Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
2000-12-26
2003-08-12
Huynh, Kim (Department: 2182)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S111000, C361S112000, C174S256000
Reexamination Certificate
active
06606230
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention is related to Japanese Patent Applications Nos. 11-155464, 11-210499, 11-210500, 11-341476, 2000-199651, and 2000-232208, and claims priority under 35 USC §119 to Japanese Patent Application No. 2000-199651, filed on Jun. 30, 2000, and Japanese Patent Application No. 2000-232208, filed on Jul. 31, 2000, the entire contents of all of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chip-type surge absorber for protecting an electronic circuit, a communication device, and the like from a surge such as a lightning surge or noise, and also to a method of producing such a chip-type surge absorber.
2. Discussion of the Background
In order to protect electronic circuits and communication devices from internal or external surges or noise, various types of surge absorbers have been developed and are used effectively. In general, a surge absorber is disposed at a connection point at which an electronic device such as a telephone or a modem is connected to a communication line or is disposed in parallel to a circuit such as a CRT driver circuit or a device which is subject to an electrical impulse such as a lightning surge or electrostatic discharge so as to protect the circuit or the device from the surge by shunting the surge current. In some cases, a surge absorber is disposed in a ground circuit so that a surge current is shunted to ground thereby protecting circuits.
Among various types of surge absorbers, the surge absorber
140
of the type shown in
FIG. 13
is widely used because of its good surge response and long life.
The surge absorber of this type is made up of an absorber element enclosed together with a discharge gas within a glass tube
146
sealed at both ends with cap electrodes
143
each having a slag lead
145
wherein the absorber element is made up of a cylindrical-shaped insulator
141
covered with a conductive film
142
having a discharge gap
142
A formed at the center thereof.
The principle of operation of the surge absorber
140
is as follows. When a circuit is in a normal operating state, no current flows through the surge absorber
104
because of the existence of the discharge gap
142
A formed at the center of the conductive film
142
. However, if a surge such as an indirect lightning stroke is input to the circuit, a voltage depending upon the surge is applied across the discharge gap
142
A. If this surge voltage is equal to or greater than the discharge start voltage of the surge absorber
140
, the discharge gap
142
A has an electrical breakdown, and a glow discharge occurs in the discharge gap
142
A. If the surge continues further, the temperature of the discharge gas becomes high, and the discharge gas is ionized. Thus, the glow discharge grows into an arc discharge which occurs between the cap electrodes
143
. As a result, the surge absorber becomes possible to shunt a greater surge current.
As a result, no surge is applied to the circuit or device which should be protected, and thus the circuit or device is prevented from being damaged. The surge absorber is not damaged fatally only by one discharge, but, in many cases, the surge absorber can withstand a large number of impacts of surges such as about 1000 impacts. In this respect, surge absorbers are very different from fuses which are broken by a single impact of a surge and which must be replaced whenever being broken.
However, the structure of the surge absorber
140
does not allow it to be surface-mounted on a circuit board, because lead wires are necessary for connection with an external circuit. Another problem of the surge absorber
140
is that it is difficult to reduce the size because it is necessary to enclose the cylindrical-shaped insulator
141
within the glass tube
146
.
Therefore, the conventional surge absorber
140
does not meet the requirements for electronic circuits with small sizes and high densities.
One technique to overcome the above problems while maintaining the performance of the surge absorber has been proposed in Japanese Unexamined Patent Application Publication No. 8-64336.
FIG. 14
illustrates a chip-type surge absorber
150
according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-64336 cited above. The chip-type surge absorber
150
includes an insulating substrate
151
in the shape of a rectangular parallelepiped, discharge electrodes
152
formed on the surface of the insulating substrate
151
, a discharge gap
153
formed between the discharge electrodes
152
, and a pair of terminal electrodes
154
which are electrically connected to the respective discharge electrodes
152
and which are disposed on respective ends of the insulating substrate
151
. Furthermore, a hermetic cap
156
is adhesively bonded to the insulating substrate
151
so as to form a hermetically sealed cavity
155
filled with a discharge gas in which the discharge gap
153
and a part of each discharge electrode
152
are enclosed.
The chip-type surge absorber
150
can be surface-mounted on a circuit board by electrically connecting the terminal electrodes to an external circuit via solder. In this chip-type surge absorber
150
, the glass tube and the cap electrodes for the purpose of encapsulation are not required, and thus it is possible to reduce the size. The principle of operation is basically the same as that of the surge absorber
140
, and thus the surge absorber
150
has similar performance to that of the surge absorber
140
.
Although the chip-type surge absorber
150
has the advantage that it can be surface-mounted on a circuit board, the small volume of the hermetically sealed cavity
155
in which a discharge occurs results in small surge resistance.
In the chip-type surge absorber
150
, the electrical connection between the discharge electrodes
152
and the corresponding terminal electrodes
154
is realized by forming the discharge electrodes
152
so as to extend to both ends of the insulating substrate
151
and disposing the terminal electrodes
154
directly upon the discharge electrodes
152
. To this end, it is necessary that the end faces of the hermetic cap
156
should be at locations shifted inwardly from the ends of the insulating substrate
151
so as to create areas on the insulating substrate
151
where the discharge electrodes
152
are exposed to the outside. However, this results in a reduction in the volume of the hermetically sealed cavity
155
. In the chip-type surge absorber
150
shown in
FIG. 14
, because the surge resistance is proportional to the volume of the hermetically sealed cavity
155
, the structure of the chip-type surge absorber
150
results in a reduction in the surge resistance. The above problem may be solved by increasing the surface area of the insulating substrate
151
. However, in this case, the resultant penalty is an increase in the mounting area.
The reason why the chip-type surge absorber
150
has the above-described structure is that if the terminal electrodes
154
are formed on the end faces of the insulating substrate
151
, it is not assured that the terminal electrodes
154
are electrically connected to the discharge electrodes
154
which are formed, to achieve a long life, so as to have a thickness as small as 1 &mgr;m. In particular, in the case where there area plural pairs of discharge electrodes
152
, it is very difficult to achieve electrically connection for all pairs of discharge electrodes
152
.
In order to trigger the discharge, it is required that electrons which start the discharge be emitted by a high voltage which is induced across the discharge gap by an surge applied to the surge absorber. However, in the chip-type surge absorber
150
shown in
FIG. 14
in which there is only one pair of discharge electrodes
152
and there is only one discharge gap
153
, there is no particular point where an electric field is concentrated, and thus a delay occurs in starting of the discharge.
Another probl
Harada Kouichirou
Hujiwara Kazutaka
Inaba Hitoshi
Kitahara Naoto
Nakamoto Takahiro
Huynh Kim
Mitsubishi Materials Corporation
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