GB1574078A

GB1574078A - Voltage-limiting circuit

Info

Publication number: GB1574078A
Application number: GB41646/77A
Authority: GB
Inventors: G Bergmann; A Kessler
Original assignee: Licentia Patent Verwaltungs GmbH; Telefonbau und Normalzeit GmbH; Licentia Oy
Current assignee: Licentia Patent Verwaltungs GmbH; Tenovis GmbH and Co KG; Licentia Oy
Priority date: 1976-12-01
Filing date: 1977-10-06
Publication date: 1980-09-03
Also published as: AT356187B; BE861307A; CA1083667A; SE421094B; ATA705077A; AU2901477A; DE2654419A1; NL7708817A; GR59256B; DE2654419C2; FR2373181A1; IT1090802B; SE7713151L

Description

(54) VOLTAGE-LIMITING CIRCUIT (71) We, TELEFONBAU UND NORMAL ZEIT GMBH, of Mainzer Landstrasse 128146, 6 Frankfurt/M., German Federal Republic, and LICENTIA PATENT-VERWALTUNGS- GMBH, of Theodore-Stern-Kai 1, 6 Frank furtiM. 70, German Federal Republic, both limited liability companies organised under the laws of the German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: The present invention relates to a voltage-limiting circuit, especially but not exclusively for use in telecommunications and telephone installations to protect electronic coupling elements or contacts connected with signal transmission lines against surges or over-voltages, the circuit having a conductivity, which is very low below a certain threshold voltage and high above this, and being usable as a two-pole network.

Known as over-voltage protection devices are over-voltage or surge suppressors, which comprise a gas-filled glass tube provided with two electrodes to be connected between the signal transmission line to be protected and earth. Their manner of effect is similar to that of a glow-discharge lamp, their voltage-limiting effect generally however setting in only above a voltage of about 200 volts, or at least some tens of volts in some special cases, so that over-voltage or surge suppressors of that kind are unsuitable for the protection of semiconductor circuits, since over-voltage peaks of that magnitude would lead to the breakdown of the semiconductor barrier layers usual in electronic coupling contacts.

Furthermore, so-called varistors have been used as over-voltage or surge suppressors. These comprise a semiconductor body provided with two electrodes, between which the resistance is independent of the direction of current and dependent on the applied voltage. The varistors have however the disadvantage that they form a not negligible shunt to the line or circuit to be protected even in the case of low applied voltages.

For the suppression of interference voltages, diodes and-for higher voltages series connections of diodes and/or Zener diodes have also been proposed. These are however usuable only for the suppression of relatively small interference energies Where a small space requirement is significant.

Beyond that, Zener diodes are subject to appreciable sample scattering in their switching voltage values, especially in the case of monolithic integration.

According to the present invention, there is now provided a voltage-limiting circuit comprising- a semiconductor network provided with a switching device having a control electrode and including a current path, which is controllable with the aid of the control electrode and which extends between the poles of a two-pole source of voltage to be limited, the circuit further comprising a second path including at least two diodes connected between one of the two poles and the control electrode, a control path including input means of the switching device and extending between the control electrode and the respective other of the two poles, and a shunt path extending from a point between the two diodes to the other one of the two poles and including at least one additional diode and a resistor, the or each additional diode being so connected that its preferred direction of conduction with respect to the two poles is the same as that of the diodes in the second path and the additional diodes being of such number and type that the sum of the switching voltages in the shunt path is less than the sum of the switching voltages of the control path and of the diode or diodes in the portion of the second path between said point and the control electrode, the arrangement being such that the controllable path is rendered conductive when the voltage of the source exceeds the sum of the switching voltages of the at least two diodes and of the control path.

The semiconductor network can without difficulty be so dimensioned that it is capable in the conductive state of conducting the required shunt leakage current, the series connection of several diodes in the second path for the driving of the semiconductor network having the advantage that these are easily produceable as low-loss components of small physical size, while their number alone substantially determines the value of the voltage to be limited. Since interference voltage peaks occur relatively seldom, the semiconductor network can be constructed as an integrated circuit element together with the diodes on a semiconductor chip in standard bipolar silicon technique.

In that case, it is of particular advantage that the resultant device can easily be arranged several times on a single semiconductor chip without significant mutual interference or cross-talk between circuits.

In the simplest case, the semiconductor network comprises a transistor, the base electrode of which provides the control electrode, the collector-emitter path of the transistor providing the controllable path and the base-emitter path of the transistor providing the control path. This arrangement has the advantage of being very simple and produceable with relatively little expenditure.

When the voltage-limiting circuit is required to cope with greater shunt leakage currents, then the semiconductor network to advantage comprises a Darlington transistor circuit.

With extremely small energies available for control, it may be expedient for the semiconductor network to comprise a plurality of transistors connected in cascade, the collector electrode of each transistor except the last being connected to the base electrode of the transistor following in the cascade connection, and successive ones of the transistors in the cascade connection, being of respectively opposite conductivity type and having emitter electrodes connected alternately to the one and the respective other one of the two poles.

With relatively little additional expenditure, the voltage-limiting circuit may be constructed as a symmetrical voltage limiter, for which purpose the second path includes a plurality of diodes, a further path including an equal plurality of further diodes extends between the control electrode and the other one of the two poles, the preferred direction of conduction of the diodes in the second path and in the further path being mutually opposite with respect to the two poles, the first shunt path so extends between the two poles as to include a number of the diodes of the second path, a second shunt path including at least one diode and a resistor so extends between the two poles as to include an equal number of the diodes of the further path, the respective diodes in the first and second shunt paths each being so connected that their preferred directions of conduction with respect to the two poles each are the same as those of the diodes in the second and further path respectively and the respective diodes in the first and second shunt paths each being of such number and type that the sum of the switching voltages in each of the first and the second shunt path is less than the sum of the switch voltages of the control path and of the respective portion of the second and further path not included in one of the shunt paths.

In the use of a circuit embodying the invention for surge or over-voltage protection in telecommunication and telephone installations, especially for the protection of electronic coupling elements or contacts, it is of advantage that, due to the simplicity of the construction and the small space requirement, the circuit can easily be integrated together with such electronic coupling elements or contacts.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, in which: Fig. 1 shows a schematic diagram of the basic voltage-limiting circuit embodying the present invention, Fig. 2 shows a further circuit embodying the present invention with a transistor providing the semiconductor network, Fig. 3 shows a Darlington transistor circuit as semiconductor network, Fig. 4 shows a transistor cascade connection as semiconductor network, and Fig. 5 shows a symmetrical arrangement of a voltage-limiting circuit embodying the present invention.

Referring now to the drawings, Fig. 1 shows a schematic circuit diagram of the basic voltage-limiting circuit, which as a two-pole network has two terminals 1 and 2, between which lies a controllable path of a semiconductor network 3, which is controlled by way of its control electrode 4 through one diode or, as illustrated in Fig.

1, by a series connection of several diodes 5 in a second path between the control electrode 4 and the terminal 1, in such a manner, that the semiconductor network is driven into the conducting state when the voltage across the terminals 1 and 2 lies above the total switching voltage of the diodes and the control path of the semiconductor network. The control path of the semiconductor network in the present case lies between the terminal 2 and the control electrode 4.

In the simplest case, the semiconductor network 3 comprises one transistor, as shown in Fig. 2, the collector-emitter path of the transistor providing the controllable path of the semiconductor network and the base electrode providing the control electrode. The series connection of the diodes in the second path is branched at a tap 8 and will be discussed in greater detail below.

The switching voltage of a diode or of a transistor is to be understood as that voltage across the diode or across the baseemitter path of the transistor, at which the diode or the transistor becomes conductive.

This is the case with silicon semiconductors at a voltage of about 0.5 volts at the PNjunction. The total switching voltage of the diodes and the control path of the semiconductor network is the sum of the switching voltages of the individual diodes in addition to the switching voltage of the base-emitter path of the transistor 31.

Fig. 3 shows a Darlington transistor circuit arrangement of the cascade connection of the transistors 32 and 33 in emitter follower configuration, which may take the place of the transistor 31 in Fig. 2 and is capable of suppressing interference pulses of substantially greater energy.

In another modification, the single transistor 31 of Fig. 2 may be replaced by a cascaded transistor circuit as shown in Fig. 4, in which by way of example three transistors 34. 35 and 36 of successivelv opposite conductivity type are used and each successive transistor has its emitter electrode connected alternately to terminal 1 and to terminal 2, respectively, the transistors 34 and 36 being of non-conductivity type, while the transistor 35 is of pnp-conductivity type. The controllable current path is provided by the collector-emitter path of the last transistor 36, while the control electrode 4 is provided by the base electrode of the first transistor 34, the base electrodes of the other two transistors 35 and 36 each being connected to the collector electrode of the respectively preceding transistor 34 or 35.The greater amplification made available by such a cascade connection permits the construction of a rather more sensitive limiting circuit requiring rather less energy to trigger it into a conductive state.

It is furthermore shown in Fig. 2 that a further series combination of at least one diode 7 and one resistor 6 in a shunt path is connected between the terminal 2 and the tap 8 of the diodes 51 and 52 feeding the semiconductor network, in this case the transistor 31. The diodes 7 in this shunt path are poled in the same direction as the diodes 51 and 52 driving the transistor 31 conductive and their number is so chosen that their total switching voltage is lower than the sum of the switching voltages of the control path of the transistor 31 and of the diodes 52 between the tap 8 and the base electrode of the transistor 31. The limiting or threshold voltage is determined substantially with the aid of the series combination of the diodes 51 and 52 in the second path and the base-emitter path of the transistor 31.The further series combination of the diodes 7 with the resistor 6 in the shunt path generates in the low forward current region of the diodes a shunt for the transistor 31 so that the transistor 31 is more certainly blocked for voltages below the threshold voltage. Through this further series combination, the transition from the blocked state of the transistor 31 to the conducting state is thereby substantially sharpened so that a sharper onset of the voltage-limiting effect of the circuit arrangement is attained.

A circuit as illustrated in either Fig. 1 or Fig. 2 limits a voltage which is positive at terminal 1 relative to terminal 2. When positive and negative voltages are to be limited, then it is expedient to provide a circuit comprising an anti-parallel connection of two circuits as shown in Fig. 2 or as shown in Figs. 2 and 4. This is easily attained in monolithic integration of the circuit, while a high symmetry of the onset of limitation for positive and negative overvoltages can be attained. Of course, an asymmetrical limitation is also attainable through appropriate choice of the number of diodes in the shunt paths 7 and 71, as shown in Fig. 5.

It is possible to produce transistors, which have high current amplification factors also in inverse operation and which thus allow a circuit arrangement for symmetrical limitation of over-voltages to be realised also with a single transistor. As Fig. 5 shows, the second path 51 and 52 and the shunt path 6 and 7 are each duplicated for this purpose. The second path 53 and 54 provided with a tap 81 is connected with reversed polarity to the base-emitter path of the transistor 31. The other shunt path including the resistor 61 and the diode or diodes 71 is likewise connected in opposite polarity between the tap 81 and the terminal 1, that is to say the diodes 53, 54 and 71 display a polarity opposite to that of the diodes 51, 52 and 7 of Fig. 2 or Fig. 5 with respect to the terminals 1 and 2.

When the positive voltage at the terminal 1 exceeds the predetermined threshold value relative to the voltage of the terminal 2, then the diodes 51, 52 and 7 become conductive and since the total switching voItage of the diodes 7 is lower than the total switching voltage of the diodes 52 and the base-emitter path of the transistor 31, the transistor 31 is driven into conduction. For this, it is however required that such a high voltage is present at the resistor 6, which has a value of between 10 and 100 ohms, due to the forward current already flowing through the diodes 7 that the transistor 31 is saturated, i.e., driven hard into conduction.

When a higher voltage is present at the terminal 2 than at the terminal 1, then the limitation of the voltage occurs when the diodes 53 and 54 as well as the diodes 71 become conductive and the voltage drop at the resistor 61, which has the same value of resistance as the resistor 6, has become so great that the transistor 31 is now driven in inverse operation.

The components of the illustrated circuits, especially the diodes and resistors, are required to dissipate only small loss energies so that they can be realised as small as technically possible. Consequently, the semiconductor network need not take up much space and thus the entire circuit arrangement can be arranged several times on a single semiconductor chip. This is particularly favourable when a plurality of signal transmission lines or signalling circuits are to be protected against surges or over-voltages.

The small space requirement of the described circuits even permits a circuit embodying the present invention in a telecommunications installation to be integrated together with electronic coupling or switching circuits.

A particular advantage of the circuits hereinbefore described is to be seen in the possibility of their use to all intents and purposes in a manner as if they were a passive two-pole network. Consequently, no additional operational voltages are required.

WHAT WE CLAIM IS: - 1. A voltage-limiting circuit comprising a semiconductor network provided with a switching device having a control electrode and including a current path, which is controllable with the aid of the control electrode and which extends between the poles of a two-pole source of voltage to be limited, the circuit further comprising a second path including at least two diodes connected between one of the two poles and the control electrode, a control path including input means of the switching device and extending between the control electrode and the respective other of the two poles, and a shunt path extending from a point between the two diodes to the other one of the two poles and including at least one additional diode and a resistor, the or each additional diode being so connected that its preferred direction of conduction with respect to the two poles is the same as that of the diodes in the second path and the additional diodes being of such number and type that the sum of the switching voltages in the shunt path is less than the sum of the switching voltages of the control path and of the diode or diodes in the portion of the second path between said point and the control electrode, the arrangement being such that the controllable path is rendered conductive when the voltage of the source exceeds the sum of the switching voltages of the at least two diodes and of the control path.

2. A circuit as claimed in Claim 1, wherein the semiconductor network comprises a transistor, the base electrode of which provides the control electrode, the collector-emitter path of the transistor providing the controllable current path and the base-emitter path of the transistor providing the control path.

3. A circuit as claimed in Claim 2, wherein the semiconductor network comprises a further transistor having a collector electrode connected to the collector electrode of the first-mentioned transistor and an emitter electrode connected to the base electrode of the first-mentioned transistor, the base-emitter path of the further transistor being included in the second path between the control electrode and the diode or diodes.

4. A circuit as claimed in Claim 1, wherein the semiconductor network comprises a plurality of transistors connected in cascade, the collector electrode of each transistor except the last being connected to the base electrode of the transistor following in the cascade connection, and successive ones of the transistors in the cascade connection being of respectively opposite conductivity type and having emitter electrodes connected alternately to the one end and the respective other one of the two poles.

5. A circuit as claimed in any one of the preceding claims, wherein the second path includes a plurality of diodes, a further path including an equal plurality of further diodes extends between the control electrode and the other one of the two poles, the preferred direction of conduction of the diodes in the second path and in the further path being mutually opposite with respect to the two poles, the first shunt path so extends between the two poles as to include a number of diodes of the second path, a second shunt path including at least one diode and a resistor so extends between the two poles as to include an equal number of the diodes of the further path, the respective diodes in the first and second shunt paths each being so connected that their preferred directions of conduction with respect to the two poles are the same as those of the diodes in the second and the further path respectively, and the respective diodes in the first and second shunt paths each being of such number and type that the sum of the switching voltages in each of the first

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

due to the forward current already flowing through the diodes 7 that the transistor 31 is saturated, i.e., driven hard into conduction.

When a higher voltage is present at the terminal 2 than at the terminal 1, then the limitation of the voltage occurs when the diodes 53 and 54 as well as the diodes 71 become conductive and the voltage drop at the resistor 61, which has the same value of resistance as the resistor 6, has become so great that the transistor 31 is now driven in inverse operation.

The components of the illustrated circuits, especially the diodes and resistors, are required to dissipate only small loss energies so that they can be realised as small as technically possible. Consequently, the semiconductor network need not take up much space and thus the entire circuit arrangement can be arranged several times on a single semiconductor chip. This is particularly favourable when a plurality of signal transmission lines or signalling circuits are to be protected against surges or over-voltages.

The small space requirement of the described circuits even permits a circuit embodying the present invention in a telecommunications installation to be integrated together with electronic coupling or switching circuits.

A particular advantage of the circuits hereinbefore described is to be seen in the possibility of their use to all intents and purposes in a manner as if they were a passive two-pole network. Consequently, no additional operational voltages are required.

WHAT WE CLAIM IS: -

1. A voltage-limiting circuit comprising a semiconductor network provided with a switching device having a control electrode and including a current path, which is controllable with the aid of the control electrode and which extends between the poles of a two-pole source of voltage to be limited, the circuit further comprising a second path including at least two diodes connected between one of the two poles and the control electrode, a control path including input means of the switching device and extending between the control electrode and the respective other of the two poles, and a shunt path extending from a point between the two diodes to the other one of the two poles and including at least one additional diode and a resistor, the or each additional diode being so connected that its preferred direction of conduction with respect to the two poles is the same as that of the diodes in the second path and the additional diodes being of such number and type that the sum of the switching voltages in the shunt path is less than the sum of the switching voltages of the control path and of the diode or diodes in the portion of the second path between said point and the control electrode, the arrangement being such that the controllable path is rendered conductive when the voltage of the source exceeds the sum of the switching voltages of the at least two diodes and of the control path.

2. A circuit as claimed in Claim 1, wherein the semiconductor network comprises a transistor, the base electrode of which provides the control electrode, the collector-emitter path of the transistor providing the controllable current path and the base-emitter path of the transistor providing the control path.

3. A circuit as claimed in Claim 2, wherein the semiconductor network comprises a further transistor having a collector electrode connected to the collector electrode of the first-mentioned transistor and an emitter electrode connected to the base electrode of the first-mentioned transistor, the base-emitter path of the further transistor being included in the second path between the control electrode and the diode or diodes.

4. A circuit as claimed in Claim 1, wherein the semiconductor network comprises a plurality of transistors connected in cascade, the collector electrode of each transistor except the last being connected to the base electrode of the transistor following in the cascade connection, and successive ones of the transistors in the cascade connection being of respectively opposite conductivity type and having emitter electrodes connected alternately to the one end and the respective other one of the two poles.

5. A circuit as claimed in any one of the preceding claims, wherein the second path includes a plurality of diodes, a further path including an equal plurality of further diodes extends between the control electrode and the other one of the two poles, the preferred direction of conduction of the diodes in the second path and in the further path being mutually opposite with respect to the two poles, the first shunt path so extends between the two poles as to include a number of diodes of the second path, a second shunt path including at least one diode and a resistor so extends between the two poles as to include an equal number of the diodes of the further path, the respective diodes in the first and second shunt paths each being so connected that their preferred directions of conduction with respect to the two poles are the same as those of the diodes in the second and the further path respectively, and the respective diodes in the first and second shunt paths each being of such number and type that the sum of the switching voltages in each of the first

and the second shunt path is less than the sum of the switching voltages of the control path and of the respective portion of the second and further path not included in one of the shunt paths.

6. A voltage-limiting circuit substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawing.

7. A circuit as claimed in Claim 6 and substantially as hereinbefore described with reference to and illustrated in Fig. 3 of the accompanying drawing.

9. A circuit as claimed in either Claim 6 or Claim 7 and modified substantially as hereinbefore described with reference to and illustrated in Fig. 4 of the accompanying drawing.

10. A voltage-limiting circuit comprising a plurality of circuits each as claimed in any one of the preceding claims, constructed as an integrated electronic circuit and arranged on a common semiconductor chip.

11. A telecommunications exchange installation comprising a plurality of electronic switching circuits connectable with respective signal transmission lines and at least one voltage-limiting circuit as claimed in any one of the preceding claims and constructed as an integrated electronic circuit together with the respective electronic switching circuit in such a manner as to provide surge suppression to protect that switching circuit against excessive voltages.

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