Patent Publication Number: US-2005128046-A1

Title: Over-current protection device

Description:
BACKGROUND OF THE INVENTION  
      (A) Field of the Invention  
      The present invention is related to an over-current protection device, and more specifically to an over-current protection device capable of increasing operation current.  
      (B) Description of the Related Art  
      The resistance of a positive temperature coefficient (PTC) conductive material is sensitive to temperature variation, and can be kept extremely low at normal operation due to its low sensitivity to temperature variation so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g., above 10 4  ohm.) Therefore, the over-current will be reversely eliminated and the objective of protecting the circuit device can be achieved. Consequently, PTC devices have been commonly integrated into various circuitries so as to prevent the damage caused by over-current.  
      A traditional over-current protection device comprises a current-sensitive element and two pre-preg (P/P) layers. The P/P layers are stacked on the surfaces of the current-sensitive element by hot press, and in consequence they function as an exterior protective material of the current-sensitive element to prevent moisture immersion and scratch. Moreover, the P/P layers function as insulation layers also.  
      The glass switching temperature (Tg) of P/P is commonly between 130 and 140° C., and the glass switching temperature of the so-called high Tg P/P is between 170 and 180° C. The curing temperature of the P/P layers needs to be taken into account for hot-pressing the P/P layers and the current-sensitive element, and the hot-press temperature has to be larger than the glass switching temperature of the P/P layer by 30 to 50° C. However, such high temperature process may cause the expansion of the PTC material within the current-sensitive element, or even wrinkles on the surface of the P/P layer. Under the circumstances, the above events will affect the final dimensions of the over-current protection device. Further, after the over-current protection device is hot-pressed, the resistance of the over-current protection device becomes larger than 1.2 times that before being hot-pressed, inducing the applications for the over-current protection device to be tremendously limited.  
      Moreover, for the decrease in size of the over-current device, the heat dissipation of the device becomes an important design factor. Traditionally, the heat dissipation rate of a P/P layer is between 0.3 and 0.5W/° C.-m. However, it cannot dissipate heat efficiently, and therefore the life-time and reliability of the over-current protection device are decreased. Accordingly, if the heat dissipation rate of the P/P layer can be increased, the over-current protection device will have more stable performance, and can be used in more applications.  
     SUMMARY OF THE INVENTION  
      The objective of the present invention is to provide an over-current protection device for decreasing the resistance trip ratio of the over-current protection device through hot-press, and increasing the heat dissipation rate so as to increase the operation current, so that the device can be in wide use.  
      To achieve the above-mentioned objective, an over-current protection device is disclosed. The over-current protection device comprises a current-sensitive element, two insulating layers and two electrode layers. The current-sensitive element comprises two electrode foils and a current-sensitive layer laminated between the two electrode foils, where the current-sensitive element is composed of PTC material. The two insulating layers are stacked on the upper and lower surfaces of the current-sensitive element, respectively, and the glass switching temperature thereof is between 90-120° C. or the heat dissipation rate is between 1-7W/° C.-m. The two electrode layers are connected to the two ends of the current-sensitive element, respectively.  
      As usual, the two electrode layers are composed of copper, aluminum or aluminum-copper alloy, but there may be a potential oxidation issue due to the nature of the materials. The over-current protection device can further comprise two soldering electrode layers capping the two electrode layers, where the two soldering electrode layers are composed of tin or tin-lead alloy that is capable of anti-oxidation. Consequently, the two electrode layers are not directly in contact with atmosphere, so that the oxidation of the two electrode layers can be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates the over-current protection device of the first embodiment in accordance with the present invention;  
       FIG. 2  is the cross-sectional view along the line 1-1 in  FIG. 1 ; and  
       FIG. 3  illustrates the over-current protection device of the second embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates the over-current protection device  10  of the first embodiment, and  FIG. 2  is the cross-sectional view of the line  1 - 1  in  FIG. 1 . The over-current protection device  10  comprises a current-sensitive element  13 , two insulating layers  14 , two solder-mask (S/M) layers  15  and two electrode layers  16 . The current-sensitive element  13  comprises a current-sensitive layer  11  and two electrode foils  12 , where the current-sensitive layer  11  is sandwiched between the two electrode foils  12 . The current-sensitive layer  11  is composed of polymeric PTC material. The insulating layer  14  can be composed of P/P, resin or epoxy, and the glass switching temperature is between 90 and 120° C. The two electrode layers  16  are disposed at the two ends of the current-sensitive element  13 , two insulating layers  14  and two S/M layers  15 , respectively.  
      In comparison with the insulating layers of a known over-current protection device, the insulating layers  14  of the over-current protection device  10  have lower glass switching temperature. Therefore, the dimensions of the over-current protection device  10  vary slightly through hot-press, and the resistance trip ratio through hot-press can be decreased.  
      Referring to Table 1, the over-current protection device  10 , including the insulating layer of lower glass switching temperature, has lower resistance, i.e., it can provide larger operation current.  
                       TABLE 1                                      Resistance Trip Ratio Through Hot Press                             The present invention   Known       Device   (90° C. ≦ Tg ≦ 120° C.)   (130° C. ≦ Tg ≦ 140° C.)               0603   1.07   1.31       0805   1.01   1.30       1206   1.00   1.22                  
 
      Power dissipation (Pd) of the over-current protection device can be expressed by the equation Pd=I 2 R, wherein I is current, and R is resistance. According to the above equation, the higher the power dissipation is, the higher the current is. From physical point of view, better heat dissipation rate indicates that the heat caused by current can be dissipated fast, i.e., better power dissipation efficiency. Accordingly, the device before being tripped can withstand more current. In other words, an over-current protection device of higher heat dissipation rate can be applied to the circumstances under larger operation current.  
      The test result of the over-current protection device  10  including the insulating layer  14  made of P/P, resin or epoxy with heat dissipation rate between 1-7W/° C.-m is shown in Table 2. In view of Table 2, the over-current protection device set forth in the present invention, i.e., the one having an insulating layer of higher heat dissipation rate, has higher operation current and power dissipation in comparison with a known one.  
                       TABLE 2                                      Over-Current Protection Device                             The present invention   Known       Device 0805   (1-7 W/° C.-m)   (0.3-0.5 W/° C.-m)                                 Resistance (ohm)   0.248   0.245       Operation Current (A)   1.10   0.95       Power Dissipation (W)   0.66   0.54                  
 
       FIG. 3  illustrates an over-current protection device  30  of the second embodiment in accordance with the present invention. The over-current protection device  30  comprises a current-sensitive element  33 , two insulating layers  34 , two S/M layers  35 , two electrode layers  36  and two soldering electrode layers  37 . The current-sensitive element  33  is formed by laminating a current-sensitive layer  31  between two electrode foils  32 , wherein the current-sensitive layer  31  is made of polymeric PTC material. The insulating layer  34  can be made of P/P, resin or epoxy whose glass switching temperature is between 90-120° C. or heat dissipation rate is between 1-7W/° C.-m. The two electrode layers  36  are disposed at the two ends of the current-sensitive element  33 , two insulating layer  34  and two S/M layers  35 , respectively. The two soldering electrode layers  37  cap the electrode layers  36  for being connected with leads.  
      In comparison with the over-current protection device  10 , the over-current protection device  30  further comprises the two soldering electrode layers  37  capping the two electrode layers  36 . For increasing electrical conduction, the electrode layers  36  are commonly composed of copper, aluminum and aluminum-copper alloy. If the electrode layers  36  are soldered to leads, the electrode layers  36  exposed to atmosphere will be easily oxidized. The soldering electrode layers  37  are made of tin or tin-lead alloy which has the anti-oxidation nature, so that the electrode layers  36  can be avoided to directly contact atmosphere by capping the soldering electrode layers  37  thereon.  
      The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.