LED array employing a lattice relationship

Details LED array employing a lattice relationship LED array employing a lattice relationship

1999-11-01

2001-03-13

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural series connected load devices

Christmas lights

C315S18500S, C362S259000, C362S800000

Reexamination Certificate

active

06201353

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to lighting systems, and more particularly to an improved array for light-emitting diodes used as illumination sources.
BACKGROUND OF THE INVENTION
A light-emitting diode (LED) is a type of semiconductor device, specifically a p-n junction, which emits electromagnetic radiation upon the introduction of current thereto. Typically, a light-emitting diode comprises a semiconducting material that is a suitably chosen gallium-arsenic-phosphorus compound. By varying the ratio of phosphorus to arsenic, the wavelength of the light emitted by a light-emitting diode can be adjusted.
With the advancement of semiconductor materials and optics technology, light-emitting diodes are increasingly being used for illumination purposes. For instance, high brightness light-emitting diodes are currently being used in automotive signals, traffic lights and signs, large area displays, etc. In most of these applications, multiple light-emitting diodes are connected in an array structure so as to produce a high amount of lumens.
FIG. 1
illustrates a typical arrangement of light-emitting diodes
1
through m connected in series. Power supply source
4
delivers a high voltage signal to the light-emitting diodes via resistor R
1
, which controls the flow of current signal in the diodes. Light-emitting diodes which are connected in this fashion usually lead to a power supply source with a high level of efficiency and a low amount of thermal stresses.
Occasionally, a light-emitting diode may fail. The failure of a light-emitting diode may be either an open-circuit failure or a short-circuit failure. For instance, in short-circuit failure mode, light-emitting diode
2
acts as a short-circuit, allowing current to travel from light-emitting diode
1
to
3
through light-emitting diode
2
without generating a light. On the other hand, in open-circuit failure mode, light-emitting diode
2
acts as an open circuit, and as such causes the entire array illustrated in
FIG. 1
to extinguish.
In order to address this situation, other arrangements of light-emitting diodes have been proposed. For instance, FIG.
2
(
a
) illustrates another typical arrangement of light-emitting diodes which consists of multiple branches of light-emitting diodes such as
10
,
20
,
30
and
40
connected in parallel. Each branch comprises light-emitting diodes connected in series. For instance, branch
10
comprises light-emitting diodes
11
through n
1
connected in series. Power supply source
14
provides a current signal to the light-emitting diodes via resistor R
2
.
Light-emitting diodes which are connected in this fashion have a higher level of reliability than light-emitting diodes which are connected according to the arrangement shown in FIG.
1
. In open-circuit failure mode, the failure of a light-emitting diode in one branch causes all of the light-emitting diodes in that branch to extinguish, without significantly affecting the light-emitting diodes in the remaining branches. However, the fact that all of the light-emitting diodes in a particular branch are extinguished by an open-circuit failure of a single light-emitting diode is still an undesirable result. In short-circuit failure mode, the failure of a light-emitting diode in a first branch may cause that branch to have a higher current flow, as compared to the other branches. The increased current flow through a single branch may cause it to be illuminated at a different level than the light-emitting diodes in the remaining branches, which is also an undesirable result.
Still other arrangements of light-emitting diodes have been proposed in order to remedy this problem. For instance, FIG.
2
(
b
) illustrates another typical arrangement of light-emitting diodes, as employed by a lighting system of the prior art. As in the arrangement shown in FIG.
2
(
a
), FIG.
2
(
b
) illustrates four branches of light-emitting diodes such as
50
,
60
,
70
and
80
connected in parallel. Each branch further comprises light-emitting diodes connected in series. For instance, branch
50
comprises light-emitting diodes
51
through n
5
connected in series. Power supply source
54
provides current signals to the light-emitting diodes via resistor R
3
.
The arrangement shown in FIG.
2
(
b
) further comprises shunts between adjacent branches of light-emitting diodes. For instance, shunt
55
is connected between light-emitting diodes
51
and
52
of branch
50
and between light-emitting diodes
61
and
62
of branch
60
. Similarly, shunt
75
is connected between light-emitting diodes
71
and
72
of branch
70
and between light-emitting diodes
81
and
82
of branch
80
.
Light-emitting diodes which are connected in this fashion have a still higher level of reliability than light-emitting diodes which are connected according to the arrangements shown in either
FIGS. 1
or
2
(
a
). This follows because, in an open-circuit failure mode, an entire branch does not extinguish because of the failure of a single light-emitting diode in that branch. Instead, current flows via the shunts to bypass a failed light-emitting diode.
In the short-circuit failure mode, a light-emitting diode which fails has no voltage across it, thereby causing all of the current to flow through the branch having the failed light-emitting diode. For example, if light-emitting diode
51
short circuits, current will flow through the upper branch. Thus, in the arrangement shown in FIG.
2
(
b
), when a single light-emitting diode short circuits, the corresponding light-emitting diodes
61
,
71
and
81
in each of the other branches are also extinguished.
The arrangement shown in FIG.
2
(
b
) also experiences other problems. For instance, in order to insure that all of the light-emitting diodes in the arrangement have the same brightness, the arrangement requires that parallel connected light-emitting diodes have matched forward voltage characteristics. For instance, light-emitting diodes
51
,
61
,
71
and
81
, which are parallel connected, must have tightly matched forward voltage characteristics. Otherwise, the current signal flow through the light-emitting diodes will vary, resulting in the light-emitting diodes having dissimilar brightness.
In order to avoid this problem of varying brightness, the forward voltage characteristics of each light-emitting diode must be tested prior to its usage. In addition, sets of light-emitting diodes with similar voltage characteristics must be binned into tightly grouped sets (i.e—sets of light-emitting diodes for which the forward voltage characteristics are nearly identical). The tightly grouped sets of light-emitting diodes must then be installed in a light-emitting diode arrangement parallel to each other. This binning process is costly, time-consuming and inefficient.
Various light-emitting diode arrangements were proposed in Applicant's co-pending applications, designated as Attorney Docket Numbers 755-003 and 755-004, both of which are incorporated herein by reference as fully as if set forth in their entirety. However, there exists a further need for an improved light-emitting diode arrangement which does not suffer from the problems of the prior art.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, a lighting system comprises a plurality of light-emitting diodes. The lighting system further comprises a power supply source for driving a current signal through a plurality of parallel disposed, electrically conductive branches. Each light-emitting diode in one branch together with corresponding light-emitting diodes in the remaining branches define a cell unit. In each cell, the anode terminal of each light-emitting diode in one branch is coupled to the cathode terminal of a corresponding light-emitting diode of an adjacent branch via a shunt. Each shunt further comprises another light-emitting diode. The branches along with the shunts are coupled in a lattice arrangement.
According to one embodiment, a plurality of K cells are coupled together in a cas

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