GB2336728A

GB2336728A - Overvoltage protection for telephone equipment

Info

Publication number: GB2336728A
Application number: GB9808739A
Authority: GB
Inventors: Michael John Maytum; Stephen Wilton Byatt
Original assignee: Power Innovations Ltd
Current assignee: Power Innovations Ltd
Priority date: 1998-04-25
Filing date: 1998-04-25
Publication date: 1999-10-27
Anticipated expiration: 2018-04-25
Also published as: GB2336728B; GB9808739D0

Abstract

An overvoltage protector for telephone equipment has first second and third protection elements 132, 134, 136 with a first electrode of each element connected to a common node 138 and the other electrodes of elements 132, 134, 136 being respectively connected to a tip line 18, a ring line 20 and ground. The first and third elements 132, 136 each have a breakdown conduction characteristic in one direction and a diode conduction characteristic in the other direction, and the second element 134 has a breakdown conduction characteristic in each direction. Elements 132, and 134 may be reverse conducting unidirectional thyristor diodes with their cathodes connected to the common node 138. The breakdown voltages of the elements 132, 136 may be different. The element 134 may be a bidirectional thyristor diode and may have different breakdown voltages in the two different directions. The protector may be formed as an integrated monolithic semiconductor device (Fig.7) in which the protection elements comprise diodes and thyristors, and the elements may be isolated from one another by continuous wrap around p-layers.

Description

1 Overvoltage Protection 2336728 The present invention is concerned with

overvoltage protection, and particularly with overvoltage protection in electrical equipment for use in telephony.

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 -20OV and the peak positive voltage level being in the recion of + 1 OOV.

c 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.

C 2 Overcurrent protection is provided by means of resistors 36, 38 on the TIP and Ring wires 18, 20 respectively.

Overvoltage protection is provided onthe POTS line by means of aprotector 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 TISPPBL2 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 3 0 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 3 ltage. Therefore, the the TIP wire 18 will only have a maximum of half of the ring vo overvoltage protector 30 could have limiting voltages of +125V and -150V.

When the MOS switches are in the On state, they will on: ly conduct current levels of up to about 20OmA. 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 turn-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 1 70V. As the SLIC can be offsetting one switch terminal by -50V, the protector limiting voltages should be +120V and -1 70V. Such limiting voltage levels would cause system malfunction, since the negative ringing signal (-20OV) would be clipped by the limit of - 1 70V set by the overvoltage protector 30.

It is an object of the invention to provide a protector which ameliorates for the problem identified above as existing in the protector described above.

According to the invention, there is provided an overvoltage protector for protecting conductors, the overvoltage protector comprising first, second and third at least bipolar protection elements, a first electrode of each protection element being connected to a common node and a second electrode of the first protection element and a second electrode of the second protection element being suitable for connection to a conductor to be protected, the first and third elements having breakdown conduction characteristic in one direction and diode conduction characteristic in the other, and the second element having breakdown 4 conduction characteristic in both directions, wherein the respective firs't'electrodes of the first and third elements are of the same polarity.

In that way, two overvoltage protection paths can be defined by the protector according to the invention, one through the first and third elements and the other through the second and third elements.

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.

In that way 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.

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 overvoltag ge 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; and Figure 7 shows a perspective view of a structure diagram illustrating a manner of implementation of the overvoltage protector illustrated in figure 2.

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, 1334, 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 13 8 form contacts for the connection respectively to the TIP 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.

6 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 13 6 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 1 will be the sum of the Vbo of the third element 13 6 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 - 1 60V, 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 voltaae Vt of the third element 136 ( about 2V in practice). Therefore, the a.c. protection level would be - 1 62V, which would protect the On state MOS switches.

A specific integration of the above described arrangement is shown in figure 7. Diodes are 0 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 7 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.

8

Claims

Claims

1. An overvoltage protector suitable for protecting conductors, the overvoltage protector comprising first, second and third at least bipolar protection elements, a first electrode of each protection element being connected to a common node and a second electrode of the first protection element and a second electrode of the second protection element being suitable for connection to a conductor to be protected, the first and third elements having breakdown conduction characteristic in one direction and diode conduction characteristic in the other, and the second element having breakdown conduction characteristic in both directions, wherein the respective first electrodes of the first and third elements are of the same polarity.

2. An overvoltage protector in accordance with claim 1 wherein the first and third elements have predetermined breakdown voltages, the breakdown voltages being different.

3. An overvoltage protector in accordance with claim 1 or claim 2 wherein the second element has different breakdown voltages in the two different conduction directions.

4. An overvoltage protector in accordance with any preceding claim 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.

5. An overvoltage protector in accordance with any preceding claim wherein the protector is integrated in a monolithic semiconductor device.

6. An overvoltagge protector in accordance with claim 5 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.

7. An overvoltage protector in accordance with claim 6 wherein the protection elements comprise diodes and thyristors.

9 8. An overvoltage protector in accordance with claim 7 wherein each thyristor can be designed independently of others, by the introduction of intermediate diffused layers in junction of the thyristor.

9. An overvoltage protector suitable for protecting conductors, substantially in accordance with Figures 2 to 7 of the drawings appended hereto.