Transient protection circuit

Abstract: A transient protection circuit comprising using a circuit connected to operate as a diode while presenting lower forward voltage drop for the same area and current as in pn junction diodes to provide greater efficiency. The circuit is a standard merged SCR circuit wherein a resistive path R1 is provided between the base and collector of the pnp transistor 11 and R2 between the base and emitter of the npn transistor 13.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a transient protector, particularly for protection of electronic circuits from lightning strikes and, more specifically, to a system for protecting equipment at a telephone central station from damage due to lightning strikes on the external telephone lines. BRIEF DESCRIPTION OF THE PRIOR ART Devices and circuits for protection from lightning strikes and particularly such circuits for protection against lightning strikes in electronic circuits are well known in the prior art. It is known that lightning strikes can be of either positive or negative polarity relative to the reference voltage of the protected circuit. It is therefore necessary that any protection circuit be equipped to handle both positive and negative lightning strikes. The above noted problem with lightning is especially acute in the case of telephone circuits and particularly such circuits at telephone central stations. In such circuits, the subscriber is connected to the central station, often through many miles of telephone line or cable. Lightning is often attracted to the exposed telephone lines and causes a transient, positive or negative, to travel down the line to circuits at the central station, such as, for example, the subscriber line interface circuit (SLIC), wherein such transients can burn out semiconductor circuit components therein. SLIC circuits generally include a ground line, a tip line and a ring line, both the tip and ring lines generally being negative with respect to the ground line. Circuits are located between the ground line and the tip and/or ring lines. It is therefore readily apparent that transients caused by a lightning strike can and often do travel through such circuits and cause burn out therein. In the prior art, the above noted lightning problem has been handled by, for example, the use of a reference voltage controlled trip circuits, one such trip circuit coupled between the tip line and ground and the other trip circuit being coupled between the ring line and ground. Each trip circuit controls a separate SCR having its anode coupled to ground and its cathode coupled to the tip or ring line. In addition, diodes were coupled across the tip line and ground and across the ring line and ground with the cathode of each diode at ground. In circuits of this type, current from positive-going transients due to lightning travelled through the diode to ground whereas negative-going transients caused an increase in current travelling to the trip circuit, thereby causing the SCR to conduct when a predetermined threshold was reached, this threshold generally being the battery voltage at the central station. Circuits of the above described type perform the function for which they are designed. However, in order to obtain semiconductor diodes capable of passing current of the magnitudes required without burning out themselves, it has been necessary that these diodes be very large and occupy a large area of the semiconductor chip. It therefore follows that chip packing density is sacrificed, this being an important consideration in semiconductor circuit design. Accordingly, it is desirable to provide circuit designs which lend themselves to greater component packing density. In addition, due to the large amounts of current potentially being drawn by such protective circuits, hot spots developed at regions of high current flow due to the voltage drop created in the diode. It is highly desirable that such hot spots be eliminated or at least minimized. Another problem encountered with the prior art circuits is that, when the tip or ring line goes negative with respect to Vs, the central office battery voltage which is negative with respect to ground, an abnormal situation arises since Vs is the most negative voltage normally in the circuit, and trip current will be drawn. This causes a reverse current to be drawn in the central office battery. In the prior art, a filter, such as a capacitor, has been used to support that current during transient and prevent large currents from travelling through the lateral transistor at the trip circuit. It would be highly desirable to minimize the current drawn from the battery to minimize the risk of overcurrents in the trip circuit lateral transistor, thereby minimizing and possibly eliminating the need for the filter. In US-A-4 408 248 there is described a protection circuit for a telephone line having a thyristor and a bridge rectifier connected to the two conductors of the line and to ground in such a way that a voltage pulse of either polarity relative to ground on either line is applied with a fixed polarity across the thyristor. An overvoltage detection circuit is connected from the anode to the gate of the thyristor. Overcurrent detection circuits in the conductors are coupled optically to the thyristor. The thyristor is caused to conduct by the overvoltage circuit in response to an excessive voltage pulse or by the overcurrent in response to an excessive current in a conductor. In the overvoltage detection circuit current passing through a breakdown diode due to a voltage pulse serves to turn on a thyristor-connected pair of transistors which together turn on a third transistor to feed a current to the gate of the thyristor to turn it on. In EP-A-0 032 046 there is described a protection circuit for protecting the input of a semiconductor device against static electricity, the circuit comprising two bipolar transistors of complementary conductivity types in thyristor-connection with a resistor connected between the bases of the transistors so that the circuit operates in a similar way to a diode but with a more rapid response to voltages causing conduction and more effective limiting of those voltages. SUMMARY OF THE INVENTION It is an object of the present invention to provide a transient protector circuit which minimizes the above problems of the prior art. According to the present invention there is provided a transient protection circuit which comprises: first and second electrical lines each said line normally being at different voltage levels, the second of said lines being at a reference voltage; a controlled rectifier circuit having an anode, a cathode and a control electrode, said anode being coupled to the second of said lines and said cathode coupled to the first of said lines; and circuit means for controlling the operation of said controlled rectifier circuit in response to excessive voltage on one of said lines; characterised in that the circuit includes a unidirectional semiconductor device having its anode coupled to a first of said lines and its cathode coupled to the second of said lines; and said circuit means controlling operation of said controlled rectifier circuit includes a first semiconductor device having a control terminal and a controlled current path with its current path coupled between said control electrode of said controlled rectifier circuit and a source of voltage having a voltage difference from said first of said lines at least equal to that of said second of said lines, biasing means coupled between said control electrode and said cathode of said controlled rectifier circuit, a second semiconductor device having a current path connected to the control terminal of the first semiconductor device and being responsive to the voltage on said first of said lines to conduct current in its current path and when the current in that current path exceeds a predetermined threshold current it initiates conduction of current in said current path of said first semiconductor device, thereby causing said controlled rectifier circuit to conduct. Briefly, in accordance with a first embodiment of the present invention, the diode of the prior art protector circuit is replaced by an SCR circuit which performs the diode function, yet has much greater efficiency than a p-n junction due to the conductivity modulation of the SCR, thus allowing the SCR to have a lower forward voltage drop for the same area and current. The circuit includes a pnp transistor with the emitter thereof functioning as the anode of the diode and an npn transistor with the emitter thereof functioning as the cathode. The pnp transistor base is coupled directly to the npn transistor collector and to the npn transistor base through a first resistor. The pnp transistor collector is coupled directly to the npn transistor base, the latter being coupled to the cathode via a second resistor. In accordance with a second embodiment of the invention, the trip circuit for the SCR of the prior art protector circuit is modified to avoid large currents in the lateral transistor thereof. This circuit includes the prior art circuit which includes a pnp lateral transistor having its emitter coupled to the battery Vs and its base and collector coupled together and to the tip or ring line via a first resistor. The additional circuit components are provided to shift large currents to an alternate circuit path, these components including a second resistor in series with the first resistor and the pnp transistor base. Also provided is a second npn transistor having its base coupled to the base of the first transistor, the collector coupled to the ground line and the emitter coupled to the junction of the first and second resistors. In this way, transient current travels through the first transistor alone until the voltage across the first resistor turns on the second transistor, whereupon all further current is drawn through the second transistor. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a typical prior art transient protector circuit; FIGURE 2 is a circuit diagram of a circuit for providing a diode function in accordance with the present invention; FIGURE 3 is an SCR trip circuit in accordance with the present invention; and FIGURE 4 is a circuit diagram of a SLIC circuit using the protector circuit in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURE 1, there is shown a typical prior art SLIC protection circuit for transient protection circuit for use in SLIC circuits at telephone central stations. The circuit includes a ground line 1 at one reference voltage and a tip or ring line 3 which is generally negative with respect to the ground line, it being understood that a ring or tip line is provided which is a mirror image of line 3 with respect to the ground line. Accordingly, all discussions herein relative to a tip line 3 apply as well to the ring line (not shown in FIGURE 1). The circuit of FIGURE 1 provides protection against both positive and negative transients with respect to ground to include all possible forms of lightning transients. The circuit includes a trip circuit 5 coupled between the ground line 1 and the tip line 3 and controlled by Vs, another reference voltage derived from a battery at the central station which is normally at a more negative voltage than the tip line. A diode 7 in the form of a p-n junction type semiconductor device is coupled between the ground line 1 and the tip line 3 with its anode coupled to the tip line and its cathode coupled to the ground line. In this manner, the diode 7 will shunt current directly to ground when large positive with respect to ground voltage transients appear on the tip line 3. The circuit also includes an SCR 9 having a gate electrode coupled to and controlled by the trip circuit 5. The anode of the SCR is coupled to the ground line 1 and the cathode thereof is coupled to the tip line 3. For negative with respect to ground voltage transients on the tip line 3, it is desirable that the protective circuit not come into operation until the voltage of the tip line goes negative relative to the battery Vs at the central station to prevent or avoid reverse current in the battery. Under this condition, the diode 7 is reverse biased. Current will flow from the reference voltage source Vs until a predetermined current level has been reached in the trip circuit whereupon the trip circuit will fire the SCR 9 and provide a current path for the current away from the SLIC circuit, thereby providing the required protection for both positive and negative transients. Referring now to FIGURE 2, there is shown a circuit for use in place of the diode 7 which has large current capability relative to a p-n junction diode and therefore can be formed in less space for the same current carrying capacity with diminished forward voltage drop in power dissipation . The circuit includes a pnp transistor 11 with the emitter thereof functioning as the anode 19 of the diode and an npn transistor 13 with the emitter thereof functioning as the cathode. The pnp transistor base is coupled directly to the npn transistor collector and to the npn transistor base through a first resistor 15. The pnp transistor collector is coupled directly to the npn transistor base, the latter being coupled to the cathode 21 via a second resistor 17. In operation, as positive voltage is applied to the anode 19 relative to the cathode 21, the base-emitter junction of transistor 11 becomes forward biased and the circuit appears as a diode in series with resistors 15 and 17 at that time. As the current in the circuit of the base-emitter of transistor 11 and resistors 15 and 17 increases, the voltage across resistor 17 increases and eventually, when the voltage across resistor 17 is sufficiently high, turns on transistor 13. At this time, transistor 11 will also turn to provide the low impedance path through transistor 11 and resistor 17 to the cathode. The circuit will continue to conduct in this manner until the voltage across resistor 17 is below the threshold voltage for transistor 13. Referring now to FIGURE 3, there is shown a preferred embodiment of an SCR trip circuit for control of the SCR 9 of FIGURE 1. The SCR 9 is provided in the form of a pnp transistor 51 and an npn transistor 53 with resistor 55 in standard manner. The gate of transistor 53 corresponds to the gate of the SCR 9 of FIGURE 1. The trip circuit for the SCR of the prior art protector circuit is modified to avoid large currents in the lateral transistor 57 thereof. This circuit includes the prior art circuit which includes pnp lateral transistor 57 having its emitter coupled to the battery Vs and its base and collector coupled together and to the tip or ring line 3 via a first resistor 61. The additional circuit components are provided to shift the excess of large currents to an alternate circuit path, these components including a second resistor 63 in series with the first resistor 61 and the pnp transistor 57 base. Also provided is a second npn transistor 59 having its base coupled to the base of the first transistor 57, the collector thereof being coupled to the ground line 1 and the emitter thereof coupled to the junction of the first and second resistors 61 and 63. In this way, transient current travels through the first transistor alone until the voltage across the second resistor 63 turns on the transistor 59, whereupon all further current is drawn through the second transistor. Resistor 63 must be small enough to prevent transistor 59 from conducting with a predetermined amount of current. The SCR 9 is turned on when the voltage across resistor 61 reaches the appropriate threshold as is well known to provide an effective short circuit across lines 1 and 3. The SCR is turned off when the holding current through resistor 61 is insufficient to maintain the SCR on. Referring now to FIGURE 4, there is shown a schematic diagram of an actual SLIC transient protection circuit utilizing the features in accordance with the present invention. As can be seen the circuit includes the ground line 1, the tip line 3 and the ring line 4. The circuits denoted as 81 and 83 are diode circuits of the type set forth in FIGURE 2. The circuits denoted as 85 and 87 are SCR circuits of the type shown as 9 in FIGURE 3 and the circuits denoted as 89 and 91 are the trip circuit 5 as shown in FIGURE 3.

Claims: A transient protection circuit which comprises: first and second electrical lines (3 or 4,1), each said line normally being at different voltage levels, the second of said lines (1) being at a reference voltage; a controlled rectifier circuit (9,85,87) having an anode, a cathode and a control electrode, said anode being coupled to the second of said lines and said cathode coupled to the first of said lines; and circuit means (57 to 63,89,91) for controlling the operation of said controlled rectifier circuit in response to excessive voltage on one of said lines; characterised in that the circuit includes a unidirectional semiconductor device (7) having its anode coupled to a first of said lines and its cathode coupled to the second of said lines; and said circuit means controlling operation of said controlled rectifier circuit includes a first semiconductor device (59,Q11,Q21) having a control terminal and a controlled current path with its current path coupled between said control electrode of said controlled rectifier circuit and a source (1 or V s ) of voltage having a voltage difference from said first of said lines (3 or 4) at least equal to that of said second of said lines (1), biasing means (61,R24) coupled between said control electrode and said cathode of said controlled rectifier circuit, a second semiconductor device (57,Q12,Q23) having a current path connected to the control terminal of the first semiconductor device and being responsive to the voltage on said first of said lines to conduct current in its current path and when the current in that current path exceeds a predetermined threshold current for initiating conduction of current in said current path of said first semiconductor device, thereby causing said controlled rectifier circuit to conduct. A circuit according to claim 1 in which said second semiconductor device (57,Q12,Q22) is connected as a diode in a series path with said control terminal and an end terminal of said controlled current path of said first semiconductor device (59,Q11,Q21) from said control electrode of said controlled rectifier circuit to said source of voltage (V s ). A circuit according to claim 1 or claim 2 wherein said controlled rectifier circuit comprises a silicon controlled rectifier. A circuit according to any one of the preceding claims wherein said first semiconductor device (59) is an npn bipolar transistor and said second semiconductor device (57) is a pnp bipolar transistor. A circuit according to any one of claims 1 to 3 wherein said first semiconductor device (Q11,Q21) is a pnp bipolar transistor and said second semiconductor device (Q12,Q22) is an npn bipolar transistor. A circuit according to any one of the preceding claims wherein said biasing means is resistive means (61,R24) connected between said control electrode and the cathode of said controlled rectifier circuit. A circuit according to claim 4 including resistive means (63) coupled between the control terminal of said first transistor and the control electrode of said controlled rectifier circuit. A circuit according to any one of the preceding claims wherein said unidirectional semiconductor device comprises a pnp transistor (11) having an emitter electrode coupled to the first of said lines (3 or 4) and an npn transistor (13) having an emitter electrode coupled to the second of said lines (1), the control electrode of said pnp transistor being coupled to the collector electrode of said npn transistor and the collector electrode of said pnp transistor being coupled to the control electrode of said npn transistor, resistive means (15) coupling the control electrode and the collector electrode of said pnp transistor (11) and resistive means (17) coupling the control electrode and the emitter electrode of said npn transistor (13).

Inventors: CULP NORMAN L, KWAN STEPHEN D, CULP, NORMAN L., KWAN, STEPHEN D.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1989
Main classification: H04M3
IPC codes: H04M3, H02H9