Abstract:
An improved over-voltage and over-current protection device is provided. The device includes: a first over-current protection device disposed between a first electrically conductive terminal and a second electrically conductive terminal, wherein the first over-current protection device creates an open circuit when a current exceeding a certain level flows between the first terminal and the second terminal; a first over-voltage protection device electrically coupled to the first terminal, wherein the first over-voltage protection device clamps voltages applied to the first terminal below a specified level; and a second over-voltage protection device electrically coupled to the second terminal, wherein the second over-voltage protection device clamps voltages applied to the second terminal below a specified level.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an improved over-voltage and over-current protection device for protecting electronic circuits from relatively high voltages and/or currents that may otherwise damage the electronic circuits.  
         [0003]     2. Description of Related Art  
         [0004]     Our contemporary society enjoys the convenience and utility offered by the plethora of modem electronic devices available to industry, businesses and consumers. Electronic devices, however, often contain circuitry or components that may be sensitive to certain levels of current or voltage. Spikes or otherwise higher-than-nominal voltage or current levels are often referred to as over-voltage or over-current conditions. The occurrence of over-voltage or over-current conditions may result in damage to or destruction of the circuitry or components of the electronic device. As a result, designers often utilize fuses, varistors, thyristors or other devices to shield the circuitry from such conditions.  
         [0005]     Fuses are well known and widely used for over-current protection of electronic circuits. Many current limited fuses are made of metal wires, metal sheets, or metal films as the fusing elements. When the electrical current passing through the fusing element exceeds a certain level, the heat generated by the electrical current will melt the fusing element and create an open circuit, thereby preventing further current flow. Occasionally, however, when the fuse element melts or breaks an arcing effect occurs and allows undesired current levels to reach the circuit to be protected, potentially causing damage to the circuit. Therefore, the fusing elements are typically surrounded by arc suppressing or arc shielding materials. Many types and designs of fuses are known in the art and such fuses are described, for example, in U.S. Pat. Nos. 6,590,490; 6,005,470; 5,726,621; 5,479,147; 5,453,726; 5,296,833; 5,245,308; 5,228,188; and 2,864,917.  
         [0006]     Over-voltage protection devices such as varistors, for example, are also well known and widely used for protecting electronic circuits from above-nominal voltage levels. A varistor is an electronic component designed to protect circuits against excessive voltage. The most common type is a metal oxide varistor (MOV). Similar to a capacitor, a varistor typically includes two metal plates or electrodes separated by an insulator. The insulator materials are typically semi-conducting materials, which have high resistance when a crossing voltage is low and have low resistance when the crossing voltage is high. When the voltage between the two electrodes reaches a certain value, the insulator breaks down and admits the flow of current (i.e., the breakdown current). Varistors have a capacitance and could be called capacitors; likewise, all capacitors have a breakdown voltage. The difference is that in most capacitors, breakdown is highly undesirable, and usually results in the destruction of the device. Varistors on the other hand are designed to repeatedly withstand breakdown.  
         [0007]      FIG. 1  illustrates a circuit diagram of a conventional over-current and over-voltage protection device or module  100  having a fuse  102  located on one side of a varistor  104 . Such protection circuits are disclosed, for example, in U.S. Pat. Nos. 6,636,404 and 6,510,032. As illustrated in  FIG. 1 , a power supply  106  is connected to a first terminal of the fuse  102 . A second terminal of the fuse  102  is electronically connected to an electronic circuit  108  to be protected. A first electrode of the varistor  104  is connected to the second terminal of the fuse  102  and to the electronic circuit  108 . A second electrode of the varistor  104  is connected to ground. Appropriate terminals (not shown) of the power supply  106  and the electronic circuit  108  are also connected to ground.  
         [0008]     Because the protection module  100  described above and illustrated in  FIG. 1  has the fuse  102  located on only one side of the varistor  104 , the circuit&#39;s design is not symmetrical. Therefore, if the protection circuit  100  is implemented as a surface mount device or component, the orientation of the device when utilized in a printed circuit (PC) board, for example, is critical to the proper operation of the protection device  100 . For example, a voltage pulse coming from the power supply  106  will have a different impact on the protection device  100  than a voltage pulse coming from the circuit  108 . A voltage pulse coming from the power supply  106  will generate a break through current through the varistor  104 . All of this break through current will pass through the fuse  102 . If the current is high enough, it could blow the fuse  102 . On the other hand, if a voltage pulse is generated from the circuit  108 , the entire break through current of the varistor  104  will pass through the varistor  104  only, without giving extra stress on the fuse  102 . Thus, in order to make sure that the protection module  100  is correctly inserted and oriented onto the PC board as intended by a circuit designer, a marking must be placed on the packaging of the protection module  100  to indicate the location of the varistor  104  and proper orientation of the protection module  100 . This adds extra cost and difficulties during the manufacturing and automatic packaging of the protection module  100 , as well as during assembly of the protection module  100  onto a PC board.  
         [0009]     Furthermore, if an undesired voltage surge or spike is output by the power supply  106 , in order for the varistor  104  to serve its intended function and clamp the voltage surge, the entire breakdown current of the varistor  104  must pass through the fuse  102 . This places a significant limitation on the design of the protection circuit  100  because the fuse must have a high enough current rating to withstand the breakdown current generated by the voltage protection function of the varistor  104 . In view of this limitation, prior protection circuits typically utilized a single layer or hollow tube varistor, which has a much smaller ratio of current carrying capacity to volume than that in a multilayer varistor. Such single-layer or hollow tube varistors are well known in the art and described, for example, in Japanese patent nos. 04-359403 and 05-013205.  
         [0010]     In view of the above deficiencies associated with prior over-voltage and over-current protection circuits, there is a need for an improved over-voltage and over-current protection circuit that overcomes these deficiencies.  
       SUMMARY OF THE INVENTION  
       [0011]     The invention addresses the above and other needs by providing an over-voltage and over-current protection device wherein the breakdown current of a varistor, or other over-voltage protection device, need not all pass through a fuse of the protection device during voltage protection operation of the varistor (i.e., when it is in its breakdown state) or other over-voltage protection device. Thus, the current rating of the fuse may be designed in accordance with the current rating of the electronic circuit to be protected without being overly concerned with the breakdown current of the varistor.  
         [0012]     In one embodiment, the over-voltage and over-current protection device of the present invention is characterized by a symmetrical design wherein a fuse is positioned between two varistors. A first terminal of the fuse is electrically connected to a first electrode of a first varistor and a second terminal of the fuse is electrically connected to a first electrode of a second varistor. Second electrodes of the first and second varistors are connected to ground. Thus, the orientation of the protection circuit on a PC board, for example, is not important because the protection circuit will behave the same regardless of its orientation when placed between a power source and an electronic circuit to be protected. In this way, an external marking on the package of the protection circuit is no longer required because the orientation of the protection circuit does not need to be taken into account during manufacturing of the protection module and assembly of the protection module onto a PC board.  
         [0013]     In further embodiments, two or more parallel varistors (i.e., a multi-layer varistor) may be coupled to each terminal of one or more fuses, wherein if more than one fuse is utilized, the fuses are configured in parallel with one another.  
         [0014]     In one embodiment, the protection circuit of the present invention is implemented as a multilayer surface mount component adapted for use on a printed circuit (PC) board. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  illustrates a diagram of an equivalent circuit of a prior art over-voltage and over-current protection device coupled between a power supply and an electronic circuit.  
         [0016]      FIG. 2  illustrates a diagram of an equivalent circuit of an over-voltage and over-current protection device in accordance with one embodiment of the invention. The protection device is coupled between a power supply and an electronic circuit to be protected.  
         [0017]      FIG. 3  illustrates a perspective view of an over-voltage and over-current protection device implemented as a surface mount component or module, in accordance with one embodiment of the invention.  
         [0018]      FIG. 4  illustrates a cross-sectional side view of the surface mount component of  FIG. 3 , in accordance with one embodiment of the invention.  
         [0019]      FIG. 5  illustrates a cross-sectional top view of the surface mount component of  FIG. 3 , in accordance with one embodiment of the invention.  
         [0020]      FIG. 6  illustrates a diagram of an equivalent circuit of an over-voltage and over- current protection device in accordance with one embodiment of the invention.  
         [0021]      FIG. 7  illustrates a cross-sectional side view of a surface mount component incorporating the over-voltage and over-current protection circuit of  FIG. 6 , in accordance with one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The invention, in accordance with various preferred embodiments, is described in detail below with reference to the figures, wherein like elements are referenced with like numerals throughout. In the embodiments discussed below, varistors are described as the over-voltage protection device used in conjunction with one or more fuses. However, it is understood and appreciated that other types of over-voltage protection devices may be implemented in the invention by those of skill in the art without undue experimentation. For example, instead of varistors, other known over-voltage protection devices such as thyristors, diodes, electric static discharge (ESD) protection devices (e.g., polymer composite devices such as those described in U.S. Pat. Nos. 6,642,297, 6,160,695 and 5,476,714), and well known gas discharge tube devices may be utilized in the present invention.  
         [0023]      FIG. 2  illustrates an equivalent circuit diagram of a protection device or module  200  coupled to a power supply  106  at a first side of the module  200  and to an electronic circuit  108  at a second side. The protection module  200  includes a fuse  102  having a first end of its fuse element electrically coupled to a first terminal  103   a  of the module  200  and a second end of the fuse element connected to a second terminal  103   b  of the module  200 . As shown in  FIG. 2 , varistors  104   a  and  104   b  are each connected to a respective terminal  103   a  or  103   b.  A first electrode (A) of a first varistor  104   a  is connected to terminal  103   a,  and hence electrically coupled to the first end of the fuse element of the fuse  102 , and a first electrode (B) of a second varistor  104   b  is connected to terminal  103   b,  and hence electrically coupled to the second end of the fuse element of the fuse  102 . A second electrode (C) of each of the varistors  104   a  and  104   b  is connected to ground.  
         [0024]     Terminal  103   a  of the protection device or module  200  is connected to a first terminal of the power supply  106  and terminal  103   b  of the protection device or module  200  is connected to a first terminal of the electronic circuit  108 . Appropriate terminals of the power supply  106  and the electronic circuit  108  are also connected to ground.  
         [0025]     During operation, if the power supply  106  outputs a voltage surge  202  that is above a pre-specified voltage level, the first varistor  104   a  will breakdown and allow a breakdown current  204  to flow through it, thereby clamping or reducing the voltage surge  202  below a certain level. In this way, the circuit  108  is protected from the voltage surge  202 . Additionally, it should be noted that the entire breakdown current  204  of the varistor  104   a  need not pass through the fuse  102  when the device  200  is functioning as an over-voltage protection device. Rather, only a fraction of the breakdown current (e.g., approximately 50%) need pass through the fuse  102 , which provides significantly more flexibility in designing the fuse  102 .  
         [0026]      FIG. 3  illustrates a perspective view of a protection circuit implemented as a surface mount component or module  300 , in accordance with one embodiment of the invention. The protection module  300  includes a first contact terminal  103   a,  which corresponds to the terminal  103   a  illustrated in  FIG. 2 , and a second contact terminal  103   b  located on an opposite end of the module  300  from the first contact terminal  103   a,  which corresponds to the terminal  103   b  of  FIG. 2 . When placed and assembled onto a PC board, the contact terminals  103   a  and  103   b  provide electrical contacts for the fuse  102  and varistors  104   a  and  104   b  contained within the module  300  to external circuits and/or components (e.g., a power supply and/or integrated circuit chip), which are also assembled onto or otherwise coupled to the PC board.  
         [0027]     The protection module  300  further includes a pair of side ground terminals  302  located on opposite sides of the module  300  from one another. These ground terminals  302  are adapted to provide an electrical conduction path to ground for the fuse  102  and varistors  104   a  and  104   b  contained within the module  300 .  
         [0028]      FIG. 4  illustrates a cross-sectional side view of the protection circuit module  300  of  FIG. 3 , in accordance with one embodiment of the invention. In the embodiment illustrated, the module  300  comprises multiple layers of a semiconducting and/or insulating material. Various types of semiconducting and insulating materials, which may be utilized in the present invention, are known in the art. Such materials are collectively referred to herein as an “insulator” or “insulating material.” 
         [0029]     As shown in  FIG. 4 , the fuse  102  is placed on a top surface of a first insulator layer  402 . A first end of the fuse  102  is electrically coupled to a first contact terminal  103   a  and a second end of the fuse  102  is electrically coupled to a second contact terminal  103   b.  In one embodiment, an arc suppressant material  404  surrounds or encloses the fuse element of the fuse  102  in order to suppress arcing and cut off the current through the arc, which may otherwise damage the electronic circuit to be protected. When the electrical current passing through the fusing element exceeds a certain level, the heat generated by the electrical current will melt the fusing element and create metal vapors, which in turn can generate high-current arcing. In order to quench or suppress the arc, several materials such as ceramic powder, glass, organic materials, etc., are known and used to enclose the fusing element and absorb the metal vapor that results when the fusing element melts and vaporizes. By absorbing the metal vapor, the arc suppressant material  404  prevents arcing and cuts off high current levels from reaching the electronic circuit to be protected.  
         [0030]     Composite fusing elements wherein an arc suppressant material encloses or “sandwiches” a metal or alloy conducting material between two or more layers of arc suppressant material are known in the art. Such encapsulated or “sandwiched” composite fusing elements may be used in accordance with the invention. In other embodiments, an improved fuse element made from a composite mixture of conductive particles (e.g., a powder) and arc suppressant particles, or particles of one material coated with a film of the other material, may be utilized in the present invention. Such improved fuse elements, and methods of making same, are described in a concurrently-filed and commonly-owned U.S. patent application entitled, “Composite Fuse Element and Methods of Making Same,” attorney docket no. 38666-2000100, the entirety of which is incorporated by reference herein.  
         [0031]     Referring again to  FIG. 4 , a first electrode  406  comprises a metal and/or alloy conductive plate having one end in electrical contact with the terminal  103   a.  This first electrode corresponds to electrode A of varistor  104   a  in  FIG. 2 . A second electrode  408  comprises a metal or alloy conductive plate having one end in electrical contact with the second terminal  103   b.  This second electrode  408  corresponds to electrode B of the second varistor  104   b  in  FIG. 2 . A third electrode  410  comprising a metal or alloy conductive plate is disposed between electrodes  406  and  408 . This third electrode  410  is in electrical contact with one or both of the side ground terminals  302  ( FIG. 3 ) and corresponds to a common ground electrode (C) shared by both of the varistors  104   a  and  104   b.  A layer of insulating material separates each of the electrodes  106 ,  108  and  110 . Thus, the electrode  106  (A), the electrode  110  (C) and an insulating layer between these electrodes make up the varistor  104   a  ( FIG. 2 ). The electrode  108  (B), the electrode  110  (C) and an insulating layer therebetween make up the varistor  104   b.    
         [0032]     When the voltage between the electrodes  106  and  110  of varistor  104   a  reaches a certain value, the insulator between the electrodes will break down and allow the flow of current (i.e., the breakdown current). In this way, varistor  104   a  will clamp the voltage between its electrodes below a predetermined breakdown voltage. Similarly, when the voltage between electrodes  108  and  110  of varistor  104   b  reaches a certain value, the insulator between the electrodes will break down and allow the flow of current between the electrodes, thereby clamping the voltage across varistor  104   b.  It should be noted that the figures provided herein are not necessarily drawn to scale.  
         [0033]      FIG. 5  illustrates a cross-sectional top view of the protection circuit module  300 . The first electrode  406  (A) is in electrical contact with the first terminal  103   a  and extends across the length of the module  300  but does not reach the second terminal  103   b.  The second electrode  408  (B) is diposed below the first electrode  406  and below intermediate insulating layers as indicated by dashed lines. The second electrode  408  is in electrical contact with the second terminal  103   b  and extends partially across the length of the module  300  but does not make electrical contact with the first terminal  103   a.  The end of the second electrode  408  is indicated by the dashed line  408   b  in  FIG. 5 .  
         [0034]     Sandwiched between the electrodes  406  and  408  and between two insulating layers (not shown) is the third electrode  410  (C). As illustrated by dashed lines in  FIG. 5 , the third electrode  410  extend outwardly to make electrical contact with each of two opposing side contact terminals  302 . In alternative embodiments, the third electrode  410  need only make electrical contact with one of the side contact terminals  302 . As discussed above, the terminals  103   a,    103   b  and  302  provide the electrical contacts for surface mount component  300  ( FIG. 3 ) so that the component  300  is easily assembled onto a PC board (not shown) using well known surface mount assembly techniques.  
         [0035]     As discussed above with respect to  FIGS. 2-5 , in one embodiment, the over-voltage and over-current protection circuit has a symetrical design. In other words, a power supply or electronic circuit to be protected may be connected to either terminal  103   a  or  103   b  because the circuit configuration and functionality is the same either way the protection circuit module  300  is oriented between a power supply and a circuit to be protected.  
         [0036]     Additionally, an over-voltage pulse from either side of the fuse  102  will mainly generate current in a corresponding varistor  104   a  or  104   b  coupled to that same side of the fuse  102 , reducing the breakdown current of the varistor through the fuse  102 . In an extreme case, the fuse  102  has approximately zero resistance. Thus, the varistors  104   a  and  104   b  approximate a pair of varistors connected in parallel. During an over-voltage protection state, current passing through the fuse  102  is approximately equal to half of the normal breakdrown current through a single varistor because the pair of varistors  104   a  and  104   b  will share the current load. Thus, the current rating of the fuse  102  can be dictated mostly by the current limiting protection requirments of the electronic circuit to be protected, without being substantially limited by the breakdown current generated by the varistor  104   a  or  104   b  for clamping an over-voltage pulse or spike. Thus, the design of the protection device  100  of the present invention provides greater flexibility than prior art designs of over-voltage and over-current protection devices.  
         [0037]      FIG. 6  illustrates an equivalent circuit diagram of an over-voltage and over-current protection device  600 , in accordance with another embodiment of the invention. The protection device  600  is substantially similar to the protection device  200  of  FIG. 2  except that additional varistors  105   a  and  105   b  are placed in parallel with respective varistors  104   a  and  104   b  on both sides of the fuse  102 . By adding multiple varistors in parallel, the breakdown current can be increased and the protection device  600  can handle higher current levels when it is performing its over-voltage protection function. It is noted that multiple single-layer varistors connected in parallel are equivalent to a single multi-layer varistor.  
         [0038]     In further embodiments (not illustrated), multiple fuses  102  can be connected in parallel between the contact terminals  103   a  and  103   b.  In this way, the current rating of the over-current protection function can also be increased.  
         [0039]      FIG. 7  illustrates a cross sectional, side view of the protection circuit  600  of  FIG. 6  when implemented as a multi-layer, surface mount component  700 . This device  700  is substantially similar to the protection device or module  300  described above with respect to  FIGS. 2-5 . Elements  102 ,  103   a,    103   b,    402 ,  404 ,  406 ,  408  and  410  are identical to the elements referenced with the same numerals in  FIG. 4 . Therefore, the reader is directed to the previous discussion of these elements. However,  FIG. 7  further illustrates the additional electrodes and insulating layers that form the additional parallel varistors  105   a  and  105   b  of  FIG. 6 .  
         [0040]     As shown in  FIG. 7 , an additional electrode  406 ′ (A′) is in electrical contact with the terminal  103   a  and an additional electrode  408 ′ (B′) is in electrical contact with the terminal  103   b.  An additional ground electrode  410 ′ (C′) is disposed between the electrodes  406 ′ and  408 ′ and separating each of the electrodes  406 ′,  408 ′ and  410 ′ is a layer of insulating material  402 .  
         [0041]     Thus, the electrode  406 ′, the electrode  410 ′ and the insulating layer between these electrodes form the varistor  105   a  ( FIG. 6 ). Because electrodes  406  and  406 ′ are both connected to terminal  103   a  and electrodes  410  and  410 ′ are both connected to one or more ground terminal  302  ( FIGS. 3 and 5 ), varistor  105   a  is electrically connected in parallel with the varistor  104   a.  Similarly, the electrode  408 ′, the electrode  410 ′ and the insulating layer between these electrodes form the varistor  105   b.  Since the electrodes  408  and  408 ′ are both connected to the terminal  103   b  and the electrodes  410  and  410 ′ are both connected to one or more ground terminals  302 , varistor  105   b  is electrically connected in parallel with varistor  104   b.  It is appreciated that parallel varistors  104   a  and  105   a  can be viewed as a single multi-layer varistor structure. Similarly, varistors  104   b  and  105   b  can be viewed as a single multi-layer varistor structure.  
         [0042]     In further embodiments, one or multiple parallel varistors  104  can be placed on either side of one or multiple parallel fuses  102 , depending on the desired breakdown current of the varistor(s) and/or current rating of the fuse element(s).  
         [0043]     In one embodiment, the fuse element of a fuse  102  is located near the center of the module  300 ,  700 , as illustrated in  FIGS. 4 and 7 , for example. In alternative embodiments, the fuse  102  can be located off-center of the module  300 ,  700 . When multiple fuses  102  are implemented in the design, corresponding fuse elements can be located close to one another or separated from one another by varistor electrodes and insulating layers.  
         [0044]     Devices in accordance with the embodiments described above can be manufactured using various known techniques, such as a dry sheet process, a wet coating process, a screen printing process, or a UV forming process. The subsequent cutting, sintering, termination, and plating processes are similar to those widely adopted in the multilayer ceramic component manufacturing industry. These processes are well known by those of skill in the art.  
         [0045]     Various preferred embodiments of the invention have been described above. However, it is understood that these various embodiments are exemplary only and should not limit the scope of the invention as recited in the claims below. Various modifications of the preferred embodiments described above can be implemented by those of ordinary skill in the art, without undue experimentation. For example, alternative over-voltage protection devices (e.g., thyristors, diodes, etc.) may be used instead of the varistors described above. These various modifications are contemplated to be within the spirit and scope of the invention as set forth in the claims below.