Abstract:
A structure for protecting a circuit connected to first and second rails of a telephone connection against overvoltages, including: first and second diodes in anti-series between the first and second rails; a first capacitor in parallel with a first resistor between a first node common to the first and second diodes and a low voltage reference node; and a protection element capable of removing fast overvoltages between any of the rails and the low reference voltage node when these overvoltages exceed a first threshold associated with the voltage of the first node.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 10/54504, filed on Jun. 8, 2010, entitled “ANTI-LIGHTNING PROTECTION FOR TELEPHONE CONNECTION,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure for protecting an electronic circuit connected to a telephone connection line against fast overvoltages, for example due to lightning. 
     2. Discussion of the Related Art 
       FIG. 1  schematically shows an electronic circuit  1  connected to rails  3  and  5  of a telephone connection  7 . Circuit  1  is capable of transmitting and/or of receiving signals, respectively V TIP  and V RING , over rails  3  and  5 . Signals V TIP  and V RING  for example are speech signals, ringing signals, etc. Circuit  1  is further connected to a power supply terminal  9  of high voltage V H  and to a power supply terminal  11  of low voltage V L . Circuit  1  for example is a SLIC-type circuit (“Subscriber Line Interface Circuit”), capable of behaving as an interface between an analog telephone connection line and digital telephone network equipment. 
     Short and abrupt overvoltages, for example due to lightning, may occur on rails  3  and/or  5 . Such overvoltages are capable of damaging components of circuit  1 . It is thus generally provided to connect to telephone line  7 , between rails  3  and  5 , a protection structure  13 , capable of rapidly draining off significant currents that may appear when an overvoltage occurs on rail  3  and/or on rail  5 . 
     In an example, structure  13  comprises thyristors  15 ,  17 ,  19 , and  21 , respectively forward-connected between the ground and rail  3 , between rail  3  and the ground, between the ground and rail  5 , and between rail  5  and the ground. Structure  13  further comprises zener diodes  23 ,  25 ,  27 , and  29 , respectively forward-connected between a cathode gate of thyristor  15  and the ground, between the ground and an anode gate of thyristor  17 , between a cathode gate of thyristor  19  and the ground, and between the ground and an anode gate of thyristor  21 . 
     It should be noted that term “ground” here designates a reference potential common to all the device elements, for example, a potential close to 0 V. In practice, structure  13  may be grounded via a ground terminal of circuit  1 , or via a ground rail (not shown) comprised in connection  7 . In the following description, “positive potential” and “negative potential” will be used to designate potentials respectively greater than the ground potential and smaller than the ground potential, and each time digital potential values will be given as an example, these values will be considered to refer to a ground potential equal to 0 V. 
     In case of a positive overvoltage on rail  3 , thyristor  17 , which is forward biased, is capable of being turned on. If the overvoltage exceeds a given threshold, zener diode  25  turns on by avalanche effect. A current then flows between rail  3  and the ground, through the PN junction, between the anode and the anode gate of thyristor  17 , and through zener diode  25 . Thyristor  17  is thus turned on and the overvoltage is removed towards the ground. 
     In case of a negative overvoltage on rail  3 , thyristor  15  is capable of being turned on. If the overvoltage exceeds a given threshold, zener diode  23  turns on by avalanche effect for the negative overvoltage. A negative current then flows between rail  3  and the ground, through the PN junction between the cathode gate and the cathode of thyristor  15 . Thyristor  15  is thus turned on and the overvoltage is removed towards the ground. 
     In case of a positive or negative overvoltage on rail  5 , a similar removal scheme applies through thyristors  21  or  19  and zener diodes  29  or  27 . Thus, structure  13  enables to remove any overvoltage that may occur on rails  3  and  5 . 
     The turn-on threshold for a positive overvoltage is thus equal to the avalanche voltage of a zener diode ( 25  or  29 ) plus the forward voltage drop of a PN junction (on the order of 0.6 V). The turn-on threshold for a negative overvoltage is equal to the opposite of the avalanche voltage of a zener diode ( 23  or  27 ) minus the forward voltage drop of a PN junction. 
     A disadvantage of this type of structure is that the avalanche voltages of the zener diodes should be adapted to the maximum and minimum values that may be taken by signals V TIP  and V RING  in a normal operation of the device. This actually results in selecting avalanche voltages much greater than the normal excursions of signals V TIP  and V RING  with respect to the ground, to take into account the component dispersion. 
       FIG. 2  is an electric diagram corresponding to the diagram of  FIG. 1 , where protection structure  13  with fixed turn-on thresholds has been replaced with a protection structure  31  having its turn-on thresholds associated with the power supply voltages of circuit  1 . Structure  31  is connected not only to rails  3  and  5 , but also to power supply terminals  9  and  11  of circuit  1  to be protected. It comprises thyristors  15 ,  17 ,  19 , and  21 , connected as in  FIG. 1 . Structure  31  further comprises an NPN transistor  33 , a PNP transistor  35 , an NPN transistor  37 , and a PNP transistor  39 . The emitters of transistors  33 ,  35 ,  37 , and  39  are respectively connected to the cathode gate of thyristor  15 , to the anode gate of thyristor  17 , to the cathode gate of thyristor  19 , and to the anode gate of thyristor  21 . The collectors of these transistors are all grounded. The bases of transistors  33  and  37  are connected to low power supply terminal  11  (V L ), and the bases of transistors  35  and  39  are connected to high power supply terminal  9  (V H ). 
     In case of a positive overvoltage on rail  3 , thyristor  17 , which is forward biased, is capable of being turned on. The overvoltage is transferred by the PN junction between the anode and the anode gate of thyristor  17  onto the emitter of transistor  35 . If the overvoltage exceeds a given threshold, the voltage of the emitter of transistor  35  exceeds the base voltage of this transistor (that is, V H ), which turns on. A current then flows between rail  3  and the ground, from the anode to the anode gate of thyristor  17 , and through transistor  35 . Thyristor  17  is thus turned on and the overvoltage is removed towards the ground. 
     The other overvoltage polarities are similarly removed by one of the other thyristors, noting that negative overvoltages are referenced to a threshold associated with low power supply voltage V L . 
     The turn-on threshold for a positive overvoltage is thus equal to high power supply voltage V H  plus twice the forward voltage drop of a PN junction (on the order of 1.2 V). The turn-on threshold for a negative overvoltage is equal to low power supply voltage V L  minus twice the forward voltage drop of a PN junction. 
     This type of structure is used when the power supply voltages V H  and V L  of the circuit to be protected, present on terminals of the circuit, correspond to the maximum and minimum values that may be taken, in normal operation, by signals V TIP  and V RING . This type of structure is also used when the circuit to be protected comprises reference terminals between which a reference voltage corresponding to the voltage level of the signals present on the line is established. However, structure  31  has the disadvantage of disturbing the voltages of the reference terminals to which it is connected. 
     In certain cases, the circuit to be protected receives on its access terminals a power supply voltage of much lower level than the voltage level of signals V TIP  and V RING . The circuit to be protected then comprises converters for providing voltage levels adapted to the telephone line, and these voltage levels are not accessible from access terminals of the circuit. 
     SUMMARY OF THE INVENTION 
     Thus, an embodiment of the present invention is to provide a structure for protecting a circuit connected to a telephone line, which overcomes at least some of the disadvantages of existing solutions. 
     An embodiment of the present invention is to provide a self-adaptive protection structure, that is, a structure having turn-on thresholds which automatically adapt to the voltage level of the signals which run through the telephone line in a given operation mode. 
     An embodiment of the present invention is to provide such a structure which requires no previous knowledge of the voltage levels of the signals conducted by the telephone line. 
     An embodiment of the present invention is to provide such a structure which does not require to be connected to reference terminals defining the voltage level of the signals conducted by the telephone line. 
     Thus, an embodiment provides a structure for protecting a circuit connected to first and second rails of a telephone connection against overvoltages, comprising: first and second diodes in anti-series between the first and second rails; a first capacitor in parallel with a first resistor between a first node common to the first and second diodes and a low voltage reference node; and a protection element capable of removing fast overvoltages between any of the rails and the low reference voltage node when these overvoltages exceed a first threshold associated with the voltage of the first node. 
     According to an embodiment, the first threshold is a low threshold lower than the low reference voltage. 
     According to an embodiment, the protection structure further comprises: third and fourth diodes in anti-series between the first and second rails, the first and fourth diodes being series-connected, and the second and third diodes being series-connected; and a second capacitor in parallel with a second resistor between a second node common to the third and fourth diodes and said low voltage node, wherein the protection element is capable of removing fast overvoltages between any of the rails and the low voltage node when such overvoltages exceed a second threshold associated with the voltage of the second node. 
     According to an embodiment, the second threshold is a high threshold, higher than the low reference voltage. 
     According to an embodiment, the protection element comprises: first to fourth thyristors forward-connected, respectively, between the low voltage node and the first rail, between the first rail and the low voltage node, between the low voltage node and the second rail, and between the second rail and the low voltage node; and first and second NPN transistors, and first and second PNP transistors, having their emitters respectively connected to a cathode gate of the first thyristor, to a cathode gate of the third thyristor, to an anode gate of the second thyristor, and to an anode gate of the fourth thyristor, and having their collectors connected to the low voltage node, the bases of the PNP transistors being connected to the second common node, and the bases of the NPN transistors being connected to the first common node. 
     According to an embodiment, the protection element comprises: first and second thyristors forward-connected, respectively, between the low voltage node and the first rail, and between the low voltage node and the second rail; first and second NPN transistors having their emitters respectively connected to a cathode gate of the first thyristor and to a cathode gate of the second thyristor, having their collectors connected to the low voltage node, and having their bases connected to the first common node; and third and fourth diodes forward-connected, respectively, between the first rail and the low voltage node and between the second rail and the low voltage node. 
     According to an embodiment, the first capacitor has a capacitance ranging between 100 and 300 nF, and the first resistance ranges between 3 and 6 MΩ. 
     Another embodiment provides a subscriber line interface circuit associated with a protection structure of the above-mentioned type. 
     Another embodiment provides a method for protecting an integrated circuit connected to first and second rails of a telephone line against overvoltages, comprising the steps of: storing, in an initialization phase, the maximum or minimum voltage level of the signals applied to the first and second rails; and triggering a protection element connected between the first and second rails and to a low reference voltage node, when a fast overvoltage exceeding a threshold associated with said maximum or minimum level occurs between any of the rails and the low reference voltage node. 
     The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , previously-described, schematically shows a device comprising a circuit connected to rails of a telephone line, and a structure for protecting this circuit against overvoltages; 
         FIG. 2 , previously described, schematically shows another example of a structure for protecting a circuit connected to rails of a telephone line against overvoltages; 
         FIGS. 3A to 3C  are timing diagrams schematically illustrating examples of the variation of signals conducted by the rails of a telephone line in normal operation; 
         FIG. 4  schematically shows an embodiment of a self-adaptive structure for protecting a circuit connected to rails of a telephone line; and 
         FIG. 5  schematically shows another embodiment of a self-adaptive protection structure, specifically adapted to the case where only negative signals are applied to the telephone line. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings. 
     In a telephone network, several operating modes are possible, where signals V TIP  and V RING  conducted by rails  3  and  5  of telephone line  7  ( FIGS. 1 and 2 ) are at very different voltage levels. 
       FIGS. 3A to 3C  are timing diagrams schematically illustrating examples of the variation of signals V TIP  and V RING  during the normal operation of the telephone network. 
       FIG. 3A  corresponds to an operating mode where line  7  conducts a ringing signal addressed to a subscriber of the network. It will be spoken hereafter of a “ringing mode”. In this case, signal V TIP  is a sinusoidal signal having a 284-V peak-to-peak amplitude, centered on −46 V, of 50-Hz frequency. Signal V RING  is a D.C. −2-V signal. 
       FIG. 3B  corresponds to the case where line  7  is idle, that is, no communication is completed or about to be completed. It will be spoken hereafter of a “stand-by mode”. In this case, signals V TIP  and V RING  are D.C. signals, respectively of −46 V and −2 V. 
       FIG. 3C  corresponds to an operating mode in which line  7  conducts a speech signal. It will be spoken hereafter of a “speech mode”. In this case, signal V TIP  is a modulated sound signal having a 2.8-V peak-to-peak amplitude, centered on −46 V. Signal V RING  also is a signal with a peak-to-peak amplitude equal to 2.8 V, but centered on −2 V. 
     Other operating modes, not shown, may also be encountered, where signals V TIP  and V RING  take different shapes and/or voltage levels. 
       FIG. 4  schematically shows an embodiment of a self-adaptive structure  41  for protecting a circuit  1  connected to rails  3  and  5  of a telephone line  7  against overvoltages. Circuit  1  is for example capable of transmitting and/or of receiving, over rails  3  and  5 , signals V TIP  and V RING  of the type described in relation with  FIGS. 3A to 3C . In this example, circuit  1  is connected to power supply terminals  43  of high voltage V DD  and  45  of low voltage (here, the ground). Voltage V DD  for example approximately is 5 V. 
     Structure  41  comprises a protection element  31  similar to that described in relation with  FIG. 2 . This protection element is connected between rails  3  and  5 , as in  FIG. 2 . The bases of transistors  33  and  37  of element  31  are connected to a node N, and the bases of transistors  35  and  39  of element  31  are connected to a node P. Protection element  31  is capable of rapidly draining off significant currents when a positive overvoltage, having a level greater than a threshold associated with the voltage at node P, or a negative overvoltage, having a level smaller than a threshold associated with the voltage at node N, occurs on rail  3  or on rail  5 . 
     The turn-on threshold for a positive overvoltage is thus equal to the voltage at node P plus twice the forward voltage drop of a PN junction (on the order of 1.2 V). The turn-on threshold for a negative overvoltage is equal to the voltage at node N minus twice the forward voltage drop of a PN junction. 
     Protection structure  41  further comprises an element  49  capable of storing the voltage level of signals V TIP  and V RING  conducted by rails  3  and  5 , during a normal operation of the network. 
     Storage element  49  comprises diodes  51 ,  53 ,  55 , and  57  forward-connected, respectively, between rail  5  and node P, between rail  3  and node P, between node N and rail  3 , and between node N and rail  5 . Element  49  further comprises a resistor  59  in parallel with a capacitor  61 , between node P and the ground, and a resistor  63  in parallel with a capacitor  65 , between node N and the ground. 
     When protection structure  41  is not connected, or when rails  3  and  5  are left floating, capacitors  61  and  65  are discharged. Thus, at a time t 0  when the network portion comprising circuit  1 , rails  3  and  5 , and structure  41 , is started up, the voltages at nodes N and P are substantially equal to the ground voltage. 
     Diodes  51  and  53  are conductive for positive signals V RING  and V TIP  having a level greater than their forward voltage drop V F  (on the order of 0.6 V), and non-conductive for signals having a level lower than V F . 
     Diodes  55  and  57  are conductive for negative signals V TIP  and V RING  having a level smaller than −V F  and non-conductive for signals having a level greater than −V F . 
     If at least one of signals V TIP  and V RING  reaches a positive level greater than V F , capacitor  61  progressively charges to a value V MAX −V F , V MAX  being the maximum level reached by the most positive of signals V TIP  and V RING . Thus, after an initialization time, the voltage at node P settles to V MAX −V F . 
     However, if signals V TIP  and V RING  both remain lower than V F , the voltage at node P is maintained equal to the ground voltage. 
     If at least one of signals V TIP  and V RING  reaches a negative level lower than −V F , capacitor  65  progressively charges to a value V MIN +V F , V MIN  being the minimum level reached by the most negative of signals V TIP  and V RING . Thus, after an initialization time, the voltage at node N settles to V MIN +V F . 
     However, if signals V TIP  and V RING  both remain greater than −V F , the voltage at node N is maintained equal to the ground voltage. 
     As an example, when the device is in ringing mode (see  FIG. 3A ), after an initialization period, the voltage at node P settles to V MAX −V F , and the voltage at node N settles to V MIN +V F . The high and low turn-on thresholds of protection structure  41  in case of an abrupt overvoltage thus respectively settle to:
 
 V   MAX   −V   F +2 V   F   =V   MAX   +V   F (=96.6 V in this example), and  V   MIN   +V   F −2 V   F   =V   MIN   −V   F (=−188.6 V in this example).
 
     When the device is in stand-by mode (see  FIG. 3B ), after an initialization period, the high and low turn-on thresholds of protection structure  41  in case of an abrupt overvoltage respectively settle to:
 
 V   GND +2 V   F , where  V   GND  is the ground voltage, and  V   MIN   +V   F −2 V   F   =V   MIN   −V   F ,
 
     that is, 1.2 V et −46.6 V in this example. 
     When the device is in speech mode (see  FIG. 3C ), after an initialization period, the high and low turn-on thresholds of protection structure  41  in case of an abrupt overvoltage respectively settle to:
 
 V   GND +2 V   F (=1.2 V in this example), and  V   MIN   +V   F −2V F   =V   MIN   −V   F (=−47.4 V=−0.6 V=−48 V in this example).
 
     The provided protection structure enables to remove any type of abrupt overvoltage, be it positive or negative, capable of occurring on the telephone line. 
     An advantage of such a structure is that the protection turn-on thresholds automatically adapt, after a short initialization period, to the voltage level of the signals conducted by the line, and this while the protection structure is only connected to rails  3  and  5  of the line and to ground. 
     This protection structure can thus be used whatever the circuit to be protected, even when the minimum and maximum voltage levels of the signals capable of conducted by the telephone line are not known previously. 
     The capacitances of capacitors  61  and  65  should be selected to be low enough to enable a fast charge on starting up of the device or in a change of operating mode. Further, capacitor  61  and resistor  59 , on the one hand, and capacitor  65  and resistor  63 , on the other hand, should define sufficiently high products R*C to result in a relatively slow discharge of the capacitors. In particular, in ringing mode, capacitors  61  and  65  should not significantly discharge between two peaks of the 50 Hz sinusoidal signal. The present inventors have determined that capacitors having a capacitance ranging between 100 and 300 nF, and capable of withstanding a voltage on the order of 250 V, and resistors from 3 to 6 MΩ, generally are a good compromise. Such values are further compatible with an integration of the capacitors. The present invention is however not limited to these specific values. 
       FIG. 5  schematically shows a simplified embodiment of a self-adaptive protection structure  71 , specifically adapted to the case where the telephone line only conducts negative signals V TIP  and V RING . 
     Structure  71  comprises a protection element  73 , connected between rails  3  and  5  and to a node N, capable of rapidly draining off significant currents when a positive overvoltage greater than the forward voltage drop of a diode, or a negative overvoltage of lower level than a threshold associated with the voltage at node N, occurs on rail  3  or on rail  5 . 
     In this example, protection element  73  comprises thyristors  75  and  77  forward-connected, respectively, between the ground and rail  3 , and between the ground and rail  5 , and diodes  79  and  81  forward-connected, respectively, between rail  3  and the ground, and between rail  5  and the ground. Protection element  73  further comprises NPN transistors  83  and  85 . The emitters of transistors  83  and  85  are respectively connected to the cathode gate of thyristor  75 , and to the cathode gate of thyristor  77 . The collectors of these transistors are grounded, and their bases are connected to node N. 
     The turn-on threshold for a positive overvoltage is equal to forward voltage drop V F  of a diode ( 79  or  81 ). The turn-on threshold for a negative overvoltage is equal to the voltage at node N minus twice the forward voltage drop of a PN junction. 
     Protection structure  71  further comprises an element  87  capable of storing the most negative voltage level of signals V TIP  and V RING  conducted by rails  3  and  5 , during a normal operation of the network. 
     Storage element  87  comprises diodes  91  and  93  forward-connected, respectively, between node N and rail  5 , and between node N and rail  3 . Element  87  further comprises a resistor  95  in parallel with a capacitor  97 , between node N and the ground. 
     Thus, at a time t 0  when the telephone network portion comprising circuit  1 , rails  3  and  5 , and structure  71 , is started up, capacitor  97  is discharged, and the voltage at node N is substantially equal to the ground voltage. 
     Capacitor  97  progressively charges to a value V MIN +V F , V MIN  being the minimum level reached by the most negative of signals V TIP  and V RING . Thus, after an initialization time, the high and low turn-on thresholds of protection structure  71  in case of an abrupt overvoltage on rail  3  or on rail  5  respectively settle to V F , and V MIN +V F −2V F =V MIN −V F . 
     Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, structures of protection against overvoltages comprising the following elements have been described herein: 
     an element for storing the maximum (and/or minimum) voltage level of the signals conducted by a telephone line, and 
     a protection element capable of removing fast overvoltages on the line, when the overvoltages exceed the stored level. 
     The present invention is not limited to the above-mentioned examples of protection elements. It will be within the abilities of those skilled in the art to implement the desired operation by using other protection elements capable of removing overvoltages when the overvoltages exceed a threshold associated with a reference voltage applied to a terminal of the protection element. 
     Further, a telephone line protection structure is generally formed of an independent chip, capable of being connected to the telephone line on the side of the circuit to be protected. However, the provided structure may also be integrated to the circuit which is desired to be protected. 
     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.