Abstract:
A lighting system comprising a plurality of light-emitting diodes and a current driver for driving current through a plurality of parallel disposed, electrically conductive branches, wherein the branches comprise at least one cell. In each cell, each branch has a light-emitting diode with an anode terminal and a cathode terminal. The anode terminal of each light-emitting diode is coupled to the cathode terminal of a light-emitting diode of an adjacent branch via a shunt. The shunt further comprises a light-emitting diode. In each cell, each light-emitting diode may have a different forward voltage characteristic, while still insuring that all of the light-emitting diodes in the arrangement have the same brightness. Upon failure of one light-emitting diode, the remaining light-emitting diodes in the lighting system are not extinguished.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject matter of this application is related to application Ser. No. 09/431,585 filed on Nov. 1, 1999 by the inventor herein and Shaomin Peng for LED ARRAY EMPLOYING A SPECIFIABLE LATTICE RELATIONSHIP. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to lighting systems, and more particularly to an improved array structure 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, traffics 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 effecting 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   25  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. 
     Therefore, there exists a need for an improved light-emitting diode arrangement which does not suffer from the problems of the prior art, as discussed above. 
     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 current driver 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. Thus, each cell may comprise two branches, thereby having four light-emitting diodes, or may have more than two branches. 
     The arrangement of light-emitting diodes according to the present invention enables the use of light-emitting diodes having some different forward voltage characteristics, while still insuring that all of the light-emitting diodes in the arrangement have substantially the same brightness. Advantageously, the lighting system of the present invention is configured such that, upon failure of one light-emitting diode in to a branch, the remaining light-emitting diodes in that branch are not extinguished. In another embodiment, the lighting system comprises at least two cells which are cascading, wherein the cascading cells are successively coupled such that the cathode terminal of each light-emitting diode in a branch is coupled to is an anode terminal of a light-emitting diode of the same branch in a next successive cell. 
     In a preferred embodiment, each branch of the lighting system includes a current-regulating element, such as a resistor, coupled for example, as the first and the last element in each branch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be further understood from the following description with reference to the accompanying drawings, in which: 
     FIG. 1 illustrates a typical arrangement of light-emitting diodes, as employed by a lighting system of the prior art; 
     FIG.  2 ( a ) illustrates another typical arrangement of light-emitting diodes, as employed by a lighting system of the prior art; 
     FIG.  2 ( b ) illustrates another typical arrangement of light-emitting diodes, as employed by a lighting system of the prior art; 
     FIG. 3 illustrates an arrangement of light-emitting diodes, as employed by a lighting system, according to one embodiment of the present invention; and 
     FIG. 4 illustrates an arrangement of light-emitting diodes, as employed by a lighting system, according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 illustrates an arrangement  100  of light-emitting diodes, as employed by a lighting system, according to one embodiment of the present invention. The lighting system comprises a plurality of electrically-conductive branches. Each branch has diodes connected in series. A set of corresponding light-emitting diodes of all branches defines a cell. The arrangement shown in FIG. 3 illustrates cascading cells  101 ( a ),  101 ( b ) through  101 ( n ) of light-emitting diodes. It is noted that, in accordance with various embodiments of the present invention, any number of cells may be formed. 
     Each cell  101  of arrangement  100  comprises a first light-emitting diode (such as light-emitting diode  110 ) of branch  102  and a first light-emitting diode (such as light-emitting diode  111 ) of branch  103 . Each of the branches having the light-emitting diodes are initially (i.e.—before the first cell) coupled in parallel via resistors (such as resistors  105  and  106 ). The resistors preferably have the same resistive values, to insure that an equal amount of current is received via each branch. 
     The anode terminal of the light-emitting diode in each branch is coupled to the cathode terminal of a corresponding light-emitting diode in an adjacent branch. For example, the anode terminal of light-emitting diode  110  is connected to the cathode terminal of light-emitting diode  111  by a first shunt (such as shunt  114 ) having a light-emitting diode (such as light-emitting diode  112 ) connected therein. In addition, the anode terminal of light-emitting diode  111  is connected to the cathode terminal of light-emitting diode  110  by a second shunt (such as shunt  115 ) having a light-emitting diode (such as light-emitting diode  113 ) connected therein. Power supply source  104  provides a current signal to the light-emitting diodes via resistors  105  and  106 . Additional resistors  107  and  108  are employed in arrangement  100  at the cathode terminals of the last light-emitting diodes in the arrangement shown. 
     As shown in FIG. 3, branches  102  and  103  have respective input nodes a 1  and b 1 , and nodes a 2 , a 3  and b 2 , b 3  which are respective nodes in each branch between adjoining cells. 
     Light-emitting diodes which are connected according to the arrangement shown in FIG. 3 have a higher level of reliability compared to light-emitting diodes which are connected according to the arrangement shown in FIG.  2 ( b ). This follows because, in open-circuit failure mode, an entire branch does not extinguish because of the failure of a light-emitting diode in that branch. Instead, current flows via shunts  114  or  115  to bypass a failed light-emitting diode. For instance, if light-emitting diode  110  of FIG. 3 fails, current still flows to (and thereby illuminates) light-emitting diode  120  via lower branch  103  and light-emitting diode  113 . In addition, current from the upper branch still flows to the adjacent branch via shunt  114 . 
     Furthermore, in short-circuit failure mode, light-emitting diodes in other branches and shunts do not extinguish because of the failure of a light-emitting diode in one branch. This follows because the light-emitting diodes are not connected in parallel. For example, if light-emitting diode  110  short circuits, current will flow through upper branch  102 , which has no voltage drop, and will also flow through light-emitting diode  112  in shunt  114 . Light-emitting diode  112  remains illuminated because the current flowing through it drops only a small amount, unlike that which occurs in the arrangement of FIG.  2 ( b ). Light-emitting diodes  111  and  113  also remain illuminated because a current flow is maintained through them via branch  103 . 
     In addition, arrangement  100  of light-emitting diodes also alleviates other problems experienced by the light-emitting diode arrangements of the prior art. For instance, light-emitting diode arrangement  100  of the present invention, according to one embodiment, insures that all of the light-emitting diodes in the arrangement have the same brightness without the requirement that the light-emitting diodes have tightly matched forward voltage characteristics. For instance, light-emitting diodes  110 ,  111 ,  112  and  113  of the arrangement shown in FIG. 3 may have forward voltage characteristics which are not as tightly matched as the forward voltage characteristics of light-emitting diodes  51 ,  61 ,  71  and  81  of the arrangement shown in FIG.  2 ( b ). This follows because, unlike the arrangements of the prior art, the light-emitting diodes in cell  101  of arrangement  100  are not parallel-connected to each other. 
     Because light-emitting diodes in each cell are not parallel-connected, the voltage drop across the diodes does not need to be the same. Therefore, forward voltage characteristics of each light-emitting diode need not be equal to others in order to provide similar amounts of illumination. In other words, the current flow through a light-emitting diode having a lower forward voltage drop will not increase in order to equalize the forward voltage of the light-emitting diode with the higher forward voltage of another light-emitting diode. 
     Because it is not necessary to have light-emitting diodes with tightly matched forward voltage characteristics, the present invention alleviates the need for binning light-emitting diodes with tightly matched voltage characteristics. Therefore, the present invention reduces the additional manufacturing costs and time which is necessitated by the binning operation of prior art light-emitting diode arrangements. 
     It is also noted that the present invention, according to one embodiment thereof, may employ cells having more than two branches. FIG. 4 illustrates an arrangement  200  of light-emitting diodes, as employed by a lighting system, according to another embodiment of the present invention. This lighting system also comprises a plurality of electrically-conductive branches, each having light-emitting diodes connected in series. A set of corresponding light-emitting diodes of all of the branches define a cell unit. The arrangement shown in FIG. 4 illustrates cascading cells  101 ( a ),  101 ( b ) through  101 ( n ) of light-emitting diodes. It is noted that, in accordance with various embodiments of the present invention, any number of cells may be formed. 
     As shown in FIG. 4, when connected successively, each cell  201  of arrangement  200  comprises a plurality of corresponding light-emitting diodes (such as light-emitting diodes  210 ,  211  and  216 ). The branches of the plurality of light-emitting diodes are initially (i.e.—before the first cell) coupled in parallel via current regulating elements such as resistors (e.g.—resistors  205 ,  206  and  207 ). 
     In a preferred embodiment, resistor  205  has the same resistive value as resistor  207 , while resistor  208  has the same resistive value as resistor  209 ( b ). In addition, resistor  206  advantageously has a resistive value which is two-thirds of the resistive values of either resistors  205  or  207 . Similarly, resistor  209 ( a ) advantageously has a resistive value which is two-thirds of the resistive values of either resistors  208  or  209 ( b ). The lower relative resistive values of resistors  206  and  209 ( a ) are due to the fact that they are coupled to branch  203 , which provides current to three light-emitting diodes in each cell, while resistors  205  and  208 , and resistors  207  and  209 ( b ), which are coupled to branches  202  and  204 , respectively, provide current to only two light-emitting diodes in each cell. 
     In addition, the anode terminal of the light-emitting diode in each branch is coupled to the cathode terminal of a corresponding light-emitting diode in an adjacent branch. For instance, the anode terminal of light-emitting diode  210  is connected to the cathode terminal of light-emitting diode  211  by shunt  214 . Shunt  214  has light-emitting diode  212  connected therein. In addition, the anode terminal of light-emitting diode  211  is connected to the cathode terminal of light-emitting diode  210  by shunt  215 . Shunt  215  has light-emitting diode  213  connected therein. 
     Furthermore, the anode terminal of light-emitting diode  211  is also connected to the cathode terminal of light-emitting diode  216  by shunt  219 ( a ). Shunt  219 ( a ) has light-emitting diode  217  connected therein. In addition, the anode terminal of light-emitting diode  216  is connected to the cathode terminal of light-emitting diode  211  by shunt  219 ( b ). Shunt  219 ( b ) has light-emitting diode  218  connected therein. Power supply source  204  provides current to the light-emitting diodes via resistors  205 ,  206  and  207 . Additional resistors  208 ,  209 ( a ) and  209 ( b ) are employed in arrangement  200  at the cathode terminals of the last light-emitting diodes in the arrangement. 
     Light-emitting diodes which are connected according to the arrangement shown in FIG. 4 also have a high level of reliability. In open-circuit failure mode, no other light-emitting diodes in a branch are extinguished upon the failure of a light-emitting diode in that branch. Instead, current flows via shunts  214  or  215 , or via shunts  219 ( a ) or  219 ( b ), to bypass a failed light-emitting diode, and the remaining light-emitting diodes in the same cell, as well as the remaining light-emitting diodes in the adjacent cascading cells, are not extinguished. For instance, if light-emitting diode  211  of FIG. 4 fails, current still flows to (and thereby illuminates) light-emitting diode  221  via shunts  214  and  218 . In addition, current still flows to the light-emitting diodes of the adjacent branches. 
     Furthermore, in short-circuit failure mode, no other light-emitting diodes in a cell are extinguished when any light-emitting diode short circuits. Current continues to flow through each of the other light-emittting diodes in the cell. For instance, if light-emitting diode  211  short circuits, current will flow through upper branch  203 , which has no voltage drop, and will also flow through light-emitting diodes  213  and  217  in shunts  215  and  219 ( a ). Light-emitting diode  112  remains illuminated because the current flowing through it drops only a small amount, unlike that which occurs in the arrangement of FIG.  2 ( b ). Light-emitting diodes  210 ,  212 ,  216  and  218  also remain illuminated because a current flow is maintained through them via branches  202  and  204 . 
     The light-emitting diode arrangement shown in FIG. 4, as previously discussed in connection with the light-emitting diode arrangement shown in FIG. 3, also reduces the requirement that the light-emitting diodes have tightly matched forward voltage characteristics. For instance, the light-emitting diodes in cell  201  of arrangement  200 , specifically light-emitting diodes  210  through  218 , are not parallel-connected to each other such as to cause the current flow through an light-emitting diode having a lower forward voltage to increase in order to equalize the forward voltage of the light-emitting diode with the higher forward voltage of another light-emitting diode. Again, the present invention reduces the additional manufacturing costs and time which is necessitated by the binning operation of prior art light-emitting diode arrangements. 
     While there has been shown and described particular embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications can be made therein without departing from the invention, and therefore, the appended claims shall be understood to cover all such changes and modifications as fall within the true spirit and scope of the invention.