WO2009002834A1 - Short-to-ground overvoltage protector - Google Patents

Abstract

An over-voltage protector includes at least one surge-responsive switching device that switches from an operational mode to a surge protection mode in response to an overvoltage condition, and a shunting assembly that creates a shunt path to ground, bypassing the switching device in response to heat generated by the switching devices in the surge protection mode. The shunting assembly includes a spring-loaded shorting element having a biased stand-off position and a released shorting position establishing the shunt path to ground, and a heat-responsive retention element that maintains the shorting element in the stand-off position, and that releases the shorting element to assume the shorting position in response to heat generated by the switching device. If an overvoltage condition persists, the heat generated by the current flowing through the switching device trips the shunting assembly, creating a shunt path to ground that bypasses the switching device.

Description

SHORT-TO-GROUND OVERVOLT AGE PROTECTOR

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit, under 35 U.S. C. §119(e), of US Provisional Application No. 60/945,806; filed 22 June 2007, the disclosure of which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] Not Applicable.

BACKGROUND

[0003] This disclosure relates to the field of overvoltage protection devices, particularly the class of overvoltage protection devices used in telephone circuit applications.

[0004] In copper wire telephone circuit ("telecom") applications, such as central office telephone exchanges and customer premise telephone points, there is a need to protect against voltage surges that can damage equipment, and, in rare, extreme cases, cause injury. These surges are the result of electrical disturbances stemming from three main sources: lightning strikes, accidental AC line contact with a telephone circuit line, and accidental induction between an AC power line and a telephone circuit line.

[0005] Lightning strike impulses are typically handled with overvoltage protection devices in the telephone circuitry that very quickly switch from a high impedance to a very low impedance so as to create a veiy low impedance path ("short"), either to ground or to another part of the circuitry where the impulse can be safely handled. The short duration of this type of impulse means that the energy can be dissipated in a time frame usually measured in milliseconds.

[0006] The impulses from both of the aforementioned AC events, on the other hand, may have durations lasting from several seconds to several hours, or even several days. Typical overvoltage protection devices usually cannot withstand currents exceeding about 5 amps for prolonged periods of time. If a surge event results in a condition that exceeds the device's capability for a prolonged period of time, the resulting power dissipation could allow an unsafe condition to occur.

[0007] Accordingly, there is a need for an overvoltage protection device for telephone circuits that is sufficiently robust to withstand exposure to high currents for prolonged periods of time, and yet that can provide a rapid response at the onset of the overvoltage event. Furthermore, there is a need for such a device that can accommodate multiple lines and multiple short-to- ground configurations within a telephone circuit.

SUMMARY OF THE DISCLOSURE

[0008] Broadly, the present disclosure relates to an over-voltage protector, comprising an overvoltage switching device configured to switch from an operational mode to a surge protection mode in response to an overvoltage fault condition; and a heat-responsive shunting assembly configured to create a shunt path to ground bypassing the switching device in response to heat generated by the switching device in the surge protection mode. The shunting assembly comprises a spring-loaded shorting element having a biased stand-off position and a released shorting position establishing a shunt path to ground; and a heat-responsive retention element that maintains the shorting element in the stand-off position, and that releases the shorting element to assume the shorting position in response to heat generated by the switching device.

[0009] The switching device functions normally to direct short-duration overvoltage excursions or surges to ground. If an overvoltage condition persists, the heat generated by the ground fault current flowing through the switching device causes the shunting assembly to trip, creating a shunt patli to ground that bypasses the switching device, and thereby avoiding potentially damaging heat build-up in the equipment in which overvoltage protector is installed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1 is a perspective view of a multi-line short-to-ground overvoltage protector in accordance with a preferred embodiment of the invention, wherein the overvoltage protector includes three surge-responsive switching devices; [0011] Fig. 2 is a perspective view of the overvoltage protector of Fig. 1, showing the surge- responsive switching devices and the substrate (circuit board) of the overvoltage protector, with the spring-loaded shorting element removed for clarity;

[0012] Fig. 3 is an exploded perspective view of the shorting element and the retention element used in the overvoltage protector of Fig, 1;

[0013] Fig. 4 is a perspective view of the assembled shorting element and retention element shown in Fig. 3;

[0014] Fig. 5 is a cross-sectional view taken along line 5 - 5 of Fig. 1, showing the overvoltage protector in its "untripped" (non-shorting) configuration;

[0015] Fig. 6 is a cross-sectional view, similar to that of Fig. 5, showing the overvoltage protector in its "tripped" (shorting) configuration;

[0016] Fig.7 is a perspective view of the circuit board of the overvoltage protector of Fig. 1, with the components removed therefrom to show the metallized contact pads on the upper surface thereof;

[0017] Fig. 8 is a bottom perspective of the overvoltage protector of Fig. 1;

[0018] Fig. 9 is a schematic diagram of a protection circuit incorporating the overvoltage protector of the present invention, with the shorting element shown in an "untripped" (non- shorting) state; and

[0019] Fig. 10 is a graph of return current versus time-to-trip for an exemplary embodiment of the overvoltage protector of Fig. 1. DETAILED DESCRIPTION

[0020] Referring to Figures 1-6, a multi-line short-to-ground overvoltage protector 10, in accordance with a preferred embodiment of the present invention, comprises a substrate 12. such as a circuit board, having a first or upper surface 14 and a second or lower surface 15. A plurality of semiconductor overvoltage switching devices 16a, 16b, 16c, responsive to overvoltage surges, are mounted on the upper surface 14. The switching devices 16a, 16b, 16c are packaged as surface-mount devices (SMDs), and they may advantageously be appropriately- rated surge-responsive devices, such as thyristors, triacs, or any suitable functional equivalent that may suggest itself to those skilled in the pertinent arts. As will be seen from the ensuing description, there are optimally three surge-responsive switching devices 16a, 16b, 16c in a preferred embodiment of an overvoltage protector 10 for use in a telephone tip-and-ring circuit. It will be appreciated, however, that a single line short-to-ground overvoltage protector, in accordance with the present invention, would include a single surge-response switching device. Furthermore, a dual-line overvoltage protector having two switching devices is contemplated within the scope of the present invention, as is a multi-line overvoltage protector having four or more switching devices.

[0021] As shown in Figs. 2 and 7, the substrate 12 has a tab 17 extending from its rear edge. The purpose of the tab 17 will be explained below. As best shown in Fig. 7, the upper surface 14 of the substrate 12 includes metallization areas that serve as contact pads for the switching devices 16a, 16b, 16c. Specifically, the terminals of the first switching device 16a are respectively fixed (as by soldering) to first and second contact pads 18, 20; the terminals of the second switching device 16b are respectively fixed to third and fourth contact pads 22, 24; and the terminals of the third switching device 16c are respectively fixed to fifth and sixth contact pads 26, 28. The sixth contact pad 28 is connected by conductive traces 30, 32 respectively to the first and third contact pads 18, 22 to form a substantially Y-shaped metallized area. The second and fourth contact pads 20, 24 may be respectively provided with first and second metallized through-holes 34a, 34b in which conductive connection pins 36a, 36b are respectively installed and fixed. A third metallized through hole 34c is provided between the first and second metallized through-holes 34a, 34b, and a third conductive connection pin 36c is advantageously installed therein. As shown in Fig. 8, the internal metallization of the third metallized through- hole 34c is connected by an embedded conductor 38 in the substrate 12 to a large metallized pad that serves as a ground plane 40 when the third connection pin 36c is connected to ground. A fourth metallized through-hole 34d is provided in the fifth contact pad 26, whereby the internal metallization of the fourth through-hole 34d extends to and connects with the ground plane 40,

[0022] As shown in Figure 1, the overvoltage protector 10 further includes a heat-responsive shunting assembly that comprises a spring-loaded shorting element 42 and a stand-off or retention element 44, both of which are shown in greater detail in Figures 3 and 4. The shorting element 42 is made of a resilient, conductive metal that is formed (as by stamping and bending) into a spring clip configuration. Thus, in an exemplary embodiment, the shorting element 42 has an upper portion comprising a pair of forwardly-extending parallel arms 46, each terminating in a downwardly-extending finger 48. Each of the amis 46 is connected to a respective longitudinal base member 50 by a connecting portion 52. Each of the connecting portions 52 advantageously has an inwardly-directed nub 53 that engages the rearwardly-extending tab 17 of the substrate 12 to maintain the proper orientation of the shorting element 42.

[0023] The amis 46 are connected to each other by a first or front lateral member 54 and a second or rear lateral member 56, whereby the lateral members 54, 56 and the amis 46 define the perimeter of an opening 58. The longitudinal base members 50 are joined near the front of the shorting element 42 by a lateral base member 60. Extending rearwardly from the lateral base member 60 is a central contact finger 62 terminating in an upwardly bent distal tip 64 that is located and configured to enter the fourth metallized through-hole 34d from the lower surface 15 of the substrate 12, thereby contacting the metallized internal surface of the fourth through-hole 34d. The central contact finger 62 thus provides a connection to the ground plane 40 from the fifth contact pad 26, while the embedded conductor 38 provides a connection to the ground plane 40 from the third connection pin 36c.

[0024] The opening 58 in the shorting element 42 accommodates the stand-off or retention element 44, as shown in Figures 3 and 4. The retention element 44, preferably formed of a unitary piece of stamped and formed thermally conductive metal, comprises a main body 66 that extends transversely across the opening 58 below the level of the amis 46 of the shorting element 42. Extending upwardly from the front and rear edges of the main body 66 are front and rear vertical members 68, 70, terminating in front and rear horizontal tabs 72, 74, respectively. The front tab 72 is attached to the first or front lateral member 54, and the rear tab 74 is attached to the second or rear lateral member 56. The attachment of the tabs 72, 74 to the lateral members 54, 56 is by means of a thermally-detachable bonding agent, preferably a eutectic solder, that releases the shorting element 42 from the retention element 44 in response to heat generated by the switching devices 16a, 16b, 16c when they remain in the surge protection mode for longer than a predetermined time.

[0025] As shown in Figure 1, the shorting element 42, with the stand-off or retention element 44 soldered to it (as described above), is installed on the substrate 12 (to which the switching devices 16a, 16b, 16c have been fixed, as described above) as follows: The shorting element 42 is fitted over a rear edge 76 of the substrate 12. so that the connecting portions 52 abut or are closely adjacent to the rear board edge 76, with the anus 46 extending forwardly over the upper surface 14 of the substrate 12 and the switching devices 16a, 16b, 16c. The longitudinal base members 50 of the shorting element 42 extend across, and are in contact with, the ground plane 40 on the lower surface 15 of the substratel2, with the distal tip 64 of the central contact finger 62 extending into the fourth through-hole 34d to contact the metallized surface therein (Fig. 8). This brings the bottom surface of the stand-off or retention element 44 into contact with the switching devices 16a, 16b, 16c, as shown in Fig. 5. As installed, the downwardly depending fingers 48 of the amis 46 of the shorting element 42 are spaced above and separated from the second and fourth contact pads 20, 24. respectively. With the shorting element 42 in this position, best illustrated in Fig. 5, the overvoltage protector 10 is in its untripped or operational state.

[0026] The shorting element 42 functions as a spring clip that maintains the stand-off or retention element 44 in intimate contact with the surfaces of the overvoltage switching devices 16a, 16b, 16c. The stand-off or retention element 44 also maintains the separation between the fingers 48 of the shorting element 42 and the second and fourth contact pads 20, 24. During short-duration overvoltage events (e.g., a lightning strike surge), the overvoltage switching devices 16a, 16b, 16c function as designed; i.e., they switch to a surge protection mode in which they route the surge to ground. Upon conclusion of the surge event, the switching devices 16a, 16b, 16c revert or switch back to their normal operational mode. If a prolonged overvoltage event (such as one of the AC events described above) is experienced, the switching devices 16a, 16b, 16c remain in their surge protection mode, but the current passing through them to ground generates heat that, unless quickly dissipated, can cause equipment damage, or possibly even injury. In the overvoltage protector 10 of the present invention, however, this heat is transmitted through the stand-off or retention element tabs 72, 74 and the lateral members 54, 56, respectively, of the shorting element 42. If the event lasts long enough, the heat so generated heats the thermally-responsive bonding agent between the tabs 72, 74, and their respective lateral members 54, 56 beyond the melting point of the bonding agent, thereby releasing the stand-off or retention element 44 from the shorting element 42. This, in turn, releases the shorting element 42 to allow its spring action to bring the fingers 48 into contact with the second and fourth contact pads 20, 24 (see Fig. 6), thereby creating a permanent path to ground. Furthermore, the pressure applied by the spring-loaded shorting element 42 in its closed (tripped or shorting) position clamps the stand-off or retention element 44 against the switching devices 16a, 16b, 16c, thereby preventing them from falling off the substrate 12 when the solder joints connecting these devices to their respective contact pads melt due to dissipated heat from the devices.

[0027] The shorting element 42 is advantageously provided with a trapping stub or finger 78 projecting laterally from one of the arms 46 into the opening 58, whereby the loose stand-off or retention element 44 is trapped between the shoring element 42 and the substrate 12, and thus is prevented from falling out of the opening 58, if for example, the overvoltage protector 10 is oriented upside-down. This keeps the loose stand-off or retention element 44 from creating a short or damaging another component or circuit.

[0028] Specifically, it can be seen from the drawings that, upon closure of the spring-loaded contact fingers 48 against the second and fourth contact pads 20, 24, an electrical path is created directly from the second and fourth contact pads 20, 24, to the ground plane 40 through the shorting element 42. This path creates a direct-to-ground shunt that directs an overvoltage current from the first contact 18 to ground, and from the third contact 22 to ground, bypassing the switching devices 16a, 16b, 16c. This is shown schematically in Fig. 9, in which three surge- responsive switching devices 16a, 16b, 16c, are arranged in a T-configuration in a typical tip- and-ring circuit having a first terminal A and a second terminal B. Specifically, the first switching device 16a and the second switching device 16b are connected in series between the terminals A and B. The third switching device 16c is connected in series between the first switching device 16a and ground, and between the second switching device 16b and ground. In Fig. 9, the contact fingers 48 of the shorting element 42 of the overvoltage protector 10 are shown as movable switch contacts operable to close against the stationary switch elements represented by the second and fourth contact pads 20, 24. Thus, in response to a sustained overvoltage fault condition, the contact fingers 48 create a first shunt path 80a to ground bypassing the first switching device 16a and the third switching device 16c, and a second shunt path 80b to ground, bypassing the second switching device 16b and the third switching device 16c. This result obtains whether an overvoltage fault condition occurs on a single line to ground (A to ground or B to ground), both lines to ground (A and B to ground), or line-to-line (A to B). Thus, if at least one switching device of the three experiences a prolonged overvoltage event or condition, sufficient heat is generated to release the retention or stand-off element 44 and thus trip the spring-loaded shorting element 42 as described above.

[0029] hi the circuit configuration of Fig. 9, it may be advantageous for the first and second switching devices 16a, 16b to have substantially equal current ratings, while the third switching device 16c would typically have a higher current rating, because the sum of the currents from the first and second switching devices 16a, 16b would flow through the third switching device 16c.

[0030] The shunting assembly (the shorting element 42 and the retention element 44) can be designed to trip into the shunting mode after a period of time in the surge protection mode that depends on the operational parameters of the overvoltage switching devices, the melting point of the bonding agent that joins the retention element 44 to the shorting element 42, and the overall heat dissipation qualities of the telecom circuit and equipment in which the overvoltage protector 10 is installed. This period may represent an overvoltage fault condition of less than one hundred milliseconds to several minutes in duration, hi a specific embodiment of the overvoltage protector 10, the bonding agent between the retention element tabs 72, 74 and their respective lateral members 54, 56 of the shorting element 42 is a eutectic solder having a melting point of about 218° C. The relationship between return current and failsafe time to trip for such a device, operated at 50 Hz AC at 230V, is shown in Fig. 10.

[0031] The embodiment disclosed herein is exemplary only, and other embodiments, configurations, variations, and modifications may suggest themselves to those skilled in the pertinent arts. For example, as mentioned above, an overvoltage protector in accordance with the present invention may include one, two, three, or more surge-responsive switching devices, and the modifications needed to accommodate any particular number of switching devices will readily suggest themselves to those of ordinary skill in the pertinent arts. Such other embodiments, configurations, variations, and modifications should be considered within the spirit and scope of the invention, as defined in the claims that follow.

Claims

WHAT IS CLAIMED IS:

1. An overvoltage protector, comprising: at least one surge-responsive switching device configured to switch from an operational mode to a surge protection mode in response to an overvoltage fault condition; and a heat-responsive shunting assembly configured to create a shunt path to ground bypassing the at least one switching device in response to heat generated by the switching device in the surge protection mode.

2. The overvoltage protector of claim 1, wherein the shunting assembly comprises: a spring-loaded shorting element having a biased stand-off position and a released shorting position establishing a shunt path to ground; and a heat-responsive retention element that maintains the shorting element in the stand-off position, and that releases the shorting element to assume the shorting position in response to heat generated by the switching device.

3. The overvoltage protector of claim 1, wherein the at least one surge-responsive switching device includes a plurality of surge-responsive switching devices.

4. The overvoltage protector of claim 3, further comprising: a substrate having first and second opposed surfaces; first, second, and third pairs of electrical contacts on the first surface; and a ground plane conductor on the second surface; wherein the plurality of switching devices comprises first, second, and third switching devices mounted on the first surface, whereby the first switching device connects the first pair of contacts, the second switching device connects the second pair of contacts, and the third switching device connects the third pair of contacts.

5. The overvoltage protector of claim 4, wherein the shorting element is mounted on the substrate in contact with the ground plane conductor, and so that, in its stand-off position, it is spaced from all of the first, second, and third pairs of contacts, and in its shorting position it establishes contact with one of the first pair of contacts and one of the second pair of contacts.

6. The overvoltage protector of claim 2, wherein the retention element comprises a thermally- conductive metal element having a first portion in contact with at least one of the switching devices and a second portion that is attached to the shorting element by a thermally-detachable bonding agent, whereby the shorting element is maintained in its stand-off position when the retention element is attached to the shorting element, and wherein the shorting element is released to assume its shorting position when the retention element is detached from the shorting element.

7. The overvoltage protector of claim 6, wherein the shorting element includes a trapping member configured to trap the retention element after the retention element is detached from the shorting element.

8. An overvoltage protector, comprising: a substrate having first and second opposed surfaces; first, second, and third pairs of electrical contacts on the first surface; a ground plane conductor on the second surface; first, second, and third overvoltage switching devices mounted on the first surface, each of the switching devices being configured to switch from an operational mode to a surge protection mode in response to an overvoltage fault condition, the first switching device being connected between the first pair of contacts, the second switching device being connected between the second pair of contacts, and the third switching device being connected between the third pair of contacts; a spring-loaded shorting element mounted on the substrate in contact with the ground plane conductor, the shorting element being operable between a biased stand-off position, in which it is spaced from all of the first, second, and third pairs of contacts, and a released shorting position, in which it establishes contact with one of the first pair of contacts and one of the second pair of contacts so as to establish a shunt path to ground; and a heat-responsive retention element that maintains the shorting element in the stand-off position, and that releases the shorting element to assume the shorting position in response to heat generated by the switching devices when the switching devices are in the surge protection mode.

9. The overvoltage protector of claim 8, wherein the retention element comprises a thermally- conductive metal element having a first portion in contact with at least one of the switching devices and a second portion that is attached to the shorting element by a thermally-detachable bonding agent, whereby the shorting element is maintained in its stand-off position when the retention element is attached to the shorting element, and wherein the shorting element is released to assume its shorting position when the retention element is detached from the shorting element.

10. The overvoltage protector of claim 8, wherein the shorting element includes a trapping member configured to trap the retention element after the retention element is detached from the shorting element.

11. A method of protecting a circuit from overvoltage conditions, the circuit having first and second terminals, the method comprising: connecting a plurality of overvoltage switching devices into the circuit between the first and second terminals, at least one of the switching devices being connected to ground, each of the switching devices being operable to switch from an operational mode to a surge protection mode in response to an overvoltage fault condition; and shunting an overvoltage current to ground around the switching devices in response to an overvoltage fault condition having a duration greater than a predetermined duration.

12. The method of claim 11, wherein the shunting step is performed in response to heat generated by the switching devices when they are in their surge protection mode.

13. The method of claim 11, wherein the plurality of switching devices comprises first, second, and third switching devices, wherein the first switching device and the second switching device are connected in series between the first and second terminals, and wherein the third switching device is connected in series between the first switching device and ground, and between the second switching device and ground.

14. The method of claim 13, wherein the first and second switching devices have substantially equal current ratings, and wherein the third switching device has a current rating that is higher than the current rating of the first and second switching device.