Power supply for light emitting diode array

Abstract

An apparatus (10) for supplying regulated voltage d.c. electrical power to an LED array (12) includes a rectifier (32) responsive to a.c. power for generating rectified d.c. power and a power factor correcting and voltage regulating buck / boost switchmode converter (38) responsive to the rectified d.c. power for generating regulated voltage d.c. power to illuminate the LED array (12). A battery backup system (62) receives the a.c. power applied to the rectifier (32) for charging a rechargeable battery (66) and sensing an a.c. power failure. A switch-over relay (82) is connected between the battery backup system (62) and the rectifier. Upon sensing a failure of the a.c. power, the battery backup system (62) controls the switch-over relay (82) to connect the battery backup system (62) to the rectifier (32) to provide d.c. power to the switchmode converter (38) to illuminate the LED array (12). A half wave power detector (88) causes the apparatus (10) to reduce regulated d.c. power to dim the LED array (12).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001]This application is the parent of a continuation reissue application filed Mar. 5, 2008, and accorded application Ser. No. 12 / 074,723.<?insert-end id="INS-S-00001" ?>BACKGROUND OF THE INVENTION[0002]The present invention relates generally to an apparatus for generating power to a light emitting diode array and, in particular, to a power supply for operating light emitting diode array traffic signals.[0003]Light emitting diode (LED) arrays are becoming more common in many applications as they are used to replace less efficient incandescent lamps. Status annunciators, message boards, liquid crystal display back lights and traffic signals are common applications for LED arrays. In most of these uses, electrical power is obtained from a.c. mains (120 v.a.c., 60 Hz) and some form of power supply converts the alternating line voltage to d.c., or pulsing d.c., for powering the plurality of LEDs.[0004]LEDs typically exhibit forward voltage...

AI Technical Summary

Benefits of technology

[0017]The apparatus according to the present invention also includes an adaptive clamp circuit connected to the rectifier input for eliminating leakage current problems. The adaptive clamp circuit has an input adapted to be connected to a pair of a.c. power lines, a pair of clamp circuit output lines connected to the adaptive clamp circuit input, a voltage sensing means connected across the adaptive clamp circuit input, and a controlled load means connected across the clamp circuit output lines and to the voltage sensing means. The voltage sensing means is responsive to a magnitude of a.c. voltage at the adaptive clamp circuit input lower than a predetermined magnitude for turning on the controlled load means to connect a low impedance load in the controlled load means across the clamp circuit output lines and the voltage sensing means is responsive to a magnitude of the a.c. voltage at the adaptive clamp circuit input equal to or greater than the predetermined magnitude for turning off the controlled load means to disconnect the low impedance load from the clamp circuit output lines.

[0018]It is also an objective of the present invention to eliminate leakage current problems by providing an adaptive clamp circuit.

Problems solved by technology

The highly dissipative nature of such linear regulators makes such use questionable in heat sensitive apparatus such as LED signals however.

Such highly reactive power supplies exhibit very poor power factors however, and may be disallowed by power utilities.

Several problems are associated with these prior art simple circuit topologies.

The input current wave forms are generally badly distorted and the power factor is poor.

Reasons for the poor power factor and high distortion relate to the discontinuous conduction of the diodes in the circuit feeding large capacitors.

This phenomenon is well understood, and plagues many small off line power supplies.

In fact, many utilities are placing limits on permissible power factor and distortion behavior of electrical devices connected to their lines.

Also, in many parts of the country, electrical generating capacity is at its limits, and new capacity cannot be added because of environmental concerns.

This strong interest in LED signals as an important energy conservation resource is clouded however by the poor power factor performance of commercially available signals.

However in aggregate, a large number of small devices can have a significant impact on the power grid.

Another factor that directly influences the amount of power (apparent VA) that needs to be delivered to a given load is the total harmonic distortion of the current waveform supplying the device.

True power factor is adversely affected by current or voltage distortion, and the significance of this influence is only now being widely accepted.

Harmonic distortion or deviation from true sinusoidal wave forms not only gives rise to further wasted energy, but increases the electromagnetic interference potential of the load.

Radiated and conducted interference is a concern because of the interference potential with other services (radio communications for example).

Harmonic distortion is becoming more prevalent in power supplies as these devices are converted from inefficient linear operation to far more efficient switchmode operation.

A consequence of these solid state approaches is increased harmonic distortion and poor power factor behavior.

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