JP2006128277A - Ptc element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer PTC element which can protect a circuit without using a fuse together even if a conductive polymer element is broken when a voltage higher than a rating voltage is applied to the polymer PTC element. <P>SOLUTION: The PTC element is provided with a conductive polymer element (3) having a PTC characteristic, two electrode foils (3a and 3b) pinching the conductive polymer element (3) and two terminals (1 and 2) arranged on the surface of the two electrode foils, respectively. In this case, conductive adhesive agents (4 and 5) whose electric resistance irreversibly increase as a temperature rises are used to join the terminals (1 and 2) with surfaces of the electrode foils (3a and 3b) so as to manufacture the PTC element. Metallic thin films (1a, 2a, 3c, and 3d) may be formed on the surfaces of the electrode foils and the terminals to improve the adhesive force of the conductive adhesive agent. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention particularly relates to a PTC element, an overcurrent protection element, an overheat protection element, and the like provided as a polymer PTC element.

The “PTC element” refers to a thermistor having a positive temperature coefficient as known in the field of electric / electronic circuit technology. A PTC element has a low electrical resistance (or impedance) under relatively low temperature conditions (for example, at room temperature), but when the temperature exceeds a certain temperature (hereinafter referred to as trip temperature), the electrical resistance increases rapidly. In the present specification, the former state of the PTC element is referred to as a low state, and the latter state is referred to as a high state. Further, when the PTC is in a tripped state and is further applied with a voltage and exceeds a certain temperature (hereinafter, referred to as a thermal runaway start temperature), the PTC is thermally runaway. Here, thermal runaway means that after the trip, the increase in electrical resistance is saturated and the rate of increase in electrical resistance is reduced. As a result, the power consumption accompanying the voltage increase (P = V ² / R: where P is The power consumption, V is the applied voltage, and R is the resistance value of the PTC element), and the temperature of the PTC element rises. The PTC will be destroyed if it becomes hot.

  A polymer in which an element made of a conductive polymer having PTC characteristics (hereinafter, this polymer is referred to as “PPTC”) is sandwiched between two electrode foils, and two terminals are soldered to each of the two electrode foils. PTC elements (hereinafter also referred to as “PPTC elements”) are widely known and are used in various fields. This element has a characteristic that when its temperature exceeds a predetermined temperature, its electric resistance value increases abruptly and current flow is substantially cut off. Utilizing this characteristic, PPTC elements are used in electric circuits for the purpose of protecting electric circuits. When an overcurrent flows through the circuit to which the PPTC element is connected and the temperature of the PPTC element rises, the resistance of the PPTC element increases and the current flow is interrupted, thereby protecting the circuit.

  When the conventional PPTC element operates due to an overcurrent (that is, the electric resistance increases), the voltage of the entire circuit is substantially applied to the PTC element. At this time, when the voltage applied to the PPTC element is a very large voltage exceeding the rated voltage, there is a possibility that the element is destroyed due to thermal runaway of the PPTC element and the electrode foils are brought into contact with each other and short-circuited. In order to avoid such a situation, the PPTC element is selected or designed such that its maximum rated voltage is larger than the voltage of the circuit (or system). However, even if the rated voltage of the PPTC element is selected in consideration of the maximum voltage of the circuit, a large voltage that cannot be predicted may be inevitably applied to the circuit. In that case, it is desirable from the viewpoint of circuit protection to prevent a short circuit between the two electrode foils in the PPTC element. For this reason, in order to protect the circuit from overcharge, overheat, or overcurrent, a fuse is used together as a backup in a battery pack or the like.

The fuse is connected in series with the PPTC element. The fuse is generally one that melts by heat and cuts (opens) the circuit, but may be one that increases resistance by heat. For example, Patent Document 1 proposes a conductive paste that forms a hardened paste layer that constitutes a switching mechanism by a function of changing conductivity in response to a thermal change, and mentions the use of this as a fuse. However, since the conductive paste described in Patent Document 1 inherently has a switching function and does not irreversibly increase the electrical resistance value, it prevents a short circuit when the ambient temperature decreases. I can't.
JP-A-6-163203

  As described above, when a fuse is used in combination, there is a problem that the fuse operates first due to a large current such as a lightning surge or the fuse is destroyed by an impact or the like. The present invention has a structure capable of suppressing the performance degradation of the PPTC element as a whole without using a fuse even when a conductive polymer element having PTC characteristics is destroyed due to thermal runaway with a very low probability. It is an object to provide a PPTC element.

  In order to solve the above problems, the present invention provides a conductive polymer element having PTC characteristics, two electrode foils sandwiching the conductive polymer element, and two electrode foils disposed on the surfaces of the two electrode foils, respectively. A PTC element including a terminal, wherein the terminal is bonded to the surface of the electrode foil via a conductive adhesive having a characteristic that its electrical resistance increases irreversibly with a temperature rise. .

  The PTC element of the present invention is characterized by using a conductive adhesive that irreversibly increases in electrical resistance when the temperature rises due to heating or the like to electrically connect the terminal and the electrode foil. To do. Here, the temperature rise of the conductive adhesive can be known from the surrounding temperature rise. The temperature of the conductive adhesive can be measured using, for example, a thermocouple.

  As described above, the PPTC element of the present invention operates due to its PTC characteristics when an overcurrent flows or when the ambient temperature rises, and becomes a high resistance, thereby rapidly reducing the amount of current passing therethrough. At this time, the voltage of the entire circuit is substantially applied to PPTC. When this voltage is an overvoltage, that is, an excessive voltage exceeding the rating, the surface temperature of the PPTC rises and thermal runaway may occur, leading to destruction. Heat generation during the PPTC thermal runaway or heat generation at the time of destruction heats the conductive adhesive to a high temperature and irreversibly increases its electrical resistance. As a result, even after the PPTC element is destroyed, the conductive adhesive becomes a high resistance body, so even if the electrode foils of the PPTC element come into contact with each other, a short circuit in the PPTC element is effectively prevented. Thus, the circuit can be protected. In other words, the conductive adhesive that can be a high-resistance element serves as a backup fuse. Therefore, it is preferable that the conductive adhesive used in the present invention has a sufficiently high electric resistance in a short time from when an overvoltage is applied to the PPTC element until destruction occurs. The conductive adhesive also preferably has the property of increasing its electrical resistance so that it has the same or higher electrical resistance than that of the tripping PPTC in such a short time.

  The conductive adhesive has a high electrical resistance at a low rate of increase in electrical resistance (Ω / ° C) in the temperature range lower than the PPTC thermal runaway start temperature, and has a large electrical resistance in the temperature range above the thermal runaway start temperature. It is preferable that the electrical resistance increases at an increasing rate (Ω / ° C.). The PPTC element should function as a conductor having a low electrical resistance unless the PPTC is thermally runaway, and if the resistance of the conductive adhesive is increased below the thermal runaway start temperature, it may adversely affect the circuit. There is. In other words, since the increase in the electrical resistance of the conductive adhesive means the deterioration of the conductive adhesive, the PPTC has the property of being hardly deteriorated as long as the PPTC is in a normal state or tripped. Is preferred. This is also required for repeated use of PPTC elements.

When the PPTC element of the present invention is used, even when PPTC breaks down due to thermal runaway, the conductive adhesive becomes a high resistance body, so that a short circuit occurs between the electrode foils disposed on both sides of the PPTC. Can be prevented. Therefore, the PPTC element of the present invention can protect a circuit by itself without using a fuse even when a voltage higher than the rated voltage is applied, and exhibits a higher circuit protection function. In addition, the ratio (R _B / R _A ) between the resistance (R _B ) at the start of thermal runaway and the resistance (R _A ) at room temperature due to the stress acting on the PTC element, etc., decreases the withstand voltage. Even when the voltage is lowered, the PPTC element having the configuration of the present invention can protect the circuit satisfactorily. Furthermore, since the PPTC element of the present invention eliminates the need for a fuse, the use of this makes it possible to save circuit space, reduce assembly costs, and reduce component costs. Furthermore, in the PPTC element of the present invention, the electrode foil and the terminal are connected by a conductive adhesive, and soldering is not required, so that the reflow furnace required for soldering is not necessary, and the equipment cost is reduced. Can be reduced. Further, nitrogen used at the time of soldering is not necessary, so that it is possible to reduce the cost of nitrogen and the equipment cost for using nitrogen. Furthermore, since the product and jig cleaning steps can be eliminated, the cost of cleaning liquid and cleaning equipment can be reduced. About the obtained PTC element, since the heat history in a manufacturing process decreases compared with the case where soldering is used when using a conductive adhesive, the resistance of the PTC element can be reduced.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a plan view of an example of a PPTC element of the present invention, and FIG. 2 is a cross-sectional view thereof. The PPTC element 10 of the present invention includes a conductive polymer element 3 having PTC characteristics, electrode foils 3a and 3b disposed on both sides thereof, and terminals 1 and 2. Terminals 1 and 2 are joined to electrode foils 3a and 3b by conductive adhesives 4 and 5, respectively. In the illustrated PPTC element 10, metal thin films 1a and 2a are formed on the surfaces of terminals 1 and 2, respectively, and metal thin films 3c and 3d are formed on the surfaces of electrode foils 3a and 3b, respectively.

  The conductive polymer element 3 is, for example, a polymer resin body configured by kneading polyolefin or a fluorine-based resin and carbon black and then crosslinking by irradiation with radiation. In the production of the PPTC element having the illustrated configuration, the electrode foils 3a and 3b are generally integrated by thermocompression bonding after the kneading and before irradiation with radiation. In the interior of the conductive polymer element 3, a large number of conductive paths through which current flows are formed because carbon black particles are connected in a normal temperature environment, whereby the conductive polymer element 3 has good conductivity. Demonstrate. When the conductive polymer element 3 is thermally expanded due to the excess of the current flowing through the conductive path, the inter-particle distance of the carbon black is expanded, the conductive path is cut, and the resistance value increases rapidly (positive resistance temperature characteristic; PTC ). In the illustrated form, the conductive polymer element is a rectangular plate. The dimensions are, for example, about 3 to 10 mm for vertical, 7 to 15 mm for horizontal, and about 0.2 to 1 mm for thickness. The conductive polymer element may have other forms conventionally employed, for example, a disk form.

  The electrode foils 3a and 3b disposed on the two opposing surfaces of the conductive polymer element 3 are metal foils. The metal foil is, for example, a nickel foil, a copper foil, or a nickel-plated copper foil. The thickness of the metal foil is generally about 0.025 to 0.040 mm. For example, as described above, the metal foils 1 and 2 are integrated with the conductive polymer element 3 by thermocompression using a press or a roller after the kneading of the conductive polymer element and before crosslinking.

  Terminals 1 and 2 are used as mounting leads. The terminals 1 and 2 are made of, for example, nickel, brass, iron plated with copper or SUS plated with nickel. In the illustrated form, the terminal is a rectangular strip, and the dimensions thereof are, for example, 2-8 mm in length, 16-20 mm in width, and about 0.1-0.3 mm in thickness. The terminal may have other shapes, for example, a plate-like body.

  The conductive adhesives 4 and 5 include a synthetic resin and a conductive powder, and include additives for adjusting viscosity as necessary. Examples of the synthetic resin that can be used include thermoplastic resins such as vinyl acetate resin, polyvinyl alcohol resin, acrylic resin, and vinyl urethane resin. In addition, thermosetting resins such as urea resin, melamine resin, phenol resin, resorcinol resin, epoxy resin, silicone resin, α-olefin maleic anhydride resin, polyamide resin, and polyimide resin can be used. Further, two or more kinds of resins selected from the resins listed here can be mixed and used. As the conductive powder, for example, a gold powder, a silver powder, a nickel powder, a carbon powder, a palladium powder, or a powder having at least a surface conductive can be used. When any material is selected, the conductive adhesive needs to be configured so that its electrical resistance increases irreversibly due to temperature rise. The conductive adhesives 4 and 5 are formed between the electrode foils 3a and 3b and the terminals 1 and 2 so that the conductive polymer elements 3 act as high resistances that shield current when the conductive polymer element 3 is thermally runaway. It is necessary to apply in such an amount that it is formed by. The specific thickness is selected according to the type and size of the conductive polymer element 3 and the area to which the conductive adhesive is applied. For example, when using a conductive polymer element and a terminal having the above dimensions, it is preferable to apply a conductive adhesive so that a layer having a thickness of about 0.01 to 0.1 mm is formed.

  The metal thin films 1a, 2a, 3c and 3d are for preventing the oxidation of the surface of each member and further strengthening the adhesion by the adhesives 4 and 5 while ensuring good electrical conductivity. These metal thin films are formed, for example, by plating gold, or by plating using an appropriate conductive material such as palladium silver or copper. The metal thin film is not necessarily formed. For example, the metal thin film may be formed only on one or both terminals, or may be formed only on the surface of one or both electrode foils.

  Even when the conductive polymer element 3 is destroyed due to thermal runaway due to an overvoltage being applied to the polymer PTC element, the heat released during the thermal runaway of the conductive polymer element 3 and / or Alternatively, the temperature of the conductive adhesives 4 and 5 rises due to the heat released by the conductive polymer element 3 at the time of breakdown, and the electrical resistance increases. Thereby, even if the electrode foils 3a and 3b come into contact with each other because the conductive polymer element 3 is broken, the conductive adhesives 4 and 5 become resistors, and the gap between the terminals 1 and 2 is reduced. The amount of current flowing is preferably kept as small as before the polymer PTC element is thermally runaway. By the action of the conductive adhesives 4 and 5, the safety of the circuit in which the polymer PTC element is placed and the equipment incorporating the circuit are maintained.

  In the PPTC element of the present invention, even when a short circuit occurs between the electrode foils due to the breakage of the conductive polymer element due to thermal runaway, the conductive adhesive exhibits a high electric resistance and the amount of current is reduced. Since the state can be maintained, it can be used to more safely protect various devices or devices such as battery packs and other general electronic devices, communication devices and computers.

FIG. 1 is a plan view of an example of the PTC element of the present invention. FIG. 2 is a cross-sectional view of an example of the PTC element of the present invention.

Explanation of symbols

1, 2 ... Terminal, 1a, 2a ... Metal thin film, 3 ... Conductive polymer element, 3a, 3b ... Electrode foil, 3c, 3d ... Metal thin film, 4, 5 ... Conductive adhesive, 10 ... PTC element.

Claims (4)

  In a PTC element comprising a conductive polymer element having PTC characteristics, two electrode foils sandwiching the conductive polymer element, and two terminals respectively disposed on the surfaces of the two electrode foils, the terminal Is bonded to the surface of the electrode foil via a conductive adhesive, and the conductive adhesive irreversibly increases its electrical resistance due to a temperature rise.   2. The PTC element according to claim 1, wherein the temperature of the conductive adhesive is increased by heat generation during thermal runaway of the conductive polymer element and / or heat generation during breakdown of the conductive polymer element. 3.   The conductive adhesive increases in temperature due to heat generated during thermal runaway of the conductive polymer element and / or heat generated during breakdown of the conductive polymer element, and when the conductive polymer element having a PTC characteristic is tripped, the electrical resistance is increased. The PTC element according to claim 2, wherein the PTC element has the same or higher electrical resistance. The conductive adhesive has an electrical resistance increase rate in a low temperature region lower than the thermal runaway start temperature of the conductive polymer element than an electrical resistance increase rate in a high temperature region higher than the thermal runaway start temperature of the conductive polymer element. The PTC element according to any one of claims 1 to 3, wherein the PTC element is small.

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