Apparatus for protecting against overvoltage

An apparatus for protecting against overvoltage includes a semiconductor switching element and a trigger element. The semiconductor switching element is formed of a PNPN junction including a plurality of PN junctions and the trigger element has a characteristic similar to that of a PN junction Zener diode. The apparatus for protecting against overvoltage is normally nonconductive; however, once an overvoltage not lower than a defined level is applied, the trigger element attains a state so as to have a gate current flow from a gate electrode thereof to the semiconductor switching element. In response to this, the semiconductor switching element causes a so-called thyristor phenomenon, thereby protecting a load connected in parallel to the apparatus for protecting against overvoltage, i.e., an object to be protected, from being provided with the overvoltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to apparatuses for protecting 
against overvoltage and, more particularly, to apparatuses for protecting 
electrical apparatuses, circuits and the like driven by direct current 
power sources against detrimental overvoltage. 
2. Description of the Background Art 
A fuse is one representative of conventional apparatuses for protecting 
electrical apparatuses and circuits against overcurrents applied thereto. 
The fuse, which is connected in series to a load, is melted by Joule heat 
generated when an overcurrent higher than a rated current flows through a 
circuit, and thus is employed to cut energization applied from a power 
source. In order to achieve this purpose, the fuse has a rated current 
defined therein, so that the fuse is to be selected in accordance with its 
own purpose for practical use. 
The fuse is less expensive with its simple structure, and thus has been 
widely used as a representative of safety devices for electrical 
apparatuses. However, since a fuse can not be reused once it has been 
melted, there has been a need for a spare fuse. 
Meanwhile, other protective apparatuses are proposed such as a circuit 
breaker and a protector. These proposed apparatuses can be used 
repetitively if a cause of the overcurrent is removed; however, they are 
large-scaled with complicated structures due to the requirement of a 
switch disengaging mechanism which operates in response to the 
overcurrent, and thus are expensive and require a large space to be 
installed. Therefore, they cannot be employed as easily as the fuse. 
Moreover, the circuit breaker and the fuse both employ energy caused by the 
overcurrent, and require much time and energy of a certain level or more 
to achieve their own purposes. 
Meanwhile, as in the case of a computer, circuits of low operating voltage 
and small operating, i.e., decreased power requirements have been 
developed since the appearance of semiconductor elements. An electronic 
apparatus employing such a circuit should be protected against overvoltage 
rather than overcurrent. Therefore, it often occurs, for example, that an 
overvoltage V.sub.S which has a pulse waveform generally called a spike, 
which is instantaneously applied at the level exceeding the level of a 
rated voltage V.sub.CC of a load (e.g., 5V), as shown in FIG. 5, results 
in an abnormal operation of the load and destruction of the circuit. In 
such a case, energy generated instantaneously is relatively small, so that 
a power consumption type protective apparatus, such as the described fuse 
or the breaker, cannot cope with the applied overvoltage. A Zener diode 
and a varistor serve as apparatuses for protecting against overvoltage 
which cope with such a problem. However, in the case of employing the 
Zener diode or the varistor as the apparatus for protecting the electrical 
apparatus and circuit against overvoltage, the following disadvantages 
occur, and thus it is impossible to realize a sufficiently quick response 
to an applied overvoltage. 
FIG. 6 shows one example of an electrical circuit diagram employing a Zener 
diode as the apparatus for protecting against overvoltage. 
As shown in this figure, V.sub.R is a variable voltage, R is a resistor, 
Z.sub.D is a Zener diode, and L is a load connected in parallel to the 
Zener diode Z.sub.D. The Zener diode Z.sub.D, which is also called a 
constant voltage diode, has its both ends supplied with a constant voltage 
so as to protect the load L against a surge. However, when a voltage equal 
to or higher than a Zener voltage is applied to the load L by a surge 
current due to an overvoltage applied by the voltage V.sub.R (for example, 
an overvoltage V.sub.S shown in FIG. 5), the load L is caused to 
malfunction. 
Furthermore, there is another problem that due to a current flowing in the 
state that the overvoltage is applied, the Zener diode Z.sub.D has its own 
heat value increased and thus burns out. In addition, there also arises a 
problem that since the overvoltage is kept applied to the load L connected 
in the period that the overvoltage is applied, an apparatus corresponding 
to the load L causes malfunction and thus a burnout. 
Further, in the case that the varistor is substituted for the Zener diode 
Z.sub.D shown in FIG. 6, the same disadvantage as in the Zener diode is 
caused with respect to the applied overvoltage. Moreover, in the case of 
employing the varistor as the apparatus for protecting against 
overvoltage, it is impossible to form the varistor with other circuit 
elements as a semiconductor integrated circuit. In detail, a manufacturing 
method of the varistor is complicated because various kinds of oxides are 
sintered in a reduction atmosphere in the manufacturing process, so that 
it is impossible to integrate the varistor on the semiconductor integrated 
circuit. 
Therefore, implementation of a small and inexpensive apparatus for 
protecting against overvoltage has been desirable, which can also be used 
as easily as a fuse, Zener diode, the varistor or the like, and has a 
sufficiently quick response even to an overvoltage applied in such a short 
period as described above. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus for 
protecting against overvoltage, having a sufficiently quick response to an 
overvoltage applied in an extremely short period. 
It is another object of the present invention to provide an apparatus for 
protecting against overvoltage, which can be used as easily as a fuse, 
Zener diode, a varistor or the like. 
It is a further object of the present invention to provide a small-scaled 
apparatus for protecting against overvoltage, which can be manufactured at 
a low cost. 
The apparatus for protecting against overvoltage according to the present 
invention, comprising a semiconductor switching element formed of a 
plurality of stages of PN junctions, and a semiconductor element for 
controlling conduction of the semiconductor switching element at a 
predetermined voltage level, detects the level of a supply voltage 
exceeding the predetermined level by the semiconductor element connected 
to one electrode of a power source line, and instantaneously renders the 
semiconductor switching element connected to the semiconductor element 
conductive. This conduction causes a short-circuit between both electrodes 
of the power source line, and thus this short-circuit protects a load from 
being provided with an overvoltage not lower than the predetermined level. 
Since the apparatus for protecting against overvoltage according to the 
present invention is manufactured through the same process as the other 
semiconductor elements, the apparatus is extremely small-scaled, also 
acceptable in a container for a conventional fuse contained in a glass 
tube, and can easily be connected to the power source line of an 
electrical apparatus. 
The apparatus for protecting against overvoltage according to the present 
invention is normally in a non-conducting state and is rendered conductive 
by application of the overvoltage; however, since the apparatus recovers 
from the conducting state to the non-conducting state due to the fact that 
the voltage applied to the apparatus is thereafter interrupted, the 
apparatus can easily be used repetitively. 
As described above, the present invention is provided for implementing by a 
simple means the apparatus for protecting against overvoltage, having a 
quick response even to an overvoltage applied in a short period through 
the power source line. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention. However, it should be understood 
that the detailed description and specific examples, while indicating 
preferred embodiments of the present invention, are given by way of 
illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiment of the present invention will now be described in detail 
with reference to the figures. 
FIG. 1 is a schematic block diagram illustrating the configuration of an 
apparatus for protecting against overvoltage according to one embodiment 
of the present invention. The apparatus 1 for protecting against 
overvoltage in FIG. 1 comprises a semiconductor switching element 1a and a 
trigger element 1b. The semiconductor switching element 1a is formed of 
PNPN junction including a plurality of PN junctions and comprises a first 
electrode P1, a second electrode N1, a third electrode P2 and a fourth 
electrode N2. The trigger element 1b has a characteristic similar to a PN 
junction Zener diode and comprises a cathode N3 and an anode P3. The 
semiconductor switching element 1a and the trigger element 1b have the 
respective third electrode P2 and anode P3 connected to each other to be 
integrally formed as an integrated circuit. 
The semiconductor switching element 1a causes a so-called thyristor 
phenomenon; that is, a forward bias voltage is first applied to the 
semiconductor switching element 1a, namely, a voltage V.sub.cc is applied 
from the first electrode P1 as a positive polarity to the fourth electrode 
N2 as a negative polarity. Further, a bias voltage V.sub.g is applied 
forwardly from the third electrode P2 as the positive polarity to the 
fourth electrode N2 as the negative polarity. A gate current I.sub.g then 
flows in the direction of the fourth electrode N2 with the third electrode 
P2 used as a gate electrode. As a result, a portion between the first 
electrode P1 and the fourth electrode N2 becomes conductive (turn-on). 
The trigger element 1b has a characteristic similar to that of a PN 
junction Zener diode. When a Zener voltage V.sub.z, which is a 
predetermined voltage level, is reversely applied to the trigger element 
1b, a portion between the cathode N3 and the anode P3 break down, and the 
gate current I.sub.g flows from the cathode N3 through the anode P3 so as 
to make the third electrode P2 of the semiconductor switching element 1a 
of positive polarity. Accordingly, the portion between the first electrode 
P1 and the fourth electrode N2 of the semiconductor switching element 1a 
experiences the thyristor phenomenon and becomes conductive, as described 
above. This conduction is normally achieved in an extreme instant of 
several nS (nanoseconds) after generation of the Zener voltage V.sub.z. 
Therefore, even if the voltage V.sub.s, which is an instant pulse waveform 
as shown in FIG. 5, is applied to the apparatus 1 for protecting against 
overvoltage, the semiconductor switching element 1a quickly becomes 
conductive in response to the applied voltage V.sub.s, and is kept 
conductive while the voltage V.sub.cc is applied thereto. 
An integral structure of the semiconductor switching element 1a and the 
trigger element 1b is implemented by being integrated on a substrate such 
as of a silicon wafer by such manufacturing methods as alloying, 
diffusion, an epitaxial process or the like. The Zener voltage V.sub.z of 
the trigger element 1b is set to be a desired value through the process of 
the integration of the integral structure. 
A description will be given on the integral structure formed of integration 
of the semiconductor switching element 1a and the trigger element 1b, with 
reference to FIG. 2. FIG. 2 is a schematic diagram illustrating a 
sectional structure of an integrated circuit of the apparatus for 
protecting against overvoltage according to one embodiment of the present 
invention. 
As shown in this figure, the integrated circuit of the apparatus for 
protecting against overvoltage comprises an n.sup.- type substrate 2a, a 
p.sup.+ type impurity layer 2b, a p.sup.- type impurity layer 2c and an 
n.sup.+ type impurity layer 2d. 
A description will be given in regard to the correspondence of the 
respective electrodes of the semiconductor switching element 1a and of the 
trigger element 1b to the respective impurity layers shown in FIG. 2. 
The cathode N3 corresponds to the n.sup.+ type impurity layer 2d, the anode 
P3 and the third electrode P2 correspond to the p.sup.- type impurity 
layer 2c, the first electrode P1 corresponds to the p.sup.+ type impurity 
layer 2b, the second electrode N1 corresponds to the n.sup.- type 
substrate 2a in contact with the p.sup.+ type impurity layer 2b and to the 
p.sup.- type impurity layer 2c, and the fourth electrode N2 corresponds to 
the n.sup.- type substrate 2a in contact with the p.sup.- type impurity 
layer 2c. The Zener voltage V.sub.z of the trigger element 1b is 
controlled by the impurity concentration of the PN junction with the lower 
concentration, i.e., the impurity concentration of the p.sup.- type 
impurity layer 2c. 
As mentioned above, the apparatus 1 for protecting against overvoltage 
according to the present invention is manufactured through the same 
process as the other semiconductor elements, so that it becomes extremely 
small-scaled. In addition, since only two external connection terminals 
are connected to the apparatus as will be described later, the apparatus 
is also acceptable in a container for a conventional fuse contained in a 
glass tube and can be easily connected to a power source line of an 
electrical apparatus. 
FIG. 3 is an electric circuit diagram illustrating the apparatus for 
protecting against overvoltage in use according to one embodiment of the 
present invention. As shown in FIG. 3, the electric circuit comprises the 
apparatus 1 for protecting against overvoltage, a commercial power source 
AC, a power source portion PS, a load L, a fuse F, and power source lines 
l1 and l2. The above devices are interconnected via the power source lines 
l1 and l2. The apparatus 1 for protecting against overvoltage is connected 
in parallel to the load L, and the fuse F is connected in series to the 
described parallel circuit. Further, an alternating voltage supplied from 
the commercial power source AC to the circuit is applied to the power 
source portion PS to be converted into a direct current voltage, and a 
rated voltage V.sub.cc (e.g. 5V) of the load L is then provided with the 
power source line l1 used as the positive polarity and the other power 
source line l2 as the negative polarity. The rated voltage V.sub.cc 
derived from the power source portion PS is applied to a series circuit 
formed of the load L and the fuse F serving as means for preventing an 
overcurrent, so that the load L is supplied with power. The load L is, for 
example, a load circuit formed such as of a CPU (a central processing 
unit) and a memory. 
Referring to this figure, the protective apparatus 1 has the first 
electrode P1 of the semiconductor switching element 1a and the cathode 
electrode N3 of the trigger element 1b connected in common to the power 
source line l1, and the fourth electrode N2 of the switching element 1a 
connected to the other power source line l2. Accordingly, the 
semiconductor switching element 1a is forward-biased between the power 
source lines l1 and l2, while the trigger element 1b is reverse-biased. 
The Zener voltage V.sub.z of the trigger element 1b in the apparatus 1 for 
protecting against overvoltage is set to be V.sub.z &gt;V.sub.cc, and it is 
assumed that, for example, the value V.sub.z =6V. Furthermore, the 
switching element 1a is nonconductive in the normal state, and it is 
assumed that the load L is provided with the rated voltage V.sub.cc via 
the fuse F. 
When an overvoltage V.sub.s, as shown in FIG. 5, exceeding the rated 
voltage V.sub.cc is applied in the direction of the arrow A between the 
power source lines 11 and l2, and thus the relationship between the 
overvoltage V.sub.s and the Zener voltage V.sub.z become V.sub.s &gt;V.sub.z, 
the trigger element 1b instantaneously breaks down and the gate current 
I.sub.g flows in the third electrode P2, or the gate electrode of the 
semiconductor switching element 1a, so that the switching element 1a 
becomes conductive. Thus, the both ends of the load L are short-circuited 
via the apparatus 1 for protecting against overvoltage, and at the same 
time, a short-circuit current I.sub.s flows in the apparatus 1 through the 
fuse F. The fuse F is melted by the short-circuit current I.sub.s to 
prevent application of the supply voltage V.sub.cc. Accordingly, the load 
L is prevented from being provided with the overvoltage V.sub.s, so that 
the load L is protected against the overvoltage V.sub.s. After the fuse F 
is melted, a lowering of the supply voltage V.sub.cc causes the apparatus 
1 for protecting against overvoltage to recover from the conducting state 
to the non-conducting state. 
Since the fuse F is melted within an instantaneous allowable time period 
even if the apparatus 1 becomes conductive to have the short-circuit 
current I.sub.s flow therethrough, the apparatus 1 for protecting against 
overvoltage is not destroyed due to the short-circuit current I.sub.s, but 
is available for repetitive use. 
The operation of the apparatus 1 for protecting against overvoltage, having 
the load L protected against overvoltage will be described in further 
detail with reference to FIGS. 4A and 4B. 
FIG. 4A is a graph for illustrating a voltage level to be applied to the 
load in the case that the overvoltage is applied to the electric circuit 
shown in FIG. 6. 
FIG. 4B is a graph for illustrating a voltage level to be applied to the 
load in the case that the overvoltage is applied to the electric circuit 
shown in FIG. 3. 
In either of FIGS. 4A or 4B, the ordinate shows the voltage level V to be 
applied to the load, while the abscissa shows the time T. 
Conventionally, in the case of employing a Zener diode Z.sub.d as the 
apparatus for protecting the load L against overvoltage, as shown in FIG. 
6, when an overvoltage not lower than the Zener voltage V.sub.z, as shown 
in FIG. 4A is applied from a variable voltage V.sub.R, there flows a surge 
current due to this overvoltage. Therefore, the connected load L is 
provided with the overvoltage not lower than the Zener voltage V.sub.z, so 
that the load L cannot be protected against the overvoltage. 
Meanwhile, in the electric circuit shown in FIG. 3 according to the present 
invention, when the voltage not lower than the Zener voltage V.sub.z as 
shown in FIG. 4A is applied, the apparatus 1 for protecting against 
overvoltage causes a voltage to be applied to the load L to attain the 
voltage level shown in FIG. 4B, so that the load L is protected against 
the application of the overvoltage. In detail, the voltage level to be 
applied to the circuit becomes instantaneously high, and the Zener voltage 
V.sub.z is applied to the trigger element 1b at the time T1, so that the 
gate current I.sub.g starts flowing from the trigger element 1b to the 
semiconductor switching element 1a. In response to this, the semiconductor 
switching element 1a changes from the OFF state to the ON state. A voltage 
developing across the anode and cathode of the semiconductor switching 
element 1a attains the value .perspectiveto.0V at the time T2 after an 
extremely short time period required for this change, i.e., a turn-on 
time. That is, when the overvoltage is applied, the voltage to be applied 
to the load L attains the value .perspectiveto.0V, so that the apparatus 1 
for protecting against overvoltage is able to protect the load L against 
being applied by the overvoltage. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.