OVERVOLTAGE PROTECTION CIRCUITRY

Abstract: An overvoltage protector suitable for protecting conductors comprises first, second and third at least bipolar protection elements (132, 134, 136). A first electrode of each protection element is connected to a common node (138). A second electrode of the first protection element (132) and a second electrode of the second or third protection elements (134, 136) are suitable for connection to a conductor to be protected. The first and third elements (132, 136) have breakdown conduction characteristic in one direction and diode conduction characteristic in the other, and the second element (134) has breakdown conduction characteristic in both directions, wherein the respective first electrodes of the first and third elements (132, 136) are of the same polarity.

Description: OVERVOLTAGE PROTECTION CIRCUITRY Background of the Invention Field of the Invention The present invention is concerned with overvoltage protection, and particularly with overvoltage protection in electrical equipment for use in telephony. Description of the related art The present invention is suitable for use in the overvoltage protection of POTS (Plain Old Telephone System) equipment. In use, a POTS line from an exchange to a subscriber can carry various signals. For example, the line can carry, among others, a ringing signal to indicate an incoming call, information signals exchanged during a call, a line test signal, metering signals, or caller ID signals. At the exchange, these signals need not originate from the same source. In fact, for convenience, the signals can each be generated by different means and switched onto the line by relay switches. An example of an exchange organised in that manner is illustrated schematically in Figure 1 of the drawings. The exchange 10 illustrated therein comprises a Subscribe Line Interface Circuit (SLIC) 12 which is connected, via SLIC protection 14 and SLIC relay switches 16, to a TIP wire 18 and Ring wire 20 of a POTS line. Information signals are sent and received by the SLIC, at voltage levels in the region of IV rms with a d.c. bias of -3V to -50V on the two line conductors 18, 20. The SLIC 12 is usually implemented on an IC (Integrated Circuit). A ring generator 22 operative to generate a ringing signal is connected to the TIP wire 18 and the Ring wire 20 via ring relay switches 24. Typically, the ringing signal is asymmetrical, the peak negative voltage being in the region of -200V and the peak positive voltage level being in the region of +100 V. Test equipment 26 operative to generate test levels is connected to the TIP wire 18 and the Ring wire 20 via test relay switches 28. Test levels are usually lower in amplitude than the ringing signal. Overcurrent protection is provided by means of resistors 36, 38 on the TIP and Ring wires 18, 20 respectively. Overvoltage protection is provided on the POTS line by means of a protector 30, comprising overvoltage protectors 32 and 34 connected between the respective wires and ground. In use, the POTS line can be subjected to unwanted voltages which are conducted, coupled or radiated from lightning strokes or fault conditions on a.c. power lines. These voltages can exceed the ringing voltage levels. To protect the ring and test generators, the overvoltage protectors 32, 34 are placed on the incoming line. The overvoltage limiting levels of these protectors 32, 34 are usually too high for the SLIC to withstand, and so the SLIC has its own protection 14. In figure 1, the SLIC protection 14 is illustrated as comprising diodes and thyristors connected in opposing bias to ground, with transistors acting as amplifying gates on the thyristors. The protection 14 can suitably be implemented by device TISPPI3L2 produced by Power Innovations Ltd of Bedford, England. In order to reduce size, and to improve reliability, semiconductor replacements to the relay switches 16, 24, 28 are gradually being introduced. These are called SSRs (Solid State Relays) or LCASs (Line Card Access Switches). A typical LCAS is the L7582 produced by Lucent Technologies. In the L7582, the ring feed switch 24 is a triac which is turned on by a gating signal and turned off by the a.c. component of the ring signal. Because of the d.c. carried by the other switches 16, 28, these switches are implemented by bidirectional MOS technology. The triac switch 24 is relatively robust and has a higher voltage capability than the MOS switches. Therefore, the incoming line protection 30 is in place to protect the MOS switches. In the OFF state. MOS switches can be made to withstand voltages of about ±300V. Protection to this voltage level can be achieved using normally available protectors. Since the SLIC may hold one end of the pair of SLIC switches 16 at a voltage of around -50V, the protection should be asymmetric, and not exceed +250V or -350V. The switches on the TIP wire 18 will only have a maximum of half of the ring voltage. Therefore, the overvoltage protector 30 could have limiting voltages of +125V and -150V. When the MOS switches are in the On state, they will only conduct current levels of up to about 200mA. Under these excessive current levels, the MOS switches act as constant current elements, which means that large voltages will be developed across the conducting MOS switches. For short duration lightning impulses, and provided that the voltage levels are limited by protectors 30, 32, the MOS switches will not be damaged. In contrast, under a.c. overvoltage conditions, the long power loss periods can overheat the MOS switches, under which condition they are likely to fail. To overcome this problem, an over-temperature tum-off circuit, otherwise known as a thermal shut-down circuit, is normally included. However, the thermal shut-down circuit takes a finite period of time to respond; during that finite period the switch can fail through excessive temperature rise. In the On state, the MOS switch may only withstand a peak voltage of ±170V. As the SLIC can be offsetting one switch terminal by -50V, the protector limiting voltages should be +120V and -170V. Such limiting voltage levels would cause system malfunction, since the negative ringing signal (-200V) would be clipped by the limit of -170V set by the overvoltage protector 30. Summary of the Invention The present invention seeks to provide an improved overvoltage protector. Accordingly, the present invention provides an overvoltage protector suitable for protecting conductors, the overvoltage protector comprising: first, second and third at least bipolar protection elements; wherein: a first electrode of each protection element is connected to a common node; the first and third elements have breakdown conduction characteristic in one direction and diode conduction characteristic in the other; the second element has breakdown conduction characteristic in both directions; and the respective first electrodes of the first and third elements are of the same polarity. In one preferred form of overvoltage protector a second electrode of the first protection element and a second electrode of the second protection element are suitable for connection to a conductor to be protected. In that way, two overvoltage protection paths can be defined by the protector, one through the first and third elements and the other through the second and third elements.- In a further preferred form of overvoltage protector a second electrode of the first protection element and a second electrode of the third protection element are suitable for connection to a conductor to be protected. In a particular embodiment of the invention, the first and third elements have predetermined breakdown voltages, the breakdown voltages being different. The second element may have breakdown voltages in the two different conduction directions. The circuit designer can design a specific overvoltage protector with the ability to select the limits to which the protector will protect a specific conductor from overvoltage. The overvoltage protector may comprise further protection elements connected by a first electrode to the common node, a second electrode thereof being suitable for connection to a conductor for overvoltage connection thereof. Description of the drawings A particular embodiment of the invention will now be described, by way of example, with reference to the drawings appended hereto, in which: Figure 1 shows a schematic circuit diagram of a POTS exchange and line, as described above; Figure 2 shows a specific embodiment of an overvoltage protector in accordance with the invention; Figure 3 shows a graph of the characteristic of a reverse conducting unidirectional thyristor as included in the overvoltage protector illustrated in figure 2; Figure 4 shows a graph of the characteristic of a bidirectional thyristor diode as included in the overvoltage protector illustrated in figure 2; Figure 5 shows a graph of the characteristic of two reverse conducting unidirectional thyristors as included in the overvoltage protector illustrated in figure 2; Figure 6 shows a graph of the characteristic of a reverse conducting unidirectional thyristor in series with a bidirectional thyristor diode as included in the overvoltage protector illustrated in figure 2; Figure 7 shows a perspective view of a structure diagram illustrating a manner of implementation of the overvoltage protector illustrated in figure 2; Figure 8 shows a further specific embodiment of an overvoltage protector in accordance with the invention; and Figure 9 shows a perspective view of a structure diagram illustrating a manner of implementation of the overvoltage protector illustrated in figure 8. The overvoltage protector 130 illustrated in figure 2 can be used in the POTS exchange 10 illustrated in figure 1, in place of the overvoltage protector illustrated therein. The overvoltage protector 130 comprises three protection elements 132, 134, 136 in star formation, with a common node 138. The first element 132 is a reverse conducting unidirectional thyristor diode, with conduction characteristic as illustrated in figure 3. The second element 134 is a bidirectional thyristor diode with the characteristic illustrated in figure 4. The third element is a reverse conducting unidirectional thyristor diode similar to the first element 132. In each case, the protector voltage limiting value is termed Vbo, in the normal way. The cathodes of the unidirectional thyristor diodes 132, 136 are connected to the common node. The anode of the first unidirectional thyristor diode (the first element) 132, and the one electrode of the bidirectional diode (the second element) 134 not connected to the common node 138 form contacts for the connection respectively to the TLP wire 18 and the Ring wire 20 illustrated in figure 1. The anode of the third unidirectional thyristor diode (the third element) 136 forms a contact for connection to ground. When the overvoltage protector is connected in the suggested manner, the composite characteristic of the behaviour of the connection between the TIP wire and ground is illustrated in figure 5. In the positive polarity, the limiting voltage will be the sum of the Vbo of the first element 132 and the forward diode voltage of the third element 136. In the negative polarity, the limiting voltage will be the sum of the Vbo of the third element 136 and the forward diode voltage of the first element 132. If the two Vbo values are different, then the limiting voltage will be asymmetrical. Without the operation of the first element 132, the Ring to Ground limiting characteristics will be determined by characteristics of the second and third elements 134, 136. In the positive polarity, the limiting voltage will be the sum of the (+ve) Vbo of the second element 134 and the forward diode voltage of the third element 136. In the negative polarity, the limiting voltage Vbo I will be the sum of the Vbo of the third element 136 and the (-ve) Vbo of the second protector 134. If the third element 136 is switched by current fed from the reverse conducting first element 132, the negative polarity protection voltage Vbo2 becomes the sum of the Vbo of the second protector and the low voltage On-state voltage Vt of the third protector. The protection characteristics of the Ring to Ground terminal pair is illustrated in figure 6. In normal operation, the Ring to Ground negative limiting voltage is high (Vbol). For example, if the Vbo for the second element 134 is - 160V, and the Vbo for the third element 136 is -130V, Vbol will be -290V. That value will protect the circuit without causing clipping of ring signals. Under a.c. overvoltage conditions, the protection voltage would drop to Vbo2 which would be the sum of the Vbo of the second element 134 and the low voltage On state voltage Vt of the third element 136 ( about 2V in practice). Therefore, the a.c. protection level would be - 162V, which would protect the On state MO S switches. A specific integration of the above described arrangement is shown in figure 7. Diodes are formed on a substrate by pn-n+ layers, and thyristors by n+pn-p layers. All terminal connections are made on one surface, and the individual elements 132, 134, 136 are isolated from each other by wrap around p-layers. The third element 136, connecting to Ground, carries the sum of the currents through the first and second elements 132, 134. Hence, the current capacity handling capability of the third element 13 6 should preferably be higher than those of the first and second elements 132, 134. The limiting voltage of the individual thyristors can be set by locally reducing the n-type material resistivity at the pn junction by a modifying diffusion. These local reductions are represented by the four dotted areas 140 indicated in figure 7. Whilst n-type material is shown as the starting material it will be appreciated that a complementary version could be made with all the material doping types reversed. In the structure of Figure 7 all three elements, two unidirectional and one bidirectional, are isolated from each other by vertical diffusions. Figure 8 shows a further specific embodiment of an overvoltage protector in accordance with the invention in which the elements are connected to the TlPwire 18, Ringwire 20 and ground in a different arrangement to that of Figure 2. In some balanced line communication systems it is desirable to limit the voltage from the wires and ground at a much higher value than the voltage between the wires. One example is when a power frequency noise may occur if the wires are connected to multiple grounds around a system. In this case the line may be allowed to "float". To prevent insulation breakdown (sparkover) a high voltage protection (typically about 500 V) may be connected from ground to the line. The protection between the line wires is likely to limit the voltage to a much lower level. In some cases the network loses its ground connection. There is a possibility that the equipment may become live and feed dangerous a.c. levels on to the line, via the line-to- ground protectors. To stop this, the ground protector can have a protection voltage higher than the peak of the a.c. supply. The voltage between the line wires may be limited to a much lower value. In a telephone exchange, there will be a protection applied where the telephone lines enter the building. This is called the primary protection and is meant to remove most of the incoming voltage surges. The electronics that drives the line is also likely to have protection - secondary protection. It is important to ensure that the secondary protection does not by- pass the primary protection and take all the energy that the primary should take. For digital systems (ISDN, ADSL - Asymmetrical Digital Subscriber Line) which use transformer coupling the following secondary protection approach can be used. Protection is applied across the line wires appropriate to the line voltages. Protection of the line to ground is done at a voltage that is higher than the primary protection voltage to ensure that normally the primary will conduct before the secondary. The secondary then only operates when the primary develops a fault. These three situations require a higher wire to ground voltage than wire to wire voltage. A very economical way of making this in a single piece of silicon is a star connected combination of two unidirectional and one asymmetric bidirectional breakover triggered thyristors. The example shows a protector that has a line to ground voltage of 550V and a wire to wire voltage of 320V. Assuming a forward diode voltage drop of IV, the two unidirectional protectors 132, 136 which are series connected across the wires will have a breakover voltage of 320V - IV = 319V and a diode voltage of IV. The asymmetrical protector 134 from the star point 138 to ground will have voltages of 550V - IV = 549V and 550V - 319V = 23 IV to give 550V from ground to line wire in both polarities. • A suggested implementation of the protector of Figure 8 is shown in Figure 9. In practice as the asymmetrical protector carries the sum of the currents from the unidirectional protectors 132, 136 it should be made with double the current capability of the unidirectional protectors.

Claims: Claims 1 An overvoltage protector suitable for protecting conductors, the overvoltage protector comprising: first, second and third at least bipolar protection elements (132, 134, 136); wherein: a first electrode of each protection element (132, 134, 136) is connected to a common node (138); the first and third elements (132, 136) have breakdown conduction characteristic in one direction and diode conduction characteristic in the other; the second element (134) has breakdown conduction characteristic in both directions; and the respective first electrodes of the first and third elements (132, 136) are of the same polarity. 2 An overvoltage protector as claimed in claim 1 wherein a second electrode of the first protection element (132) and a second electrode of the second protection element (134) are suitable for connection to a conductor ((18, 20) to be protected. 3 An overvoltage protector as claimed in claim 2 wherein a second electrode of the third protection element (136) is suitable for connection to ground. 4 An overvoltage protector as claimed in claim 1 wherein a second electrode of the first protection element (132) and a second electrode of the third protection element (136) are suitable for connection to a conductor (18, 20) to be protected. 5 An overvoltage protector as claimed in claim 4 wherein a second electrode of the second protection element (134) is suitable for connection to ground. 6 An overvoltage protector as claimed in any of the preceding claims wherein the first and third elements(132, 136) have predetermined breakdown voltages, the breakdown voltages being different. 7 An overvoltage protector as claimed in any of the preceding claims wherein the second element (134) has different breakdown voltages in the two different conduction directions. 8 An overvoltage protector as claimed in any of the preceding claims wherein the overvoltage protector comprises further protection elements connected by a first electrode to the common node, a second electrode thereof being suitable for connection to a conductor for overvoltage connection thereof. 9 An overvoltage protector as claimed in any of the preceding claims wherein the protector is integrated in a monolithic semiconductor device. 10 An overvoltage protector as claimed in claim 9 wherein the protection elements are constructed of diffused areas of a semiconductor, the elements being isolated from each other by means of a continuous layer. 11 An overvoltage protector as claimed in claim 9 wherein the protection elements are constructed of diffused areas of a semiconductor, the second element (134) being isolated from the first and third elements (132, 136) by means of a continuous layer. 12 An overvoltage protector as claimed in claim 10 or 11 wherein the protection elements (132, 134, 136) comprise diodes and thyristors. 13 An overvoltage protector as claimed in claim 12 wherein each thyristor can be designed independently of others, by the introduction of intermediate diffused layers in junction of the thyristor.

Inventors: BYATT STEVEN WILTON, MAYTUM MICHAEL JOHN, BYATT, STEVEN, WILTON, MAYTUM, MICHAEL, JOHN
Applicants: BYATT STEVEN WILTON, MAYTUM MICHAEL JOHN, POWER INNOVATIONS LTD, BYATT, STEVEN, WILTON, MAYTUM, MICHAEL, JOHN, POWER INNOVATIONS LTD.
Application year: 1999
Main classification: H04M3
IPC codes: H02H9, H04M3, H04Q3