Patent Application
20080007885
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
a through 2c are diagrams of a multi-LED implementation of a light source and the effect of a failure of a single LED in the light source;
a and 3b are diagrams of a light element with an apparatus for bypassing a failed LED, according to a preferred embodiment of the present invention;
a though 4c are diagrams of exemplary light sources, according to a preferred embodiment of the present invention;
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The present invention will be described with respect to preferred embodiments in a specific context, namely a projection display system and method utilizing a light source comprising multiple LEDs. The present invention can be used in any type of projection display system, such as projection display systems utilizing one of a wide variety of microdisplays, including digital micromirror devices, deformable mirrors, liquid crystal displays, liquid crystal on silicon, and so forth. The invention may also be applied, however, to other systems, wherein there is a need for bypassing a failed component in a sequence of components and maintaining operation of the sequence, such as a sequence of lights, and so forth. For example, the present invention can be utilized in a light source wherein sequences of standard lights are used in the light source.
In addition, the present invention can find use in applications that involve large arrays of high power LEDs, such as in automotive applications (LED brake lights or headlights, for example), traffic lights, LED lamp fixtures that are designed to replace household incandescent light bulbs, and so forth. In the case of LED lamp fixtures, long strings of serially connected LEDs can be wired with household 120 VAC supplies that perform direct conversion of AC to DC (for example, a simple four-diode full-wave bridge rectifier can be used). The long string of serially connected LEDs can be directly driven and cost savings can be achieved by negating the need for a switching mode buck regulator or some other power conversion circuitry. In these types of applications, the present invention can be useful for increasing reliability.
With reference now to
In order to provide the needed amount of light for use in a projection display system, the LEDs typically require a large current. As a result of the large current, the LEDs can get hot. Furthermore, a common way to illuminate the LEDs is to rapidly pulse the LEDs on and off. For example, it may be required to pulse the LED 305 with short duration pulses when it is desired to display a minimum amount of light. The rapid current pulses and the resulting heating, can force the LED 305 through a large number of thermal cycles, which includes heating up and then cooling down. The rapid succession of heating/cooling can subject the LED 305 to thermal shock. Additionally, conductors inside the LED 305 as well as solder connections, bond wires, and so forth, which connect the LED 305 to a circuit board, also undergo the thermal cycles and the attendant thermal shock. Therefore, the conductors, solder connections, and bond wires can fail. A failure of the conductors and/or the solder connections can have the same net result as the failure of the LED 305.
The antifuse 310, while the LED 305 is operating within design specifications, can have a resistance that is significantly higher than the resistance of the LED 305 so that the majority of the current, preferably almost all of the current, passes through the LED 305. The antifuse 310 can be made from a material that contains a powered conductive material that will normally have a large resistance, for example.
Alternatively, the antifuse 310 can be made from a multilayered member that typically lies across a pair of electrical terminals. The antifuse 310 can contain a layer of a highly resistive material, such as a resistive film, that will provide the high resistance when the LED 305 is operating properly. Then, if the LED 305 fails as an open circuit, the current flowing through the resistive film would increase and cause the resistive film to heat up. Another layer of the multilayered member may be made from a material that will deform when heated. Therefore, when excessive current flows through the antifuse 310 (the resistive film layer), the heat sensitive layer will deform and physically (mechanically) attach the deformable layer of the multilayered member to the electrical terminals, creating a low-resistance current path.
In yet another alternative, the antifuse 310 can contain a layer of a material that is electrostrictive in nature, and when an electric field of sufficient magnitude is applied (as from increased current flow due to the failed LED 305), the electrostrictive layer can deform and can create a low-resistance current path around the failed LED 305. Additionally, if properly selected, a current of sufficient magnitude can cause a spot weld and permanently affix the antifuse 310 in the position where it would provide a low-resistance current path. Other materials that can be used include piezoelectric materials and ferroelectric materials.
The antifuse 310 can also be implemented as a solid-state device. One such example of a solid-state antifuse would be a silicon controlled rectifier (SCR). The SCR (functioning as the antifuse 310) can be placed parallel to the LED 305 and while the LED 305 is operating properly, the SCR would remain substantially inoperative. However, when the LED 305 has an open circuit failure, the SCR would see an increased voltage drop and would begin to conduct current. Although the SCR is a temporary device, wherein the SCR would reset itself should power be removed, if the LED 305 remains an open circuit, then the SCR would return to its current conducting state shortly after the power is reapplied to the LED 305 and SCR combination.
When the LED 305 has an open circuit failure, which may be a result of a failure of the LED 305 and/or a failure of conductors and/or solder connections of the LED 305, the current path 315 through the LED 305 is no longer present. Therefore, the current that originally flowed through the LED 305 will now flow through the antifuse 310 (as shown as current path 320 (
Although the antifuse 310 is shown in
With reference now to
Generally, when an LED, such as the LED 305, has an open circuit failure and an associated antifuse, such as the antifuse 310, short circuits, the current flowing through the remaining LEDs in the sequence 405 changes (is increased). The increased current flow will, at least, alter the amount of light emitted by the remaining LEDs, or at worst, shorten the life of the remaining LEDs. Therefore, the power supply 415 should ideally adjust to alter (or maintain) the current flow so that the current flowing through the remaining LEDs does not increase significantly so as to shorten the useful life of the remaining LEDs.
The diagram shown in
Although the heretofore-discussed failure of an LED (plus potentially, attendant connections, such as circuit board leads, conductors, and so forth) results in the LED turning into an open circuit, another common failure of LEDs can result in the LED turning into a short circuit. In such a situation, the failed LED will continue to conduct. Although the short circuit failure of an LED will not cause an entire string of serially connected LEDs to fail, the change in the current flowing through the serially connected string can expedite failures of other LEDs in the serially connected string.
With reference now to
The light element 470 includes an LED 305 and an antifuse 310 arranged in a parallel configuration as in the light element 410. However, connected in series with the LED 305 is a fuse 475, which can have a typical fuse behavior, wherein the fuse 475 will open circuit when a current exceeding some specified amount flows through the fuse 475. Therefore, when the LED 305 fails and turns into a short circuit, the current flowing through the light element 470 will increase (due to the decreased resistance) and cause the fuse 475 to become an open circuit (blow).
The blowing of the fuse 475 will open circuit the portion of the light element 470 containing the fuse 475 and the LED 305, leading to an increased current flowing through the antifuse 310. When the current flowing through the antifuse 310 exceeds a specified amount, the antifuse 310 will short circuit and reconnect the serially connected string of LEDs that contains the light element 470.
Alternatively, if a light source includes a large number of strings of LEDs, a single fuse can be used with each string of LEDs. In this situation, when an LED fails as a short circuit, the fuse in the string can blow and turn off the entire string of LEDs. While this may reduce the number of operating LEDs, the removal of the string of LEDs containing the failed LED can allow the remaining strings of LEDs to operate in a normal manner and at normal conditions, which could help to prevent the failure of additional LEDs.
With reference now to
With reference now to
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
