Protection of a telephone line against overvoltages

Abstract

A structure protects a SLIC telephone line interface against overvoltages lower than a negative threshold or higher than a positive threshold. The structure includes at least one thyristor connected between each conductor of the telephone line and a reference potential. For all of the included thyristors, a metallization corresponding to the main electrode on the gate side is in contact, by its entire surface, with a corresponding semiconductor region. Furthermore, the gate of each thyristor is directly connected to a voltage source defining one of the thresholds.

Claims

1. A structure for protecting a first line and a second line against overvoltages lower than a negative threshold or higher than a positive threshold, comprising: at least one thyristor connected between each of the first and second lines and a reference potential; wherein, for all thyristors: a metallization corresponding to an electrode on a gate side, wherein that metallization is in contact, by its entire surface, with a surface of a corresponding semiconductor region; and the gate is directly connected to a voltage source providing a DC voltage defining one of said negative and positive thresholds.

2. The structure of claim 1, wherein the positive threshold is zero, comprising: each of the first and second lines is coupled to the anode of a diode and to the cathode of a cathode-gate thyristor, and cathodes of the diodes and the anodes of the thyristors are coupled to the reference potential; a common negative DC voltage source connected to the two gates of the cathode-gate thyristors.

3. The structure of claim 2, wherein the cathode-gate thyristors and the diodes are formed in a same monolithic component.

4. The structure of claim 1, wherein: each of the first and second lines is connected to the cathode of a cathode-gate thyristor and to the anode of an anode-gate thyristor, the anodes of the cathode-gate thyristors and the cathodes of the anode-gate thyristors being coupled to the reference potential; the gates of the cathode-gate thyristors are directly connected to a common negative DC voltage source defining the negative threshold; and the gates of the anode-gate thyristors are directly connected to a common positive DC voltage source defining the positive threshold.

5. The structure of claim 4, wherein all thyristors are formed in a same monolithic component.

6. The structure of claim 1, wherein at least one of the voltage sources is a power supply source of a Subscriber Line Interface Circuit (SLIC) for the first and second lines.

7. The structure of claim 1, wherein at least one of the voltage sources comprises at least one battery.

8. A structure for protecting a first line and a second line against overvoltages lower than a negative threshold or higher than a positive threshold, comprising: at least one thyristor connected between each of the first and second lines and a reference potential; wherein: all thyristors have no emitter short circuits; and all thyristors have a semiconductor gate region maintained at one of a negative and positive DC potential by a gate electrode.

9. The structure of claim 8, wherein the positive threshold is zero, comprising: each of the first and second lines is coupled to the anode of a diode and to the cathode of a cathode-gate thyristor, and cathodes of the diodes and the anodes of the thyristors are coupled to the reference potential; a common negative potential is maintained at the two gates of the cathode-gate thyristors.

10. The structure of claim 8, wherein the positive threshold is zero, comprising: each of the first and second lines is coupled to the anode of a diode and to the cathode of a cathode-gate thyristor, and cathodes of the diodes and the anodes of the thyristors are coupled to the reference potential; a common negative potential is maintained at the two gates of the cathode-gate thyristors.

11. The structure of claim 8, wherein the at least one thyristor is: conductive when the voltage between one of the first and second line and the reference potential is greater than one of the positive DC potential and negative DC potential; and non-conductive when the voltage between one of the first and second line and the reference potential is less than one of the positive DC potential and negative DC potential.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein:

(2) FIG. 1, previously described, shows a circuit of protection against overvoltages lower than a negative threshold or higher than a positive threshold, connected to a telephone line;

(3) FIG. 2A shows an embodiment of a circuit of protection against overvoltages lower than a negative threshold or higher than a positive threshold, connected to a telephone line;

(4) FIG. 2B is a cross-section view of an example of a monolithic component implementing the circuit of FIG. 2A;

(5) FIG. 3A shows a structure of protection against overvoltages lower than a negative threshold or higher than zero, connected to a telephone line, according to another embodiment;

(6) FIG. 3B is a cross-section view of an example of a monolithic component implementing the circuit of FIG. 3A.

DETAILED DESCRIPTION

(7) The same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and are detailed.

(8) In the following description, when reference is made to terms qualifying absolute positions, such as terms high, low, left-hand, right-hand, reference is made to the orientation of the drawings in a normal position of use.

(9) FIG. 2A shows an embodiment of a circuit of protection against overvoltages lower than a negative threshold or higher than a positive threshold, connected to a telephone line, on the SLIC side. The two conductors 3 and 5 of the line, at voltages V.sub.TIP and V.sub.RING, are connected to a protection structure 30. Structure 30 comprises two cathode-gate thyristors 32, 34 having gates 36 and 38, and having their anodes connected to ground GND. Cathode 40 of thyristor 32 is connected to conductor 3 and cathode 42 of thyristor 34 is connected to conductor 5. Structure 30 also comprises two anode-gate thyristors 44 and 46, having gates 48 and 50, and having their cathodes connected to ground GND. Anode 52 of thyristor 44 is connected to conductor 3, and anode 54 of thyristor 46 is connected to conductor 5.

(10) A positive potential V.sub.H is supplied by a voltage source 56 and a negative potential V.sub.L is supplied by a voltage source 58. Each voltage source may supply or absorb a current while maintaining its potential at a value close to V.sub.H or V.sub.L. Gates 36 and 38 of the cathode-gate thyristors are directly connected to voltage source 58 and gates 48 and 50 of the anode-gate thyristors are directly connected to voltage source 56. Voltage sources 56 and 58 may possibly be power supply sources of the SLIC, for example, batteries or stabilized D.C. power supplies.

(11) All thyristors are deprived of emitter short-circuits, that is, in each of the thyristors, the metallization area corresponding to the main electrode on the gate side is in contact, by its entire surface, with the corresponding semiconductor region, and is not in partial contact with the layer having the gate connected thereto.

(12) In normal operation, the voltages of conductors 3 and 5 remain between V.sub.L and V.sub.H and the thyristors are off.

(13) If a negative overvoltage, more negative than V.sub.L, occurs on conductor 3, the voltage of cathode 40 becomes lower than gate voltage 36. A current flows from gate 36 to cathode 40. Thyristor 32 having no emitter short-circuit, it is highly sensitive and turns on rapidly, which enables to discharge the overvoltage to ground.

(14) At the end of the overvoltage, first thyristor 32 is still on, and conducts a current flowing from the ground to conductor 3. Since the thyristor comprises no emitter short-circuits, its hold current is low. Thus, the current which flows through thyristor normally remains higher than its hold current. However, since the thyristor gate is maintained at potential V.sub.L, lower than the cathode potential, part of the current originating from the ground is deviated towards voltage source 58 instead of flowing towards the cathode, which turns off the thyristor. In other words, the charges present in the gate layer during the thyristor conduction are absorbed by voltage source 58. Since the charges are no longer available to maintain the thyristor on, the latter turns off. This operation is possible since voltage source 58 is a real voltage source capable of maintaining potential V.sub.L while absorbing part of the current. This operation would be impossible if potential V.sub.L was defined, for example, by a Zener diode as described in the two above-mentioned patents.

(15) In the case of a negative overvoltage, more negative than potential V.sub.L, on line 5, the operation is identical and implies cathode-gate thyristor 34.

(16) In the case of a positive overvoltage greater than V.sub.H on line 3 or 5, the operation is similar, and implies the corresponding anode-gate thyristor 44 or 46.

(17) In the foregoing, it should be understood that the comparisons of the overvoltages with potential values may be to within a forward diode voltage drop.

(18) FIG. 2B is a cross-section view of an example of a monolithic component using circuit 30 of FIG. 2A. The monolithic component is formed from a lightly-doped N-type semiconductor substrate 60, the semiconductor for example being silicon. The front surface of the component is at the top of the cross-section and the rear surface is at the bottom. The component is divided into two symmetrical portions with respect to the axis of the drawing. The left half contains thyristors 32 and 44, and the right half contains thyristors 34 and 46. Only the left half will be detailed herein, the right half being symmetrical and having an identical operation.

(19) The thyristors have a vertical structure and, at the back of the component, a metallization 62 defines reference potential GND.

(20) Anode-gate thyristor 44 is located between a metallization 64 at the front surface and metallization 62. It comprises a P-type anode region 66 in contact with metallization 64, an N-type gate area 68 with no direct contact with metallization 64 and which contains region 66, a P-type well 70 which contains area 68, a portion of N-type substrate 60, and a heavily-doped N-type cathode layer 72 in contact with metallization 62. The P/N avalanche voltage between anode region 66 and gate area 68 is greater than V.sub.H. Gate area 68 is in contact, via a heavily-doped N-type area 74, with a metallization 76.

(21) Cathode-gate thyristor 32 is located between a front surface metallization 78 and metallization 62. It comprises an N-type cathode region 80 in contact with metallization 78, a P-type gate area 82 with no direct contact with metallization 78 and which contains region 80, a portion of N-type substrate 60, a P-type rear surface well 84, in contact with metallization 62 via a heavily-doped P-type anode layer 86. The P/N avalanche voltage between gate area 82 and cathode region 80 is greater than V.sub.L. Gate area 82 is in contact, via a heavily-doped P-type area 88, with a metallization 90. Conventionally, the component comprises channel stop regions 92.

(22) Metallizations 64 and 78 are intended to be connected together to conductor 3 (V.sub.TIP).

(23) Gate metallization 76 of thyristor 44 is intended to be in direct contact with voltage source 56 of potential V.sub.H. Gate metallization 90 of thyristor 32 is intended to be in direct contact with voltage source 58 of potential V.sub.L.

(24) FIG. 3A shows another embodiment of a protection structure adapted to the cases where the positive threshold is zero, that is, at the potential of ground GND, to within the forward diode voltage drop. The two conductors 3 and 5 of the telephone line are connected to a protection structure 100. Negative potential V.sub.L is supplied by a voltage source 58. Structure 100 comprises thyristors 32 and 34, connected in the same way as in previously-described protection structure 30. Structure 100 also comprises diodes 102 and 104 having their anodes respectively connected to conductors 3 and 5, and their cathodes connected to ground GND.

(25) In case of a negative overvoltage, the operation of protection structure 100 is similar to that of protection structure 30 of FIG. 2A. In case of a positive overvoltage on conductor 3, greater than the forward voltage drop of a PN junction, the overvoltage is deviated to ground through diode 102. The operation is similar with diode 104 in case of a positive overvoltage on conductor 5.

(26) FIG. 3B is a cross-section view of an example of a monolithic component using the circuit of FIG. 3A. The component is symmetrical with respect to the axis of the drawing, and only the left-hand portion will be described. This component is formed from a lightly-doped N-type substrate 60, provided with a rear surface metallization 62. The left-hand portion comprises diode 102 and thyristor 32, and the right-hand portion comprises diode 104 and thyristor 34. Thyristors 32 and 34 are formed as described in FIG. 2B. Diode 102 is formed between a front surface metallization 106 and metallization 62, its anode area corresponds to a P-type well 108, in contact with metallization 106 via a heavily-doped P-type layer 110, its cathode area corresponds to a portion of substrate 60 and to a heavily-doped N-type layer 112. Metallization 106 is in contact with metallization 78 and is intended to be connected to conductor 3.

(27) An advantage of the protection structures described herein, where the thyristors have no emitter short-circuits, is that the operation requires no transistor, which enables to decrease the component surface area, and no external components (diodes or transistors), which decreases the cost and the bulk.

(28) Another advantage is that thyristors comprising no emitter short-circuits are highly sensitive, which allows a fast start and thus improves the quality of the protection.

(29) Further, the characteristics of the thyristors may be selected independently from the maximum current capable of flowing through the telephone line. Thus, telephone lines having different characteristics may be protected by protection structures having the same characteristics.

(30) Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, although the forming of the monolithic components has been described from an N-type substrate, it should be clear to those skilled in the art that they may be formed from P-type substrates.

(31) Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.