Solid state overvoltage protection circuit

Abstract: An overvoltage protection circuit (16) for electronic equipment, such as telecommunications equipment (10), computers and instrumentation equipment, having tip and ring lines (TR) includes a first voltage sensitive switch (30) having first and second terminals. The first terminal of the first switch is connected to the tip line (T). A second voltage sensitive switch (32) includes first and second terminals. The first terminal thereof is connected to the second terminal of the first voltage sensitive switch (30), and the second terminal thereof is connected to the ring line (R). A third voltage sensitive switch (34) includes first and second terminals. The first terminal is connected to the second terminal of the first voltage sensitive switch and to the first terminal of the second voltage sensitive switch. The second terminal of the third voltage sensitive switch is connected to ground potential. The switches are conductive when a respective voltage threshold is exceeded. In one embodiment, the sum of the voltage thresholds of the first and third voltage sensitive switches is approximately equal to the sum of the voltage thresholds of the second and third voltage sensitive switches, such that the circuit is conductive when either of the thresholds of the first and third voltage sensitive switches or the second and third voltage sensitive switches are exceeded. In another embodiment, the holding current of the third voltage sensitive switch is lower than that of either the first or second switch. In this combination, the third switch turns on first, then both the first and second switches turn on in unison and hold the lines in balance during switching.

Description: TECHNICAL FIELD This invention relates to electrical protection of electronic equipment, and more particularly to a solid state overvoltage protection circuit. BACKGROUND OF THE INVENTION Transmission lines may carry foreign electrical energy from, for example, transient voltage surges which can be generated by lightning strikes, industrial equipment, the utilities themselves, as well as other equipment within a communications system. In order to safeguard electronic equipment, such as telecommunications equipment, computers, and instrumentation equipment, connected to such lines, traditional protective methods have included the use of gas discharge tubes, carbon blocks, fuses, heat coils, MOVs and other types of devices and wiring schemes. These methods do not provide a sufficient level of protection required for solid state circuits used in present day electronic equipment. These prior methods are slow to respond, allow significant voltage spikes above the clamping voltage before they begin to conduct, and respond with voltage overshoot. Protection circuits have also been developed which include semiconductor elements, such as the protection circuit described and claimed in U.S. Patent No. 4,408,248. However, such circuits have not proven satisfactory because they have required numerous components which are costly and which require excessive physical space. Other protection circuits have used a single solid state switch formed on N-type silicon material, such as RCA's Surjector, where once the breakover voltage is exceeded, there is a positive slope on the current-voltage diagram. Such a protection circuit dissipates additional energy until it turns on. The voltage overshoot is typically 5 to 15 volts or more, depending on the speed of the transient that the device is trying to protect. A need has thus arisen for an overvoltage protection circuit that has a fast reaction time, offers tight voltage control, and provides for a lower power dissipation during switching. Additionally, a need has arisen for an overvoltage protection circuit that is economical and which has reduced component requirements, reduced physical size, while maintaining high surge current capacities. FR- A-2 433 845 discloses an electronic protection circuit possessing a combination of zener diodes and regular diodes in a three stage configuration. The individual stages become conductive when the voltage applied to the stage exceeds the threshold voltage level. The threshold voltage for two stages between the circuit line and ground must be exceeded in order for the circuit line to conduct to ground. SUMMARY OF THE INVENTION In accordance with the present invention, a solid state transient voltage protection circuit for electronic equipment is provided. The circuit comprising: a first bidirectional voltage sensitive switch, a second bidirectional voltage sensitive switch, and a third bidirectional voltage sensitive switch, said first switch connected to one line, said second switch connected to a second line, and the third switch connected between a ground potential and a common node between said first and second switches, the combined switching voltages of said first and third switches being substantially equal to the combined switching voltages of said second and third switches, such that the circuit conducts on either the first line to ground or the second line to ground when the voltage on the respective line exceeds the threshold voltage levels, characterized by : at least one of said switches possessing a negative-sloping voltage with increasing current after said switch exceeds the respective voltage threshold. In accordance with one embodiment of the invention, the holding current of the third voltage sensitive switch (ground) is lower than that of either the first or second voltage sensitive switch. In this embodiment, the third switch turns on first, then both the first and second switches turn on in unison and hold the lines in balance during switching. In accordance with another embodiment of the invention, at least one voltage sensitive switch in the tip and ground and ring and ground is made on P-type silicon material and has the characteristic of a negative slope with increasing current after exceeding the breakover voltage. As an example, only the voltage sensitive switch to ground, or both the voltage sensitive switch to tip and the voltage sensitive switch to ring, or all three voltage sensitive switches are made on P-type material. The combined switch of this protection circuit has the desirable characteristic of low switching power loss on continuous AC circuits and also safety conducting "sneak" currents to ground, even though they may be less than the holding current. "Sneak" current refers to the current at a high voltage that sneaks or passes by the protector before the device turns on. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Description taken in conjunction with the accompanying Drawings in which: FIGURE 1 is a block diagram of a protection circuit in accordance with the present invention connected between electronics equipment and a transmission line; FIGURE 2 is a block diagram of the protection circuit of the present invention; FIGURE 3 is a diagram of the current versus the voltage of a sidac used in the tip or ring line of one embodiment of the present invention; FIGURE 4 is a diagram of the current versus the voltage of a sidac used in the ground line of one embodiment of the present invention; FIGURE 5 is the combined current-voltage diagram for one embodiment of the present invention using three sidacs; FIGURES 6A and 6B are current-voltage diagrams of the reaction of one embodiment of the invention to a small overvoltage; and FIGURE 7 is a current-voltage diagram of the reaction of one embodiment of the invention to a large current surge. DETAILED DESCRIPTION Referring to FIGURE 1, electronic equipment 10 is connected via tip (T) line 12, 18 and ring (R) line 14, 20 to a protection circuit 16. Electronic equipment 10 may comprise, for example, telecommunications equipment such as central office, telephone, terminal or other station devices. Electronic equipment 10 may also comprise computers and instrumentation equipment, for example. Protection circuit 16 is connected to tip and ring lines 18 and 20 of a transmission or line. Protection circuit 16 is also interconnected to ground potential 22. Electronic equipment 10 is thereby protected from foreign electrical energy due to a fault or transient voltage surge on tip and ring lines 18 and 20, respectively. FIGURE 2 illustrates the protection circuit 16 which includes three bidirectional voltage sensitive switches (V ss ) 30, 32, and 34. Voltage sensitive switch 30 is disposed in the tip line 12, 18 voltage sensitive switch 32 is disposed in the ring line 14, 20 and voltage sensitive switch 34 is disposed in the ground line 22. In application of the circuit 16 to a normal telephone line, the switch 34 requires at least the normal DC battery voltage, 52 volts, to initiate conduction. Protection circuit 16 creates a balanced protection system in that if either of the voltage sensitive switches 30 or 32 in the tip and ring lines, respectively, conduct to ground, there is an identical conduction of the other side of the tip or ring line to ground. Protection circuit 16 becomes conductive on both the tip and ring to ground line whenever the voltage on either the tip or ring to ground line is exceeded, provided the voltage on the tip or ring line not first grounded, exceeds the remaining blocking voltage in the other of the tip or ring line. This balanced aspect of protection circuit 16 prevents a voltage differential between the tip and ring lines which can cause damage to electronic equipment 10. Foreign electrical energy on tip or ring lines is thereby dissipated rather than affecting electronics equipment 10. The balanced switching may take place on both tip and ring lines within 8 microseconds. Voltage sensitive switches 30, 32, and 34 may comprise, for example, a Model P2353A sidac manufactured and sold by Teccor Electronics, Inc. of Irving, Texas. Additionally, voltage sensitive switches 30, 32, and 34 may comprise, for example, triacs and a silicon controlled rectifier bridge with the voltage threshold of the triac controlled by a zener diode. In one embodiment of the present invention, at least one of the voltage sensitive switches 30, 32, and 34 in the tip to ground and in the ring to ground circuit may be fabricated from a P-type semiconductor material having the characteristic of a negative slope with increasing current after exceeding breakover voltage. In this embodiment, lower switching currents are found than in designs, now on the market and giving the device the ability to ground AC sneak currents without device degradation or adding more circuitry as shown in U.S. Patent No. 4,408,248. As one example, only the ground voltage sensitive switch 34 may be fabricated on P-type semiconductor material, such as silicon. In another example, both tip voltage sensitive switch 30 and ring voltage sensitive switch 32 may be made on P-type semiconductor material, or all three switches 30, 32, 34 may be made from such material. The combined breakover of two switches in a line to ground current exhibits a negative slope with increasing current after exceeding V BO due to selecting the proper P-type chip even if the companion voltage sensitive switch has a positive slope. The combined threshold voltages of voltage sensitive switch 30 and voltage sensitive switch 34 and the combined threshold voltages of voltage sensitive switch 32 and voltage sensitive switch 34 are approximately equal and chosen for a predetermined specification. As an example, the switches 30, 32 and 34 may be three sidacs having a V BO of ± 110 volts so there is ± 220 volts V BO between tip and ground and ring and ground. At least one voltage sensitive switch in both the tip to ground and ring to ground path has a holding current which is less than the other and chosen for a predetermined specification. In one embodiment of the present invention, the holding current of voltage sensitive switch 34 is lower than the holding current of voltage sensitive switch 30 and voltage sensitive switch 32, such that voltage sensitive switch 34 conducts first so that both voltage sensitive switches 30 and 32 conduct in unison to hold the tip and ring lines in balance during switching. In such an embodiment, balanced switching may take place within 0.5 microseconds. The holding current of voltage sensitive switch 34 may comprise, for example, 20 mAmps. In a further modification of this embodiment, at least one of the switches 30, 32, 34 in a line to ground circuit is made of P-type semiconductor material, such as silicon, such that the combined breakover voltages of two switches in a line to ground circuit exhibit the characteristic of a negative slope with increasing current after V BO exceeded, as described above. Voltage sensitive switches 30, 32, and 34 may be housed in a single three leaded plastic package such as, for example, a TO-92, TO-202, TO-220, TO-218, or TO-220A. The interconnection between voltage sensitive switches 30, 32, and 34 is internal to the package and the three leads comprise interconnections to the tip, ring and ground lines. FIGURE 3 illustrates the current-voltage diagram for the tip or ring chip of one embodiment, a three-element protection circuit 16 in which the voltage sensitive switches 30, 32 and 34 are sidac chips. Both the tip and ring sidac chips have a relatively high I h , e.g., 300 mils, block a voltage of ± 100 volts and have the capability of passing a maximum surge of 100A. 10x1000 µs. The three-element design provides three ± 110 volt chips in the same package connected to offer a ± 220 volt drop between tip to ground, ring to ground and tip to ring. The sidac chips are predetermined to insure that the chip in the ground leg turns on first when the applied voltage exceeds ± 220 volts. When the ground chip turns on the potential to common for both tip and ring lines 12, 18 and 14, 20, respectively, drops to 100 volts. Since both tip and ring exceed 100 volts, the sidac chips start to conduct together, dropping the voltage to zero volts and maintaining balance between the tip and ring circuits. FIGURE 4 illustrates the sidac chip for the voltage sensitive switch 34 in the ground leg of one embodiment of a three-element protection circuit 16 with sidacs for the voltage sensitive switches 30 and 32 as having the current-voltage diagram shown in FIGURE 3 and described hereinabove. The sidac chip in the ground line needs to take two times the rated surge, two times 100A, as both tip and ring switches 30, 32 fire together. The I h for sidac chip in the switch 34 must be lower than the tip or ring sidac chip I h , but high enough to meet certain predetermined applications, e.g., 150 mils I h . FIGURE 5 illustrates the current-voltage diagram of both tip to ground and ring to ground of the embodiment three-element protection circuit 16 as illustrated in FIGURES 3 and 4 and described above. Referring to FIGURE 5, A to B represents the switching voltage in both the tip and ring legs of the circuit, e.g., 110 volts. B to C is the switching voltage in the ground leg, e.g., 110 volts. A to C is the switching voltage of the combined three-element circuit, 220 volts, measuring ± tip to ground or ± ring to ground. C to D is the switching current of the sidac chip 34 in the ground line, 150 mils, and illustrates the negative slope due to selecting the proper P-type chip as described above. D to E is the voltage drop in both tip and ring circuits when the ground chip, switch 34, turns on. E to F is the switching current of each of the switches 30, 32 in the tip and ring legs, 300 mils, and again illustrates the negative slope due to selecting the proper P-type chip. F to G is the voltage drop when all the sidac chips turn on measured to ground in either line. G to H is the voltage increasing slightly as the sidac chips conduct the high current surges. In operation of this embodiment, when an overvoltage spike appears to the circuit 16, the voltage-current path proceeds A, C, D, E, F, G, H and when the spike passing the current-voltage path proceeds H, G, F, E, D, C, A. FIGURE 6A illustrates the current-voltage reaction of an embodiment of the three-element circuit 16 of FIGURES 3-5 to a small overvoltage of 230 volts. At 20 volts before peak voltage is reached, tip⁰ is at 210 volts and ring⁰ is at 160 volts. The ring has -48 to -52 volt DC on that line. No current flows at this point as the breakover voltage C of 220 volts has not been reached. Referring to FIGURE 6B, the voltage increases to 220 volts and the ground chip quickly follows the path C-D-E as it turns on. At this instant, tip¹ is moving from E to F depending on the available current. If enough current is available to drive tip¹ past F, the tip chip will turn on. At this instant, the ring line does not increase to 170 volts, since the ground chip shorted on it is only being blocked by the chip in the ring line defined by A-B-F-G-H. Since the voltage is over the A-B voltage of 110 volts, the ring voltage drops to B and current increases up to B and then moves up to a point ring¹ between B-F and follows the B-F-G-H line depending on the available current. FIGURE 7 illustrates the current-voltage diagram for the embodiment of the circuit 16 illustrated in FIGURES 3-5 and described above when a large current is present, e.g., 100 A. Both tip² and ring² would move on up the path E to F, switch F to G, then move G to H. When the current surge is over, both ring and tip voltages would recover down the path H-G-F-E-D-C, and both tip and ring lines are in the blocking mode with all three sidac chips recovered from the surge. The present protection circuit creates a balanced protection system with minimal components, such that when either the tip or ring line conducts to ground, the other of the tip or ring line also conducts to ground. It also provides a degree of AC protection at low levels when fabricated with "P" type materials as described while remaining economical to manufacture. Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.

Claims: A solid state transient voltage protection circuit (16) for electronic equipment, comprising: a first bidirectional voltage sensitive switch (30), a second bidirectional voltage sensitive switch (32), and a third bidirectional voltage sensitive switch (34), said first switch (30) connected to one line (18), said second switch (32) connected to a second line (20), and the third switch (34) connected between a ground potential (22) and a common node between said first (30) and second (32) switches, the combined switching voltages of said first (30) and third (34) switches being substantially equal to the combined switching voltages of said second (32) and third (34) switches, such that the circuit conducts on either the first line (18) to ground (22) or the second line (20) to ground (22) when the voltage on the respective line exceeds the threshold voltage levels, characterized by : at least one of said switches possessing a negative-sloping voltage with increasing current after said switch exceeds the respective voltage threshold. The circuit of Claim 1 wherein both the first line (18) to ground (22) and the second line (20) to ground (22) conduct when the voltage on the first line (18) or the second line (20) to ground (22) exceeds the necessary threshold voltage levels and the voltage on the other line to ground (22) exceeds the remaining blocking voltage in the other line whereby a balance voltage protection circuit is provided. The circuit of Claim 1 wherein the first line (18) is a tip line the second line (20) is a ring line. The circuit of Claim 1 wherein said voltage sensitive switches (30, 32, 34) each include a plurality of triacs each having a zener diode for controlling the voltage threshold level of said voltage sensitive switch. The circuit of Claim 1 wherein the holding current of said third voltage sensitive switch (34) is lower than the holding current of either of said first (30) or said second (32) voltage sensitive switches. The circuit of Claim 1 wherein at least one of said voltage sensitive switches (30, 32, or 34) include P-type semiconductor material. The circuit of Claim 6 wherein the holding current of said third voltage sensitive switch (34) is lower than the holding current of either of said first (30) or said second (32) voltage sensitive switches. The circuit of Claim 1 wherein said voltage sensitive switches are disposed within a single housing having three terminals comprising the first terminal of said first voltage sensitive switch (30), said second terminal of said second voltage sensitive switch (32) and said third terminal of said third voltage sensitive switch (34). The circuit of Claim 3 wherein said third voltage sensitive switch (34) requires at least the normal DC battery voltage on the ring line (20) to conduct. The circuit in Claim 1 wherein the first (30), second (32), and third (34) voltage sensitive switches are sidacs. The overvoltage protection circuit of Claim 10 wherein the holding current of said third sidac (34) is lower than the holding currents of said first (30) and second (32) sidacs. The overvoltage protection circuit of Claim 10 wherein said third sidac (34) is made on P-type silicon material and has the characteristic of a negative slope in a current-voltage diagram as current increases past the breakover voltage. The overvoltage protection circuit of Claim 10 wherein two of the three sidacs (30, 32, 34) are made on P-type silicon material and both P-type sidacs have the characteristic of a negative slope in a current-voltage diagram as current increases past the breakover voltage.

Inventors: WEBB MONTY F, WEBB, MONTY F.
Applicants: TECCOR ELECTRONICS INC, TECCOR ELECTRONICS, INC.
Application year: 1989
Main classification: H04M3
IPC codes: G06F1, H04M3, H02H9