Transient protector

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 is provide between the base and collector of the pnp transistor and between the base and emitter of the npn transistor. In addition, there is provided a trip circuit wherein current is shunted away from the lateral transistor after a predetermined threshold current is passed through said transistor to minimize current drawn through the later transistor.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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 the 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 since Vs is the most negative voltage normally in the circuit, 
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. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a transient protector circuit is 
provided which minimizes the above problems of the prior art. 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 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 FIG. 1). The circuit of FIG. 
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 FIG. 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 -9 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 FIG. 3, there is shown a preferred embodiment of an SCR 
trip circuit for control of the SCR 9 of FIG. 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 FIG. 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 57 alone until the voltage across the second resistor 63 
turns on the transistor 59, whereupon all further current is drawn through 
the second transistor 59. 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 FIG. 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 FIG. 2. The circuits 
denoted as 85 and 87 are SCR circuits of the type shown as 9 in FIG. 3 and 
the circuits denoted as 89 and 91 are the trip circuit as shown in FIG. 3. 
Though the invention has been described with respect to specific preferred 
embodiments thereof, many variations and modifications will immediately 
become apparent to those skilled in the art. It is therefore the intention 
that the appended claims be interpreted as broadly as possible in view of 
the prior art to include all such variations and modifications.