Electric heating – Heating devices – With current connection and/or disconnection means
Reexamination Certificate
2000-03-07
2002-02-05
Paschall, Mark (Department: 3742)
Electric heating
Heating devices
With current connection and/or disconnection means
C219S481000, C219S548000
Reexamination Certificate
active
06344633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a protective device in which a heating element is energized during a malfunction, whereby the heating element is heated and a low-melting metal element is fused.
2. Related Art of the Invention
The conventional current fuses in which low-melting metal element composed of lead, tin, antimony, or the like are fused by overcurrent are widely known as protective devices for cutting off such overcurrent. Protective devices comprising heating elements and low-melting metal elements are also known as protective devices capable of preventing not only overcurrents but also overvoltages (Japanese Patent No. 2,790,433; Japanese Patent Application Laid-Open No. 8-161990, etc.).
FIG. 9
is a circuit diagram of an overvoltage prevention device featuring such a protective device
1
p.
FIG.
10
A and
FIG. 10B
are respectively a plane view and a cross sectional view of the protective device
1
p.
The protective device
1
p
is obtained by the sequential stacking of the following elements on a substrate
2
: a heating element
3
(formed by applying or otherwise spreading a resistance paste), an insulating layer
4
, and a low-melting metal element
5
composed of a fuse material. In the drawing, the numerals
6
a
and
6
b
are electrodes for the heating element, and the numerals
7
a
and
7
b
are electrodes for the low-melting metal element. In addition, the numeral
8
is an inside seal composed of solid flux or the like and designed to seal the low-melting metal element
5
in order to prevent the surface of this low-melting metal element
5
from being oxidized; and the numeral
9
is an outside seal composed of a material whose melting point or softening point is higher than that of the low-melting metal element
5
and designed not to allow molten material to flow outside the device during the fusion of the low-melting metal element
5
.
In the overvoltage prevention device shown in FIG.
9
and obtained using the protective device
1
p,
the electrode terminals of, for example, a lithium ion battery or other device to be protected are connected to terminals A
1
and A
2
; and the electrode terminals of, for example, a charger or other device connected to the device to be protected are connected to terminals B
1
and B
2
. With this overvoltage prevention device, when the lithium ion battery is charged and a reverse voltage higher than the breakdown voltage is applied to a Zener diode D, base current ib flows in an abrupt manner, substantial collector current ic greater than the base current ib is caused to flow across the heating element
3
, and the heating element
3
is heated. This heat is transmitted to the low-melting metal element
5
on the heating element
3
, the low-melting metal element
5
is fused, and the application of overvoltage to the terminals A
1
and A
2
is prevented.
With the overvoltage prevention device in
FIG. 9
, however, current continues to flow through the heating element
3
even after the low-melting metal element
5
has been fused by the overvoltage. An overvoltage prevention device whose circuitry is shown in
FIG. 11
is also known. FIG.
12
A and
FIG. 12B
are respectively a plane view and a cross sectional view of the protective device
1
q
used in this overvoltage prevention device. In this protective device
1
q,
two heating elements
3
are connected by means of an intermediate electrode
6
c,
and a low-melting metal element
5
is disposed thereon so as to allow an insulating layer
4
to intervene therebetween.
In the overvoltage prevention device shown in
FIG. 11
, the heat generated by the heating elements
3
fuses the low-melting metal element
5
at two locations (
5
a
and
5
b
), completely cutting off electric power to the heating elements
3
following this type of fusion.
Also known is a protective device
1
r
in which the arrangement in which a heating element
3
and low-melting metal element
5
are stacked so as not to allow an insulating layer
4
to intervene therebetween, is replaced by an arrangement in which a heating element
3
and a low-melting metal element
5
are arranged in a planar configuration on a substrate
2
, as shown in FIG.
13
. In the drawing, the numerals
6
d,
6
e,
6
f,
and
6
g
are electrodes, and the numeral
8
is an inside seal consisting of a flux coating film (Japanese Patent Application Laid-open Nos. 10-116549 and 10-116550).
In situations such as those encountered with the protective device
1
p
or
1
q
shown in
FIGS. 10A and 10B
or
FIGS. 12A and 12B
, stacking the heating element
3
and the low-melting metal element
5
so as to allow the insulating layer
4
to intervene therebetween makes it difficult to reduce the operating time (that is, the time from the energizing of the heating element
3
to the fusing of the low-melting metal element
5
) because the heat-up of the low-melting metal element
5
is slowed down by the delay in heat transfer due to the presence of the insulating layer
4
during the heating of the heating element
3
. When glass components are used for the insulating layer
4
, the insulating layer
4
flows during heating, creating a risk that fusion characteristics will be adversely affected.
In a structure in which a heating element
3
and a low-melting metal element
5
are arranged in a planar configuration on a substrate
2
(as in the protective device
1
r
in FIG.
13
), the planar configuration of the elements cannot be miniaturized because separate planar spaces are required for arranging the heating element
3
and the low-melting metal element
5
. Consequently, the protective device
1
r
is larger than the above-described protective device
1
p
or
1
q,
which are obtained by stacking the heating element
3
and the low-melting metal element
5
so as to allow the insulating layer
4
to intervene therebetween.
Merely reducing the size of the protective device
1
r
in this case will result in a smaller surface area for the electrodes, making it impossible to fuse the low-melting metal element
5
because of low rated current or insufficient heat generation.
Another feature of the protective device
1
r
is that the heat from the heating element
3
during heating is transferred via the electrode
6
g
and the substrate
2
, slowing down the heat-up of the low-melting metal element
5
and hence increasing the operating time. Mounting the protective device
1
r
on the base circuit substrate with the aid of solder in order in an attempt to enhance the thermal conductivity of the substrate
2
(and thus to eliminate the delay in the operating time) is disadvantageous because the mounting solder melts before the fusion of the low-melting metal element
5
, and the protective device
1
r
separates from the base circuit substrate. In addition, lowering the melting point of the low-melting metal element
5
in order to eliminate the delay in the operating time has an adverse effect on the reflow resistance of the protective device
1
r
during mounting, makes it impossible to use automatic mounting, and turns the protective device
1
r
into a hand-mounted component.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the shortcomings of prior art and to make it possible to miniaturize the devices and to reduce the operating time without reducing the rated current in a protective device in which a low-melting metal element is fused by the energizing of a heating element.
The inventor perfected the present invention upon discovering that to cause fusion in a protective device in which a heating element and a low-melting metal element are formed on a substrate, and the low-melting metal element is fused by the heat generated by the heating element, it is important that adequate space be provided for the low-melting metal element to wet the surface and to spread thereover during melting, resulting in fusion; that the fusion of the low-melting metal element can be facilitated by making it easier for the molten low-melting metal elem
Oliff & Berridg,e PLC
Paschall Mark
Sony Chemicals Corp.
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