Patent Application
20120194958
1. Field
The present invention relates generally to electronic protection circuitry. More, specifically, the present invention relates to an electrically activated surface mount circuit protection device.
2. 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.
Further disadvantages include size and versatility. Circuit protection devices are often too tall to meet the height constraints for circuit board mounted devices. Circuit protection devices also often do not provide the versatility to allow the circuit protection device to activate under all the conditions necessary to adequately protect the circuit.
Thermal fuses described in U.S. patent application Ser. No. 12/383,595, filed Mar. 24, 2009 and published as US 2010/0245022 and U.S. application Ser. No. 12/383,560, filed Mar. 24, 2009 and published as US 2010/0245027—the entirety of each of which are 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.
A circuit protection device includes a substrate with first and second electrodes connected to the circuit to be protected. The circuit protection device also includes a heater element between the first and second electrodes. A sliding contact is connected by a sensing element to the first electrode, second electrode, and heater element, thereby bridging and providing a conductive path between each. A spring element is held in tension by, and exerts a force parallel to a length of the substrate against, the sliding contact. A flux material is provided around the sensing element. Upon detection of an activation condition, the sensing element releases the sliding contact and the force exerted by the spring element moves the sliding contact to another location on the substrate at which the sliding contact no longer provides a conductive path between the first electrode, second electrode, and heater element. The flux allows the sliding contact to move without dragging the sensing material.
a is a bottom view an assembled circuit protection device.
b is a top view the assembled circuit protection device shown in
a is a circuit protection device with the sliding contact in the closed position.
b is the circuit protection device of
The substrate 102 may include a printed circuit board (PCB). For the sake of explanation, the substrate 102 is described as a multilayer PCB including a top PCB 114 and a bottom PCB 116. It will be understood that the substrate 102 may also be fabricated as a single layer.
The top PCB 114 includes an opening 118 that receives the heater element 104. The height of the top PCB 114 may be set to allow the top of the heater element 104, when placed in the opening 118, to be co-planar with the top surface of the substrate 102, i.e., with the top surface of the top PCB 114. In another embodiment shown in
The top PCB 114 may also include another opening 120 for receiving a cantilever portion 122 of the sliding contact 108. The opening 120 in
The top PCB 114 includes pads/electrodes, 124, 126 and 128. The electrodes 124 and 126 may be positioned on opposite sides of the opening 118 along a width of the top PCB 114. The electrode 128 may be positioned on a side of the opening 118 opposing the side the opening 120 is located on opposite sides of the opening 118. As shown in
The bottom PCB 116 includes pads 130, 132 and 134 corresponding to the location of the electrodes 124, 126 and 128, respectively, of the top PCB 114. The bottom PCB 116 may also include pad 136 corresponding to the location of the heater element 104. As shown in
As noted, the heater element 104 fits into the opening 118 in the substrate 102. The heater element 104 may also constitute another electrode of the circuit protection device 100. The heater element 104 may be a positive temperature coefficient (PTC) device, such as the PTC device disclosed in U.S. application Ser. No. 12/383,560, filed Mar. 24, 2009, the entirety of which is incorporated herein by reference. Other heater elements, such as a conductive composite heater, that generate heat as a result of current flowing through the device, may be utilized in addition to or instead of the PTC device. In another example, the heater element 104 may be zero temperature coefficient element or constant wattage heater. As shown in
The sliding contact 108 may be a conductive and planar element with the cantilever portion 122. The cantilever portion 122 fits into the opening 120. The spring element 106 is located between the cantilever 122 and a side of the opening 120. The sliding contact 108 may be fused to the heater element 104 and electrodes 124, 126 with, for example, a low melt-point sensing element (not shown). When the sensing element changes state, e.g., melts at a threshold temperature, the sliding contact 108 is no longer fused to the electrodes 124, 126 and heater element 104, and the spring element 106 expands and pushes the sliding contact 108 down the channel 120. The sensing element may thus provide mechanical, and electrical, contact between the sliding contact 108 and the electrodes 124, 126 and heater element 104.
The sensing element may be, for example, a low melt-point metal alloy, such as solder. For the sake of explanation, the sensing element is described herein as a solder. It will be understood that other suitable materials may be used as the sensing element such as, for example, a conductive thermoplastic having a softening point or melting point.
With the sliding contact 108 soldered to the heater element 104 and electrodes 124, 126, the spring element 106 between the cantilever 122 and the side of the opening 120 is held in a compressed state. When the solder that holds the sliding contact 108 to the heater element and electrodes 124, 126 melts, the spring element 106 is allowed to expand, pushing against the cantilever 122 and causing it to slide down the opening 120, which in turn pushes the sliding contact 108 off the heater element 104 and electrodes 124, 126. In this manner, the electrical connection between the heater element 104, electrode 124 and electrode 126 is broken.
The spring element 106 may be a coil spring made of copper, stainless steel, plastic, rubber, or other materials known or contemplated to be used for coil springs. The spring element 106 may be of other compressible materials and/or structures known to those of skill in the art. For the sake of explanation, the spring element 106 is described as being held under tension in a compressed state by the sliding contact 108. It will be understood that a spring element may also be configured to be held under tension in an expanded or stretched state, such as if the spring element comprises an elastic material. In this example, when an activation condition is detected and the solder melts, the spring element may pull the sliding contact off a heater element and electrodes of the substrate.
The circuit protection device 100 is configured to open under at least three conditions. The solder can be melted by an over current condition, i.e., by a current through electrodes 124 and 126. When a current passing through the electrodes 124 and 126 reaches a threshold current, i.e., a current that exceeds a designed hold current, Joule heating will cause the solder to melt, or otherwise lose resilience, and the sliding contact 108 to move to the open position by being pushed open by the spring element 106.
The solder can be melted by an over temperature condition where the temperature of the device 100 exceeds, such as by an overheating FET or high environmental temperatures, the melting point of the solder holding the sliding contact 108 to the electrodes 124, 126 and the heater element 104. For example, the ambient temperature surrounding the circuit protection device 100 may reach a threshold temperature, such as 140° C. or higher, that causes the solder to melt or otherwise lose resilience. After the solder melts, the sliding contact 108 is pushed down the channel 120 and into an open position, thus preventing electrical current from flowing between the electrodes 124, 126 and the heater element 106.
The solder can also be melted by a controlled activation condition where the heater element 104 is activated by a control current supplied by the circuit into which the circuit protection device 100 is installed. For example, the circuit protection device may pass a current to the heater element 104 upon detection of an overvoltage in the circuit, causing the device to act as a controlled activation fuse. As the current flowing through the heater element 104 increases, the temperature of the heater element 104 may increase. The increase in temperature may cause solder to melt, or otherwise lose resilience, more quickly, resulting in the sliding contact 108 moving to an open position.
The circuit protection device 100 also includes a restraining element (not shown) that holds the sliding contact 108 in the closed position during reflow. During a reflow process, the solder holding the sliding contact 108 to the heater element 104 and electrodes 124, 126 can melt, which would result in the sliding contact 108 moving to the open position due to the force of the compressed spring 106. 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 element 106 to prematurely move the sliding contact 108 to the open position.
To prevent the force applied by the spring element 106 from opening the circuit protection device 100 during installation, the restraining element may be utilized to maintain the holding sliding contact 108 in place and resist the expansion force of the spring 106. After the reflowable thermal fuse is installed on a circuit or panel and passed through a reflow oven, the restraining element may be blown by applying an arming current through the restraining element. This in turn arms the reflowable thermal fuse.
A spacer 110 may be placed on the substrate 102. The spacer 100 is an insulating material, such as a ceramic, polymeric, or glass, or a combination of thereof. For example, the spacer 100 may be made of a fiber or glass-reinforced epoxy. The spacer 100 includes an opening that forms a channel that allows the sliding contact 108 to slide under the conditions discussed above. The spacer 110 may have a height slightly greater than a height of the sliding contact 108 such that when the cover 112 is placed on the circuit protection device 100, the underside of the cover abuts with the spacer 110, allowing the sliding contact 108 to slide freely and avoiding any friction between the sliding contact 108 and the cover 112.
A flux 138 may be applied to the top PCB 114 near the location where the sliding contact 108 is soldered to the electrodes 124, 126 and the heater element 104. The flux 138 may be a thermoplastic flux or other material characterized by a viscosity of less than 150 centipoise, and a melting point less than the melting point of the solder holding the sliding contact 108 to the heater element 104 and electrodes 124, 126. The flux 138 may also be a material characterized by an acid number of at least 30. The flux 138 may be, for example, a carboxylic acid. As another example, the flux 138 may include a mixture of carboxylic acid or other like material with a wax, e.g. a polyethylene wax. The ratio of carboxylic acid or other like material to wax is selected to increase the melting point of the mixture, relative to the melting point of the carboxylic acid or other like material alone, closer to the melting point of the solder without exceeding the melting point of the solder.
After application of the flux 138, the flux 138 is heated to its melting point. The flux 138 melts and spreads over the adjacent area.
During operation after the circuit protection device 100 is armed, the flux 138 will melt before the solder holding the sliding contact 108 in place will melt in that the flux 138 is a material characterized by a melting point less than that of the solder. In other words, when an activation condition is detected and the solder melts, allowing the sliding contact 108 to slide, the flux 138 will have already melted as well. The melted flux 138 allows the sliding contact to smoothly slide away from the heater element 104 and electrodes 124, 126 without dragging the melted solder. Solder dragged by the sliding contact 108 can result in the solder bridging the sliding contact 108 and heater element 104 and electrodes 124, 126, resulting in an electrical connection between the heater element 104 and electrodes 124, 126 even after the circuit is intended to be open. As noted, the flux 138 described herein allows the sliding contact 108 to slide without dragging the solder and causing the bridging effect without interfering with the normal operation of the device 100.
Described below is an exemplary process for assembling the circuit protection device 100. The substrate 102 may be fabricated by a PCB panel process, where circuit board pads form primary terminals, and plated vias make the connection from these terminals to surface mount pads. Slots may be cut using known drill and router processes. As an alternative, discrete, injection-molded parts with terminals that are insert-molded, or installed in a post-molding operation, may be used.
After the substrate 102 is fabricated and patterned, the heater element 104 may be installed in the substrate 102, such as by soldering the bottom of the heater element 104 to the substrate 102. The spring element 106 is inserted into the channel 120. The sliding contact 108 is inserted and slid to place the spring element 106 in a compressed state between the cantilever 122 and a side of the channel 120. The sliding contact 108 is soldered to the heater element 104 and the electrodes 124, 126.
The restraining element is attached to the sliding contact 108 on one end, and to the electrode 128 on the other end. Alternatively, one end of the restraining element may be attached to the sliding contact 108 before the sliding contact is soldered to the heater element 104 and electrodes 124, 126. In this example, the other end of the restraining element is attached to the electrode 128 after soldering of the sliding contact 108. The restraining element may be attached by resistance welding, laser welding, or by other known welding techniques.
The flux 138 is applied to the top PCB 114 and then heated to the flux's melting point. The melted flux 138 spreads out over the heater element 104 and electrodes 124, 126. The melted flux 138 is then cooled, forming a film over the heater element 104 and electrodes 124, 126 and adjacent areas. The film may be located around the solder connection between the sliding contact 108 and heater element 104 and electrodes 124, 126. The flux 138 is applied and melted after the solder connection has been made between the sliding contact 108 and the heater element 104 and electrodes 124, 126, as well as after attachment of the restraining element which holds the sliding contact 108 in place before the circuit device 100 is armed. In this manner, if while heating and melting the flux 138 the temperature reaches the melting point of the solder, the restraining element will hold the sliding contact 108 in place until the solder cools again.
The spacer 110 may then be placed on top of the substrate 102, the opening within the spacer having a width sufficient for the sliding contact 108 to fit within. The cover 112 may then be installed to keep the various parts in place.
a-2b show bottom and top views, respectively, of an assembled circuit protection device 200. The bottom of the circuit protection device may include terminals 202, 204, 206, 208 that facilitate electrical connection of the electrodes 124, 126, 128 and the heater element 106, respectively, to external circuit board elements. In this manner the terminals 202, 204, 206, 208 may be utilized to mount the circuit protection device 200 to a surface of a circuit panel (not shown) and bring the heater element 106, electrodes 124, 126, 128 into electrical communication with circuitry outside of the device 200.
In order to achieve a low profile, the height of the circuit protection device 200 may be 1.5 mm or less. The width of the circuit protection device 200 may be 3.8 mm or less. The length of the circuit protection device 200 may be 6.0 mm or less. In one embodiment, the circuit protection device may be 6.0 mm×3.8 mm×1.5 mm. Due to the expansion force of the spring element being parallel to the plane of the substrate surface, which results in the sliding contact also sliding parallel to the plane of the substrate, a substantially thin circuit protection device 200 is achieved.
a-3b show a circuit protection device 300 with the sliding contact 302 in the closed and open positions, respectively. In the closed position the sliding contact 302 bridges and provides an electrical connection between the electrodes 304, 306 and the heater element 308. In the open position, when the solder holding the sliding contact 302 to the electrodes 304, 306 and heater element 308 melts, the force of an expanding spring element pushes the sliding contact 302 down the channel 310 in the substrate 312, severing the electrical connection between the electrodes 304, 306 and heater element 308. As discussed above, the circuit protection device 300 is a three-function reflowable thermal fuse that is configured to open under three conditions: over current, over temperature, and controlled activation.
a also shows the restraining element 314 discussed above. The restraining element 314 may be a welded, fusible restraining wire that holds the sliding contact 302 in place during reflow. In particular, the restraining element 314 is adapted to secure the sliding contact 302 in a state that prevents it from sliding down the channel 310 during reflow. For example, the restraining element 314 may enable keeping the spring element in a compressed state even with the solder or other material holding the sliding contact 302 to the electrodes 304, 306 and heater element 308 melts, thereby preventing the spring element from expanding and pushing the sliding contact 302 down the channel 310.
The restraining element 314 may made of a material capable of conducting electricity. For example, the restraining element 314 may be made of copper, stainless steel, or an alloy. The diameter of the restraining element 314 may be sized so as to enable blowing the restraining element 314 with an arming current. The restraining element 314 is blown, such as by running a current through the restraining element 314, after the device 300 is installed. In other words, sourcing a sufficiently high current, or arming current, through the restraining element 314 may cause the restraining element 314 to open. In one embodiment, the arming current may be about 2 Amperes. However, it will be understood that the restraining element 314 may be increased or decrease in diameter, and/or another dimension, allowing for higher or lower arming currents.
To facilitate application of an arming current, a first end 314a and second end 314b of the restraining element 314 may be in electrical communication with various pads disposed about the housing. The first end 314a may be connected to the electrode 316, which corresponds to the electrode 128 in the embodiment of
a-3b also shows a flux 318, such as the flux 138 described above with respect to
Described below is an exemplary process for installing the three-function reflowable circuit protection devices described herein. The circuit protection device is placed on a panel. Solder paste may be printed on a circuit board before the circuit protection device is positioned. The panel, with the circuit protection device, is then placed into a reflow oven which causes the solder on the pads to melt. After reflowing, the panel is allowed to cool.
An arming current is run through pins of the circuit protection device so as to blow the restraining element. Referring to
The underside of the cover 804 (shown in
The spring element 802 may be installed into the cover 804 through a side of the cover 804. A cap 812 may then be inserted into the side of the cover 804 to hold one end of the spring element 802 in place such that when the spring element 802 expands under of the activation conditions described herein, the resulting force will push the cantilever portion 808 down the channel 902. The cap 812 includes a protrusion 814 that is tapered on one end and normal to the length of the cap 812 on the other end. In this manner, the cap 812 may be inserted into a hole on the side of the cover 804 with a snap-fit connection. It will be understood that other methods may be used to insert the spring element 802 into the cover 804.
While the three-function reflowable 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 three-function 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.
This application is a continuation-in-part of, and claims the benefit of priority from, U.S. application Ser. No. 13/019,976, filed Feb. 2, 2011, which is incorporated herein by reference. This application is related to U.S. application Ser. No. 13/019,983, filed Feb. 2, 2011, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13019976 | Feb 2011 | US |
| Child | 13209146 | US |
