like. 
     This protective element  20 A is made of one PTC material  1 ′ as mentioned above, and although it can be manufactured easily, the first electrode  11   a,  the second electrode  11   b  and the PTC material  1 ′ between them can be operated as a first PTC element  1   a,  and the second electrode  11   b,  the third electrode  11   c  and the PTC material  1 ′ between them can be operated as a second PTC element  1   b.    
     Moreover, the electrodes  11   a,    11   b  and  11   c  are provided on the surface on one side of the PTC material  1 ′, so this protective element  20 A is suitable for surface-mounting. 
     FIG. 2 is a circuit diagram showing a protective circuit of the present invention, using the above-described protective element  20 A. In this protective circuit, 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. 
     Moreover, in this protective circuit, an FET is used as the switching element, source terminal S/drain terminal D of the FET and the second PTC element  1   b  are connected in series, and these are connected in parallel to the first PTC element  1   a.    
     On the other hand, the gate terminal G of the FET is connected to an IC for detecting the terminal voltage of the circuit to be protected, and this IC controls the potential of the gate terminal G of the FET in accordance with the value of the detected terminal voltage of the circuit to be protected. In other words, if the IC does not sense any abnormality, that is, when the terminal voltage of the circuit to be protected is lower than a predetermined voltage, the IC turns the FET on, and the FET allows current to flow to the second PTC element  1   b.  Consequently, at this time, current flows through both the first PTC element  1   a  and the second PTC element  1   b,  in accordance with their resistance. When current flows through both PTC elements  1   a  and  1   b  in this manner and an overcurrent occurs, then the current is curbed due to tripping of the PTC elements  1   a  and  1   b.    
     On the other hand, when the IC senses abnormality, that is, when it senses an overvoltage where the terminal voltage of the circuit to be protected exceeds a predetermined voltage, the IC turns the FET off, and the FET interrupts the current to the second PTC element  1   b.  As a result, when abnormality occurs, current flows only through the first PTC element  1   a,  and the first PTC element  1   a  trips quickly because of overcurrent. 
     With the protective circuit of the present invention, a device to be protected can be protected from overcurrent and overvoltage with an extremely simple configuration. 
     In the protective circuit of the present invention, the resistance for regular operation of the first PTC element  1   a  and the second PTC element  1   b  or the current value to cause tripping occurs can be chosen as appropriate, but it is preferable if the resistance of the first PTC element  1   a  is higher than the second PTC element  1   b.  In this case, the second PTC element  1   b  trips at large currents, and the first PTC element  1   a  trips also at relatively small currents, so that the first PTC element  1   a  can be caused to trip extremely quickly when the IC has detected an overvoltage. 
     It is also possible to realize the protective circuit in FIG. 2 by mounting two conventional PTC elements on a substrate, as long as the first PTC element  1   a  and the second PTC element  1   b  are connected in parallel as shown in the drawing, and it is not always necessary to use the protective elements  20 A of the present invention. 
     FIG. 3A is a plane view and FIG. 3B is a cross-sectional view of a protective element  20 B in another embodiment of the present invention. In this protective element  20 B, the second electrode  11   b,  which was provided substantially in the middle of the PTC material  1 ′ in the protective element  20 A in FIGS. 1A and 1B, is provided on the opposite side of the third electrode  11   c.  With this protective element  20 B the first electrode  11   a,  the third electrode  11   c  and the PTC material  1 ′ between them can be operated as a first PTC element  1   a,  and the second electrode  11   b,  the third electrode  11   c  and the PTC material  1 ′ between them can be operated as a second PTC element  1   b.    
     FIG. 4A is a plane view and FIG. 4B is a cross-sectional view of a protective element  20 C in still another embodiment of the present invention. This protective element  20 C is further provided with a fourth electrode lid on the surface of the PTC material  1 ′ on the side opposite the surface on which the electrodes are formed in the protected element  20 A in FIGS. 1A and 1B. With this protective element  20 C, the resistance between the electrode  11   b  and the electrode  11   c  can be made lower than in the protective element  20 A in FIGS. 1A and 1B. 
     FIG. 5A is a plane view and FIG. 5B is a cross-sectional view of a protective element  20 D in still another embodiment of the present invention. With this protective element  20 D, the resistance between the electrode  11   b  and the electrode  11   c  can be made lower than in the protective element  20 C in FIGS. 4A and 4B. 
     The protective element and the protective circuit of the present invention are not limited to the above-noted examples, and various embodiments are possible. The shape of the PTC material  1 ′, the number of electrodes formed thereon, the shape of the electrodes, etc. can all be chosen as appropriate. For example, in the protective elements in the FIGS. 3A,  3 B,  4 A,  4 B,  5 A and  5 B, at least two electrodes are provided on one surface of the PTC material, and a further electrode is provided on the other side of the PTC material, but for this case, how many electrodes are provided on which side of the PTC material can be chosen as appropriate. 
     Moreover, in the protective circuit of the present invention, it is also possible to use a bipolar transistor or a relay or the like instead of an FET for the switching element. 
     EXAMPLES 
     The present invention will be explained in more detail by referring to the following example. 
     Example 1 
     A protective element with the configuration shown in FIGS. 4A and 4B was made. For this example, a polymer PTC was used for the PTC material  1 ′, and its size was 9 mm×3 mm×0.3 mm (thickness). The electrodes were formed by sputtering 3 mm×1.5 mm. 
     In the resulting PTC element  20 C, the resistance between the first electrode  11   a  and the second electrode  11   b  (first PTC element  1   a ) was 300 mΩ and the resistance between the second electrode  11   b  and the third electrode  11   c  (second PTC element  1   a ) was 30 mΩ. 
     When current flows in parallel through the PTC element  1   a  and the PTC element  1   b,  the PTC element  1   a  and the PTC element  1   b  trip simultaneously at 5 A(ampere). On the other hand, when current flows only through the PTC element  1   a,  the PTC element  1   a  trips at 0.8 A(ampere). 
     With the present invention, a protective element having the functions of at least two PTC elements can be manufactured of one PTC material, easily and at low cost. Moreover, using the same, a circuit to be protected can be protected from overcurrent and overvoltage. 
     The entire disclosure of the specification, claims, drawings and summary of Japanese Patent Application No. 11-215979 filed Jul. 29, 1999 is hereby incorporated by the reference.