Abstract:
A circuit protection device includes a housing, which includes first and second electrodes. The device includes a conductive slider inside the housing. At a first location within the housing, the slider provides an electrical connection between the first and second electrodes. At a second location within the housing, the slider does not provide the electrical connection. A spring is secured to and stretched between the slider and an inner side of the housing such that the spring is held in tension in an expanded state. The slider is held at the first location by a solder between the slider and the first and second electrodes. After the device is armed, detection of an over-temperature condition causes the solder to begin to melt and the spring to compress and pull the slider to the second location within the housing, thus severing the electrical connection between the first and second electrodes.

Description:
BACKGROUND 
     I. Field 
     The present invention relates generally to electronic protection circuitry. More, specifically, the present invention relates to a reflowable surface mount circuit protection device, which may also be adapted to a weldable or pluggable installation. 
     II. Background Details 
     Protection circuits are often times utilized in electronic circuits to isolate failed circuits from other circuits. For example, the protection circuit may be utilized to prevent electrical or thermal fault condition in electrical circuits, such as in lithium-ion battery packs. Protection circuits may also be utilized to guard against more serious problems, such as a fire caused by a power supply circuit failure. 
     One type of protection circuit is a thermal fuse. A thermal fuse functions similar to that of a typical glass fuse. That is, under normal operating conditions the fuse behaves like a short circuit and during a fault condition the fuse behaves like an open circuit. Thermal fuses transition between these two modes of operation when the temperature of the thermal fuse exceeds a specified temperature. To facilitate these modes, thermal fuses include a conduction element, such as a fusible wire, a set of metal contacts, or set of soldered metal contacts, that can switch from a conductive to a non-conductive state. A sensing element may also be incorporated. The physical state of the sensing element changes with respect to the temperature of the sensing element. For example, the sensing element may correspond to a low melting metal alloy or a discrete melting organic compound that melts at an activation temperature. When the sensing element changes state, the conduction element switches from the conductive to the non-conductive state by physically interrupting an electrical conduction path. 
     In operation, current flows through the fuse element. Once the sensing element reaches the specified temperature, it changes state and the conduction element switches from the conductive to the non-conductive state. 
     One disadvantage of some existing thermal fuses is that during installation of the thermal fuse, care must be taken to prevent the thermal fuse from reaching the temperature at which the sensing element changes state. As a result, some existing thermal fuses cannot be mounted to a circuit panel via reflow ovens, which operate at temperatures that will cause the sensing element to open prematurely. 
     Thermal fuses described in U.S. patent application Ser. No. 12/383,595, filed Mar. 24, 2009 and published as U.S. Publication No. 2010/0245022, and U.S. application Ser. No. 12/383,560, filed Mar. 24, 2009 and published as U.S. Publication No. 2010/0245027—the entirety of each of which is incorporated herein by reference—address the disadvantages described above. While progress has been made in providing improved circuit protection devices, there remains a need for improved circuit protection devices. 
     SUMMARY OF THE INVENTION 
     A circuit protection device includes a housing, which includes first and second electrodes. The device includes a conductive slider inside the housing. At a first location within the housing, the slider provides an electrical connection between the first and second electrodes. At a second location within the housing, the slider does not provide the electrical connection. A spring is secured to and stretched between the slider and an inner side of the housing such that the spring is held in tension in an expanded state. The slider is held at the first location by a solder between the slider and the first and second electrodes. After the device is armed, detection of an over-temperature condition causes the solder to begin to melt and the spring to compress and pull the slider to the second location within the housing, thus severing the electrical connection between the first and second electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a reflowable surface mount circuit protection device prior to being armed. 
         FIG. 2  shows a cross sectional view of the device shown in  FIG. 1  in a closed position. 
         FIG. 3  shows a cross sectional view of the device shown in  FIG. 1  in an open position. 
         FIG. 4 a    is a circuit representation of an exemplary circuit protection device for protecting a circuit external to the device. 
         FIG. 4 b    is a circuit representation of the circuit of  FIG. 4 a    with the fusible link blown and the slider in the closed position. 
         FIG. 4 c    is a circuit representation of the circuit of  FIG. 4 b    with the slider in the open position. 
         FIGS. 5 a -5 f    illustrate exemplary assembly steps a circuit protection device. 
         FIG. 6  is another example of a reflowable circuit protection device. 
         FIG. 7  shows an example of a weldable circuit protection device. 
         FIG. 8  shows another example of a weldable circuit protection device. 
         FIG. 9  shows yet another example of a weldable circuit protection device. 
         FIG. 10  shows an example of the subassembly structure inside the device of  FIG. 8 . 
         FIG. 11  shows an example of a pluggable circuit protection device. 
         FIGS. 12 a - d    illustrate selected parts of a reflowable circuit protection device. 
         FIG. 13  shows a cross-section of a circuit protection device including a capillary break. 
         FIG. 14  shows a zoomed-in view of the electrode of the device shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a reflowable surface mount circuit protection device  100  prior to being armed. The device  100  includes a slider  102 , spring  104 , and a fusible element  106  inside of a housing  108 . In  FIG. 1 , the spring  104  is a helical tension spring. The housing  108  includes an arming pin  110  and electrodes  112 ,  114 . The electrodes may be, for example, surface mount pads for connecting the device  100  to the circuit to be protected. The housing  108  includes an arm  116 . A bottom surface of the end of the arm  116  includes an arming pad that is electrically connected to the arming pin  110  through the housing  108 . An arming current (discussed below) is applied to the arming pin  110  via the arming pad. 
     The slider  102  may be made of a conductive material such as copper. In the embodiment shown in  FIG. 1 , the slider  102  includes two protrusions  118  extending from an upper surface of the slider  102 . The fusible element  106  includes two openings that fit over the protrusions  118 , securing the fusible element  106  to the slider  102 . While  FIG. 1  shows a slider having two protrusions, it will be understood that in other embodiments the slider may include a different number of protrusions, and the fusible element may include a number of openings to match the number of protrusions in the slider. Other attachment methods may be used including laser welding, and mechanical fasteners such as with an adhesive, screws, rivets, etc. In some embodiments in which other attachment methods are used, the slider  102  may omit the protrusions  118 . 
     The device  100  also includes a fusible link  120  and an arming pin connector  122  connected to the fusible link  120 . The fusible link  120  may be made of the same material and be integrally connected with the fusible element  106 . The arming pin connector  122  includes a loop, or opening, that hooks over the arming pin  110 , providing an electrical connection between the arming pin and the fusible link  120 . The fusible link  120  provides an electrical and mechanical connection between the fusible element  106  and the arming pin  110  until the fusible link  120  is blown (discussed below). 
     The slider  102  includes a pocket in which a portion of the spring  104  is inserted. In  FIG. 1  the pocket is a depression defined in the slider  102  that is sufficiently deep such that all or a substantial part of the portion of the spring  104  inserted in the pocket is below the upper surface of the slider  102 . It will be appreciated that in other embodiments, the pocket may be more shallow and receive a portion of the head of the spring  104 , such as in  FIG. 6 . In  FIG. 1  the spring  104  is shown to be in tension in an expanded state. One end  124  of the spring  104  is inserted into the pocket of the slider  102 . The other end  126  of the spring  104  is stretched to and inserted into an overmold portion  128  of the housing  108 . The fusible element  106  may include a portion that covers part of the spring  104  to help hold the spring  104  in place. 
     The slider  102  may be soldered to the bottom of the inside of the housing  108 , which holds the slider  102  in place (resisting the compression force of the spring  104  held in tension) after the device  100  is installed in a circuit to be protected. The slider  102  provides an electrical connection between the electrodes  112  and  114 . 
     The melting point of the solder holding the slider  102  in place may be lower than a reflow temperature. The fusible link  120 , which is made of a material that allows it to open at a temperature higher than that of the reflow temperature and thus may have a melting point higher than that of the reflow temperature, is provided to hold the slider  102  and fusible element  106  in place during reflow. After reflow and when the device  100  is installed in the device to be protected, an arming current is applied to the arming pin  110  and through the fusible link  120  that causes the fusible link  120  to open. With the fusible link  120  open, the device  100  is armed. If the circuit to be protected overheats, causing the solder holding the slider  102  in place to begin to melt, the force of the spring  104  pulls the slider  102  to an open position in which there is no longer an electrical connection between the electrodes  112  and  114 , thus protecting the circuit from overheating. 
     The following are examples of dimensions for the device. The device  100  may be approximately 11.6 mm long, approximately 8.2 mm wide on the end of the device  100  with the arm  116 , approximately 6.2 mm wide on the other end of the device  100 , and approximately 3.4 mm in height. The arm  116  of the housing may be approximately 1.4 mm wide. 
     It will be appreciated that the arming pad (located at the bottom surface of the arm  116  in  FIG. 1 ) may be located at different locations on the housing  108 . For example, the arming pad may be located between the electrodes  114  and  112  with an electrical connection to the arming pin  122 . In this example, the housing  108  may omit the arm  116 . 
       FIG. 2  shows a cross sectional view of the device  100  in a closed position. For the purposes of illustration, certain elements of the device  100 , e.g., the fusible element  106 , are not shown. The slider  102  provides a conductive path between the electrodes  112  and  114 . 
       FIG. 3  shows a cross sectional view of the device  100  in an open position. If, for example, the circuit to which the device  100  is connected overheats to an overtemperature condition, causing the solder holding the slider  102  in place to begin to melt, the spring  104  pulls the slider  102  in the direction indicated by the arrow  300 . In this manner, the electrical connection between the electrodes  112  and  114  is severed, thus protecting the outside circuit from overheating. Element  130  indicates where the solder is provided above the electrode  112 . While not visible in  FIG. 3 , solder is similarly provided above the electrode  114 . 
       FIGS. 4 a -4 c    are a circuit representation  400  of an exemplary circuit protection device for protecting a circuit external to the device. The circuit  400  includes electrodes  402  and  404 , which may correspond to the electrodes  112  and  114 , respectively, shown in  FIG. 1 . Electrode  406  corresponds to the arming pin  110  shown in  FIG. 1 . The circuit  400  also includes a fusible link  408  connected to the electrode  406  (arming pin  110 ). An arming current may be applied to the fusible  408  through the electrode  406 . The circuit  400  also includes a conductive element  410  between the electrodes  402 ,  404 , which may correspond to the slider  102  shown in  FIG. 1 . For the sake of explanation, the circuit protection device can be positioned in series between circuit components to be protected, such as one or more FETs. It will be understood that the circuit protection device may be used in other circuit configurations. 
       FIG. 4 a    shows the circuit  400  before the fusible link  408  is blown, i.e., before the device is armed.  FIG. 4 b    shows the circuit  400  after the fusible link  408  is blown. Further, in  FIGS. 4 a -4 b    the slider  410  is in the closed position, thus bridging and providing an electrical connected between electrodes  402 ,  404 .  FIG. 4 c    shows the circuit  400  in the open position in which the electrical connected between the electrodes  402 ,  404  is severed, such as after an over-temperature condition is detected. 
       FIGS. 5 a -5 f    illustrate exemplary assembly steps a circuit protection device, such as the device  100  shown in  FIG. 1 .  FIG. 5 a    illustrates that a slider  500  is provided. The slider  500  may be made of a conductive material, such as copper. The slider  500  includes a pocket  502  shaped to accept a spring (see  FIG. 2 b   ). The slider  500  also includes protrusions  504  that extend up from an upper surface of the slider  500 . Other attachment methods may be used including laser welding, and mechanical fasteners such as with an adhesive, screws, rivets, etc. 
       FIG. 5 b    shows that a spring  506  is placed in the pocket  502 . The spring  506  may be a coil spring or other spring element having elasticity and being capable of being brought into tension through expansion. 
       FIG. 5 c    shows that a fusible element  508  is placed on top of at least a part of the slider  500 . The fusible element  508  includes two openings that fit over the protrusions  504  extending from the slider  500 . The fusible element  508  may be joined onto the slider  500  using known stamping techniques. A fusible link  510  is connected to the fusible element  508  at a side of the fusible element  508  opposite to the side of element  508  near the openings. An arming pin connector  512  is connected at the end of the fusible link  510  that opposite to the end of the fusible link  510  connected to the fusible element  508 . The arming pin connector  512  connects to an arming pin  522  that is part of the device housing (see  FIG. 5 e   ). 
     The fusible element  508  may be attached to the slider  500  via the openings  510  and protrusions  504 . In particular, the fusible element  508  may be secured to the slider  500  via known crimping techniques performed on the protrusions  504  to hold the fusible element  508  down and prevent the element  508  from sliding back up the protrusions  504 . Other techniques may include, depending on the material used for the slider  500  and/or the fusible element  508 , laser or resistance welding, or high temperature adhesion, mechanical fasteners such as screws or rivets. 
     The fusible element  508  may be made of a material capable of conducting electricity. For example, the fusible element  508  may be made of copper, stainless steel, or an alloy. The diameter of the fusible link  510  may be sized so as to enable blowing the fusible link  510  with an arming current. The fusible link  510  is blown, such as by running a current through the fusible link  510 , after the device is installed in a circuit to be protected. In other words, sourcing a sufficiently high current, or arming current, through the fusible link  510  may cause the fusible link  510  to open. In one embodiment, the arming current may be about 2 Amperes. However, it will be understood that the fusible link  510  may be increased or decrease in diameter, and/or another dimension, allowing for higher or lower activating currents. 
       FIG. 5 d    shows an inside of a housing  514  in which the slider  500 , spring  506 , and fusible element  508  will be placed. At the bottom of the housing  514  there are provided solder preforms  516 ,  518 . An underside of the housing  514  may include electrodes, e.g., surface mount pads, corresponding to teach of the solder preforms  516 ,  518 , thus providing an electrical connection between the circuit to be protected and the slider that will be placed inside the housing  514 . The housing  514  also includes an arming pin  520  through which an arming current is provided to the fusible link  510 . The arming pin  520  includes a hook-like protrusion  522  over which the arming pin connector  512  may be paced. 
       FIG. 5 e    shows that the assembly including the slider  500 , spring  506 , and fusible element  508  is placed in the housing  514 . In particular, the arming pin connector  512  is secured to the arming pin  520 . The bottom of the slider  500  is soldered to the solder preforms  516 ,  518 . Once cooled, the solder holds the slider in place when the spring  506  is stretched (see  FIG. 5 f   ). 
       FIG. 5 f    shows that the spring  506  is then stretched. The end of the spring  506  not inserted in the slider  500  is stretched to an overmold section  524  at the opposite end of the housing. As shown in  FIGS. 5 b -5 f   , the ends of the spring  506  have a wider diameter than the middle portion of the spring  506  to allow the ends of the spring  506  to fit into the overmold  524  and the pocket  502  and remain in tension. 
     The resulting device is shown, for example, in  FIG. 1 , which is then subject to reflow in a reflow oven. During a reflow process, the solder holding the slider  500  to the outside electrodes, which would result in the slider  500  moving to an open position due to the force of the spring  506  held in tension. For example, the melt point of the solder may be approximately 140° C., while the temperature during reflow may reach more than 200° C., for example 260° C. Thus, during reflow the solder would melt, causing the spring  506  to prematurely pull the slider  500  to the open position. To prevent the force applied by the spring  506  from opening the circuit protection device during installation, the fusible link  510 , which has a higher melting point than the solder, may be utilized to maintain the slider  500  in place and resist the compression force of the spring  506 . 
     A cap (not shown) is placed over the housing using, for example, a snap-fit connection and the device is ready to be installed in a circuit to be protected. Once installed, the device is armed by applying an arming current, as discussed above, to the fusible link  510  through the arming pin  520 . The fusible link  510  opens and the device is armed. 
       FIG. 6  is another example of a reflowable circuit protection device  600 . The device  600  differs from the device  100  of  FIG. 1  in that the fusible element is omitted. In  FIG. 6 , the fusible link  602  is part of the slider  604 . For example, the slider  604  and fusible link  602  may be one contiguous part stamped out of copper. In this example, the slider  604  may include an arming pin connector  606  that hooks over (in one embodiment) or otherwise connects to the arming pin of the housing  608 . The slider  604  may be made of a copper material, and the fusible link  602  being a thin strand of copper connected between the body  610  of the slider  604  and the arming pin connector  606 . The fusible link  602  portion of the slider  604  is coated by an epoxy. In this example, a higher arming current, relative to the arming current required to arm the device of  FIG. 1 , may be required to arm the device  600  after reflow due to the lower resistance of the copper link  602 . In  FIG. 6 , the slider  604  includes a grip portion  612  that holds one end of the spring  614  in place above the slider  604 . 
     Similar to the device of  FIG. 1 , the fusible link  602  holds the slider  604  in place during reflow. After reflow, the device  600  is armed by applying an arming current through the fusible link  602 . Once the device is armed, if the device overheats the solder between the slider  604  and the electrodes  616 ,  618  melts, causing the force of the extended spring to pull the slider  604  towards the overmold portion  620 . 
       FIG. 7  shows an example of a weldable circuit protection device  700 . The device  700  is shown including the cap  702  that fits over the housing. The structure inside the cap/housing may be, for example, the structure shown in  FIG. 1  or  FIG. 6 , or  FIG. 10  as described below. For a weldable device  700 , the electrodes  704 ,  706  (i.e., lead frames) are extended relative to those of the surface mount device shown in  FIG. 1  or  FIG. 6 . The weldable device allows the customer to install the device  700  using, for example, resistance welding. In one embodiment, the weldable device  700  may not include an arming pin or fusible link connected between the fusible element and the arming pin. 
       FIGS. 8-9  show other examples weldable devices  800  and  900 . Each of the devices  800  and  900  include electrodes  802 ,  804  and  902 ,  904 , respectively, having different shapes according to a client&#39;s needs. 
       FIG. 10  shows an example of the subassembly structure inside the device  900 . As noted above, in one embodiment the weldable device  700  may not include an arming pin or fusible link connected between the fusible element and the arming pin, which is illustrated in  FIG. 10 . The device  900  includes a slider  906  and a spring  908 . The slider  906  includes a grip portion  910  that holds one end of the spring  908  to the slider  906 . The other end of the spring  908  is held by the overmold portion  912  of the housing  914 . 
       FIG. 11  shows an example of a pluggable circuit protection device  1100 . The device  1100  is shown including the cap  1102  that fits over the housing. The structure inside the cap/housing may be, for example, the structure show in  FIG. 1, 6 , or  10 . The pluggable circuit protection device  1100  includes electrodes  1104 ,  1106  structured to be able to be plugged into a receptacle on a circuit board or other circuit. The pluggable device  1100  may be a single-use fuse structured to be plugged into a fuse box. 
       FIGS. 12 a - d    illustrate selected parts of a reflowable circuit protection device.  FIG. 12 a    shows a slider subassembly  1200  of the device including a stamped slider  1202 , a fusible element  1204 , and a helical tension spring  1206 . The subassembly  1200  includes an arming pin connector  1208  and a fusible link  1210  connected between the fusible element  1204  and the arming pin connector  1208 . Similar to  FIG. 1 , the slider  1202  may be made of copper. The fusible element  1204  in this example is attached to the slider  1202  by laser welding. The slider of in the device of  FIG. 1  included a pocket in which a substantial portion of the spring was inserted. In the subassembly  1200  of  FIG. 12 a   , the slider  1202  may also include a smaller pocket that receives a portion of the end of the spring  1206  to allow the length of the spring  1206  over the fusible element  1204  to lay flush with the fusible element  1204 . 
       FIG. 12 b    illustrates that the subassembly  1200  of  FIG. 12 a    is inserted into the housing  1212 .  FIG. 12 b    also shows two solder preforms  1214 ,  1216  applied above the electrodes  1218 ,  1220 . The subassembly  1200  is inserted after the solder preforms  1214 ,  1216  are applied. 
       FIG. 12 c    illustrates that a cap  1222  is placed over the housing  1212 . In this example, the cap  1222  snaps onto the housing  1212 . Before the cap  1222  is snapped onto the housing, the spring  1206  is stretched and the end of the spring  1206  not secured to the slider  1202  is inserted into the overmold portion  1224  of the housing  1212  to place the spring  1206  in tension. In addition, a solder paste may be applied to arming pin  1226  of the housing. A purpose of solder paste is to ensure high reliability conductive connection between between the arming pin and the arming pin connector. The arming pin may also be pre-tinned. 
       FIG. 12 d    shows the assembled device  1228 . After assembly, the device  1226  may be subject to reflow in a reflow oven. 
       FIG. 13  shows a cross-section of a circuit protection device  1300  including a capillary break. The device  1300  includes a slider  1302 , spring  1304 , fusible element  1306 , fusible link  1308  within a housing  1310 . The device  1300  also includes electrodes  1312  and  1314  mounted on a circuit board  1316 . 
       FIG. 14  shows a zoomed-in view of the electrode  1314  of  FIG. 13 . The sides of the electrodes  1312  and  1314  each include a cutout portions  1318  forming a stepwise contour to the bottom sides of the electrodes  1312  and  1314 , thereby creating a space  1320 , i.e., capillary break, between the bottom surface of the housing  1310  and the circuit board  1316 . The capillary break prevents liquid flux on the circuit board  1316  that may melt during reflow from following, by capillary force, the capillary path  1322 . 
     While the circuit protection device has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the claims of the application. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from its scope. Therefore, it is intended that the reflowable circuit protection device is not to be limited to the particular embodiments disclosed, but to any embodiments that fall within the scope of the claims.