Over-current protection device

An over-current protection device comprises a PTC device and a heating element operable to heat the PTC device. The PTC device contains crystalline polymer and metal or ceramic conductive filler dispersed therein. The PTC device has a resistivity less than 0.1 Ω·cm. The over-current protection device has the relation: It (heating)<Ih (60° C.)×10%, where Ih (60° C.) is a hold current of the over-current protection device at 60° C. when the heating element is not activated; It (heating) is a trip current of the over-current protection device when the heating element is activated to heat the PTC device. The PTC device has high hold current, thereby allowing a battery containing the device can be fast charged with a large current. In a specific situation, the heating element heats the PTC device to decrease the hold current of the over-current protection device of low resistivity, and accordingly the PTC device can trip by a small current.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present application relates to an over-current protection device and, and more specifically, to an over-current protection device with protection by tripping a positive temperature coefficient (PTC) device.

(2) Description of the Related Art

Over-current protection devices are used for circuit protections to prevent circuits from being damaged due to over-current or over-temperature events. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has PTC characteristic; that is, the resistance of the PTC material remains extremely low at a normal temperature; however when an over-current or an over-temperature occurs in the circuit, the resistance instantaneously increases to a high resistance state (i.e., trip) to diminish the current for circuit protection. When the temperature decreases to room temperature or over-current no longer exists, the over-current protection device returns to low resistance state so that the circuit operates normally again. Because the PTC over-current protection devices can be reused, they can replace fuses and are widely applied to high-density circuitries.

In general, the PTC conductive composite material contains crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer. The crystalline polymer is usually a polyolefin polymer such as polyethylene. The conductive filler usually contains carbon black powder. However, carbon black exhibits low electrical conductivity and therefore is unsatisfactory to the demands of low resistivity applications. Therefore, a PTC conductive composite material containing a conductive filler of low resistivity such as metal or conductive ceramic filler is devised to obtain lower resistivity than a material containing carbon black, so as to develop a so-called low-rho over-current protection device.

In battery quick charge applications, a PTC device has to have a high hold current from a room temperature to 60° C., allowing quick charge to a battery with a large current even if the temperature goes up to 60° C. In such a case, for example, an action that needs one hour normal charge can speed up to 20 minutes by quick charge. Quick charge needs to comply with specific safety specifications. The PTC device has to rapidly sever a charging current to protect the battery when over-charge, instantaneous voltage change, over-voltage, or over-temperature occurs. At ambient temperature of 80° C., the PTC device has to trip within 60 seconds when a current of 8 amperes is applied thereto, thereby effectively providing over-current protection to relevant circuits or apparatuses.

SUMMARY OF THE INVENTION

To resolve the problem that the over-current protection device of low resistivity is not easily tripped at a specific temperature, the present application devised an over-current protection device in which a heating element is embedded therein to speed up trip of the PTC device of low resistivity so as to effectively provide over-current protection.

In accordance with a first embodiment of the present application, an over-current protection device comprises at least one PTC device and at least one heating element. In an exemplary embodiment, the PTC device and the heating element are stacked. The PTC device contains crystalline polymer and metal or ceramic conductive filler dispersed therein. The PTC device is the so-called low-rho PTC device having a volume resistivity less than 0.1 Ω·cm, or 0.05 Ω·cm. The heating element is operable to heat the PTC device. The over-current protection device has the relation: It (heating)<Ih (60° C.)×10%, where Ih (60° C.) is a hold current of the over-current protection device at 60° C. when the heating element is not activated; It (heating) is a trip current of the over-current protection device when the heating element is activated to heat the PTC device. The heating element has a resistance sufficient to effectively heat up the PTC device to decrease the hold current of the PTC device to induce trip. In an exemplary embodiment, the heating element has a resistance sufficient to induce trip within 60 seconds when a current of 8 amperes is applied to the PTC device at ambient temperature of 80° C. Preferably, the heating element has a resistance larger than or equal to 0.1Ω.

In an exemplary embodiment, the heating element may connect to a switch to receive a signal from a sensor. When the sensor detects a voltage drop in the circuit or a temperature exceeds to a threshold value, the switch turns on to allow a current flowing through the heating element to heat up the PTC device.

In an exemplary embodiment, the heating element may contain a circuit of two resistors in serial connection to increase efficiency of the heating element.

In an exemplary embodiment, the PTC device contains crystalline polymer of a melting point greater than 150° C. for high temperature applications. For example, the crystalline polymer comprises polyvinylidene difluoride (PVDF).

In an exemplary embodiment, the heating element may be a ceramic PTC heater, a polymeric PTC heater element or a traditional resistor-type heater. A polymeric PTC heater may comprise polymer of a melting point greater than 150° C., e.g., PVDF, for high temperature applications.

In an exemplary embodiment, the heating element of the over-current protection device is disposed between two PTC devices, and those two PTC devices are in parallel connection.

In an exemplary embodiment, two ends of the PTC device electrically connect to a first electrode and a second electrode, and two ends of the heating element electrically connect to a third electrode and a fourth electrode. The first, second, third and fourth electrodes are formed at a lower surface of the over-current protection device as interfaces for surface-mounting to a circuit board. In such a case, the PTC device and the heating element have no common electrode.

In an exemplary embodiment, two ends of the PTC device electrically connect to a first electrode and a second electrode, and two ends of the heating element electrically connect to the second electrode and a third electrode. The first, second and third electrodes are formed at a lower surface of the over-current protection device as interfaces for surface-mounting to a circuit board. Accordingly, the PTC device and the heating element use a common electrode, i.e., the second electrode.

In an exemplary embodiment, the PTC device comprises a PTC material layer, a first metal foil and a second metal foil. The first metal foil is formed on an upper surface of the PTC material layer, whereas the second metal foil is formed on a lower surface of the PTC material layer. The heating element comprises a heating layer, a first conductive layer and a second conductive layer. The first conductive layer is formed on an upper surface of the heating layer, and the second conductive layer is formed on a lower surface of the heating layer. On a structural basis of the PTC device and heating element design, in an embodiment, the first electrode electrically connects to the first metal foil, the second electrode electrically connects to the second metal foil and the first conductive layer, and the third electrode electrically connects to the second conductive layer. The first, second and third electrodes are formed at a lower surface of the over-current protection device as interfaces for surface-mounting. A first conductive connecting member extends vertically to connect to the first electrode and the first metal foil. A second conductive connecting member extends vertically to connect to the second electrode, the second metal foil and the first conductive layer. At least one conductive hole extends vertically to connect to the third electrode and the second conductive layer. Both the first and second conductive layers are separated from the first conductive connecting member. In another embodiment, the first electrode electrically connects to the first metal foil, the second electrode electrically connects to the second metal foil, the third electrode electrically connects to the first conductive layer, and the fourth electrode electrically connects to the second conductive layer. The first, second, third and fourth electrodes are formed at a lower surface of the over-current protection device as interfaces for surface-mounting. A first conductive connecting member extends vertically to connect to the first electrode and the first metal foil. A second conductive connecting member extends vertically to connect to the second electrode and the second metal foil. A third conductive connecting member extends vertically to connect to the third electrode and the first conductive layer. A fourth conductive connecting member extends vertically to connect to the fourth electrode and the second conductive layer. Both the first and second conductive layers are separated from the first and second conductive connecting members.

In an exemplary embodiment, the PTC device comprises a PTC material layer, a first metal foil and a second metal foil. The first metal foil is formed on an upper surface of the PTC material layer, whereas the second metal foil is formed on a lower surface of the PTC material layer. The heating element comprises a heating layer, a first conductive layer, a second conductive layer and a third conductive layer. The first conductive layer is formed on an upper surface of the heating layer, and the second and third conductive layers are formed on a lower surface of the heating layer. On a structural basis of the PTC device and heating element design, in an embodiment, the first electrode electrically connects to the first metal foil, the second electrode electrically connects to the second metal foil, the third electrode electrically connects to the second conductive layer, and the fourth electrode electrically connects to the third conductive layer. The first, second, third and fourth electrodes are formed at a lower surface of the over-current protection device as interfaces for surface-mounting. A first conductive connecting member extends vertically to connect to the first electrode and the first metal foil. A second conductive connecting member extends vertically to connect to the second electrode and the second metal foil. At least one first conductive hole extends vertically to connect to the third electrode and the second conductive layer. At least one second conductive hole extends vertically to connect to the fourth electrode and the third conductive layer. The second conductive layer is separated from the third conductive layer, and the first, second and third conductive layers are separated from the first and second conductive connecting members.

The over-current protection device of the present application sustains high hold current at a specific temperature, e.g., 60° C., allowing to conduct quick charge with a large current. When a voltage drop in a circuit or an ambient temperature exceeds a threshold value, the heating element is activated to heat the PTC device. Accordingly, the hold current of the PTC device decreases so as to induce or accelerate trip of the PTC device. The over-current protection device of the present application has low resistivity, high hold current and meets safety criteria of trip within 60 seconds when a current of 8 amperes (8 A) is applied thereto, and therefore it is suitable for low-rho PTC applications.

DETAILED DESCRIPTION OF THE INVENTION

The making and using of the presently preferred illustrative embodiments are discussed in detail below. It should be appreciated, however, that the present application provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific illustrative embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

FIGS. 1A and 1Bshow an over-current protection device10which is a hexahedron that could be used for surface-mounting.FIG. 1Ais a lateral view of the over-current protection device10illustrating essential structures and conductive paths. The over-current protection device10is a laminated structure comprising conductive layers, insulating layers and at least one PTC material layer extending in a horizontal direction, those layers associating with vertical conductive connecting members to constitute a circuitry as desired. For the ease of describing the circuitry, upper and lower surfaces of the over-current protection device10, and individual conductive layers are shown inFIG. 1B. The shadow parts are notches those are removed by etching in circuitry manufacturing for separation. The core of the over-current protection device10comprises a PTC device11and a heating element21. The PTC device11comprises a PTC material layer13, a first metal foil12and a second metal foil14. The first metal foil12and the second metal foil14are formed on upper and lower surfaces of the PTC material layer13, respectively. The heating element21may be a ceramic PTC heater or polymeric PTC heater, or may be a traditional resistor heater with a certain resistance. In an embodiment, the heating element21has a resistance greater than 0.1Ω or 0.2Ω. In an embodiment, the heating element21comprises a heating layer17, a first conductive layer15and a second conductive layer16. The first conductive layer15and the second conductive layer16are formed on upper and lower surface of the heating layer17, respectively. Insulating layers18,19and20are disposed on or between the PTC device11and the heating element12, and may contain prepreg or other insulating materials. An upper surface of the insulating layer18is provided with a solder mask28, and a lower surface of the insulating layer20is provided with a first electrode22, a second electrode23and a third electrode26. The third electrode26is disposed between the first electrode22and second electrode23, and gaps are formed therebetween for separation. The first metal foil12of the first PTC device11electrically connects to the first electrode22through a first conductive connecting member24extending in a vertical direction. The second metal foil14electrically connects to the second electrode23through a second conductive connecting member25extending in a vertical direction. In an embodiment, the first and second conductive connecting members24and25may be semicircular holes made by mechanical drilling followed by electroplating conductive films thereon. The first conductive layer15on the heating layer17electrically connects to the second electrode23through the second conductive connecting member25, whereas the second conductive layer16electrically connects to the third electrode26through conductive holes27.

The PTC material layer13may comprise crystalline polymer and metal or conductive ceramic fillers dispersed therein, and accordingly has low resistivity. Because the use of conductive filler of low resistivity, the resistivity of the PTC device11could be less than 0 Ω·cm, or 0.05 Ω·cm. The crystalline polymer of the PTC material layer13may include polyolefin such as high density polyethylene (HDPE) and low density polyethylene (LDPE). The crystalline polymer may completely or partially contain crystalline polymer of a high melting point, e.g., >150° C., for example, polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), polytetrafluoroethylene (PTFE), polychlorotrifluoro-ethylene (PCTFE), so as to increase the melting point of the PTC material layer13for high-temperature applications. The metal or conductive ceramic filler may comprise nickel, cobalt, copper, iron, tin, lead, silver, gold, platinum, titanium carbide, tungsten carbide, vanadium carbide, zirconium carbide, niobium boride, tantalum carbide, molybdenum carbide, hafnium carbide, titanium boride, vanadium boride, zirconium boride, niobium boride, molybdenum boride, hafnium boride, zirconium nitride, and combinations thereof, e.g., mixture, solid solution or core-shell.

In an embodiment, if the heating element21is a polymeric PTC device, the polymer may comprise PVDF, PVF, PTFE or PCTFE of which a melting point greater than 150° C. for high-temperature applications. In particular, the resistivity of the heating element21, in which carbon black may be used as conductive filler, is greater than that of the PTC device11. As such, when over-voltage and over-temperature is detected by voltage or temperature sensors, a switch turns on to allow current to flow through the heating element21. The heating element21has high resistivity, and therefore it can heat up rapidly to heat the PTC device11effectively. To meet the criteria of quick charge, the over-current protection device10can trip within 60 seconds at ambient temperature of 80° C. when a current of 8 A is applied thereto.

In an embodiment, a solder mask29may be formed on a lower surface of the over-current protection device10to cover a portion of the third electrode26, thereby exposing the first electrode22, the second electrode23and partially exposing the third electrode26, as shown inFIG. 1C. The first electrode22, the second electrode23and the third electrode26uncovered by the solder mask29serve as interfaces for surface-mounting the over-current protection device10to a circuit board.

The equivalent circuit of the over-current protection device10is shown inFIG. 1D. The two ends of the PTC device11connect to the first electrode22and the second electrode23. Two ends of the heating element21connect to the second electrode23and the third electrode26. As such, an end of the PTC device11and an end of the heating element21commonly connect to the second electrode23. In an embodiment, the third electrode26may connect to a switch92such as a field effect transistor (FET), and the switch92further connects to a sensor91to receive signals detected. The terminals A1and A2may connect to circuits or apparatuses to be protected, whereas terminals B1and B2may connect to a power source such as a battery. In an embodiment, the sensor91is capable of detecting voltage drops or temperatures. If a voltage drop or a temperature reaches or exceeds a predetermined value, the switch92turns on to allow a current to flow through the heating element21to heat the PTC device11. Accordingly, the hold current of the PTC device11decreases to speed up the trip of the PTC device11.

Because the over-current protection device of low resistivity has a high hold current at a specific temperature, e.g., 80° C., it is not easily tripped. According to the present application, the heating element21heats the PTC device11to speed up the trip of the device, so as to meet the specification that the device has to trip within 60 seconds at ambient temperature of 80° C. when 8 A is applied thereto.

FIGS. 2A and 2Bshow an over-current protection device30in accordance with a second embodiment of the present application, which is a hexahedron that could be used for surface-mounting to a circuit substrate.FIG. 2Ashows a lateral view of the over-current protection device30andFIG. 2Bshows another lateral view of the device30to illustrate essential structures and conductive paths thereof. The over-current protection device30is a laminated structure comprising conductive layers, insulating layers and a PTC material layer extending in a horizontal direction, those layers associating with vertical conductive connecting members to form a circuitry as desired. For the ease of describing the circuitry, upper and lower surfaces of the over-current protection device30, and individual conductive layers are shown inFIG. 2C. The shadow parts represent notches those are removed by etching in circuitry manufacturing for separation. The core of the over-current protection device30comprises a PTC device31and a heating element41. The PTC device31comprises a PTC material layer33, a first metal foil32and a second metal foil34. The first metal foil32and the second metal foil34are formed on upper and lower surfaces of the PTC material layer33, respectively. The heating element41may be a PTC heater or other heaters. In an embodiment, the heating element41comprises a heating layer37, a first conductive layer35, a second conductive layer36and a third conductive layer36′. The first conductive layer35is formed on an upper surface of the heating layer37, and the second and third conductive layers36and36′ are formed on a lower surface of the heating layer37. Insulating layers38,39and40are disposed on or between the PTC device31and the heating element41, and may comprise prepreg or other insulating materials. An upper surface of the insulating layer38is provided with a solder mask48, and a lower surface of the insulating layer40is provided with a first electrode42, a second electrode43, a third electrode46and a fourth electrode49. The third electrode46and the fourth electrode49are disposed between the first electrode42and second electrode43, and gaps are formed therebetween for separation. A separation is formed between the third electrode46and the fourth electrode49. The first metal foil32of the PTC device31electrically connects to the first electrode42through a first conductive connecting member44extending in a vertical direction. The second metal foil34electrically connects to the second electrode43through a second conductive connecting member45extending in a vertical direction. In an embodiment, the first and second conductive connecting members44and45may be semicircular holes made by mechanical drilling followed by electroplating conductive films thereon. The first conductive layer35on the heating layer37is separated from the first and second conductive connecting members44and45, and the second conductive layer36on the heating layer37is separated from the first and second conductive connecting members44and45as well. Moreover, the second conductive layer36is separated from the third conductive layer36′. Accordingly, a current path from the second conductive layer36, the heating layer37, the first conductive layer35, the heating layer37to the third conductive layer36′ is formed. This current path contains two heating resistors. That is, the heating element41contains two resistors in serial connection to further increase heating efficiency. The second conductive layer36connects to the third electrode46through the first conductive hole47, and the third conductive layer36′ electrically connects to the fourth electrode49through the second conductive hole47′. The first electrode42may further comprise an electrode42′ formed on a surface of the insulating layer38, and the second electrode43may further comprise an electrode43′ formed on the insulating layer38. The solder mask48is placed between electrodes42′ and43′.

In an embodiment, a solder mask50may cover separations among electrodes42,43,46and49but still expose electrodes42,43,46and49as interfaces for surface-mounting to a circuit board, as shown inFIG. 2D.

The equivalent circuit of the over-current protection device30of the second embodiment is depicted inFIG. 2E. In this embodiment, the heating element41comprises two resistors in serial connection to enhance heating efficiency. Similar toFIG. 1D, the heating element41may connect to a switch, upon a voltage drop or a temperature detected by a sensor, to determine whether to allow current to flow through the heating element41so as to enable the heating element41to heat and trip the PTC device31.

FIGS. 3A and 3Bshow an over-current protection device60in accordance with a third embodiment of the present application, which is a hexahedron that could be used for surface-mounting.FIG. 3Ashows a lateral view of the over-current protection device60andFIG. 3Bshows another lateral view of the device60, illustrating essential structures and conductive paths thereof. The over-current protection device60is a laminated structure comprising conductive layers, insulating layers and a PTC material layer extending in a horizontal direction, associating with vertical conductive connecting members to form a circuitry as desired. For the ease of describing the circuitry, upper and lower surfaces of the over-current protection device60, and individual conductive layers are shown inFIG. 3C. The shadow parts represent notches those are removed by etching in circuitry manufacturing for separation. The over-current protection device60essentially comprises a PTC device61and a heating element71. The PTC device61comprises a PTC material layer63, a first metal foil62and a second metal foil64. The first metal foil62and the second metal foil64are formed on upper and lower surfaces of the PTC material layer63, respectively. In an embodiment, the heating element71may be a PTC heater which comprises a heating layer67, a first conductive layer65and a second conductive layer66. The first conductive layer65is formed on an upper surface of the heating layer67, and the second conductive layer66is formed on a lower surface of the heating layer67. Insulating layers68,69and70are disposed on or between the PTC device61and the heating element71, and may comprise prepreg or other insulating materials. A surface of the insulating layer68is provided with a solder mask78, and a lower surface of the insulating layer70is provided with a first electrode72, a second electrode73, a third electrode76and a fourth electrode79. The third electrode76and the fourth electrode79are disposed between the first electrode72and second electrode73, and gaps are formed therebetween for separation. A separation is formed between the third electrode76and the fourth electrode79. The first metal foil62of the PTC device61electrically connects to the first electrode72through a first conductive connecting member74extending in a vertical direction. The second metal foil64electrically connects to the second electrode73through a second conductive connecting member75extending in a vertical direction. The first conductive layer65on the upper surface of the heating element67electrically connects to the third electrode76through a third conductive connecting member81. The second conductive layer66on the lower surface of the heating element67electrically connects to the fourth electrode79through a fourth conductive connecting member82. A separation is between the first conductive layer65and the fourth conductive connecting member82, and a separation is between the second conductive layer66and the third conductive connecting member81. Accordingly, if the third electrode76and the fourth electrode79connect to an electrical source, a current flows through the heating layer67to form a circuit containing a resistor. In an embodiment, the first, second, third and fourth conductive connecting members74,75,81and82may be semicircular holes electroplated with conductive films. The first electrode72may further comprise an electrode72′ formed on a surface of the insulating layer68, and the second electrode73may further comprise an electrode73′ formed on the insulating layer68. The solder mask78is placed between electrodes72′ and73′. A solder mask80forms a part of a lower surface of the over-current protection device60, and exposes electrodes72,73,76and79serving as interfaces for surface-mounting to a circuit board.

The equivalent circuit of the over-current protection device60of the third embodiment is depicted inFIG. 3D. In this embodiment, the heating element71comprises one resistor. Similar toFIG. 1D, the heating element71may connect to a switch, upon a voltage drop or temperature detected by a sensor, to determine whether to allow current to flow through the heating element71so as to enable the heating element71to heat and trip the PTC device61.

FIGS. 4A to 4Cshow an over-current protection device100in accordance with a fourth embodiment of the present application. The over-current protection device100contains two PTC devices in parallel connection to decrease resistance thereof.FIG. 4Ashows a top view of the over-current protection device100, andFIGS. 4B and 4Cshow cross-sectional views along lines1-1and2-2, respectively.FIG. 4Dis an equivalent circuit diagram of the over-current protection device100. The over-current protection device100is a laminated structure containing two PTC devices and a heating element. The over-current protection device100comprises a PTC device101, a PTC device111and a heating element105, and the heating element105is disposed between the PTC device101and PTC device111. As such, the heating element105can heat the PTC devices101and111simultaneously. The PTC device101comprises a PTC material layer103, a first metal foil102and a second metal foil104. The first metal foil102and the second metal foil104are formed on upper and lower surfaces of the PTC material layer103, respectively. The PTC device111comprises a PTC material layer109, a first metal foil108and a second metal foil110. The first metal foil108and the second metal foil110are formed on upper and lower surfaces of the PTC material layer109, respectively. For separation, an insulting layer112is disposed on an upper surface of the PTC device101, an insulating layer113is disposed between the PTC device101and the heating element105, and an insulating layer114is disposed on a lower surface of the PTC device111. The PTC material layers103and109may contain low-resistivity conductive filler as mentioned above to meet the requirement of low resistance of the device. In an embodiment, the heating element105may be a resistor, e.g., a PTC heater containing carbon black as conductive filler. The heating element105comprises a first conductive layer106, the first metal foil108and a heating layer107disposed therebetween. Because the use of carbon black, the heating element105has a higher resistance than the PTC devices101and111, and therefore it can effectively generate heat when current flows therethrough to heat the PTC devices101and111simultaneously. In this embodiment, the first metal foil108of the PTC device111also serve as a lower metal foil of the heating element105, that is, the first metal foil108is a common electrode. The second metal foil104of the PTC device101and the second metal foil110of the PTC device111electrically connect to a first electrode115formed on upper and lower surfaces of the device100through a vertical conductive connecting member121which may be a semicircular holes plated with a conductive film. Likewise, the first metal foil102of the PTC device101and the first metal foil108of the PTC device111electrically connect to a second electrode116formed on upper and lower surfaces of the device100through a vertical conductive connecting member122. As such, the PTC devices101and111are connected in parallel. The first conductive layer106of the heating element105electrically connects to a third electrode131on upper and lower surfaces of the over-current protection device100through a third conductive connecting member123. The insulating layers112and114among the electrodes115,116and131are covered by solder masks117and118. As the equivalent circuit diagram shown inFIG. 4D, the over-current protection device100contains two PTC devices101and111in parallel connection associating with only one heating element105. As long as the device100is not too thick, its resistance can be further decreased.

Test results of the over-current protection devices of the present application are shown in Table 1. The over-current protection devices of the embodiments Em 1-6 have various sizes and comprise a single PTC layer (a single PTC device), e.g., the aforementioned first embodiment, or two PTC layers, e.g., the aforementioned fourth embodiment. The data include initial resistances of the PTC devices “Ri (PTC)”, initial resistances of the heating elements, “Ri (heating),” surface temperatures (° C.) of the heating elements when 6V and 1 A are applied to the device, hold currents at 60° C. when the heating elements are not activated “Ih (60° C.)”, and trip currents when the heating elements are activated “It (heating)”. For comparison, comparative examples Comp 1 and 2 show the test results of the over-current protection devices without heating elements. In Em 1 to Em 6, the PTC devices use titanium carbide as conductive fillers. Alternatively, tungsten carbide and nickel powder may be used. The heating elements contain carbon black. The compositions and ratio of Em 1 to Em 6 are the same. Comp 1 and Comp 2 use the same PTC material as Em 1 to Em 6, but they do not have heating elements.

The over-current protection devices of Em 1 to Em 3 have areas of 12 mm2, 17.28 mm2and 47.5 mm2, respectively, and contain one PTC device. The over-current protection devices of Em 4 to Em 6 have areas of 12 mm2, 17.28 mm2and 47.5 mm2, respectively, and contain two PTC devices in parallel connection. Because parallel connection of two PTC devices, the initial resistance Ri (PTC) of Em 4 to Em 6 only about half those of Em 1 to Em 3 with same areas to obtain over-current protection devices of lower resistance. In Em 1 to 6, the resistances of the heating elements “Ri (heating)”, e.g., 0.1-0.6Ω, are much larger than the resistances of PTC devices “Ri (PTC)”, e.g., 0.0008-0.006Ω, by 50 to 70 times. The surface temperature of the heating element is about 80 to 110° C. when 6V/1 A is applied to the over-current protection devices. It appears that the heating element can effectively heat the PTC devices nearby after it is activated. It is observed that hold current at 60° C. when the heating element is not activated, i.e., “Ih (60° C.)”, is large and about 4-10 A. Even if battery temperature reaches 60° C., the included over-current protection device still allows high current charging for quick charge applications. However, a small current of only 0.1-0.3 A is able to trip the over-current protection device if the heating element is activated. To the contrary, Comp 1 and 2 without heating mechanism, they need 3 A to trip the over-current protection device. In summary, the over-current protection device of the present application has the relation: It (heating)<Ih (60° C.)×10%, where Ih (60° C.) is a hold current of the over-current protection device at 60° C. when the heating element is not activated; It (heating) is a trip current of the over-current protection device when the heating element is activated to heat the PTC device. In other words, the over-current protection device of the present application can sustain high hold current at high temperatures, and only need a small current to trip so as to effectively provide over-current protection. In Comp 1 and 2, It (heating) is about 0.4 to 0.6 times Ih (60° C.). That is, it needs large current to trip the over-current protection device and may be not able to timely provide over-current protection. In Table 1, Em 1-6 comply with the relation: It (heating)<Ih (60° C.)×8%, or It (heating)<Ih (60° C.)×5%, in particular.

Because the PTC device of low resistivity has high hold current at high temperatures, it is not easily tripped. In the present application, the heating element heats the PTC device for specific situations to decrease hold current of the PTC device to induce or accelerate trip. Accordingly, the problem that the PTC device of low resistivity is not easily tripped can be resolved. The over-current protection device of the present application has the features of low resistivity, high hold current and quick trip within 60 seconds at 60° C. when 8 A is applied thereto.