Patent Publication Number: US-2020279701-A1

Title: Thermal Metal Oxide Varistor Circuit Protection Device

Description:
TECHNICAL FIELD 
     Embodiments relate to the field of circuit protection devices. More particularly, the present embodiments relate to a surge protection device with a thermal disconnect system configured to provide fast response to overheating. 
     DISCUSSION OF RELATED ART 
     Over-voltage protection devices are used to protect electronic circuits and components from damage due to over-voltage fault conditions. These over-voltage protection devices may include metal oxide varistors (MOVs) connected between the circuits to be protected and a ground line. MOVs have a unique current-voltage characteristic allowing them to be used to protect such circuits against catastrophic voltage surges. Often, these devices utilize thermal links where the thermal links can melt during an abnormal condition to form an open circuit. In particular, when a voltage larger than the nominal or threshold voltage is applied to the device, current flows through an MOV, resulting in the generation of heat. This heat causes the thermal link to melt. Once the link melts, an open circuit is created, preventing the over-voltage condition from damaging the circuit to be protected. However, these existing circuit protection devices do not provide an efficient heat transfer from the MOV to the thermal link, thereby delaying response times. Additionally, after an open circuit condition is established, arcing may take place between components in close proximity to one another. In addition, existing circuit protection devices are complicated to assemble, increasing manufacturing costs. Accordingly, improvements may be useful in present day circuit protection device employing metal oxide varistors. 
     SUMMARY 
     Exemplary embodiments of the present disclosure are directed to a circuit protection device. In an exemplary embodiment, the circuit protection device may include a housing defining a cavity and a metal oxide varistor disposed within said cavity. The circuit protection device may further include a movable electrode attached to a first side of the metal oxide varistor by a solder connection, an arc shield disposed within the housing on the first side of the metal oxide varistor and adjacent the movable electrode, and a spring attached to the arc shield, wherein the arc shield is mechanically biased against the movable electrode along a surface direction parallel to the first side when the spring is in a compressed state. 
     In another exemplary embodiment, a circuit protection device includes a housing defining a cavity and a metal oxide varistor disposed within said cavity. The circuit protection device may further include an insulator pad disposed on a first side of the metal oxide varistor and a movable electrode disposed on the insulator pad and electrically connected to the metal oxide varistor. In addition, the circuit protection device may include an arc shield comprising an electrical insulator and being disposed within the housing on the insulator pad and adjacent the movable electrode; and a spring attached to the arc shield, wherein the arc shield is mechanically biased against the movable electrode along a surface direction parallel to the first side when the spring is in a compressed state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a circuit protection device in accordance with an embodiment of the present disclosure. 
         FIG. 1B  is a cut-away perspective view of the circuit protection device of  FIG. 1A  with a portion of the housing removed, according to an embodiment of the present disclosure. 
         FIG. 1C  is a side cross-sectional view of the circuit protection device of  FIG. 1A . 
         FIG. 1D  is a cut-away perspective view a partially assembled circuit protection device according to embodiments of the disclosure. 
         FIG. 2A  is a perspective view of an exemplary insulator pad according to embodiments of the disclosure. 
         FIG. 2B  is a perspective view of components of a circuit protection device according to embodiments of the disclosure. 
         FIG. 2C  is another perspective view of the components of a circuit protection device of  FIG. 2B . 
         FIG. 2D  is a bottom perspective view of the components of a circuit protection device of  FIG. 2B . 
         FIG. 3A  is a cut-away perspective view of a configuration of the circuit protection device of  FIG. 1B  during normal operation. 
         FIG. 3B  is a cut-away perspective view of a configuration of the circuit protection device of  FIG. 1B  after actuation of a fault condition in accordance with an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present embodiments will now be described more fully hereinafter with reference to the accompanying drawings, where preferred embodiments are shown. These embodiments, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
     In the following description and/or claims, the terms “on,” “overlying,” “disposed on” and “over” may be used in the following description and claims. “On,” “overlying,” “disposed on” and “over” may be used to indicate two or more elements are in direct physical contact to one other. However, “on,”, “overlying,” “disposed on,” and over, may also mean two or more elements are not in direct contact with one another. For example, “over” may mean one element is above another element but not contact one another and may have another element or elements in between the two elements. Furthermore, the term “and/or” may mean “and”, may mean “or”, may mean “exclusive-or”, may mean “one”, may mean “some, but not all”, may mean “neither”, and/or may mean “both”, although the scope of claimed subject matter is not limited in this respect. 
       FIG. 1A  to  FIG. 1D  illustrate various views of a circuit protection device  100  according to embodiments of the disclosure. In particular,  FIG. 1A  is a perspective view of the circuit protection device  100  after assembly, not showing internal components. The circuit protection device  100  as shown includes a first terminal, shown as a first contact lead  104  and a second contact lead  106 . The first contact lead  104  and second contact lead  106  extend outside a housing  102 , where the housing  102  may be an insulating material such as a known plastic material or other polymeric material. As discussed below, the first contact lead  104  and second contact lead  106  may extend inside the housing  102  to form electrical contact with a metal oxide varistor (MOV). The circuit protection device  100  may also include a pair of electrically conductive indicator pins shown as the indicator pins  108 . In various embodiments the indicator pins may be electrically connected to an electrical indicator (not shown) external to the circuit protection device  100 , such as a light or other device. 
       FIG. 1B  is a cut-away perspective view of the circuit protection device  100  with a portion of the housing  102  removed. The circuit protection device  100  may include a metal oxide varistor  110 , where the metal oxide varistor  110  may have a flat shape, such as a rectangular disc or a circular disk. The embodiments are not limited in this context. The circuit protection device  100  may include an insulator pad  112  disposed on the first side (upper side parallel to the X-Y plane in  FIG. 1B ) of the metal oxide varistor  110  as shown. The insulator pad  112  may be a printed circuit board (PCB) in various embodiments. In the present embodiments, a PCB may comprise a known material used for forming the body of a printed circuit board. The PCB may be planar in shape and may have any appropriate thickness for use in a circuit protection device. In various embodiments, the PCB may further include features such as openings or electrically conductive material disposed on the surface of the PCB or in openings extending through the PCB, for example. 
     As further shown in  FIG. 1B , the circuit protection device  100  may include a movable electrode  122  disposed on the insulator pad  112 , where the operation of movable electrode  122  is discussed below. The circuit protection device  100  may further include a flexible conductive wire  118  connected to the movable electrode  122  on a first end and connected to the first contact lead  104  on a second end. In various embodiments, the first contact lead  104 , second contact lead  106  and/or flexible conductive wire  118  may be composed of a metal such as copper. The circuit protection device  100  may further include an arc shield  114  disposed within the housing  102  on the first side of the metal oxide varistor  110  and adjacent the movable electrode  122 . The operation of the arc shield  114  is also described below. In addition, the circuit protection device may include a spring  120 , or a plurality of springs, as shown in  FIG. 1B . The spring(s)  120  may be attached to the arc shield  114 , or may otherwise engage the arc shield  114  as shown. As illustrated in  FIG. 1B , as assembled, the spring  120  may be in a compressed state. As detailed below, this compressed state may cause the arc shield  114  to be mechanically biased against the movable electrode  122  along a surface direction parallel to the first side of the metal oxide varistor  110  (i.e., along the Y-axis of the Cartesian coordinate system shown). 
     Turning now to  FIG. 1C  there is shown a side-cross sectional view along the direction A-A (in the X-Z plane) for the circuit protection device  100 . As illustrated, the metal oxide varistor  110  is disposed within the housing  102  and may have a first side  150  supporting the insulator pad  112 , as well as a second side  152 . In plan view (X-Y plane) the metal oxide varistors  110  may be rectangular in shape, in accordance with the shape of the housing  102 , in this embodiment. As will be appreciated, alternative shapes of metal oxide varistor  110  may also be employed and housing  102  may likewise have an alternative shape to accommodate the particular shapes of a metal oxide varistor  110 . The insulator pad  112  may be disposed directly on the metal oxide varistor  110  as further shown in the cut-out perspective view of  FIG. 1D . 
     The insulator pad  112 , such as a PCB, may function not only to insulate the moveable electrode and MOV but also as a protection shield to the mechanical moving system, since in the event of a high short circuit current, a possible flame generated from an MOV may damage the disconnect system if no shield is present. 
     Additionally, the arc shield  114  may be disposed over a portion of the insulator pad  112  as shown. In particular, the length L of the arc shield along the direction parallel to the Y-axis is less than the size of the cavity  130  along the Y-axis. As detailed below this relatively smaller size of the arc shield  114  allows displacement of the arc shield  114  along the surface of the insulator pad  112  in the direction parallel to the Y-axis, facilitating the ability to prevent arcs during a fusing event. In some embodiments, as further shown in  FIG. 1C , the arc shield  114  may include protrusions  128  The protrusions  128  may form points of contact to the surface of insulator pad  112 , facilitating movement of the arc shield  114  with respect to insulator pad  112  by providing less surface area for friction between arc shield  114  and insulator pad  112 . As also illustrated in  FIG. 1D , the insulator pad  112  may include an opening  132 , where the opening  132  may accommodate a solder connection, as discussed below. In the configuration of  FIG. 1D , the arc shield  114  is positioned toward one side of the cavity  130 , opposite to the side where the first contact lead  104  and second contact lead  106  enter the cavity  130  (See  FIG. 1B ). After assembly of the circuit protection device  100  for normal operation, the opening  132  of the insulator pad  112  is situated so as to not be covered by the arc shield  144 , as shown in  FIG. 1D . This opening  132  allows a solder connection to be formed between the movable electrode  122  and metal oxide varistor  110 . 
       FIG. 2A  is a perspective view of an insulator pad  112  according to embodiments of the disclosure. In this embodiment, the insulator pad may be a PCB having a known composition and structure. The shape of the insulator pad  112  may be designed according to the shape of a housing, such as a rectangular shape, or other shape. As illustrated, the insulator pad  112  includes a conductive contact pad  124  whose function has been described above, as well as an opening  132 . 
       FIG. 2B  is a perspective view of components of a circuit protection device without a housing in accordance with an embodiment of the present disclosure. The components shown in  FIG. 2A  may be used in the circuit protection device  100 , for example.  FIG. 2C  is another perspective view of the components of a circuit protection device of  FIG. 2B . In particular,  FIG. 2B  illustrates the arrangement of metal oxide varistor  110 , insulator pad  112  and first contact lead  104  and second contact lead  106 . The insulator pad  112  is disposed on the metal oxide varistors  110  and the movable electrode  122  disposed on the insulator pad  112 . The movable electrode  122  is mechanically fixed to the metal oxide varistor  110  by virtue of the solder connection  140 . As particularly shown in  FIG. 2C  the first contact lead  104  extends over the insulator pad  112 , forming a gap along the direction parallel to the Z-axis, and does not contact the insulator pad  112 . The connection of the movable electrode  122  to the first contact lead  104  via flexible conductive wire  118  facilitates movement of the movable electrode  122 . In particular, as discussed below with respect to  FIG. 3A  and  FIG. 3B , when a fault condition occurs and the movable electrode  122  is displaced away from the side  134 , the flexible conductive wire  118  may provide little mechanical resistance to movement of the movable electrode  122 . 
       FIG. 2D  presents a bottom perspective view of the components of a circuit protection device of  FIG. 2B . In this example, the second contact lead  106  may terminate in a conductive pad  107  that is electrically connected to the metal oxide varistor  110 . 
     Turning now to  FIG. 3A  and  FIG. 3B , there is shown an example of operation of the circuit protection device  100  according to embodiments of the disclosure. In  FIG. 3A  a cut-away perspective view of the configuration of the circuit protection device  100  during normal operation is shown. As shown, the arc shield  114  is positioned toward a side  134  of the cavity  130 , and includes side portions  136 , where a side portion  136  engages a spring  120 , located on either side of the arc shield  114 . When positioned toward the side  134 , the arc shield  114 , via the side portions  136 , places the spring  120  in a compressed state. As further shown in  FIG. 3A , the movable electrode  122  abuts the arc shield  114 . In some embodiments, the movable electrode  122  may include a protrusion such as a tab  138 , engaging the arc shield  114 , and preventing the arc shield  114  from moving toward side  142 . In the configuration of  FIG. 3A , the movable electrode  122  is connected to the metal oxide varistor  110  via a solder connection  140  (shown as dashed feature) extending through the opening  132  of the insulator pad  112  (see  FIG. 1D ). The solder connection  140  may be composed of a conventional low temperature solder in various embodiments, such as a low melting temperature alloy including SnIn, SnBi, or other alloy. 
     Because the movable electrode  122  prevents the arc shield  114  from moving, while the spring  120  is in a compressed state, the arc shield  114  is mechanically biased against the movable electrode  122  along the Y-axis. In other words, the arc shield  114  exerts a mechanical force against the movable electrode  122  tending to displace the movable electrode  122  toward the side  142 . 
     In accordance with various embodiments, the metal oxide varistor  110  may be a conventional metal oxide varistor (MOV) made from any appropriate composition or process. An MOV is a voltage sensitive device designed to heat up when the voltage applied across the device exceeds a rated voltage. By the way of background, MOVs may be comprised of zinc oxide granules or similar material, where the granules are sintered together to form a disc. A given zinc oxide granule may be a highly electrically conductive material, while the intergranular boundary is formed of other oxides and is highly resistive. Just at those points where zinc oxide granules meet does sintering produce a ‘microvaristor’ comparable to symmetrical Zener diodes. The electrical behavior of a metal oxide varistor results from the number of microvaristors connected in electrical series or in parallel. The sintered body of an MOV also explains its high electrical load capacity permitting high absorption of energy and thus, exceptionally high surge current handling capability. 
     Under conventional operation, the metal oxide varistor  110  may experience a voltage across the metal oxide varistor  110  below a threshold voltage of the metal oxide varistors  110 , where the threshold voltage corresponds to a voltage where metal oxide varistor  110  becomes electrically conducting. Thus, when voltage is below the threshold voltage, the metal oxide varistor  110  remains as an electrical insulator. Conversely, when voltage across the metal oxide varistor  110  exceeds the threshold voltage, the metal oxide varistor may become electrically conductive. For example, when a voltage surge condition occurs, where the voltage exceeds the threshold voltage for a sufficient duration, the metal oxide varistor  110  changes from a non-conductive state to the conductive state and current flows between first contact lead  104  and second contact lead  106 . As the voltage surge continues, the gaps and boundaries between the zinc oxide granules within the metal oxide varistor  110  are not wide enough to block current flow, and thus the metal oxide varistor  110  becomes highly conductive. This conduction generates heat, causing melting of solder at the solder connection  140 . The melting of the solder, in turn, releases movable electrode  122  from mechanical restraint formerly provided by the bonding of the movable electrode to solid solder in the solder connection  140 . 
     Once mechanical constraint is released by melting of solder in the solder connection  140 , the mechanical bias provided by arc shield  114  may displace the movable electrode  122  along the Y-axis toward the side  142 . This displacement is illustrated in  FIG. 3B   FIG. 3B , showing a cut-away perspective view of a configuration of the circuit protection device  100  after actuation of a fault condition. As illustrated, the spring  120  is now in an extended state, having released at least some of the potential energy stored in the compressed state shown in  FIG. 3A . The movable electrode  122  is now disposed toward the side  142 , while the arc shield  114  is disposed over the region of the solder connection  140 . Movement of the movable electrode  122  from the configuration of  FIG. 3A  to the configuration of  FIG. 3B  may be facilitated by the tab  138 , providing a portion of movable electrode  122  easily engaged by the arc shield  114 . Because the arc shield is displaced over the solder connection  140 , any arcing otherwise produced by the high voltage condition between the metal oxide varistor  110  and movable electrode  122 , flexible conductive wire  118 , or first contact lead  104  is suppressed. 
     While it may be possible to solder a movable electrode directly to a metal oxide varistor, for example, if the metal oxide varistor is coated with insulation material, e.g. epoxy, etc, such a design may not withstand a high short circuit current during overvoltage events as well as designs using the insulator pad  112  of the aforementioned embodiments. Accordingly, the embodiments employing an insulator pad  112  may provide better protection against flame damage caused by a high short circuit current in compared to a configuration in which the movable electrode and arc shield are directly adjacent a metal oxide varistor. 
     In various embodiments, the indicator pins  108  may be configured to provide an indication of a fault condition. As shown in  FIG. 3A  and  FIG. 3B  the indicator pins may have interior ends extending within the housing  102  and exterior ends extending outside of the housing  102 . In the configuration of  FIG. 3A , the indicator pins may extend over the arc shield  114  when the movable electrode  122  is connected to the solder connection  140  as shown. In particular, the interior ends  108 A (see  FIG. 3B ) of the indicator pins  108  may be mechanically biased downwardly along the Z-axis toward the arc shield  114 . Because the arc shield  114  is an electrical insulator, the indicator pins  108 , even if contacting the surface of the arc shield  114 , are not electrically connected to one another and accordingly do not complete an electrical path. During a fault condition where the arc shield  114  is displaced away from the side  134 , a portion of the insulator pad  112  adjacent the side  134  is exposed. In various embodiments, the insulator pad  112 , such as a PCB, may include on the outer surface an electrically conductive contact pad  124 , located towards the side  134  as shown. This location allows the indicator pins  108 , being mechanically biased toward the insulator pad  112 , to form electrical contact with the electrically conductive contact pad  124  when the movable electrode  122  is disconnected from the solder connection  140  and the arc shield is accordingly displaced toward the side  142 . The indicator pins  108  may accordingly complete an electrical path forming part of a circuit including an indicator light (not shown) or other device, and accordingly providing an indication of a fault condition. 
     In summary, the circuit protection devices of the present embodiments provide a novel configurations of components for response to an overvoltage conditions. The circuit protection devices are designed to provide a thermally driven disconnect system harnessing the heating of an MOV under a fault condition. Among other advantages, the present embodiments provide a device easy to assemble, providing lower cost. The circuit protection devices also provide fast response to overheating caused by a fault condition. In some embodiments, up to 200 kA may be passed without use of additional protection. The circuit protection devices further provide a safe disconnecting device free from arcing issues in a compact package. In addition, a convenient fault or isolation indication is provided. 
     While the present embodiments has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present embodiments, as defined in the appended claims. Accordingly the present embodiments are not to be limited to the described embodiments, but have the full scope defined by the language of the following claims, and equivalents thereof.