Patent Publication Number: US-9842676-B2

Title: Surge absorbing element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2014/063743, filed on May 23, 2014, the contents of all of which are incorporated herein by reference in their entirety. 
     FIELD 
     The present invention relates to a surge absorbing element that protects an electronic component and a circuit having the electronic component mounted thereon from a surge voltage. 
     BACKGROUND 
     A surge absorbing element has a function of causing a surge current to flow to protect a subsequent-stage circuit when a voltage equal to or higher than a predetermined value is applied. The surge absorbing element generally has a structure in which a pair of electrodes is attached to both ends of a varistor substrate made of ZnO or the like, respectively, external leads are drawn from the respective electrodes, and the varistor substrate and the electrodes are covered by an exterior member. 
     Due to a current flowing in the varistor substrate, an operation start voltage lowers. That is, a flow of a current deteriorates the function of the surge absorbing element and gradually brings the varistor substrate closer to a short-circuit state. Accordingly, when an excessive surge voltage is applied to the varistor substrate many times and the varistor substrate is further deteriorated, the excessive surge voltage finally causes a short-circuit failure. 
     For example, Patent Literature 1 describes a metal oxide varistor with bimetal, which has such a function that bimetal is incorporated in a metal oxide varistor (a surge absorbing element) for absorbing a surge voltage to be used to protect an electronic component. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Utility Model Laid-open Publication No. H1-86202 
     SUMMARY 
     Technical Problem 
     In the metal oxide varistor with bimetal described in Patent Literature 1, when a surge voltage equal to or higher than a rated value is applied to the varistor substrate including a metal oxide, the bimetal deforms due to heat generated by the varistor substrate and the surge absorbing element is brought to an open state to block a current flowing in the metal oxide varistor. When the current is blocked, the metal oxide varistor is then naturally cooled. Accordingly, the bimetal returns to its original shape and the surge absorbing element is back to the short-circuit state, so that the function of the surge absorbing element is recovered. 
     However, the metal oxide varistor with bimetal described in Patent Literature 1 does not prevent deterioration of the varistor substrate itself. Therefore, when the metal oxide varistor is naturally cooled, the bimetal returns to its original shape and the surge absorbing element is back to the short-circuit state. Accordingly, a surge voltage equal to or higher than the rated value may be applied to the metal oxide varistor (the surge absorbing element) to cause a current to flow through repeatedly and a short-circuit failure may occur, which leads to a temperature increase in the metal oxide varistor. 
     The present invention has an object of suppressing a current from flowing in a surge absorbing element of which a function of absorbing surge is deteriorated. 
     Solution to Problem 
     The present relates to a surge absorbing element including: a varistor substrate; a pair of electrodes that are electrically connected to both end faces of the varistor substrate, respectively, to sandwich the varistor substrate; external leads that electrically connect to the paired electrodes, respectively; exterior members that cover the electrodes; and a thermal expansion body that is provided between the paired electrodes and that irreversibly expands with heat generated by the varistor substrate to separate at least one of the paired electrodes from the varistor substrate. 
     Advantageous Effects of Invention 
     The present invention can suppress occurrence of a short-circuit failure in a state where a function of a surge absorbing element to absorb surge is deteriorated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view illustrating a surge absorbing element according to a first embodiment. 
         FIG. 2  is a sectional view illustrating an open state of the surge absorbing element according to the first embodiment. 
         FIG. 3  is a partial sectional view illustrating a surge absorbing element according to a second embodiment. 
         FIG. 4  is a partial sectional view illustrating an open state of the surge absorbing element according to the second embodiment. 
         FIG. 5  is a partial sectional view illustrating a surge absorbing element according to a third embodiment. 
         FIG. 6  is a partial sectional view illustrating an open state of the surge absorbing element according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for carrying out the present invention (embodiments) will be explained below in detail with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a sectional view illustrating a surge absorbing element according to a first embodiment.  FIG. 2  is a sectional view illustrating an open state of the surge absorbing element according to the first embodiment. 
     A surge absorbing element  10  has a function of causing a surge current to flow when a high voltage equal to or higher than a predetermined value is applied, that is, has a surge absorbing function. As illustrated in  FIGS. 1 and 2 , the surge absorbing element  10  according to the first embodiment includes a varistor substrate  11 , a pair of electrodes  12   a  and  12   b , external leads  13   a  and  13   b , exterior members  15   a  and  15   b , and a thermal expansion body  14 . 
     The varistor substrate  11  includes, for example, a metal oxide such as ZnO or SrTiO 3 . However, a material that can be used for the varistor substrate  11  is not limited to the metal oxides described above. The varistor substrate  11  has a pair of end faces  11 Ta and  11 Tb and a side part  11 S. The paired end faces  11 Ta and  11 Tb face each other. The side part  11 S connects the paired end faces  11 Ta and  11 Tb to each other. 
     The paired electrodes  12   a  and  12   b  electrically connect to the both end faces  11 Ta and  11 Tb of the varistor substrate  11 , respectively. Specifically, the electrode  12   a  is electrically connected to the end face  11 Ta of the varistor substrate  11  and the electrode  12   b  is electrically connected to the end face  11 Tb of the varistor substrate  11 . With this structure, the paired electrodes  12   a  and  12   b  hold the varistor substrate  11  to be sandwiched thereby and are not electrically connected to each other. 
     The external leads  13   a  and  13   b  electrically connect to the paired electrodes  12   a  and  12   b , respectively. The exterior members  15   a  and  15   b  cover the paired electrodes  12   a  and  12   b.    
     The varistor substrate  11  and the electrode  12   b  are bonded, for example, with a conductive adhesive to be electrically connected to each other. The varistor substrate  11  and the electrode  12   a  are separably and electrically connected to each other, for example, with a conductive paste. In the first embodiment, it suffices that at least one set of either the varistor substrate  11  and the electrode  12   b  or the varistor substrate  11  and the electrode  12   a  is separably and electrically connected to each other. Therefore, both the set of the varistor substrate  11  and the electrode  12   b  and the set of the varistor substrate  11  and the electrode  12   a  may be electrically connected to each other, for example, with a conductive paste. 
     The thermal expansion body  14  is provided on the side part  11 S of the varistor substrate  11  to be located between the paired electrodes  12   a  and  12   b  and be sandwiched by the paired electrodes  12   a  and  12   b . The thermal expansion body  14  irreversibly expands with heat generated by the varistor substrate  11  and separates at least one of the paired electrodes  12   a  and  12   b  from the varistor substrate  11 . In the first embodiment, because the electrode  12   b  is bonded to the varistor substrate  11  and the electrode  12   a  is connected to the varistor substrate  11  with the conductive paste or the like, the electrode  12   a  is separated from the varistor substrate  11  due to expansion of the thermal expansion body  14 . As described above, the electrode  12   b  may be separated from the varistor substrate  11  or the electrodes  12   a  and  12   b  both may be separated from the varistor substrate  11 . 
     For example, when the varistor substrate  11  is deteriorated and the operation start voltage lowers, resulting in a short-circuit failure state, a large current consequently flows in the varistor substrate  11  and accordingly the varistor substrate  11  generates heat. The heat generated in this way transmits to the thermal expansion body  14 , so that the thermal expansion body  14  irreversibly expands (thermally expands) to separate the electrode  12   a  from the varistor substrate  11 . 
     The thermal expansion body  14  is placed so as to be wound around the side part  11 S of the varistor substrate  11 . The thermal expansion body  14  is bonded to the electrodes  12   a  and  12   b , for example, with an insulating adhesive. The exterior members  15   a  and  15   b  are, for example, resin and covers the electrodes  12   a  and  12   b  and a part of the thermal expansion body  14 . In this manner, in the first embodiment, the exterior members  15   a  and  15   b  cover a part of the thermal expansion body  14  and do not entirely cover the thermal expansion body  14 . Therefore, a part of the thermal expansion body  14  not covered by the exterior members  15   a  and  15   b  can be visually recognized from outside of the surge absorbing element  10 . Although the thermal expansion body  14  expands with heat in a manner described below, prohibition of the expansion of the thermal expansion body  14  is suppressed because the external members  15   a  and  15   b  do not entirely cover the thermal expansion body  14 . 
     The thermal expansion body  14  is, for example, resin irreversibly expandable with heat. As the resin irreversibly expandable with heat, AF-3024 manufactured by Sumitomo 3M Limited is used, for example. When the thermal expansion body  14  made of resin irreversibly expandable with heat has reached a predetermined temperature, a plurality of gas cavities are formed therein to be in a foamed state and the thermal expansion body  14  expands to increase the outside dimension. Once having the gas cavities formed therein, the thermal expansion body  14  does not decrease in the volume even after cooled. The thermal expansion body  14  is irreversibly expanded in this way. That is, once the thermal expansion body  14  is expanded, it keeps the expanded state. 
     When the thermal expansion body  14  is irreversibly expanded to increase the outside dimension, the distance between the paired electrodes  12   a  and  12   b  increases. As a result, the thermal expansion body  14  separates the electrode  12   a  from the varistor substrate  11  and forms an insulating gap  16  between the varistor substrate  11  and the electrode  12   a  as illustrated in  FIG. 2 . When the electrode  12   a  is separated from the varistor substrate  11 , the surge absorbing element  10  is brought to an open state and thus no current flows in the varistor substrate  11  even when a voltage is applied to the paired electrodes  12   a  and  12   b.    
     When an excessive surge voltage is applied to the varistor substrate  11  many times and an excessive current flows therein many times, the varistor substrate  11  deteriorates to lower the operation start voltage and approaches the short-circuit failure state. That is, the surge absorbing function of the surge absorbing element  10  deteriorates. When the varistor substrate  11  approaches the short-circuit failure state, the operation start voltage lowers. Therefore, in such a case that the surge absorbing element  10  is connected between phases of power supply lines, a current flows in the varistor substrate  11  and heat is generated, resulting in a temperature increase. As a result, the temperature of the surge absorbing element  10 , more specifically, of the exterior members  15   a  and  15   b  increases. 
     The thermal expansion body  14  irreversibly expands with heat generated by the varistor substrate  11  due to a current flowing in the deteriorated varistor substrate  11 . Accordingly, once the thermal expansion body  14  is expanded, the surge absorbing element  10  keeps the state in which the insulating gap  16  is formed between the varistor substrate  11  and the electrode  12   a  as illustrated in  FIG. 2 . Therefore, once the thermal expansion body  14  is expanded, the surge absorbing element  10  keeps the open state. In the surge absorbing element  10 , because no current flows in the varistor substrate  11  after the thermal expansion body  14  is expanded, occurrence of a short-circuit failure of power supply lines, a circuit, or devices to which the surge absorbing element  10  is attached can be suppressed in a state where the surge absorbing function is lowered. Furthermore, in the surge absorbing element  10 , a temperature increase in the varistor substrate  11  and the exterior members  15   a  and  15   b  in the state where the surge absorbing function is lowered is suppressed. 
     A temperature at which the thermal expansion body  14  starts irreversible expansion is referred to as an “expansion start temperature”. The thermal expansion body  14  irreversibly expands when reaching a temperature equal to or higher than the expansion start temperature (180° C., for example). The expansion start temperature depends on specifications of resin that is irreversibly expandable with heat and thus is not limited to 180° C. described above. For example, the expansion start temperature is preferably equal to or lower than a heat-resisting temperature of the exterior members  15   a  and  15   b  and is preferably about 5° C. to 10° C. lower than the heat-resisting temperature of the exterior members  15   a  and  15   b . By changing at least one of the specifications of the expandable resin used for the thermal expansion body  14  and specifications of the exterior members  15   a  and  15   b , the expansion start temperature can be set to be equal to or lower than the heat-resisting temperature of the exterior members  15   a  and  15   b.    
     When the surge absorbing function of the surge absorbing element  10  is deteriorated, the thermal expansion body  14  irreversibly expands and the open state on a safe side is kept. As a result, a flow of a current in the surge absorbing element  10  having the deteriorated surge absorbing function is prevented, so that occurrence of a short-circuit failure in the circuit or devices to which the surge absorbing element  10  is attached can be suppressed. It is also possible to suppress a current from continuously flowing in the varistor substrate  11  of the surge absorbing element  10  in a state where the surge absorbing element is deteriorated. As a result, a temperature increase in the surge absorbing element  10  is suppressed and thus the safety is improved. Furthermore, because the thermal expansion body  14  irreversibly expands at a temperature equal to or lower than the heat-resisting temperature of the exterior members  15   a  and  15   b , the exterior members  15   a  and  15   b  can be used at a temperature equal to or lower than the heat-resisting temperature. 
     While resin that irreversibly expands with heat is used as the thermal expansion body  14  in the first embodiment, the thermal expansion body  14  is not limited to resin and any material other than resin can be used as long as it irreversibly expands with heat. For example, the thermal expansion body  14  may be shape-memory alloy that deforms so as to increase the distance between the paired electrodes  12   a  and  12   b  when reaching a temperature equal to or higher than the expansion start temperature. Alternatively, the thermal expansion body  14  may be a structure in which a vaporizing material or a material having a large thermal expansion coefficient is enclosed in a container made of a plastic deformable material. 
     Second Embodiment 
       FIG. 3  is a partial sectional view illustrating a surge absorbing element according to a second embodiment.  FIG. 4  is a partial sectional view illustrating an open state of the surge absorbing element according to the second embodiment. 
     As illustrated in  FIGS. 3 and 4 , a surge absorbing element  20  includes a varistor substrate  21 , a pair of electrodes  22   a  and  22   b , external leads  23   a  and  23   b , and exterior members  25   a  and  25   b . The varistor substrate  21  has a shape and functions identical to those of the varistor substrate  11  included in the surge absorbing element  10  according to the first embodiment. 
     The surge absorbing element  20  is different from the surge absorbing element  10  according to the first embodiment in the shape and functions of a thermal expansion body  24 . The thermal expansion body  24  is a columnar member and has a bent part  24 B between the paired electrodes  22   a  and  22   b . The bend part  24 B is sigmoidally bent. The bend part  24 B has a mark  24   a  inside a bent portion that is not viewed from outside of the surge absorbing element  20 . The mark  24   a  indicates that the surge absorbing element  20  has been brought to an open state as a result of deterioration of the varistor substrate  21  included in the surge absorbing element  20 . 
     In the second embodiment, the surge absorbing element  20  includes a plurality of the thermal expansion bodies  24 . The thermal expansion bodies  24  are sandwiched between the paired electrodes  22   a  and  22   b  and are placed outside a side part  21 S of the varistor substrate  21 . When the surge absorbing element  20  is viewed in a direction orthogonal to end faces  21 Ta and  21 Tb of the varistor substrate  21 , the thermal expansion bodies  24  are preferably placed at substantially equal intervals, respectively, along a direction in which the side part  21 S of the varistor substrate  21  extends. This placement enables the distance between the paired electrodes  22   a  and  22   b  to be uniformly increased when the thermal expansion bodies  24  irreversibly expand. As a result, the electrode  22   a  or  22   b  is reliably separated from the varistor substrate  21 . 
     While the number of the thermal expansion bodies  24  is not limited, it is preferable that the surge absorbing element  20  include at least three thermal expansion bodies  24 . This suppresses the electrode  22   a  or  22   b  from being inclined when the thermal expansion bodies  24  irreversibly expand. Accordingly, the electrode  22   a  or  22   b  is reliably separated from the varistor substrate  21  and the surge absorbing element  20  is reliably brought to the open state. 
     When the varistor substrate  21  is more deteriorated, the operation start voltage lowers and the surge absorbing element  20  approaches the short-circuit failure state. When a current flows in the varistor substrate  21  in this state and the temperature of the thermal expansion bodies  24  becomes equal to or higher than the expansion start temperature, the thermal expansion bodies  24  irreversibly expand and the bent parts  21 B become unbent. Due to irreversible expansion of the thermal expansion bodies  24 , the electrode  22   a  is separated from the varistor substrate  21  and an insulating gap  26  is formed between the varistor substrate  21  and the electrode  22   a.    
     When the bent parts  24 B of the thermal expansion bodies  24  become unbent, the marks  24   a  provided inside the bent portions become viewable from outside of the thermal expansion bodies  24 . Therefore, the surge absorbing element  20  can inform a user of the open state. The material and the expansion start temperature of the thermal expansion bodies  24  are identical to those of the thermal expansion body  14  described in the first embodiment. 
     In this manner, the surge absorbing element  20  provides actions and effects identical to those of the surge absorbing element  10  according to the first embodiment. Furthermore, the surge absorbing element  20  can inform the user of the open state and can prompt the user to replace the surge absorbing element  20 . Replacement with a new surge absorbing element  20  enables reliable protection of a subsequent-stage circuit from the surge voltage. 
     Third Embodiment 
       FIG. 5  is a partial sectional view illustrating a surge absorbing element according to a third embodiment.  FIG. 6  is a partial sectional view illustrating an open state of the surge absorbing element according to the third embodiment. 
     As illustrated in  FIGS. 5 and 6 , a surge absorbing element  30  includes a varistor substrate  31 , a pair of electrodes  32   a  and  32   b , external leads  33   a  and  33   b , exterior members  35   a  and  35   b , and a thermal expansion body  34 . The varistor substrate  31  included in the surge absorbing element  30  has a shape and functions identical to those of the surge absorbing element  10  according to the first embodiment. 
     The surge absorbing element  30  is different from the surge absorbing element  10  according to the first embodiment in that covers  34   a  and  34   b  that cover the thermal expansion body  34  are attached to the paired electrodes  32   a  and  32   b  or the exterior members  35   a  and  35   b , respectively. 
     The covers  34   a  and  34   b  are provided on surfaces of the paired electrodes  32   a  and  32   b  that face each other, respectively. The cover  34   a  is attached to the electrode  32   a  and the cover  34   b  is attached to the electrode  32   b . For example, the covers  34   a  and  34   b  may be formed by folding the corresponding electrodes  32   a  and  32   b  to be integral with the electrodes  32   a  and  32   b , respectively, or may be attached to the corresponding electrodes  32   a  and  32   b  as separate members from the electrodes  32   a  and  32   b , respectively. Alternatively, the covers  34   a  and  34   b  may be attached to the exterior members  35   a  and  35   b , respectively. 
     The covers  34   a  and  34   b  are provided outside the thermal expansion body  34  that is sandwiched between the paired electrodes  32   a  and  32   b . As illustrated in  FIG. 5 , the covers  34   a  and  34   b  overlap with each other at end parts on the opposite side from parts that are attached to the electrodes  32   a  and  32   b . With this structure, the covers  34   a  and  34   b  cover the thermal expansion body  34 . The covers  34   a  and  34   b  are configured to be spaced at the end parts on the opposite side from the parts that are attached to the electrodes  32   a  and  32   b  when the thermal expansion body  34  irreversibly expands and the distance between the paired electrodes  32   a  and  32   b  is increased. 
     When the varistor substrate  31  is more deteriorated, the operation start voltage lowers and the surge absorbing element  30  approaches the short-circuit failure state. When a current flows in the varistor substrate  31  in this state and the temperature of the thermal expansion body  34  becomes equal to or higher than the expansion start temperature, the thermal expansion body  34  irreversibly expands. Irreversible expansion of the thermal expansion body  34  separates the electrode  32   a  from the varistor substrate  31  and forms an insulating gap  36  between the varistor substrate  31  and the electrode  32   a.    
     When the thermal expansion body  34  expands, the covers  34   a  and  34   b  are spaced, so that the thermal expansion body  34  can be viewed from outside. Therefore, the surge absorbing element  30  can inform a user of the open state. The material and the expansion start temperature of the thermal expansion body  34  are identical to those of the thermal expansion body  14  described in the first embodiment. 
     In this manner, the surge absorbing element  30  provides actions and effects identical to those of the surge absorbing element  10  according to the first embodiment. Furthermore, the surge absorbing element  30  can inform the user that the surge absorbing element  30  has been brought to the open state and can prompt the user to replace the surge absorbing element  30 . Replacement with a new surge absorbing element  30  enables a subsequent-stage circuit to be reliably protected from the surge voltage. 
     It is preferable that the thermal expansion body  34  on the side of the covers  34   a  and  34   b  have a different color from that of at least either the covers  34   a  and  34   b  or the exterior members  35   a  and  35   b . This enables a user to easily visually recognize the thermal expansion body  34  when the covers  34   a  and  34   b  are spaced because the thermal expansion body  34  has a different color from that of at least either the covers  34   a  and  34   b  or the exterior members  35   a  and  35   b . As a result, the surge absorbing element  30  can reliably inform the user that the surge absorbing element  30  has been brought into the open state. 
     As a method of informing the user that the surge absorbing element  10  according to the first embodiment has been brought into the open state, for example, paint that changes color when reaching a temperature equal to or higher than the expansion start temperature is coated on an outer surface of the thermal expansion body  14  or a material that changes color when reaching a temperature equal to or higher than the expansion start temperature is used for the thermal expansion body  14 . 
     Alternatively, a user may be informed that the surge absorbing element  10  has been brought into the open state by provision of a sensor that detects expansion of the thermal expansion body  14  according to the first embodiment with heat, and an alarm unit that issues an alarm based on an output from the sensor upon detection of expansion of the thermal expansion body  14  with heat, for example, on a circuit at a subsequent stage of the surge absorbing element  10 . The sensor that detects expansion of the thermal expansion body  14  is, for example, a sensor detecting the length of the thermal expansion body  14  or a temperature sensor detecting that the temperature of the thermal expansion body  14  has reached a temperature equal to or higher than the expansion start temperature. The alarm unit may be, for example, an alarm unit that emits at least one of light and sound when the sensor has detected expansion of the thermal expansion body  14 . 
     While the first to third embodiments have been described above, the first to third embodiments are not limited to the contents described above. Furthermore, the constituent elements described above include those that can be easily anticipated by persons skilled in the art, that are substantially identical, or that are in the range of so-called equivalents. Further, the constituent elements described above can be combined with each other as appropriate. In addition, at least any one of various types of omission, replacement, and modification of the constituent elements can be made without departing from the scope of the first to third embodiments. 
     REFERENCE SIGNS LIST 
       10 ,  20 ,  30  surge absorbing element,  11 ,  21 ,  31  varistor substrate,  12   a ,  12   b ,  22   a ,  22   b ,  32   a ,  32   b  electrode,  13   a ,  13   b ,  23   a ,  23   b ,  33   a ,  33   b  external lead,  14 ,  24 ,  34  thermal expansion body,  24   a  failure indication mark,  34   a ,  34   b  cover,  15   a ,  15   b ,  25   a ,  25   b ,  35   a ,  35   b  exterior member.