Patent Publication Number: US-2023146486-A1

Title: Protection element and battery pack

Description:
TECHNICAL FIELD 
     This technology relates to a protection element that protects a circuit connected on a current path by fusing the current path and to a battery pack that uses the protection element. This application claims priority based on Japanese Patent Application No. 2020-74410 filed on Apr. 17, 2020 in Japan, and this application is hereby incorporated by reference. 
     BACKGROUND TECHNOLOGY 
     Most secondary batteries that can be charged and used repeatedly are formed into battery packs and provided to users. In particular, lithium ion secondary batteries with high energy density by weight generally have several protection circuits for overcharge protection, over-discharge protection, and the like, integrated into the battery pack with a function of cutting off battery pack output in particular cases to ensure the safety of the user and the electronic device. 
     A lot of electronic devices that use lithium ion secondary batteries perform battery pack overcharge protection or over-discharge protection operation by turning an FET switch built-in to the battery pack ON/OFF. However, even in the case where the FET switch is destroyed by a short circuit due to some cause, a lightning surge is applied and an instantaneous overcurrent flows, or output voltage is abnormally low or on the other hand excessive overvoltage is output as the battery cell reaches end of life, the battery pack or the electronic device is required to be protected from ignition or other accidents. Here, a protection element containing a fuse element having a function of cutting off a current path based on an external signal is used to safely cut off battery cell output even in such various abnormal circumstances that can be considered. 
     As such a protection element of a protective circuit for lithium ion secondary batteries and the like, a structure with a heating element in the protection element is used to fuse a fusible conductor in the electric current path by heat generated by the heating element. 
     Applications for lithium ion secondary batteries have become widespread in recent years and applications with large currents such as power tools including an electric screwdriver, hybrid vehicles, electric vehicles, transport equipment such as power-assisted bicycles, storage batteries, and the like are being investigated and some of which have started to be used. In these types of applications, a large current, ranging from several tens of amperes to over one hundred amperes, may flow during startup. To realize a protection element that can handle such a large current capacity is desired. 
     A protection element in which a fusible conductor with increased cross-sectional area is used and this fusible conductor is connected on the surface of an insulating substrate with a heating element formed thereon, has been proposed to implement a protection element that can handle such a large current. 
       FIG.  27    is a diagram illustrating a configuration example of a conventional protection element where (A) is a plan view with the cover member omitted, (B) is a cross-sectional view, and (C) is a bottom view. A protection element  100  illustrated in  FIG.  27    includes an insulating substrate  101 , first and second electrodes  102  and  103  formed on the surface of the insulating substrate  101 , a heating element  104  formed on the surface of the insulating substrate  101 , an insulating layer  105  that covers the heating element  104 , an intermediate electrode  106  laminated on the insulating layer  105  and connected to the heating element  104 , and a fuse element  107  mounted across the first electrode  102 , the intermediate electrode  106 , and the second electrode  103  via connecting solder. 
     The first and second electrodes  102  and  103  are terminals connected on the current path of the external circuit connected by the protection element  100 . The first electrode  102  is connected to a first external connecting electrode  102   a  formed on the back surface of the insulating substrate  101  via a castellation and the second electrode  103  is connected to a second external connecting electrode  103   a  formed on the back surface of the insulating substrate  101  via a castellation. With regards to the protection element  100 , by connecting the first and second external connecting electrodes  102   a  and  103   a  to connecting electrodes provided on the external circuit board that the protection element  100  is mounted on, the fuse element  107  is integrated as a part of the current path formed on the external circuit board. 
     The heating element  104  is a conductive member that has a relatively high resistance value and generates heat when energized, and is composed of a material such as nichrome, W, Mo, Ru, or the like or a material containing these materials. In addition, the heating element  104  is connected to the heating element power supply electrode  108  formed on the surface of the insulating substrate  101 . The heating element power supply electrode  108  is connected to a third external connecting electrode  108   a  formed on the back surface of the insulating substrate  101  via castellation. In the protection element  100 , the third external connecting electrode  108   a  is connected to a connection electrode provided on an external circuit board on which the protection element  100  is mounted, thereby the heating element  104  is connected to an external power source provided in an external circuit. Furthermore, the current and heat generation of the heating element  104  are continuously controlled by a switch element (not shown) or the like. 
     The heating element  104  is covered with an insulating layer  105  composed of a glass layer or the like and overlaps with the intermediate electrode  106  with the insulating layer  105  interposed therebetween by forming the intermediate electrode  106  on the insulating layer  105 . In addition, a fuse element  107  which is connected over the first and second electrodes  102  and  103 , is connected on the intermediate electrode  106 . 
     Thus, in the protection element  100 , the heating element  104  and the fuse element  107  are overlapped, thereby thermally connected, enabling to blow the fuse element  107  by heat generation through energizing of the heating element  104 . 
     The fuse element  107  is formed using a low melting point metal such as Pb free solder or a high melting point metal such as Ag, Cu or an alloy having one of these as a main component or has a multilayer structure of a low melting point metal and a high melting point metal. Furthermore, by connecting from the first electrode  102  across the intermediate electrode  106  and to the second electrode  103 , the fuse element  107  constitutes a part of the external circuit current path that the protection element  100  is incorporated into. Furthermore, the fuse element  107  blows from self-heating (Joule heat) when a current exceeding the rated value thereof is applied or is blown by heat generation of the heating element  104 , and cuts off the first and second electrodes  102 ,  103 . 
     Furthermore, when the external circuit current path needs to be cut off, the protection element  100  energizes the heating element  104  using a switch element. Thus, in the protection element  100 , the heating element  104  generates heat causing a high temperature and blows the fuse element  107  incorporated in the external circuit current path. The fuse conductor of the fuse element  107  is led to the intermediate electrode  106  with high wettability and to the first and second electrodes  102 ,  103  and blows the fuse element  107 . Therefore, the protection element  100  causes blowing of the first electrode  102  to the intermediate electrode  106  to the second electrode  103  enabling cutting off the electric current path of the external circuit. 
     Note that for the protection element, in addition to the configuration illustrated in  FIG.  27   , a configuration having two heating elements  104  as illustrated in  FIG.  28    has also been proposed. A protection element  110  illustrated in  FIG.  28    has two heating elements  104  provided in parallel between the first and second electrodes  102 ,  103  on the surface of the insulating substrate  101 . Each heating element  104  is covered by the insulating layer  105  and the intermediate electrode  106  provided on the insulating layer  105  is formed superimposed across both heating elements  104 . 
     In addition, with the protection element  110  illustrated in  FIG.  28   , a retention electrode  111  is formed on the back surface of the insulating substrate  101  and a plurality of through holes  112  are provided between the intermediate electrode  106  and retention electrode  111 . The retention electrode  111  and through holes  112  draw the fuse conductor of the fuse element  107  melted on the intermediate electrode  106  and increase the retention capacity of the fuse conductor of the fuse element  107  increased in size in accordance with a large current application and a conductive layer is formed on the inner surface of the through holes  112 . 
     CITATION LIST 
     Patent Documents 
     
         
         Patent Document 1: Japanese Unexamined Patent Application 2018-78046 
         Patent Document 2: Japanese Unexamined Patent Application 2018-156959 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In a conventional configuration such as the protection element  100  illustrated in  FIG.  27    and the protection element  110  illustrated in  FIG.  28   , if used in a protection circuit of a lithium ion secondary battery for a large current application, the lithium ion secondary battery of the large current application is used as the external power source that supplies power to the heating element  104  so an over voltage is applied to the heating element power supply electrode  108  when the protection element  100  activates. 
     Therefore, as illustrated in  FIG.  29   , with the protection element  100 , there are cases where a spark (discharge) is generated from the heating element power supply electrode  108  to the tip of the intermediate electrode  106 , damaging the intermediate electrode  106 . Furthermore, if the intermediate electrode  106  is damaged, the thermal conductivity of the damaged location to the fuse element  107  is lowered extending the time until the fuse element  107  is blown, potentially preventing rapid and safe cutting off of the current path. 
     In addition, as illustrated in  FIG.  30   , with the protection element  110 , if a spark is generated from the heating element power supply electrode  108  to the tip of the intermediate electrode  106 , damaging the intermediate electrode  106 , the thermal conductivity of the damaged location to the fuse element  107  is lowered, extending the time until the fuse element  107  is blown, potentially preventing rapid and safe cutting off of the current path. In addition, the insulating layer (glass layer)  105  is formed with a thin 10 to 40 μm thickness to enable efficient transfer of heat in the heating element  104  to the intermediate electrode  106  and fuse element  107 , therefore damage may occur thereto if the heat in the heating element  104  is applied for a long period. Furthermore, as illustrated in  FIG.  31   , a spark may be generated from the high potential side of the heating element  104  to the center portion of the intermediate electrode  106  in the damaged area of the insulating layer  105 . If the intermediate electrode  106  is damaged in this manner, due to the damage of the intermediate electrode  106  in addition to the damage of the insulating layer  105 , thermal conductivity to the fuse element  107  is lowered, extending the time until the fuse element  107  can be blown, potentially preventing rapid and safe cutting off of the current path. 
     The risk that the fuse element will remain unmelted due to electrode damage associated with this type of spark, and hinder current cut off, increases with increased fuse element size in conjunction with higher voltage and higher current as well as with increased current rated value and stronger electric fields, with closer proximity of the heating element power supply electrode  108  and the intermediate electrode  106  associated with reduced size of the protection element, and with reduced thickness of the insulating layer. 
     Therefore, in protection elements with built-in heating elements, there is a demand to handle high voltage and high current, and to provide measures to prevent electrode breakdown in the element, enabling rapid and safe operation. 
     Here, an object of the present technology is to provide a protection element that can cut off a current path safely and rapidly with less likely causing a spark when high voltage is applied, and a battery pack that uses this protection element. 
     Means to Solve the Problem 
     To resolve the problems described above, a protection element according to the present technology, includes: 
     an insulating substrate;
 
a fuse element provided on a first surface of the insulating substrate;
 
a heating element formed on the first surface of the insulating substrate that blows the fuse element by generating heat;
 
a heating element power supply electrode formed on the first surface of the insulating substrate that supplies current to the heating element for heat generation;
 
a first extraction electrode leading from the heating element power supply electrode and connected to a first end of the heating element;
 
an intermediate electrode in contact with the fuse element;
 
a heating element connecting electrode formed on the first surface of the insulating substrate between the heating element and the intermediate electrode, connecting the heating element and the intermediate electrode;
 
a second extraction electrode leading from the heating element connection electrode and connected to a second end of the heating element; and
 
an insulating layer that covers the heating element, the first extraction electrode, and the second extraction electrode, and upon which the intermediate electrode is laminated; wherein
 
the intermediate electrode does not overlap with the first extraction electrode and does overlap with the second extraction electrode with the insulating layer.
 
     In addition, a battery pack according to the present technology, includes: 
     one or more battery cells;
 
a protection element connected on the charging/discharging path of the battery cell to blow the charging/discharging path; and
 
a current control element that detects the battery cell voltage value and controls protection element energization; wherein
 
the protection element includes:
 
an insulating substrate;
 
a fuse element provided on a first surface of the insulating substrate;
 
a heating element formed on the first surface of the insulating substrate that blows the fuse element by generating heat;
 
a heating element power supply electrode formed on the first surface of the insulating substrate that supplies current to the heating element for heat generation;
 
a first extraction electrode leading from the heating element power supply electrode and connected to a first end of the heating element;
 
an intermediate electrode in contact with the fuse element;
 
a heating element connecting electrode formed on the first surface of the insulating substrate between the heating element and the intermediate electrode, connecting the heating element and the intermediate electrode;
 
a second extraction electrode leading from the heating element connection electrode and connected to a second end of the heating element; and
 
an insulating layer that covers the heating element, the first extraction electrode, and the second extraction electrode, and upon which the intermediate electrode is laminated; wherein
 
the intermediate electrode does not overlap with the first extraction electrode and overlaps with the second extraction electrode via the insulating layer.
 
     Effect of the Invention 
     With the present technology, along with leading the first extraction electrode from the heating element power supply electrode to which a high voltage is applied, the intermediate electrode does not overlap with the first extraction electrode but does overlap with the second extraction electrode; therefore, the intermediate electrode is formed in a position separated from the first extraction electrode. Thus, a discharge path between the first extraction electrode which is a high-potential portion and the intermediate electrode which is a low-potential portion is not readily formed; therefore, a spark is less likely to occur. Therefore, the insulating layer and the intermediate electrode are not damaged, the thermal conductivity to the fuse element is maintained, the fuse element can be blown quickly, and the electric current path can be cut off safely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates Embodiment 1 of a protection element where the present technology was applied. (A) is a plan view, (B) is a cross-sectional view, and (C) is a bottom view; 
         FIG.  2    is a cross-sectional view of the protection element according to Embodiment 1; 
         FIG.  3    illustrates a fuse element blown in the protection element according to Embodiment 1. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  4    is an overview diagram of the fuse element; 
         FIG.  5    is a circuit diagram illustrating an exemplary configuration of a battery pack; 
         FIG.  6    is a circuit diagram of the protection element according to Embodiment 1; 
         FIG.  7    is a plan view illustrating the relative dimensions of the protection element according to an Example; 
         FIG.  8    is a plan view illustrating the relative dimensions of the protection element according to a Comparative Example; 
         FIG.  9    illustrates Embodiment 2 of the protection element where the present technology is applied. (A) is a plan view, (B) is a cross-sectional view, and (C) is a bottom view; 
         FIG.  10    is a circuit diagram of the protection element according to Embodiment 2; 
         FIG.  11    is a cross-sectional view of the protection element according to Embodiment 2; 
         FIG.  12    illustrates the fuse element blown in the protection element according to Embodiment 2. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  13    illustrates Embodiment 3 of the protection element where the present technology is applied; 
         FIG.  14    illustrates Embodiment 4 of the protection element where the present technology is applied. (A) is a plan view, (B) is a cross-sectional view, and (C) is a lower surface view; 
         FIG.  15    is a cross-sectional view of the protection element according to Embodiment 4; 
         FIG.  16    illustrates the fuse element blown in the protection element according to Embodiment 4. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  17    illustrates Embodiment 5 of the protection element where the present technology is applied. (A) is a plan view, (B) is a cross-sectional view, and (C) is a bottom view; 
         FIG.  18    is a cross-sectional view of the protection element according to Embodiment 5; 
         FIG.  19    illustrates the fuse element blown in the protection element according to Embodiment 5. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  20    illustrates Embodiment 6 of the protection element where the present technology is applied. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  21    is a circuit diagram of the protection element according to Embodiment 6; 
         FIG.  22    is a cross-sectional view illustrating the fuse element blown in the protection element according to Embodiment 6. 
         FIG.  23    illustrates Embodiment 7 of the protection element where the present technology is applied. (A) is a plan view, and (B) is a cross-sectional view; 
         FIG.  24    is a cross-sectional view illustrating the fuse element blown in the protection element according to Embodiment 7. 
         FIG.  25    illustrates a modified example of the protection element according to Embodiment 6. (A) is a cross-sectional view illustrating before the fuse element is blown, and (B) is a cross-sectional view illustrating after the fuse element is blown; 
         FIG.  26    illustrates a modified example of the protection element according to Embodiment 7. (A) is a cross-sectional view illustrating before the fuse element is blown, and (B) is a cross-sectional view illustrating after the fuse element is blown; 
         FIG.  27    illustrates a conventional protection element. (A) is a plan view, (B) is a cross-sectional view, and (C) is a bottom view; 
         FIG.  28    illustrates a conventional protection element. (A) is a plan view, (B) is a cross-sectional view, and (C) is a bottom view; 
         FIG.  29    is a plan view illustrating a spark generated in the protection element illustrated in  FIG.  27   ; 
         FIG.  30    is a plan view illustrating a spark generated in the protection element illustrated in  FIG.  28   ; and 
         FIG.  31    is a plan view illustrating a spark generated in the protection element illustrated in  FIG.  28   . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A protection element and battery pack to which the present technology is applied are described in detail below with reference to the figures. Note that the present technology is not limited to the following Embodiments, but various modifications can be made within the scope that do not deviate from the gist of the present technology. Also, the drawings are schematic, and the proportions and the like of each dimension may defer from reality. Specific dimensions and the like should be determined with reference to the following description. In addition, needless to say, the drawings also include portions where the dimensions and proportions differ from each other. 
     Embodiment 1 
     Embodiment 1 of the protection element to which the present technology is applied is described. A protection element  1 , as illustrated in  FIG.  1    (A) through (C) and  FIG.  2   , includes: an insulating substrate  2 , a fuse element  3  provided on a front surface  2   a  side of the insulating substrate  2 , a heating element  4  formed on the front surface  2   a  side of the insulating substrate  2  blowing the fuse element  3  by generating heat, a heating element power supply electrode  5  formed on the front surface  2   a  side of the insulating substrate  2  to which the electric current to heat the heating element  4  is supplied, a first extraction electrode  6  led from the heating element power supply electrode  5  and connected to a first end  4   a  of the heating element  4 , an intermediate electrode  8  on the fuse element  3  mounted, a heating element connecting electrode  9  formed between the heating element  4  and the intermediate electrode  8  on the front surface  2   a  side of the insulating substrate  2  connecting the heating element  4  and the intermediate electrode  8 , a second extraction electrode  10  led from the heating element connecting electrode  9  and connected to a second end  4   b  of the heating element  4 , and an insulating layer  7  covering the heating element  4 , the first extraction electrode  6 , and the second extraction electrode  10 , and onto which the intermediate electrode  8  is laminated. 
     Furthermore, as illustrated in  FIG.  1 (A) , in the protection element  1 , the intermediate electrode  8  does not overlap with the first extraction electrode  6  but does overlap with the second extraction electrode  10  in plan view. With the protection element  1 , when the heating element  4  is energized by the heating element power supply electrode  5 , the first extraction electrode  6  led from the heating element power supply electrode  5  becomes high potential with respect to the second extraction electrode  10 , and the second extraction electrode  10  becomes low potential. As a result, even when a high voltage is applied to the protection element  1 , a spark (discharge) is unlikely to occur, and an electric current path can be shut off safely and quickly. This may be due to the following reason. 
     In other words, a spark is an event in which a large electric current flows instantaneously due to a dielectric breakdown occurring from a high-potential portion to a low-potential portion between electrodes facing each other with an insulating layer interposed therebetween. The heating element power supply electrode  5  connected to an external power source and applying a high voltage to the heating element  4  and the first extraction electrode  6  led therefrom are at a higher electric potential than the heating element connecting electrode  9  connected to the intermediate electrode  8  and the second extraction electrode  10  led therefrom. The heating element connecting electrode  9  does not supply electric current to the heating element  4 . 
     In the protection element  1 , along with leading the first extraction electrode  6  from the heating element power supply electrode  5  to which a higher voltage is applied, in plan view, the intermediate electrode  8  does not overlap with the first extraction electrode  6  but does overlap with the second extraction electrode  10 ; therefore, the intermediate electrode  8  is formed in a position separated from the first extraction electrode  6 . Thus, a discharge path between the first extraction electrode  6  which is a high-potential portion and the intermediate electrode  8  which is a low-potential portion is not readily formed; therefore, a spark is less likely to occur. Therefore, the insulating layer  7  and the intermediate electrode  8  are not damaged, the thermal conductivity to the fuse element  3  is maintained, the fuse element  3  can be blown quickly, and the electric current path can be cut off safely. 
     Furthermore, with the protection element  1  illustrated in  FIG.  1   , the intermediate electrode  8  overlaps with the second extraction electrode  10 , so that the intermediate electrode  8  and the second extraction electrode  10  are thermally connected through the insulating layer  7 , enabling efficient heating of the intermediate electrode  8  and the fuse element  3  mounted thereon. Therefore, the fuse element  3  can be blown quickly after electric current is applied to the heating element  4 . 
     By incorporating such protection element  1  in an external circuit, the fuse element  3  constitutes a part of the electric current path of the external circuit, and upon being blown by heat generation by the heating element  4  or an overcurrent exceeding the rated value, the electric current path is shut off. Each configuration of the protection element  1  is described in detail below. 
     Insulating Substrate 
     The insulating substrate  2  is composed of an insulating material such as alumina, glass ceramics, mullite, zirconia, and the like. In addition, a material used for a printed wiring board, such as a glass epoxy substrate and a phenolic substrate may be used for the insulating substrate  2 . 
     First and second electrodes  11  and  12  are formed on opposite ends of the insulating substrate  2 . The first and second electrodes  11  and  12  are each formed of an electrically conductive pattern of Ag, Cu, or the like. In addition, the surfaces of the first and second electrodes  11  and  12  are preferably coated with a film such as Ni/Au plating, Ni/Pd plating, or Ni/Pd/Au plating by a known method such as plating treatment. Thus, the protection element  1  can prevent oxidation of the first and second electrodes  11  and  12  and prevent fluctuations in ratings caused by an increase in conduction resistance. In addition, erosion of the first and second electrodes  11 ,  12  (solder erosion) caused by melting of the solder for connecting the fuse element  3 , can be prevented for the case in which the protection element  1  is reflow mounted. 
     In addition, the first and second electrodes  11  and  12  on the front surface  2   a  of the insulating substrate  2  are connected to first and second external connecting electrodes  11   a  and  12   a  formed on a back surface  2   b  via a castellation. With the protection element  1 , by connecting the first and second external connecting electrodes  11   a  and  12   a  formed on the back surface  2   b  of the insulating substrate  2  to connection electrodes provided on an external circuit board on which the protection element  1  is mounted, the fuse element  3  is incorporated into a part of the electric current path formed on the circuit board. 
     The first and second electrodes  11  and  12  are electrically connected by mounting the fuse element  3  with a conductive connection material such as connecting solder  14 . Also, as illustrated in  FIGS.  3    (A) and (B), the first and second electrodes  11  and  12  are cut off by flowing a large current exceeding the rated value in the protection element  1 , causing the fuse element  3  to blow due to self-heating (Joule heat), or by generating heat in the heating element  4  associated with being energized, causing the fuse element  3  to blow. 
     Heating Element 
     The heating element  4  is a conductive member that has a relatively high resistance value and generates heat when energized, and is composed of a material such as nichrome, W, Mo, Ru, or the like or a material containing these materials. The heating element  4  can be formed by mixing powders of these alloys, compositions, or compounds with a resin binder or the like to obtain a paste, forming a pattern of the paste on the insulating substrate  2  with a screen printing technology, and calcining or the like. As an example, the heating element  4  can be formed by adjusting a mixed paste of ruthenium oxide paste, silver, and glass paste according to a prescribed voltage, forming a film in a prescribed position and prescribed surface area on the front surface  2   a  of the insulating substrate  2 , then, performing a calcining treatment under appropriate conditions. The form of the heating element  4  can be suitably designed, but as illustrated in  FIG.  1   , forming a substantially rectangular shape in accordance with the shape of the insulating substrate  2  is preferable in order to maximize the heating area. 
     The heating element  4  has a first end  4   a  connected to the first extraction electrode  6  and a second end  4   b  connected to the second extraction electrode  10 . The first extraction electrode  6  is led from the heating element power supply electrode  5 , and has the same potential as the heating element power supply electrode  5  when the heating element  4  is energized. The second extraction electrode  10  is led from the heating element connecting electrode  9 , and has the same potential as the heating element connecting electrode  9  when the heating element  4  is energized. The first extraction electrode  6  is led from the heating element power supply electrode  5  along the first end  4   a  of the heating element  4 , and in the protection element  1  illustrated in  FIG.  1   , extends along a first side edge of the heating element  4  formed in a substantially rectangular shape, and the first side edge of the heating element  4  overlaps therewith. Likewise, the second extraction electrode  10  is led from the heating element connecting electrode  9  along the second end  4   b  of the heating element, and in the protection element  1  illustrated in  FIG.  1   , extends along the second side edge of the heating element  4  formed in a substantially rectangular shape, and the second side edge of the heating element  4  overlaps therewith. 
     The heating element power supply electrode  5  and heating element connecting electrode  9  are formed on opposite side edges of the insulating substrate  2  different from the side edges where the first and second electrodes  11  and  12  are provided. The heating element power supply electrode  5  is an electrode that is connected to the first end  4   a  of the heating element  4  and serves as a power supply terminal to the heating element  4 , and is continuous with a third external connection electrode  5   a  formed on the back surface  2   b  of the insulating substrate  2  via a castellation. In addition, the intermediate electrode  8  is connected to the heating element connecting electrode  9 . 
     Also, the heating element  4 , the first extraction electrode  6  and the second extraction electrode  10  are covered with an insulating layer  7 . The intermediate electrode  8  is formed on the insulating layer  7 . The intermediate electrode  8  is connected to the fuse element  3  between the first and second electrodes  11  and  12  via a bonding material such as connecting solder  14  or the like. 
     The insulating layer  7  is provided to protect and insulate the heating element  4  and efficiently transmit the heat of the heating element  4  to the intermediate electrode  8  and the fuse element  3 , and is composed of, for example, a glass layer. The insulating layer  7  is formed thin with a thickness of, for example, 10 to 40 μm in order to efficiently transmit heat of the heating element  4  to the intermediate electrode  8  and the fuse element  3 . The insulating layer  7  may, for example, be formed by coating a glass-based paste. 
     By mounting the protection element  1  on the external circuit board, the heating element  4  is connected to a current control element or the like formed in the external circuit via the third external connection electrode  5   a , and normally this regulates electric conduction and heat generation. Furthermore, the heating element  4  is energized via the third external connection electrode  5   a  and generates heat at a prescribed timing to shut off the electric conduction path of the external circuit. Herein, with the heating element  4 , the heating element power supply electrode  5  and the first extraction electrode  6  side is the high-potential portion, and the heating element connecting electrode  9 , the second extraction electrode  10 , and the intermediate electrode  8  side is the low-potential portion. The protection element  1  can blow the fuse element  3  connecting the first and second electrodes  11  and  12  by transmitting the heat of the heating element  4  to the fuse element  3  through the insulating layer  7  and the intermediate electrode  8 . A fused conductor  3   a  of the fuse element  3  aggregates on the intermediate electrode  8  and on the first and second electrodes  11  and  12 , thereby cutting off the electric current path between the first and second electrodes  11  and  12 . Note, as will be described later, when the fuse element  3  is blown, the heating element  4  stops generating heat as the electric conduction path thereof is also shut off. 
     In addition, the heating element power supply electrode  5  may be provided with a restricting wall to prevent the connection solder provided on the electrode of the external circuit board connected to the third external connection electrode  5   a  melted due to reflow-mounting and the like, from rising up the heating element power supply electrode  5  via the castellation and wet-spreading over the heating element power supply electrode  5 . Similarly, the first and second electrodes  11  and  12  may also be provided with a restricting wall. The restricting wall can be formed using an insulating material that does not have wettability to the solder, such as glass, solder resist, or insulating adhesive, and can be formed on the heating element power supply electrode  5  by printing or the like. By providing the restricting wall, preventing the molten connection solder from wet-spreading to the heating element power supply electrode  5  and maintaining the connectivity between the protection element  1  and the external circuit board is possible. 
     In addition, in the protection element  1 , the heating element  4  may be formed within the insulating layer  7 , by forming the heating element power supply electrode  5 , the first extraction electrode  6 , the second extraction electrode  10 , the heating element connecting electrode  9 , and the heating element  4  after forming the insulating layer  7  on the front surface  2   a  of the insulating substrate  2 , and by further forming the insulating layer  7  thereon. 
     Intermediate Electrode 
     Similar to the first and second electrodes  11  and  12 , the intermediate electrode  8  is formed of a conductive pattern such as Ag, Cu, or the like. In addition, the surface of the intermediate electrode  8  is preferably coated with a film such as Ni/Au plating, Ni/Pd plating, or Ni/Pd/Au plating by a known method such as plating treatment. 
     The intermediate electrode  8  has the first end connected to the heating element connecting electrode  9 , is formed on the insulating layer  7 , and partially overlaps the heating element  4  with the insulating layer  7  interposed therebetween. As described above, in plan view, the intermediate electrode  8  does not overlap the high-potential first extraction electrode  6  positioned on the upstream side of the heating element  4  in the electric current flow direction when energized, but overlaps the low-potential second extraction electrode  10  positioned on the downstream side of the heating element  4  in the electric current flow direction. As a result, in the protection element  1 , forming of a discharge path between the first extraction electrode  6  which is the high-potential portion and the intermediate electrode  8  which is a low-potential portion becomes difficult, thereby a spark (discharge) is less likely to occur even if high voltage is applied from an external circuit, and the electric current path can be cut off safely and quickly. 
     Furthermore, in the protection element  1  illustrated in  FIG.  1   , by the intermediate electrode  8  overlapping the second extraction electrode  10  which overlaps the second end  4   b  of the heating element  4 , through the second extraction electrode  10  and the insulating layer  7 , the intermediate electrode  8  and the heating element  4  are thermally connected. Thereby, the intermediate electrode  8  and the fuse element  3  mounted thereon can be efficiently heated. Therefore, the fuse element  3  can be blown quickly after electric current is applied to the heating element  4 . 
     Fuse Element 
     Next, the fuse element  3  is described. The fuse element  3  is mounted across the first and second electrodes  11  and  12 , and is blown by heat generation of the heating element  4  when energized or by self-heating (Joule heat) when electric current exceeding the rated value flows, and cuts off the electric current path between the first electrode  11  and the second electrode  12 . 
     The fuse element  3  may be any conductive material that melts due to heat generated by the heating element  4  or because of an overcurrent state, and, for example, Sn—Ag—Cu base Pb-free solder, Bi—Pb—Sn alloy, Bi—Pb alloy, Bi—Sn alloy, Sn—Pb alloy, Pb—In alloy, Zn—Al alloy, In—Sn alloy, Pb—Ag—Sn alloy, and the like can be used. 
     Further, the fuse element  3  may be a structure containing a high melting point metal and a low melting point metal. For example, as illustrated in  FIG.  4   , the fuse element  3  is a laminated structure composed of an inner layer and an outer layer, having a low melting point metal layer  15  as the inner layer and a high melting point metal layer  16  as the outer layer laminated on the low melting point metal layer  15 . The fuse element  3  is connected onto the first and second electrodes  11  and  12  and the intermediate electrode  8  via a bonding material such as a connecting solder  14  or the like. 
     The low melting point metal layer  15  is preferably solder or a metal with Sn as a main component, and is a material generally called “Pb-free solder.” The melting point of the low melting point metal layer  15  does not necessarily need to be higher than the temperature of the reflow furnace, and may melt at about 200° C. The high melting point metal layer  16  is a metal layer laminated on the surface of the low melting point metal layer  15 , and is, for example, Ag or Cu, or a metal containing one of these as a main component and has a high melting point that does not melt even if a reflow process is used when the first and second electrodes  11  and  12  and the intermediate electrode  8  are connected to the fuse element  3  or when the protection element  1  is mounted on the external circuit board. 
     Such a fuse element  3  can be formed by forming a high melting point metal layer on a low melting point metal foil using a plating technique, or by using other known lamination techniques or film-forming techniques. In this case, the fuse element  3  may have a structure in which the entire surface of the low melting point metal layer  15  is covered with the high melting point metal layer  16 , or may have a structure in which all except for a pair of opposing side surfaces are covered. The fuse element  3  may have the high melting point metal layer  16  laminated as an inner layer and the low melting point metal layer  15  as an outer layer, and can also be formed in various configurations, such as a multi-layered structure having three or more layers in which the low melting point metal layer  15  and high melting point metal layer  16  are alternately laminated, having an opening provided in a part of the outer layer to expose a part of the inner layer, and the like. 
     By laminating a low melting point metal layer  15  as an inner layer and a high melting point metal layer  16  as an outer layer, even when the reflow temperature exceeds the melting temperature of the low melting point metal layer  15 , the fuse element  3  can retain the form as the fuse element  3 , and does not blow. Therefore, the connection between the first and second electrodes  11  and  12  and the intermediate electrode  8  with the fuse element  3  and the mounting of the protection element  1  on the external circuit board can be efficiently performed using a reflow process. In addition, even with the reflow process, fluctuation in the blowout process such as not being blown at a prescribed temperature or being blown at less than a prescribed temperature due to locally increased or decreased resistance because of deformation of the fuse element  3  can be prevented. 
     In addition, the fuse element  3  does not blow due to self-heating while a prescribed rated current is flowing. Furthermore, the fuse element will melt due to self-heating when an electric current higher than the rated electric current flows, and this cuts off the electric current path between the first and second electrodes  11  and  12 . In addition, the fuse element will melt due to heat generation when the heating element  4  is energized and this cuts off the electric current path between the first and second electrodes  11  and  12 . 
     Herein, regarding the fuse element  3 , erosion (solder erosion) of the high melting point metal layer  16  by the melted low melting point metal layer  15  causes the high melting point metal layer  16  to dissolve at a temperature that is lower than the melting point thereof. Therefore, the fuse element  3  can be blown in a short time by utilizing the erosion action of the high melting point metal layer  16  by the low melting point metal layer  15 . In addition, since the fused conductor  3   a  of the fuse element  3  is separated by the physical retraction of the intermediate electrode  8  and the first and second electrodes  11  and  12 , the electric current path between the first and second electrodes  11  and  12  can be cut off quickly and securely ( FIG.  3   ). 
     In addition, in the fuse element  3 , forming of the volume of the low melting point metal layer  15  larger than the volume of the high melting point metal layer  16  is preferable. The fuse element  3  is heated by self-heating due to overcurrent or by heat generation of the heating element  4 , and melting of the low melting point metal eroding the high melting point metal causes speedy meltdown and blowout. Therefore, in the fuse element  3 , by forming the volume of the low melting point metal layer  15  larger than the volume of the high melting point metal layer  16 , this erosion action is promoted, and the path between the first and second external connecting electrodes  11  and  12  is quickly cut off. 
     In addition, since the fuse element  3  is constructed by laminating the high melting point metal layer  16  on the low melting point metal layer  15  as an inner layer, the blowout temperature can be significantly reduced compared to a conventional chip fuse containing high melting point metal. Therefore, the fuse element  3  can have a larger cross-sectional area than a chip fuse or the like of the same size, and can greatly improve the electric current rating. In addition, it can be formed smaller and thinner than a conventional chip fuse with the same current rating, and can be attained with superior rapid blowout performance. 
     In addition, the fuse element  3  can improve resistance to surges (pulse resistance) in which an abnormally high voltage is instantaneously applied to the electrical system with the protection element  1  incorporated. In other words, the fuse element  3  must not blow out even when a current of 100 A flows for several milliseconds. In this regard, since a large current that flows in an extremely short time flows through the surface layer of the conductor (skin effect), and because the fuse element  3  is provided with a high melting point metal layer  16  such as Ag plating having a low resistance value as an outer layer, the electric current applied by a surge can flow easily, and blowout due to self-heating can be prevented. Therefore, the fuse element  3  can significantly improve resistance to a surge compared with a conventional fuse made of solder alloys. 
     The fuse element  3  may be coated with flux (not illustrated) to prevent oxidation and improve wettability during fusing. The inside of the protection element  1  is protected by covering the insulating substrate  2  with a case (not illustrated). The case can be formed, for example, using members having insulating properties such as various engineering plastics, thermoplastics, ceramics, and glass epoxy substrates. In addition, the case has enough internal space to accommodate the fuse element  3  expanding into a spherical shape when blowing out on the front surface  2   a  of the insulating substrate  2  and the fused conductor  3   a  aggregating on the intermediate electrode  8  and the first and second electrodes  11  and  12 . 
     Circuit Configuration Example 
     Such protection element  1  is used by integrating in a circuit in a battery pack  20  of, for example, a lithium ion secondary battery. As illustrated in  FIG.  5   , the battery pack  20  has a battery stack  25  constituted of, for example, a total of four battery cells  21   a  through  21   d  of lithium ion secondary batteries. 
     The battery pack  20  contains the battery stack  25 , a charge/discharge control circuit  26  that controls charging/discharging of the battery stack  25 , the protection element  1  wherein the present invention is applied for cutting off the charging/discharging path of the battery stack  25  when there is an abnormality, a detecting circuit  27  for detecting the voltage of each of the battery cells  21   a  through  21   d , and a current control element  28  which is a switch element controlling operation of the protection element  1  based on the detection result of the detecting circuit  27 . 
     The battery stack  25  is formed by series-connected battery cells  21   a  to  21   d  that require control to protect against overcharging and over-discharging states, is detachably connected to a charging device  22  by a positive terminal  20   a  and a negative terminal  20   b  of the battery pack  20 , and has charging voltage from the charging device  22  applied thereto. By connecting the positive terminal  20   a  and the negative terminal  20   b  of the battery pack  20  charged by the charging device  22  to a battery-operated electronic device, the electronic device can be operated. 
     The charge/discharge control circuit  26  includes two current control elements  23   a  and  23   b  connected in series to the electric current path between the battery stack  25  and the charging device  22 , and a controller  24  that controls the operation of these current control elements  23   a  and  23   b . The current control elements  23   a  and  23   b  are constituted of, for example, field-effect transistors (hereinafter referred to as FET), and control the conduction and interruption to the charging direction and/or discharging direction of the electric current path of the battery stack  25  by controlling the gate voltage using the controller  24 . The controller  24  receives power supplied from the charging device  22  to operate and when detection results from the detection circuit  27  indicate that the battery stack is overcharging or over-discharging, the controller controls operation of the current control elements  23   a  and  23   b  so as to cut off the current path. 
     The protection element  1  is connected, for example, on a charging/discharging current path between the battery stack  25  and the charge/discharge control circuit  26 , and the operation thereof is controlled by the current control element  28 . 
     The detecting circuit  27  is connected to each battery cell  21   a  through  21   d , detects the voltage value of each battery cell  21   a  through  21   d , and supplies each voltage value to the controller  24  of the charge/discharge control circuit  26 . Moreover, the detecting circuit  27  outputs a control signal for controlling the current control element  28  when any one of the battery cells  21   a  through  21   d  reaches an overcharge voltage or an over-discharge voltage. 
     The current control element  28  is constituted of, for example, an FET, and activates the protection element  1  by a detection signal output from the detecting circuit  27  when the voltage values of the battery cells  21   a  through  21   d  exceed a prescribed overcharge or over-discharge voltage, to control cutoff of the charging/discharging current path of the battery stack  25  instead of a switch operation of the current control elements  23   a  and  23   b.    
     The protection element  1  to which the present invention is applied and is used in the battery pack  20  configured as described above has a circuit configuration as illustrated in  FIG.  6   . In other words, in the protection element  1 , the first external connection electrode  11   a  is connected to the battery stack  25  side, and the second external connecting electrode  12   a  is connected to the positive terminal  20   a  side, thereby, the fuse element  3  is connected in series on the charging/discharging path of the battery stack  25 . In addition, in the protection element  1 , the heating element  4  is connected to the current control element  28  through the heating element power supply electrode  5  and the third external connection electrode  5   a , and the heating element  4  is connected to the open end of the battery stack  25 . In this manner, a first end of the heating element  4  is connected to the fuse element  3  and the first open end of the battery stack  25  via the intermediate electrode  8 , and a second end is connected to the current control element  28  and the second open end of the battery stack  25  via the third external connection electrode  5   a , thereby a power supply path to the heating element  4  is formed in which the electric current flow is controlled by the current control element  28 . 
     Operation of Protection Element 
     When detecting circuit  27  detects an abnormal voltage in any one of battery cells  21   a  through  21   d , the circuit outputs a shutoff signal to the current control element  28 . Then, the current control element  28  controls the current for energizing the heating element  4 . In the protection element  1 , a current flows from battery stack  25  to the heating element  4 , whereby heating element  4  starts to generate heat. In the protection element  1 , the fuse element  3  is blown due to the heat generated by the heating element  4 , and the charging/discharging path of the battery stack  25  is cut off. In addition, with regards to the protection element  1 , by forming the fuse element  3  containing a high melting point metal and a low melting point metal, using the erosion action of the high melting point metal by the low melting point metal due to the melting of the low melting point metal melting before the high melting point metal melts, the fuse element  3  is blown in a short period of time. 
     At this time, in the protection element  1 , since, in plan view, the intermediate electrode  8  does not overlap with the high potential first extraction electrode  6  but overlaps with the low potential second extraction electrode  10 , the intermediate electrode  8  is formed at a position separated from the first extraction electrode  6 . As a result, in the protection element  1 , forming of a discharge path between the first extraction electrode  6  which is the high-potential portion and the intermediate electrode  8  which is a low-potential portion becomes difficult, thereby a spark (discharge) is less likely to occur even if high voltage is applied from an external circuit, and the electric current path can be cut off safely and quickly. 
     Furthermore, with the protection element  1 , the intermediate electrode  8  overlaps with the second extraction electrode  10 , so that the intermediate electrode  8  and the heating element  4  are thermally connected via the second extraction electrode  10  and the insulating layer  7 , enabling the intermediate electrode  8  and the fuse element  3  mounted thereon to be efficiently heated. Therefore, the fuse element  3  can be blown quickly after electric current is applied to the heating element  4 . 
     When the fuse element  3  is blown, the protection element  1  shuts off the power supply path to the heating element  4 , so that the heat generation of the heating element  4  is stopped. 
     In the protection element  1 , the fuse element  3  is blown due to self-heating even when an overcurrent exceeding the rated value is applied to the battery pack  20 , and the charging/discharging path of the battery pack  20  can be cut off. 
     In this manner, with the protection element  1 , the fuse element  3  is blown by heat generated by the heating element  4  or by self-heating of the fuse element  3  due to overcurrent. At this time, when the protection element  1  is reflow-mounted on the circuit board, or when the circuit board on which the protection element  1  is mounted is further exposed to a high-temperature environment such as reflow heating, by having the structure of the low melting point metal covered by the high melting point metal, deformation of the fuse element  3  can be suppressed. Therefore, fluctuation in the blow characteristics due to a fluctuation in the resistance value caused by deformation of the fuse element  3  can be prevented and being blown quickly due to heat generation from a prescribed overcurrent or heat generation of the heating element  4  is also feasible. 
     The protection element  1  according to the present invention is not limited to being used in battery packs for lithium-ion secondary batteries, but can of course be applied to various uses that require cut-off of current paths by electrical signals. 
     Examples 
     Next, an Example of the protection element  1  is described. In this Example, a protection element  1  illustrated in  FIG.  1    (Example) and a protection element illustrated in  FIG.  27    (Comparative Example) were prepared and voltages of 50 V, 100 V, and 200 V were applied to determine whether a spark was generated. In addition, the variation (standard deviation σ) of the blowout time of the fuse element when low power (43 W) and high power (180 W) were applied was obtained (50 samples) and compared. 
     The dimensions of the protection element according to the Example are illustrated in  FIG.  7   , and the dimensions of the protection element according to the Comparative Example are illustrated in  FIG.  8   . The numerical values of each size shown in  FIG.  7    and  FIG.  8    are relative values indicating the ratio when the length of the insulating substrate is set to 1. 
     Example 
     The size of the protection element according to the Example is as follows. 
     Insulation substrate: 1×0.65 
     Intermediate electrode: 0.65×0.21 
     Distance between heating element power supply electrode and intermediate electrode: 0.026 
     Overlapping width of second extraction electrode and intermediate electrode: 0.04 Heating element width between the first extraction electrode and the second extraction electrode: 0.12 
     Comparative Example 
     The size of a protection element according to the Comparative Example is as follows. 
     Insulation substrate: 1×0.65 
     Intermediate electrode: 0.65×0.21 
     Distance between heating element power supply electrode and intermediate electrode: 0.026 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 
                 Comparative Example 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Applied voltage 
                 50 
                 [V] 
                 No spark 
                 No spark 
               
               
                   
                 100 
                 [V] 
                 No spark 
                 Spark 
               
               
                   
                 200 
                 [V] 
                 No spark 
                 Spark 
               
               
                   
               
            
           
         
       
     
     The distance between the heating element power supply electrode and the intermediate electrode is 0.026 in both the Example and the Comparative Example. As indicated in Table 1, in the Example, no spark occurred when any of the voltages 50 V, 100 V and 200 V were applied. On the other hand, in the Comparative Example, no spark occurred when a voltage of 50 V was applied, but a spark occurred when voltages of 100 V and 200 V were applied. 
     Therefore, as in the protection element  1  illustrated in  FIG.  1   , along with leading the first extraction electrode  6  from the heating element power supply electrode  5  to which a higher voltage is applied, in plan view, by the intermediate electrode  8  not overlapping with the first extraction electrode  6  but overlapping with the second extraction electrode  10 , the intermediate electrode  8  can be formed at a position separated from the first extraction electrode  6 . Therefore, in the protection element  1 , forming of the discharge path between the first extraction electrode  6  which is the high-potential portion and the intermediate electrode  8  which is a low-potential portion does not readily occur, and the effectiveness of the structure for preventing a spark when high power is applied can be seen. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Applied power 
                 43 [W] 
                 Example 
                 Comparative Example 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Operating time [S] 
                 Max. 
                 12.34 
                 25.31 
               
               
                   
                 Min. 
                 8.84 
                 11.21 
               
               
                   
                 Avg. 
                 10.37 
                 17.36 
               
               
                 Variability 
                 σ 
                 0.74 
                 1.59 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 2, when the protection element according to the Example was operated at 43 W, the maximum blowout time (seconds) was 12.34 seconds, the minimum time was 8.84 seconds, and the average time of 50 samples was 10.37 seconds with a standard deviation of σ=0.74. On the other hand, when the protection element according to the Comparative Example was operated at 43 W, the maximum blowout time (seconds) was 25.31 seconds, the minimum time was 11.21 seconds, and the average time of 50 samples was 17.36 seconds with a standard deviation of σ=1.59. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Applied power 
                 180 [W] 
                 Example 
                 Comparative Example 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Operating time [S] 
                 Max. 
                 1.29 
                 1.71 
               
               
                   
                 Min. 
                 1.07 
                 1.18 
               
               
                   
                 Avg. 
                 1.18 
                 1.35 
               
               
                 Variability 
                 σ 
                 0.045 
                 0.079 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 3, when the protection element according to the Example was operated at 180 W, the maximum blowout time (seconds) was 1.29 seconds, the minimum time was 1.07 seconds, and the average time of 50 samples was 1.18 seconds with a standard deviation of σ=0.045. On the other hand, when the protection element according to the Comparative Example was operated at 180 W, the maximum blowout time (seconds) was 1.71 seconds, the minimum time was 1.18 seconds, and the average time of 50 samples was 1.35 seconds a with standard deviation of σ=0.079. 
     As indicated in Table 2 and Table 3, as in the protection element  1  illustrated in  FIG.  1   , the intermediate electrode  8  overlapping the second extraction electrode  10  results in the intermediate electrode  8  and the heating element  4  being thermally connected via the second extraction electrode  10  and the insulating layer  7 , thereby the intermediate electrode  8  and the fuse element  3  mounted thereon can be efficiently heated, enabling the fuse element  3  to be blown out quickly, and the suppressing of blowout time variations for each product even at a low power can be seen. On the other hand, in the protection element according to the Comparative Example, the blowout time was long, and the variability was large. 
     Embodiment 2 
     Next, Embodiment 2 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection element  1  described above, and the details thereof may be omitted. A protection element  30  according to Embodiment 2 as illustrated in  FIG.  9    (A) through (C), has an insulating substrate  2 , a fuse element  3  provided on a front surface  2   a  side of the insulating substrate  2 , a plurality of heating elements  4  formed on the insulating substrate  2  for blowing the fuse element  3  by heat generation, a heating element power supply electrode  5  as a power supply terminal to each of the heating elements  4 , a plurality of first extraction electrodes  6  led from the heating element power supply electrode  5  and connected to a first end  4   a  of each of the heating elements  4 , an insulating layer  7  covering the heating elements  4 , an intermediate electrode  8  formed on the insulating layer  7  and having the fuse element  3  mounted thereon, a heating element connecting electrode  9  formed between the heating element  4  and the intermediate electrode  8  on the front surface  2   a  side of the insulating substrate  2  and connecting each of the heating elements  4  and the intermediate electrode  8 , and a second extraction electrode  10  led from the heating element connecting electrode  9  and connected to a second end  4   b  of each of the heating elements  4 . 
     In the protection element  30 , a plurality of heating elements  4  are provided side by side on the front surface  2   a  of the insulating substrate  2  with a space therebetween. Each heating element  4  has the first end  4   a  connected to the heating element power supply electrode  5  via the first extraction electrode  6 , and the second end  4   b  connected to the heating element connecting electrode  9  via the second extraction electrode  10 . The heating element connecting electrode  9  is connected to the intermediate electrode  8 . In each of the heating elements  4 , when the power is applied via the heating element power supply electrode  5 , the heating element power supply electrode  5  and the first extraction electrode  6  side is the high-potential portion, and the second extraction electrode  10 , the heating element connecting electrode  9 , and the intermediate electrode  8  side is the low-potential portion. Furthermore, for the protection element  30 , the intermediate electrode  8  does not overlap with each of the first extraction electrodes  6 , but overlaps with each of the second extraction electrodes  10  in plan view. 
     Thus, with the protection element  30 , the intermediate electrode  8  is formed at a position separated from each of the first extraction electrodes  6 , so formation of a discharge path between the first extraction electrodes  6  which are the high-potential portion and the intermediate electrode  8  which is the low-potential portion becomes difficult, thereby a spark (discharge) is less likely to occur, and the electric current path can be cut off safely and quickly. 
     Furthermore, with the protection element  30 , the intermediate electrode  8  overlaps with each of the second extraction electrodes  10 , so that the intermediate electrode  8  and the second extraction electrodes  10  are thermally connected via the insulating layer  7 , resulting in enabling the intermediate electrode  8  and the fuse element  3  mounted thereon to be efficiently heated. Therefore, the fuse element  3  can be blown quickly after electric current is applied to each of the heating elements  4 . 
       FIG.  10    is a circuit diagram of the protection element  30 . In the protection element  30 , each of the first ends of the plurality of heating elements  4  are connected to a power supply to cause the heating elements  4  to generate heat via the heating element power supply electrode  5  formed on the insulating substrate  2 , and each of the second ends of the heating elements  4  are connected to the fuse element  3  via the intermediate electrode  8 . 
     Retention Electrode 
     In addition, as illustrated in  FIG.  11   , with the protection element  30 , a retention electrode  32  may be formed on the back surface  2   b  of the insulating substrate  2  to retain a fused conductor  3   a  of the fuse element  3 , and the intermediate electrode  8  and the retention electrode  32  are contiguous via a through hole  33  that passes through the insulating substrate  2 , and the fused conductor  3   a  of the blown fuse element  3  can be drawn to the retention electrode  32  side via the through hole  33 . 
     When the fuse element  3  is blown, the through hole  33  can draw the fused conductor  3   a  by capillary action and reduce the volume of the fused conductor  3   a  retained on the intermediate electrode  8 . Thus, as illustrated in  FIG.  12    (A) (B), even if melt volume is increased by the size increase of a fuse element  3  due to higher rating and capacity of the protection element, a large volume of fused conductor  3   a  can be retained by the retention electrode  32 , the intermediate electrode  8 , and the first and second electrodes  11  and  12 , guaranteeing the blowing of the fuse element  3 . 
     Similar to the intermediate electrode  8 , the retention electrode  32  can be formed by a known method such as printing using a known electrode material such as Ag, Cu, or an alloy material containing Ag or Cu as a main component. 
     A conductive layer  34  is formed on the inner surface of the through hole  33 . By forming the conductive layer  34 , the through hole  33  can readily draw the fused conductor  3   a . The conductive layer  34  is formed of, for example, any one of copper, silver, gold, iron, nickel, palladium, lead, and tin, or an alloy containing any one thereof as a main component, and the inner surface of the through hole  33  can be formed using a known method such as electroplating or printing of a conductive paste. In addition, the conductive layer  34  may be formed by inserting a plurality of metal wires or an aggregate of conductive ribbons into the through holes  33 . 
     The conductive layer  34  of the through hole  33  is contiguous with the intermediate electrode  8  formed on the front surface  2   a  of the insulating substrate  2 . With regards to the intermediate electrode  8 , in addition to supporting the fuse element  3 , since the fused conductor  3   a  aggregates when blown, the fused conductor  3   a  can be readily guided into the through hole  33  because the intermediate electrode  8  and conductive layer  34  are contiguous. 
     In addition, the conductive layer  34  of the through hole  33  is contiguous with the retention electrode  32  formed on the back surface  2   b  of the insulating substrate  2 . Therefore, when the fuse element  3  melts, the fused conductor  3   a  drawn via the through hole  33  can be aggregated on the retention electrode  32  (see  FIG.  12   ), and by drawing more of the fused conductor  3   a , the volume of the fused conductor  3   a  retained by the intermediate electrode  8  and the first and second electrodes  11  and  12  at the blown out portion of the fuse element  3  can be reduced. 
     Also, a plurality of through holes  33  may be formed in the protection element  30  to increase the number of paths for drawing the fused conductor  3   a  of the fuse element  3 , and by drawing more of the fused conductor  3   a , the volume of the fused conductor  3   a  at the blown out portion can be reduced. 
     In addition, the protection element  30  having the heating elements  4  formed on both sides of the through hole  33  in order to heat the retention electrode  32  and the intermediate electrode  8  and to aggregate and draw more fused conductor  3   a  is preferable. 
     Embodiment 3 
     Next, Embodiment 3 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection elements  1  and  30  described above, and the details thereof may be omitted. In a protection element  40  according to Embodiment 3, as illustrated in  FIG.  13   , the first extraction electrode  6 , the heating element  4 , the second extraction electrode  10 , and the intermediate electrode  8  are arranged in this order in a plan view. In the protection element  40 , the second extraction electrode  10  and the intermediate electrode  8  do not overlap in a plan view. 
     In the protection element  40 , the intermediate electrode  8  does not overlap with the second extraction electrode  10 , so the thermal connection between the intermediate electrode  8  and the second extraction electrode  10  is weaker compared to in the protection element  1 . Therefore, the protection element  40  may be inferior to the protection element  1  regarding rapid blowout capability of the fuse element  3 . 
     However, in the protection element  40 , the intermediate electrode  8  does not overlap with the first extraction electrode  6  that conducts current for heat generation in the heating element  4 , a heating element  8 , or the second extraction electrode  10 , and compared to the protection element  1 , the intermediate electrode  8  is formed at a position further separated from the first extraction electrode  6 . This makes formation of a discharge path between the first extraction electrode  6  which is a high-potential portion and the intermediate electrode  8  which is a low-potential portion difficult, thereby a spark is less likely to occur. Therefore, the insulating layer  7  and the intermediate electrode  8  are not damaged, the thermal conductivity to the fuse element  3  is maintained, the fuse element  3  can be blown quickly, and the electric current path can be cut off safely. 
     In the protection element  40 , as in the protection element  30 , a plurality of heating elements  4  and a plurality of first and second extraction electrodes  6  and  10  may be formed. In this case as well, the intermediate electrode  8  does not overlap with any of the second extraction electrodes  10  and is arranged at a position further separated from each of the first extraction electrodes  6 . Therefore, even when a high voltage is applied, a spark can be prevented. 
     Embodiment 4 
     Next, Embodiment 4 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection elements  1 ,  30 , and  40  described above, and the details thereof may be omitted. As shown in  FIG.  14    (A) through (C),  FIG.  15   , and  FIGS.  16    (A) and (B), a protection element  50  according to Embodiment 4 has, on a back surface  2   b  opposite to a front surface  2   a  of an insulating substrate  2 , a heating element  4 , a heating element power supply electrode  5 , a first extraction electrode  6 , a heating element connecting electrode  9 , a second extraction electrode  10 , and an insulating layer  7  covering these. The protection element  50  also has, on the front surface  2   a  of the insulating substrate  2 , an intermediate electrode  8  on which a fuse element  3  is mounted, a first connecting electrode  51  connecting the intermediate electrode  8  and the heating element connecting electrode  9 , a second connecting electrode  52  connected to the heating element power supply electrode  5  via a castellation, a first electrode  11 , and a second electrode  12 . 
     The first connecting electrode  51  and a second connecting electrode  52  can be formed using the same material and the same process as those of the heating element power supply electrode  5  and the heating element connecting electrode  9  described above. 
     The heating element connecting electrode  9  and the first connecting electrode  51  are contiguous via a through hole  53  passing through the insulating substrate  2 . The through hole  53  is a conductive through hole having a conductive layer formed therein, and the heating element connecting electrode  9  and the first connecting electrode  51  are electrically and thermally connected via the through hole  53 . That is, in the protection element  50 , the heating element  4  heats the intermediate electrode  8  via the insulating substrate  2 , the heat from the heating element  4  is also transferred to the intermediate electrode  8  via the highly thermal conductive heating element connecting electrode  9 , through hole  53 , and first connecting electrode  51 , and the fuse element  3  can be heated and blown out ( FIG.  16   ). 
     An insulating restricting wall  54  is formed on the second connecting electrode  52 . The restricting wall  54  prevents a fillet of a connecting solder which connects the heating element power supply electrode  5  to an external circuit board, from contacting the intermediate electrode  8  and the fuse element  3 , when wet-spreading on the second connecting electrode  52 . The restricting wall  54  can be formed, for example, by applying glass paste onto the second connecting electrode  52 . 
     In the protection element  50 , the intermediate electrode  8  overlaps with the second extraction electrode  10  via the insulating substrate  2 . As a result, the intermediate electrode  8  and the second extraction electrode  10  are thermally connected via the insulating substrate  2 , thereby assisting the heating of the intermediate electrode  8  and the fuse element  3  mounted thereon. 
     In addition, in the protection element  50 , the intermediate electrode  8  overlaps with the first extraction electrode  6  via the insulating substrate  2 . As a result, the intermediate electrode  8  and the first extraction electrode  6  are thermally connected via the insulating substrate  2  enabling assisting the heating of the intermediate electrode  8  and the fuse element  3  mounted thereon. In the protection element  50 , the intermediate electrode  8  may be formed at a position that does not overlap with the first extraction electrode  6  via the insulating substrate  2 . This makes formation of a discharge path between the first extraction electrode  6  which is a high-potential portion and the intermediate electrode  8  which is a low-potential portion difficult, thereby a spark is less likely to occur. 
     Embodiment 5 
     Next, Embodiment 5 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection elements  1 ,  30 ,  40 , and  50  described above, and the details thereof may be omitted. In contrast to the protection element  50 , a protection element  60  according to Embodiment 5 is different in having a plurality of heating elements  4 , first extraction electrodes  6 , second extraction electrodes  10 , and insulating layers  7  formed on the back surface  2   b  of the insulating substrate  2 . 
     As illustrated in  FIG.  17    (A) through (C),  FIG.  18   , and  FIGS.  19    (A) and (B), the protection element  60  has a plurality of heating elements  4  arranged side by side on the back surface  2   b  of the insulating substrate  2  with a space therebetween. Each heating element  4  has the first end  4   a  connected to the heating element power supply electrode  5  via the first extraction electrode  6 , and the second end  4   b  connected to the heating element connecting electrode  9  via the second extraction electrode  10 . Each of the heating elements  4 , each of the first extraction electrodes  6 , and each of the second extraction electrodes  10  are covered with the insulating layers  7 . 
     Retention Electrode 
     In addition, in the protection element  60 , similar to in the protection element  30 , a retention electrode  32  is formed on the back surface  2   b  of the insulating substrate  2  to retain the fused conductor  3   a  of the fuse element  3 , and the intermediate electrode  8  formed on the front surface  2   a  and the retention electrode  32  are contiguous via the through hole  33  that passes through the insulating substrate  2 , and the fused conductor  3   a  of the melted fuse element  3  is drawn to the retention electrode  32  side via the through hole  33 . Thus, even if melt volume is increased by the size increase of a fuse element  3  due to higher rating and capacity of the protection element, a large volume of fused conductor  3   a  can be retained by the retention electrode  32 , the intermediate electrode  8 , and the first and second electrodes  11  and  12 , guaranteeing the blowing of the fuse element  3 . 
     The retention electrode  32  is formed so as to overlap with the insulating layer  7  provided on the back surface  2   b  of the insulating substrate  2 . In addition, similar to the intermediate electrode  8 , the retention electrode  32  can be formed by a known method such as printing using a known electrode material such as Ag, Cu, or an alloy material containing Ag or Cu as a main component. 
     The through hole  33  passes through the intermediate electrode  8 , the insulating substrate  2 , the insulating layer  7  and the retention electrode  32 . In addition, a conductive layer  34  is formed on the inner surface of the through hole  33 . The conductive layer  34  is contiguous with the intermediate electrode  8  and the retention electrode  32 . As a result, the fused conductor  3   a  aggregated on the intermediate electrode  8  can be readily guided into the through hole  33 , and also the fused conductor  3   a  drawn via the through hole  33  can be aggregated on the retention electrode  32 , and by drawing more of the fused conductor  3   a , the volume of the fused conductor  3   a  retained by the intermediate electrode  8  and the first and second electrodes  11  and  12  at the blown out portion of the fuse element  3  can be reduced. 
     In the protection element  60 , by connecting the conductive layer  34  formed in the through hole  33  of the retention electrode  32  to the intermediate electrode  8 , a power supply path is configured from the heating element power supply electrode  5 , through the retention electrode  32  and the fuse element  3 , reaching a first electrode  11  and to the heating element  4 . In addition, in the protection element  60 , by connecting the conductive layer  34  formed in the through hole  33  of the retention electrode  32  to the intermediate electrode  8 , a thermal path transmitting the heat from the heating element  4  through the conductive layer  34  and the intermediate electrode  8  to the fuse element  3  is configured. 
     Also, a plurality of through holes  33  may be formed in the protection element  60  to increase the number of paths for drawing the fused conductor  3   a  of the fuse element  3 , and by drawing more of the fused conductor  3   a , the volume of the fused conductor  3   a  at the blown out portion can be reduced. 
     In each of the heating elements  4 , when the power is applied via the heating element power supply electrode  5 , the heating element power supply electrode  5  and the first extraction electrode  6  side is the high-potential portion, and the second extraction electrode  10 , the heating element connecting electrode  9 , and the intermediate electrode  8  side is the low-potential portion. Furthermore, for the protection element  60 , the intermediate electrode  8  does not overlap with each of the first extraction electrodes  6 , but overlaps with each of the second extraction electrodes  10  in plan view. 
     Thus, with the protection element  60 , the intermediate electrode  8  is formed at a position separated from each of the first extraction electrodes  6  in plan view, so formation of a discharge path between the first extraction electrodes  6  which are the high-potential portion and the intermediate electrode  8  which is the low-potential portion does not readily occur, thereby a spark (discharge) is less likely to occur, and the electric current path can be cut off safely and quickly. 
     Furthermore, with the protection element  60 , the intermediate electrode  8  overlaps each of the second extraction electrodes  10 , so that the intermediate electrode  8  and the second extraction electrodes  10  are thermally connected via the insulating substrate  2 , resulting in enabling the intermediate electrode  8  and the fuse element  3  mounted thereon to be efficiently heated. Therefore, the fuse element  3  can be blown quickly after electric current is applied to each of the heating elements  4 . 
     In the protection element  60 , the retention electrode  32  located on the downstream side of the electric current flow direction of the heating element  4  overlaps with the second extraction electrode  10  and does not overlap with the first extraction electrode  6 , and thereby is formed at a position separated from the first extraction electrode  6  which is the high-potential portion, thus, a spark (discharge) is less likely to occur even when a high voltage is applied, and damage due to a spark can be prevented. 
     In addition, the protection element  60  having the heating elements  4  formed on both sides of the through hole  33  in order to heat the retention electrode  32  and the intermediate electrode  8  and to aggregate and draw more fused conductor  3   a  is preferable. 
     Embodiment 6 
     Next, Embodiment 6 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection element  1 ,  30 ,  40 ,  50  and  60  described above, and the details thereof may be omitted. As illustrated in  FIGS.  20    (A) and (B), a protection element  70  according to Embodiment 6 has the heating element  4  and the intermediate electrode  8  formed on the front surface  2   a  of the insulating substrate  2  and the retention electrode  32  on the back surface  2   b  of the insulating substrate  2 , thereby constituting a blow member  71  allowing the insulating substrate  2  to draw and retain the fused conductor  3   a  of the fuse element  3  from the intermediate electrode  8  through the through hole  33  to the retention electrode  32  side, and the fuse element  3  is sandwiched by this blow member  71 . 
     The insulating substrate  2  of the blow member  71  has, on the surface  2   a , a plurality of heating elements  4 , a heating element power supply electrode  5  as a power supply terminal for each of the heating elements  4 , a plurality of first extraction electrodes  6  led from the heating element power supply electrode  5  and connected to the first end  4   a  of each of the heating elements  4 , the insulating layer  7  covering the heating element  4 , the intermediate electrode  8  formed on the insulating layer  7  and connected to the fuse element  3 , the heating element connecting electrode  9  formed between each of the heating elements  4  and the intermediate electrode  8  on the front surface  2   a  of the insulating substrate  2  and connecting each of the heating elements  4  and the intermediate electrode  8 , and the second extraction electrode  10  led from the heating element connecting electrode  9  and connected to the second end  4   b  of each of the heating elements  4 . Since the configuration and effect of electrodes  5 ,  6 ,  8 ,  9 ,  10 , the heating element  4 , and the insulating layer  7  are the same as the protection element  30  thereof described above, the details are omitted. 
     The insulating substrate  2  of the blow member  71  has the retention electrode  32  formed on the back surface  2   b  for retaining the fused conductor  3   a  of the fuse element  3 , and the intermediate electrode  8  and the retention electrode  32  are contiguous via the through hole  33  with a conductive layer  34  formed on the inner surface and passing through the insulating substrate  2 . As a result, the insulating substrate  2  draws the fused conductor  3   a  of the blown fuse element  3  from the intermediate electrode  8  to the retention electrode  32  side via the through hole  33 . The configuration and effect of the retention electrode  32 , the through hole  33 , and the conductive layer  34  are the same as that of the protection element  30  thereof described above, so details are omitted. 
     Here, the insulating substrate  2  of the blow member  71  may be provided with an auxiliary electrode  73  on the front surface  2   a , which is connected to the fuse element  3  and retains the fused conductor  3   a , together with the intermediate electrode  8 . In addition, the fuse element  3  is provided separately from the insulating substrate  2 , and is connected using a bonding material such as a connecting solder  14  to the first and second external terminals  74  and  75  which are connected to an external circuit. In other words, the first and second external terminals  74 ,  75  are electrically connected via the fuse element  3 . Similarly, the heating element power supply electrode  5  of each of the blow members  71  is also provided separately from the insulating substrate  2  and connected to a third external terminal  76  which is connected to an external circuit. 
     Note, in the protection element  70 , the blow member  71 , the fuse element  3 , and the first through third external terminals  74  through  76  are stowed in an insulating case  77 . The case  77  has a lower case  77   a  in which the blow member  71  connected to the lower side of the fuse element  3  and the first through third external terminals  74  through  76  are arranged, and a cover  77   b  which covers the lower case  77   a . The lower case  77   a  has a storage recess  78  having a size sufficient for the retention electrode  32  of the arranged blow member  71  to retain the fused conductor  3   a . In addition, the cover  77   b  stows the blow member  71  connected to the upper side of the fuse element  3 , and has sufficient internal space for the retention electrode  32  of this blow member  71  to retain the fused conductor  3   a.    
     A first end of each of the first and second external terminals  74 ,  75  is connected to the fuse element  3  inside the case  77 , and the second end is led out of the case  77  and connected to the external circuit. A first end of the third external terminal  76  is connected to the heating element power supply electrode  5  of each of the blow members  71  inside the case  77 , and the second end is led out of the case  77  and connected to the external circuit. 
     In the protection element  70 , as illustrated in  FIG.  20   , the fuse element  3  is sandwiched by a plurality of blow members  71 . In the protection element  70  illustrated in  FIG.  20   , the blow members  71  are arranged on a first surface and a second surface of the fuse element  3 , respectively.  FIG.  21    is a circuit diagram of the protection element  70 . In each of the blow members  71  arranged on the front surface and the back surface of the fuse element  3 , the first end of the heating element  4  is connected to a power source for the heating element  4  to generate heat via the first extraction electrode  6 , the heating element power supply electrode  5 , and the third external terminal  76  formed on each of the insulating substrates  2 , and the second end of the heating element  4  is connected to the fuse element  3  via the second extraction electrode  10 , the heating element connecting electrode  9 , and the intermediate electrode  8  formed on each of the insulating substrates  2 . 
     As illustrated in  FIG.  22   , in the protection element  70 , when blowing the fuse element  3  by heat from the heating element  4 , the heating element  4  of each of the blow members  71 ,  71  connected to both surfaces of the fuse element  3  generates heat to heat the fuse element  3  from both surfaces. Therefore, even when the cross-sectional area of the fuse element  3  is increased for high-current applications, the protection element  70  can quickly heat and blow out the fuse element  3 . 
     In addition, in the protection element  70 , the fused conductor  3   a  of the fuse element  3  is drawn into the through hole  33  formed on the insulating substrate  2  of the blow member  71  and aggregated on the retention electrode  32 . Therefore, even in cases where the cross-sectional area of the fuse element is increased for compatibility with large current applications where fused conductor  3   a  exceeding the intermediate electrode  8  retention capacity of fused conductor  3   a  is generated, by retaining the fused conductor  3   a  by the through hole  33  and retention electrode  32 , in addition to the intermediate electrode  8 , and by drawing the fused conductor  3   a  with a plurality of blow members  71 , the protection element  70  can reliably cause the fuse element  3  to blow. In addition, the protection element  70  can blow the fuse element  3  more quickly by drawing the fused conductor  3   a  by means of the plurality of blow members  71 . 
     The protection element  70  can quickly blow out the fuse element  3  even when the fuse element  3  has a covered structure in which a low melting point metal constituting an inner layer is covered with a high melting point metal. In other words, the fuse element  3  covered with the high melting point metal takes time to heat up to a temperature at which the outer layer of the high melting point metal melts even when the heating element  4  generates heat. Here, the protection element  70  includes a plurality of blow members  71 , and heats each of the heating elements  4  at the same time, so that the high melting point metal of the outer layer can be quickly heated to the melt temperature. Therefore, according to the protection element  70 , the thickness of the high melting point metal layer that constitutes an outer layer can be increased, and the rapid blowout characteristics can be maintained while further increasing the rating. 
     Moreover, as illustrated in  FIG.  20   , the protection element  70  is preferably connected to the fuse element  3  with a pair of blow members  71 ,  71  facing each other. As a result, the protection element  70  can simultaneously heat the same portion of the fuse element  3  from both sides by the pair of blow members  71 ,  71  and draw the fused conductor  3   a , thereby heating and blowing the fuse element  3  more quickly. 
     In the protection element  70 , the intermediate electrode  8  and the auxiliary electrode  73  formed on each of the insulating substrates  2  of the pair of blow members  71 ,  71  are preferably formed opposite from each other with the fuse element  3  interposed therebetween. As a result, the pair of blow members  71 ,  71  are symmetrically connected, so that the load applied to the fuse element  3  does not become unbalanced during reflow mounting, and the like, and resistance to deformation can be improved. 
     Embodiment 7 
     Next, Embodiment 7 wherein the protection element to which the present technology is applied is described. In the following description, the same reference codes may be given to the same configurations as those of the protection elements  1 ,  30 ,  40 ,  50 ,  60 , and  70  described above, and the details thereof may be omitted. As illustrated in  FIGS.  23    (A) and (B) and  FIG.  24   , a protection element  80  according to Embodiment 7 has the intermediate electrode  8  formed on the front surface  2   a  of the insulating substrate  2 , and the heating element  4  and the retention electrode  32  on the back surface  2   b  of the insulating substrate  2 , thereby constituting a blow member  81  allowing the insulating substrate  2  to draw and hold a fused conductor  3   a  of a fuse element  3  from the intermediate electrode  8  via a through hole  33  to a retention electrode  32  side, and the fuse element  3  is interposed by this blow member  81 . 
     The insulating substrate  2  of the blow member  81  has an intermediate electrode  8  formed on the front surface  2   a  to be connected to the fuse element  3 . Since the configuration and effect of this intermediate electrode  8  are the same as those of the protection element  60  described above, details thereof are omitted. Here, the blow member  81  may also be provided with the auxiliary electrode  73  together with the intermediate electrode  8  on the front surface  2   a  of the insulating substrate  2 , which is connected to the fuse element  3  and retains the fused conductor  3   a.    
     The insulating substrate  2  of the blow member  81  has, on the back surface  2   b , a plurality of heating elements  4 , the heating element power supply electrode  5  as a power supply terminal for each of the heating elements  4 , a plurality of first extraction electrodes  6  led from the heating element power supply electrode  5  and connected to the first end  4   a  of each of the heating elements  4 , the insulating layer  7  covering the heating elements  4 , the heating element connecting electrode  9  formed between each of the heating elements  4  and the intermediate electrode  8  on the front surface  2   a  of the insulating substrate  2  and connecting each of the heating elements  4  and the intermediate electrode  8 , the second extraction electrode  10  led from the heating element connecting electrode  9  and connected to the second end  4   b  of each of the heating elements  4 , and the retention electrode  32  retaining the fused conductor  3   a  of the fuse element  3  formed on the insulating layer  7 , and the intermediate electrode  8  and the retention electrode  32  are contiguous via the through hole  33  passing through the insulating substrate  2  and formed with a conductive layer  34  on the inner surface thereof. As a result, the insulating substrate  2  draws the fused conductor  3   a  of the blown fuse element  3  from the intermediate electrode  8  to the retention electrode  32  side via the through hole  33 , and also constitutes the electric conduction path of the heating element  4  and the thermal path to the fuse element  3 . Since the configuration and effect of each of electrodes  5 ,  6 ,  9 ,  10 , the heating element  4 , the insulating layer  7 , the retention electrode  32 , the through hole  33 , and the conductive layer  34  are the same as the protection element  60  thereof described above, the details are omitted. 
     Similar to the protection element  70 , the fuse element  3  is connected to first and second external terminals  74  and  75  connected to an external circuit by a connecting material such as connecting solder  14 . In addition, the heating element power supply electrode  5  of each of the blow members  81  is also connected to the third external terminal  76  connected to an external circuit. 
     The action of energizing the heating element  4  and blowing the fuse element  3  in the protection element  80  is the same as the protection element  70  thereof described above, so the details are omitted. 
     In the protection elements  70  and  80 , whether forming on the front surface  2   a  or the back surface  2   b  of the insulating substrate  2 , forming the heating element  4  on both sides of the through hole  33  is preferable for heating the retention electrode  32  and the intermediate electrode  8  and drawing and aggregating even more of the fused conductor  3   a.    
     In addition, as illustrated in  FIGS.  25    (A) and (B), and  FIGS.  26    (A) and (B), for the protection element  70  and  80 , after forming the insulating layer  7  on the front surface  2   a  or the back surface  2   b  of the insulating substrate  2 , the heating element power supply electrode  5 , the first extraction electrode  6 , the second extraction electrode  10 , the heating element connecting electrode  9 , and the heating element  4  may be formed, and by further forming the insulating layer  7  thereon, the heating element  4 , and the like may be formed within the insulating layer  7 . 
     REFERENCE SIGNS LIST 
       1 . Protection element,  2 . Insulating substrate,  2   a . Front surface,  2   b . Back surface,  3 . Fuse element,  3   a . Fused conductor,  4 . Heating element,  4   a . First end,  4   b . Second end,  5 . Heating element power supply electrode,  5   a . Third external connection electrode,  6 . First extraction electrode,  7 . Insulating layer,  8 . Intermediate electrode,  9 . Heating element connecting electrode,  10 . Second extraction electrode,  11 . First electrode,  11   a . First external connection electrode,  12 . Second electrode,  12   a . Second external connection electrode,  14 . Connecting solder,  15 . Low melting point metal layer,  16 . High melting point metal layer,  17 . Restricting wall,  20 . Battery pack,  20   a . Positive terminal,  20   b . Negative terminal,  21   a . through  21   d . Battery cells,  22 . Charging device,  23 . Current control element,  24 . Controller,  25 . Battery stack,  26 . Charge/discharge control circuit,  27 . Detecting circuit,  28 . Current control element,  30 . Protection element,  32 . Retention electrode,  33 . Through hole,  34 . Conductive layer,  40 . Protection element,  50 . Protection element,  51 . First connecting electrode,  52 . Second connecting electrode,  53 . Through hole,  54 . Restricting wall,  60 . Protection element,  70 . Protection element,  71 . Blow member,  73 . Auxiliary electrode,  74 . First external terminal,  75 . Second external terminal,  76 . Third external terminal,  77 . Case,  77   a . Lower case,  77   b . Cover,  80 . Protection element,  81 . Blow member