PTC fuse including external heat source

Details PTC fuse including external heat source PTC fuse including external heat source

1999-12-10

2002-12-03

Vortman, Anatoly (Department: 2835)

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

Fusible element actuated

C337S153000, C337S182000, C337S183000

Reexamination Certificate

active

06489879

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to fuse assemblies for protecting against electrical surges and/or overcharging. More particularly, an improved externally heated positive temperature coefficient (PTC) fuse assembly is described.
BACKGROUND OF THE INVENTION
In a number of electrical components, it is desirable to provide fuses to protect against electrical surges. There are currently a number of commercially available fuse styles. One available type of fuse is known as a positive temperature coefficient fuse. A representative positive temperature coefficient (PTC) fuse is illustrated in FIG.
1
A. The representative PTC fuse
50
has a top plate (top contact pad)
52
and a bottom plate (bottom contact pad)
54
that are separated by a positive temperature coefficient resistor (PTC) material
56
. The PTC fuse
50
operates such that, at a threshold temperature, the electrical resistance of the PTC material
56
increases dramatically.
As seen in the battery protection diagram
60
of
FIG. 1B
, the PTC fuse
50
may be used to protect a battery
75
during charging with a voltage source
77
. By way of example, the battery
75
may be included in a cellular telephone and the PTC fuse
50
may be implemented to protect the battery
75
from excessive current as a result of an individual using an improper voltage source, e.g. an improper power pack.
The contact pads
52
and
54
each have tabs
57
,
59
which are bent down to form sidewalls that define a cavity
61
beneath the bottom contact pad
54
. Solder posts
67
are formed on the tips of bent down tabs
57
,
59
. With this arrangement, the fuse can be mounted on a printed circuit board
69
(or other appropriate substrate) by soldering the solder posts
67
to appropriate landings
71
on the printed circuit board
69
.
In operation, the PTC material
56
often does not heat up fast enough to protect the battery
75
. As illustrated in
FIG. 1C
, in order to expedite heating of the PTC material
56
, a die
64
may be attached to the bottom surface
55
of the bottom contact pad
54
to externally heat the PTC material
56
. The die
64
may include a flip chip packaged zener diode, for example, which generates heat once a particular voltage has been reached. The die
64
also includes solder bumps
66
to mount the die
64
on the board
69
by soldering the solder bumps
66
to appropriate landings
79
.
For the battery protection diagram
80
of
FIG. 1D
, the threshold voltage for the packaged zener diode of the die
64
is then set relative to the voltage of the battery
75
. When the threshold voltage of the packaged zener diode is exceeded, it begins to draw current and externally heat the PTC material
56
. The die
64
material (i.e., silicon) typically heats up more rapidly than the PTC fuse
50
, and with the contact area between the die
64
and the bottom contact pad
54
, the PTC material
56
reaches its threshold temperature faster. In addition, when the threshold voltage of the packaged zener diode is exceeded, the diode conducts at lesser resistance than the battery
75
and PTC fuse
50
. As a result, the current is shunted away from the battery
75
, further protecting the battery
77
and increasing the heat generation in the die
64
.
In some cases, upon heating of the die
64
, the solder bumps
66
may begin to melt. In other cases, upon heating of the die
64
, the die
64
thermally expands and the solder bumps
66
crack. In either case, contact is lost, the die
64
is no longer functional and the protection circuit is unreliable.
In view of the foregoing, it should be apparent that an improved PTC device would be desirable.
SUMMARY OF THE INVENTION
To achieve the forgoing and other objects and in accordance with the purpose of the invention, an improved fuse assembly for protecting against electrical surges is described. The fuse assembly includes a PTC fuse, a die that serves as an external heat source for the PTC fuse and a lead frame based structure that facilitates electrical connection of the fuse assembly to a suitable substrate. The PTC fuse includes a PTC material which is normally conductive, but upon reaching a predetermined temperature, becomes non-conductive or substantially less conductive. To expedite heating of the PTC material, the fuse assembly includes a die package in thermal communication with the PTC material. The die package includes a die which generates heat once a threshold has been reached. To mount the die package to a substrate, the die package includes leads. The leads may be bent and surface mounted to the substrate using conventional techniques. The leads are sufficiently flexible such that the heat generated by the die does not compromise performance of the PTC protection circuit, i.e. by thermal expansion of the die package or parts thereof.
In a described embodiment, the improved PTC fuse assembly is used to protect a battery. In this embodiment, the die is placed in electrical parallel to the terminals of the battery and PTC fuse such that when a predetermined threshold voltage is reached, the current shunts through the die. In one embodiment, the die consists of one or more zener diodes which heat up after the predetermined threshold voltage has been reached. Correspondingly, the PTC material is externally heated, thereby activating the PTC fuse, which further prevents undesired current flow to the battery.


REFERENCES:
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patent: 4780598 (1988-10-01), Fahey et al.
patent: 4864624 (1989-09-01), Tichy
patent: 4891563 (1990-01-01), Ganser et al.
patent: 5712610 (1998-01-01), Takeichi et al.
patent: 5831508 (1998-11-01), Ikeda
patent: 5864458 (1999-01-01), Duffy et al.
patent: 5998051 (1999-12-01), Poirier et al.
patent: 2001-6518 (2001-01-01), None
The Illustrated Dictionary of Electronics, Sixth Edition, Stan Gibilisco, p. 176.*
Bourns, Inc., “Bourns Multifuse® Polymer PTC Resettable Fuses Help Protect Pressure Sensors From Short-Circuit-Induced Overheating”,www.bourns.com/html/mfsensor.htm, downloaded from the internet on Jun. 21, 1999 (website last updated on Apr. 6, 1999 as of download), 2 pages.
Bourns, Inc., “Polymer PTC Resettable Fuses Play An Important Role in Protecting Automotive Sound Systems”,www.bourns.com/html/apnmfspkr.htm, downloaded from the internet on Jun. 21, 1999 (website last updated on Apr. 6, 1999 as of download), 4 pages.

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