 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a controlled electronic switching device for the
suppression of transients, which can change over between two different (on
and off) states according to the state of an externally applied control
signal.
Such devices have several applications in the field of either current or
voltage protection, e.g. in telephone lines.
The need is felt for devices which can be arranged on the line between the
two leads so as to suppress transient phenomena, such as the secondary
effects of lightning, by operating as open circuits in a normal condition
and shorting the two lines together in the presence of a transient
phenomenon ("normalized" lightning) to be suppressed.
Suitable devices to provide this function are, for example, SCR switches
formed with four layers of alternate conductivity types, as the one shown
for instance in FIGS. 1 to 3. Such a device (well known per se) comprises
two terminal connections, an anode A, cathode K, and a control electrode
or gate G, and has three junctions J.sub.1,J.sub.2,J.sub.3 which govern
the device behaviour. In particular, by negatively biasing the cathode
with respect to the anode, the junctions J.sub.1 and J.sub.3 become
forward biased while junction J.sub.2 is inversely biased; consequently,
there is no current flow between the anode and cathode, and the device
will be in its off state.
On application of a signal to the control electrode G, the NPN transistor
formed by the three layers closest to the cathode begins to conduct, thus
lowering its collector voltage level which corresponds, as shown, to the
base of the second PNP transistor formed by the three layers closest to
the anode A. As a result, the second PNP transistor senses that occurrence
as a base drive and begins to conduct, thus producing a regenerative
effect. By this time, the device will be in its fully conductive state as
represented by the vertical line in the graph of FIG. 3 which also
illustrates the relationship existing between current and voltage versus
the control current to the electrode G. As is known, (refer, for instance,
to Gentry et al., "Semiconductor Controlled Rectifiers: Principles and
Application of p-n-p-n Devices", Prentice-Hall, E-E Series) in order for
the device to turn on the following condition must be met: .beta..sub.NPN
.times..beta..sub.PNP =1. As the current I.sub.A flowing through the
device decreases, the device remains on in accordance with the law
illustrated in FIG. 3, until the current reaches a minimum value called
holding current, I.sub.H.
However, that prior device has the disadvantage of becoming conductive as
the voltage across it varies rapidly even with the control current to the
electrode G below the desired value for conduction to begin. This
phenomenon, which is due to the appearance of capacitance at the
junctions, has been obviated by the device, also well known, shown in
FIGS. 4 and 5. The technique adopted (refer, for example, to R. W. Aldrich
and N. Holonyar Jr. in the article entitled "Two-terminal Asymmetrical and
Symmetrical Silicon Negative Resistance Switches", Journal of Applied
Physics, Vol. 30, No. 11, November, 1959) consists in practice in
providing a resistance between the base region of the NPN transistor and
the emitter thereof, which diverts the current generated within the
capacitor owing to a voltage variation. In particular, and as shown in
FIG. 4, on a silicon chip comprising four layers with different
conductivity types, namely a layer 1 of the P.sup.++ type, layer 2 of the
N type, layer 3 of the P.sup.+ type, and layer 4 having several regions of
the N.sup.+ type, a metal layer 5 has been deposited which part overlaps
the layers 3 and 4. The circuit equivalent of such a device is shown in
FIG. 5. In that view, one can see the transistor 7 of the PNP type formed
by the layers 1, 2, and 3 of FIG. 4, the transistor 8 formed by the layers
2,3 and 4 of the same, the capacitor 10 formed between the layers 2 and 3
of FIG. 4 (and corresponding to the junction J.sub.2), and the resistor 9
placed between the emitter and base of the transistor 8 and due to layer
5. In particular, the resistance of element 9 determines the value of the
triggering current of the gate electrode G.sub.1. That resistance, which
is selected at a very low value to avoid the capacitive current from the
capacitor 10 causing the device to turn on prematurely, also causes the
control current from the gate electrode G.sub.1 to only turn on the device
when relatively high. In particular, by selecting the value of 1.OMEGA.
for R, the triggering current of the gate electrode is fixed at 600-700 mA
for the device to turn on.
SUMMARY OF THE INVENTION
In the light of the above-outlined situation, the task of this invention is
to provide a controlled electronic switching device for the suppression of
transients, which can be turned on at lower triggering current values than
the values attainable heretofore, while keeping unaltered the device
characteristics as relates to its behaviour as voltage variation and
triggering rate.
The controlled electronic switching device according to the present
invention should be of simple design, should require no special
manufacturing techniques, and should afford comparable cost levels to
similar prior devices.
This task is achieved by a controlled electronic switching device for the
suppression of transients, comprising a main controlled solid state static
switch including several semiconductor layers having two conductivity
types alternating with one another and forming junctions, said main switch
being provided with a resistive connection between two adjacent layers of
different conductivity type, characterized in that it comprises at least a
second, auxiliary controlled solid state static switch including several
semiconductor layers having two conductivity types alternating with one
another and forming junctions, said auxiliary switch being provided with a
resistive connection between two adjacent layers of different conductivity
type, being connected in parallel with said main switch and having the
area of at least one of said junctions thereof smaller than the area of a
corresponding junction in said main switch, the resistive connection of
said auxiliary switch having a higher resistance than the resistive
connection of said main switch, thereby said device can be turned on at
low control currents.
In practice, the main switch is associated with a parallel connected
auxiliary switch which, having smaller junction areas, has a lower
capacitance value, and hence, for a given voltage variation, a lower
capacitive current, thus enabling the use of a higher resistance between
the base and emitter electrodes of a transistor in the auxiliary switch,
and consequently ensuring that the device be turned on at lower values of
the gate electrode current.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages will be more readily understood from the
following detailed description of a preferred, though not exclusive,
embodiment of this invention, with reference to the accompanying
illustrative, but not limitative, drawings, where:
FIG. 1 depicts the theoretical physical structure of a conventional SCR
switch;
FIG. 2 shows the graphic symbol for the device of FIG. 1;
FIG. 3 shows a graph illustrating the ratios between the electric
quantities of the device of FIG. 1;
FIG. 4 illustrates the physical structure of another conventional device
according to the "shorted emitter" technique;
FIG. 5 shows the wiring diagram of the conventional device of FIG. 4;
FIG. 6 shows the wiring diagram of the device of this invention; and
FIG. 7 shows the physical structure of the device of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The device according to this invention will be now described with reference
to just FIGS. 6 and 7; FIGS. 1-5 relating to conventional devices already
reviewed detailedly hereinabove.
The device of this invention comprises a main switch formed by a PNP
transistor 20, NPN transistor 21, junction capacitor 26, and resistor 24,
and an auxiliary switch including the PNP transistor 22, NPN transistor
23, junction capacitor 27, and resistor 25. Said two switches are
practically connected in parallel, and more precisely, it may be seen that
the bases of the two transistors 20 and 22 are connected together and to
the collectors of the transistors 21 and 23, while the emitters of the
transistors 20 and 22 are connected together and to the terminal of the
anode A, and the emitters of the transistors 21 and 23 are connected
together, to one terminal of the resistors 24 and 25 and to the cathode K.
Further, connected to the base of the transistor 23 is a gate electrode
G.sub.1, and a further gate electrode G.sub.2 is connected to the bases of
the transistors 20 and 22; the gate electrodes G.sub.1 or G.sub.2 being
alternatively connected to either the positive line lead or negative line
lead. Details on the device structure are shown in FIG. 7, where a first
layer 30 with conductivity of the P.sup.++ type and a second layer 31 with
conductivity of the N type may be seen; the layers 30 and 31 are shared by
both switches. Formed within the layer 31 are regions 32 and 33 with
conductivity of the P.sup.+ type, region 32 forming the collector of the
transistor 20 and base of the transistor 21 and region 33 forming
collector of the transistor 22 and base of the transistor 23. As shown in
said view, the junction region between the layer 33 and layer 31 has a
smaller area than the junction between the layer 32 and layer 31.
Consequently, the capacitance existing between the layer 33 and layer 31
is smaller (by a proportional amount to the ratio between the areas) than
the capacitance existing between the layers 32 and 31. Formed in the
layers 32 and 33 are then regions 34 and 35 with conductivity of the
N.sup.+ type; in particular, formed in the layer 32 are the emitter
regions 34 of the transistor 21, while in the layer 33, there are formed
the emitter regions 35 of the transistor 23. Partly overlapping the layers
32 and 34, respectively 33 and 35, are metal layers 36 and 37 defining the
resistors 24 and 25. Finally, a metal layer 38 is provided which
interconnects the metal layer 37 with the metal layer 36. The device
further comprises the electrodes A and K, forming the device anode and
cathode terminals, and the gate electrodes G.sub.1 and G.sub.2 which
govern the conductive state of the device. Either of said gate electrodes
is connected to a desired line lead, or possibly to the load itself. The
oxide layer 39 acts as an insulator.
The operation of the inventive device is apparent from the foregoing
description. In particular, it should be pointed out that, owing to the
junction between the layers 31 and 33 having a smaller area than the
junction between the layers 32 and 31, its equivalent capacitance C' will
be less, and accordingly, also lower will be the current generated as a
result of voltage variations across the device. Thus, the current flowing
through the equivalent resistance R', 25, will be a lower one, and since
the resistance R' is be designed with a higher ohmic value than R,
according to any known technique the behaviour of the device due to
voltage variations across it is maintained unchanged. Increase of
resistance R' allows turning-on of the device with a triggering current
from the electrodes G.sub.1,G.sub.2 lower than the lowest value attainable
heretofore with conventional devices. As evident to those skilled in the
art, triggering of the auxiliary device results then in the whole device
being triggered. Thus, the device can also operate at much lower currents
than in the past while retaining its characteristics unaltered as regards
voltage variations and triggering rate. At exhaustion of the transient
phenomenon which has turned the device on, and in particular shorted the
two leads whereto the anode and cathode terminals are connected, the
device is automatically turned off according to a similar pattern to that
shown in FIG. 3, thus again separating the two leads.
The invention as disclosed hereinabove is susceptible to many modifications
and changes. In particular, while it has been described with specific
reference to its application on telephone lines for the suppression of
normalized lightning, it could find application wherever a load is to be
protected against overvoltage and overcurrent, wherein through the use of
external circuit components, programmability of the values of the
occurring electric signals can be achieved. Furthermore, while the drawing
shows a device comprising a main switch and auxiliary switch, such
auxiliary switches may also be provided in a larger number, and, according
to a preferred embodiment, such auxiliary devices would be located outside
the main device, all around it, to ensure turning on from any sides.
* * * * *
|
|
 |
|
 |
Description |
|
