Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2002-10-07
2004-10-19
Le, Dung A. (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S232000, C438S048000
Reexamination Certificate
active
06806519
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface mountable device, and more particularly, to a surface mountable device with a three-dimensional structure.
2. Description of Related Art
FIG. 1
depicts a conventional surface mountable device
10
, comprising an upper electrode
11
, a lower electrode
12
and a current-sensitive element
13
. Nowadays, the current-sensitive element
13
is usually formed of a conductive material having Positive Temperature Coefficient (PTC). The resistance of the PTC conductive material can be kept extremely low at normal operation due to its low sensitivity to temperature variance so that the circuit can operate normally. However, if the over-current or over-temperature effect occurs, the resistance will immediately be increased thousands of times to a higher resistance state (e.g. above 10
4
ohm.) Therefore, the over current will be reversely eliminated and the objective to protect the circuit device can be achieved.
Generally, the normal resistance value of the current-sensitive element
13
by follows the conventional formula:
R
=
ρ
×
l
A
,
in which &rgr; is the conductive coefficient of the PTC conductive composition, l is the length and A is the area of the current-sensitive element
13
. Since the size of the printed circuit board of the portable electronic product decreases more and more the footprint of the surface mountable device mounted on the circuit board also needs to be reduced comparatively. Thus, according to the above formula, the normal resistance of the surface mountable device will be increased.
In addition, since the conventional surface mountable device is a planar structure, it will occupy a lot of surface area of the circuit board when the convention surface mountable device is mounted. To solve this space limitation problem, Raychem Corp. discloses a vertical surface mountable device
20
(Model TS250), comprising a conventional surface mountable device
10
, a covering
25
, an insulating material
24
, a first conductive element
26
and a second conductive element
27
, as shown in FIG.
2
. The covering
25
is used to cover the conventional surface mountable device
10
in which the surface mountable device
10
is vertically placed in the covering
25
. The first conductive element
26
and the second conductive element
27
are disposed on the bottom surface of the vertical surface mountable device
20
and contacts the first electrode
11
and the second electrode
12
of the conventional surface mountable device
10
respectively to be mounted on the circuit board (not shown.) Further, more an insulating material
24
is filled into the vacant space between the covering
25
and the two electrodes
11
,
12
. Although the mounted footprint on the surface mountable device
20
by the surface mountable device can be reduced by the above structure, the current leakage occurs more easily because the first conductive element
26
and the second conductive element
27
are disposed too closely. Because the structure is covered with the covering
25
, filling the insulating material
24
is difficult and dissipating the heat is also difficult. Moreover, the voltage endurance of the above structure has an upper limit (about 60V) which is not suitable for the products requiring high voltage endurance.
SUMMARY OF THE INVENTION
A major objective of the present invention is to provide a surface mountable device, wherein the shape of the device can be varied according to the available space of the circuit board so that the requirement of a light, thin and small circuit board can be met.
A second objective of the present invention is to provide a surface mountable device, wherein the shape of the device is a three-dimensional structure, different from the two-dimensional planar structure of the conventional surface mountable device, so as to increase the area of the PTC conductive material and reduce the normal resistance and increase the voltage endurance up to above 200V. In other words, the surface mountable device of the present invention can be applied to products requiring high voltage endurance.
A third objective of the present invention is to provide a surface mountable device, wherein the mounted direction can be varied according to the requirement. Thus, the surface space of the circuit board can be used more efficiently.
A fourth objective of the present invention is to provide a surface mountable device, which can be directly mounted onto the circuit board and be easily filled with insulating material; therefore short circuit will not occur and the heat inside the device is dissipated easily.
In order to achieve the above objectives and to avoid the disadvantages of the prior art, the present invention discloses a surface mountable device comprising a current-sensitive element and two electrodes in which the current-sensitive element is composed of a PTC conductive composite material having a positive temperature coefficient, comprising at least one polymer and a conductive filler. The present invention is characterized in that the current-sensitive element is a three-dimensional bent structure so that the shape, length and height of the element can be varied according to the requirement of mounting space and resistance and thus the surface space of the circuit board can be used more efficiently. Moreover, the area of the current-sensitive element of the present invention is larger than that of the conventional surface mountable device. Consequently, the normal resistance of the surface mountable device of the present invention is smaller than that of the conventional surface mountable device and the voltage endurance of the surface mountable device of the present invention is increased accordingly.
The foregoing and other objectives and advantages of the invention, as well as the manner in which the same are accomplished, will become clearer based on the following detailed description taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3219480 (1965-11-01), Girard
patent: 4882466 (1989-11-01), Friel
patent: 5331390 (1994-07-01), Kimura et al.
Chu Edward Fu-Hua
Ma Yun-Ching
Wang David Shau-Chew
Le Dung A.
Polytronics Technology Corporation
Seyfarth Shaw LLP
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