Abstract:
A circuit protection device includes a reflowable thermal fuse that shuts off current to a field-effect transistor if the field-effect transistor is overheating. In particular, the reflowable thermal fuse is integrated with the field-effect transistor to provide greater proximity between the field-effect transistor and the reflowable thermal fuse, thus providing efficient and accurate detection of an overtemperature condition in the field-effect transistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is an application under 35 USC 111(a) and claims priority under 35 USC 119 from Provisional Application Ser. No. 61/456,052, filed Oct. 29, 2010 under 35 USC 111(b). The disclosure of that provisional application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electronic protection circuitry. More specifically, the present invention relates to a field-effect transistor embedded with an electrically activated surface mount thermal fuse. 
     2. Introduction to the Invention 
     Protection circuits are often utilized in electronic circuits to isolate failed circuits from other circuits. For example, the protection circuit may be utilized to prevent a cascade failure of circuit modules in an electronic automotive engine controller or on a computer motherboard. Protection circuits may also be utilized to guard against more serious problems, such as a fire caused by a transistor overheating or other power supply circuit failures. 
     One type of protection circuit is a thermal fuse. U.S. Publication No. 2010/0245027, published Sep. 30, 2010, and U.S. Publication No. 2010/0245022, published Sep. 30, 2010, each of which is incorporated herein by reference in its entirety, disclose improved thermal fuses that are resettable, or reflowable. The reflowable thermal fuse is configured to, for example, open a circuit if a field-effect transistor (FET) temperature reaches an overtemperature condition. The reflowable thermal fuse may be placed close enough to the FET to provide sufficient heat transfer. The reliability of the reflowable thermal fuse to detect an overheating FET and open the corresponding circuit is improved by an increased proximity to the FET, which ensures a more efficient heat transfer from the FET to the reflowable thermal fuse. 
     SUMMARY OF THE INVENTION 
     A circuit protection device includes a reflowable thermal fuse that shuts off current to a FET if the FET is overheating. In particular, the reflowable thermal fuse is integrated with the FET to provide greater proximity between the FET and the reflowable thermal fuse, thus providing efficient and accurate detection of an overtemperature condition in the FET. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary circuit including a circuit protection device. 
         FIG. 2  shows a second exemplary circuit including a circuit protection device. 
         FIG. 3  shows a third exemplary circuit including a circuit protection device. 
         FIG. 4  shows an embodiment of the integrated circuit protection device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary circuit  100  including a circuit protection device  102  having an integrated FET  104  and reflowable thermal protection device, referred to herein as a reflowable thermal fuse (RTF)  106 . The schematic includes a power supply  108 , the circuit protection device  102 , a load resistance  110  and a control IC  112 . The circuit protection device includes a FET connected in series and integrated with a reflowable thermal fuse. 
       FIG. 2  shows a second exemplary circuit  200  including a circuit protection device  202  having an integrated FET  204  and reflowable thermal fuse  206 . The circuit protection device  202  includes an activation pin  208  to which the reflowable thermal fuse  206  is connected. The activation pin  208  is in communication with ground. The circuit protection device  202  may be used, for example, for FETs in a high-side configuration. 
       FIG. 3  shows a third exemplary circuit  300  including a circuit protection device  302  in which the FET  304  is an N-channel Power FET in a low-side configuration. In this example the reflowable thermal fuse  306  is in communication with the drain of the N-channel Power FET. 
     It will be appreciated that the reflowable thermal fuse  106 ,  206 ,  306  may also be integrated with FETs of the P-channel type. It will also be appreciated that the reflowable thermal fuse may be integrated with FETs in low and/or high side configurations. Further, it will be appreciated that the circuit protection device  102 ,  202 ,  302  may include multiple FETs integrated with and protected by the reflowable thermal fuse. While the circuits are shown and described in the context of an automotive control circuit, it will be understood that the circuit protection device may be used for circuit protection on computer motherboards or other electronics applications to protect against FET overheating. 
     The reflowable thermal fuse  106 ,  206 ,  306  is configured to open up when the FET achieves an overtemperature condition. Generally, the reflowable thermal fuse  106 ,  206 ,  306  includes a conduction element through which a load current flows, and an elastic element adapted to apply a force on the conduction element. In some embodiments, the conduction element incorporates a sensing element. When the temperature of the sensing element exceeds a threshold, the sensing element loses its resilience and becomes susceptible to deformation and/or breakage via the force on the conduction element applied by the elastic element. Eventually, the conduction element mechanically opens under the force, resulting in an open circuit condition. In other embodiments, the sensing element and the conduction element are separate and the sensing element acts to keep the conduction element in a low resistance state. 
     During a reflow process, the sensing element may lose its resilience. To prevent the force applied by the elastic element from opening the conduction element during installation, a restraining element may be utilized to maintain the elastic element in a state whereby the elastic element does not apply force on the conduction element. After the reflowable thermal fuse  106 ,  206 ,  306  is installed on a panel and/or embedded into the FET, and passed through a reflow oven, the restraining element may be blown by applying an activating current through the restraining element. This in turn activates the reflowable thermal fuse  106 ,  206 ,  306 . 
     The reflowable thermal fuse is described in more detail in U.S. application Ser. Nos. 12/383,560 and 12/383,595, each of which is incorporated herein by reference in its entirety. As described herein, the reflowable thermal fuse is integrated with the FET to provide greater proximity between the FET and the reflowable thermal fuse, thus providing greater efficiency and accuracy in detecting the overtemperature condition of the FET. 
     In one embodiment, the FET may be embedded with the reflowable thermal fuse directly into a single package or component. For example, the reflowable thermal fuse may be embedded into the FET before molding a package around the FET. In this example the reflowable thermal fuse may be in intimate thermal contact with the FET. The embedded reflowable thermal fuse may then be reflowed on a board. The structure of the reflowable thermal fuses described in U.S. application Ser. Nos. 12/383,560 and 12/383,595, allows the reflowable thermal fuses to be embedded into the FET and reflowed on a board without the molding or reflow process opening up the thermal fuse. 
     Some components in the FET structure can be used as the components of reflowable thermal fuse, or vice versa. For example, the spring-bar of an exemplary reflowable thermal fuse, as described in U.S. application Ser. Nos. 12/383,560 and 12/383,595, may be used as a clip to attach to the source or drain of the FET in place of wire bonds. The reflowable thermal fuse may be embedded below the FET, or above the FET to account for rising heat. The reflowable thermal fuse may be embedded laterally relative to the FET on the FET PCB. The structure of the reflowable thermal fuse may be configured to provide a lower resistance than the FET&#39;s R DS on . 
       FIG. 4  shows another embodiment of an integrated circuit protection device  400 . In particular,  FIG. 4  shows a reflowable thermal fuse package  402  linked to the heat sink of a separate FET package  404 , which may include multiple FETs. The reflowable thermal fuse package  402  may also include multiple reflowable thermal fuses, or may include one reflowable thermal fuse protecting one or more FETs of the FET package  404 . Leads  406  from the FET package  404  may connect with leads  408  on the reflowable thermal fuse package  402 . The reflowable thermal fuse package  402  may further include leads  410  that connect to the circuit board. While  FIG. 4  shows the FET package  404  above the reflowable thermal fuse package  402 , it will be understood that a FET package may be connected below a reflowable thermal fuse package. 
     While the integrated 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 circuit protection device including integrated FET and reflowable thermal fuse are not to be limited to the particular embodiments disclosed, but to any embodiments that fall within the scope of the claims.