# Patent

## Title
Constant brightness control for electroluminescent lamp

## Abstract

A ballast (11) includes a variable output DC converter source (12) and an inverter circuit (14) arranged to provide AC output voltage to an electro-luminescent (EL) lamp (16). A Q factor of EL lamp can characterize the age of the EL lamp. A series resonance-parallel loaded circuit and two voltage detector (18, 20) are constructed to measure the Q factor. A processor (17) controls the output of DC voltage source in accordance with a look-up table containing constant brightness algorithm based on the derived voltage and Q relationship.

## Background
This application is a continuation of U.S. application Ser. No. 12/110,426, filed on Apr. 28, 2008, which claims the benefit of the earlier filing date of U.S. Provisional Application No. 60/788,040, filed on Apr. 3, 2006 which is incorporated by reference herein in its entirety.


FIELD OF THE INVENTION
The invention relates to Electro-luminescent (EL) lamps and, more particularly, to a ballast circuit for adjusting an EL lamp to a substantially constant brightness.
BACKGROUND OF THE INVENTION
Electro-luminescent (EL) lamps are used often for backlighting. The advantages of the EL lamp include its thinness, its light weight and flexibility. The disadvantage of the use of an EL lamp is that these phosphor based lamps dim with age as the phosphor decays. To overcome this effect, the voltage or frequency applied to the lamp can be increased to cause the phosphor to light be brighter, thus compensating for the reduction in light emission from the decay.
There are several approaches for compensating for the phosphor decay. One approach is to use a constant power driver circuit to compensate life. As the EL lamp decays, the reduced capacitance result in lower deriving current and deriving power. A constant power deriving circuit could increase output voltage as the lamp ages, but it is not possible to maintain constant output brightness, since there is no measurable parameter to indicate how much the lamp has aged.
Automatic brightness control systems include optical feedback using a monitoring photocell which is currently the most effective method to measure and control the brightness output. Based on a reading, the voltage and frequency supplied to the EL lamp are adjusted. In this way, the lamp brightness output can be controlled and maintain constant. This method has many inherent problems: it is expensive, it is affected by surrounding ambient light, it needs a response calibration based on the specific application and the optical monitoring device must be appropriately mounted close to the EL lamp.
Another method to compensate for reduced brightness of an EL lamp is to configure a digital circuit which includes a microprocessor, a timer and ROM. A table in ROM represents the relationship between the appropriate derive voltage and on-time to produce the constant brightness required. The microprocessor will track the total on-time of EL lamp and select appropriate data from table from the deriving EL lamp. However, as the EL lamp changed or expired, the timer should be reset. This open loop control system is not effective for an EL lamp.
Thus, there is a need to provide a simple, measurable parameter to indicate the age of the EL lamp and use this parameter to maintain the required constant brightness.
SUMMARY OF THE INVENTION
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is obtained by providing a ballast for controlling brightness of an electro-luminescent (EL) lamp. The ballast includes a converter circuit that converts line AC voltage to a DC input voltage. An inverter circuit inverts the DC input voltage to an AC output voltage. The ballast is constructed and arranged to provide the AC output voltage to the EL lamp. An input voltage detector is constructed and arranged to measure the DC input voltage (Vin). An output voltage detector is constructed and arranged to measure the AC output voltage (Vout). A processor is constructed and arranged to adjust Vin based on measured values of Vout and Vin to thereby adjust brightness of the EL lamp.
In accordance with another aspect of the invention, a method is provided for controlling brightness of an electro-luminescent (EL) lamp. The method provides a ballast for powering the EL lamp. DC input voltage (Vin) to the ballast is measured. AC output voltage (Vout) from the ballast is measured. Based on measured values of Vout and Vin, Vin is adjusted to thereby adjust brightness of the EL lamp.
Another object of the invention is to provide a method to control of the end of lamp life enabling the ability to shut-off the lamp. In order to maintain constant output brightness of lamp, the deriving voltage increases as the lamp ages. If the deriving voltage reaches a limit of the withstanding voltage of the lamp, breakdown will occur between layers of the lamp. As the deriving voltage reaches a limit value, a controller stops compensation or shuts-off the ballast to protect the lamp.
Another object of the invention is to provide the ability to change the brightness setting at any time with an automatic recalculation of the lamp's decay parameters.
Another object of the invention is to provide the ability to utilize the ballast on different sized lamps (within the ballast specification ranges) and have the ballast automatically understand the brightness parameters.
Another object of the invention is to provide the ability to reuse the ballast as the lamps are changed or expire.
Another object of the invention is to provide the ability to receive a manual (brightness control knob or switch, etc) request to change brightness or an automated (program command) request for a required brightness setting and adjust accordingly and maintain the brightness requirement.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.



BRIEF DESCRIPTION OF THE DRAWING
The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawing in which:
FIG. 1 is a schematic of a system for a controlling brightness of an EL lamp, provided in accordance with the principles of the invention.



DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT
The object of the invention is to provide a simple and efficient system for adjusting an EL lamp to a given constant brightness. By measuring a quality “Q” factor, referring to a look up table, and using a predetermined algorithm which relates the Q factor to the brightness setting, complete control of the brightness of the EL Lamp is possible.
An LCR resonator circuit that generates a sinusoidal waveform voltage is a well known circuit in a conventional ballast to derive fluorescence of a lamp. A series resonant—parallel loaded circuit could be applied to a deriving EL lamp. The transfer function is

Vin/Vout=(1−(Ws/W0)2+(Ws/W0Q)2)1/2 

Where

Vout: output voltage of resonator,

Vin: input voltage to resonator,

Ws: deriving frequency,

W0: natural resonance frequency,

Q: quality factor of lamp,

As the lamp decays its Q value will increase because of the reduction of capacitance. If the deriving frequency is equal to natural resonance frequency, the output voltage, Vout=Vin*Q. By measuring Vout and Vin, the obtained Q factor could indicate how much the EL lamp has aged. Usually, the Q factor of the EL lamp is related to layer structure and material and is independent of lamp size. A ballast constructed with the above circuit can derive different size lamps without changing an algorithm in a microprocessor.
With reference to FIG. 1, a circuit, generally indicated at 10, for controlling brightness of an EL lamp 16, is shown in accordance with the principles of the present invention. The circuit 10 is preferably part of a ballast 11 for powering the EL lamp 16. The ballast 11 and the EL lamp 16 define an EL lamp system, generally indicated at 13. The ballast 11 has a circuit that is composed of two stages. The first stage 12 is an AC to DC converter which converts line voltage (e.g., 100-240 VAC) at input 13 to DC voltage (e.g., 30-150 VDC). The second stage is a DC to AC inverter 14 which is a half bridge, self excited, series resonance-parallel loaded circuit. The inverter 14 inverts DC voltage to AC voltage (50-380 VAC rms, at a predetermined frequency, for example 1000 Hz) that is applied to the EL lamp 16.
A look-up table 22 is established to provide a constant brightness algorithm based on the measured values of the Vin and Q relationship. Thus, the table identifies the relationship between the age of the EL lamp 16 and the Q value. In other words, once the Q value is determined, the age of the EL lamp 16 can be determined. In order to maintain a constant output brightness of an aged EL lamp 16, the output voltage (Vout) must be increased. Since Vout is proportional to Vin, a formula regarding brightness is derived from this table 22 based on the relationship between the Q value and Vin. The table 22 can be in memory in a controller or microprocessor 17 or can be in memory separate from, but accessible by the microprocessor 17. The algorithm executed by the microprocessor 17 utilizes the measured AC-DC converter 12 output voltage (Vin) and the inverter 14 output voltage (Vout), calculates the Q value, and then adjusts Vin according to the brightness required using a compensating formula. For example, every minute, the microprocessor will calculate the Q value and adjust Vin to maintain a substantially constant output brightness according to the look-up table (e.g., based age of the EL lamp 16).
A/D converters 19 and 24 convert the analog signals Vin and Vout, respectively, to digital signals that are received by the microprocessor 17. A D/A converter 21 is provided to convert the digital output of the microprocessor 17 to an analog signal.
Thus, the system 10 provides a simple and efficient way to adjust an EL lamp to given constant brightness. Once adjusted, then the parameters specific to the lamp are known and full control of the EL lamp is possible.
Another feature of the above-mentioned circuit is the control of end of lamp life by shutting-off the EL lamp 16 based on an age of the EL lamp. The compensated derived voltage to EL lamp should not exceed its withstanding voltage limit, since too high a voltage will induce breakdown and destroy the EL lamp. The microprocessor 17 will stop increasing the output voltage as the measured voltage from the A/D converter 24 reaches the limit value.
Features of the embodiment:

    
    
        Remaining lamp life at a set brightness level can be displayed on the ballast
        Control of the end of lamp life enabling the ability to
        
            Shut off the lamp
            Accelerate the decay (brightness) curve
        
        
        The ability to change the brightness setting at any time with an automatic recalculation of the lamp's decay parameters
        The ability to utilize the ballast on different sized lamps (within the ballast specification ranges) and have the ballast automatically understand the brightness parameters
        The ability to reuse the ballast as the lamps are changed or expire
        The ability to receive a manual (brightness control knob or switch, etc) request to change brightness or an automated (program command) request for a required brightness setting and adjust accordingly and maintain the brightness requirement
    
    


Thus, with the embodiment, a microprocessor controls the inverter in accordance with a table containing measuring data and appropriate derive voltage to maintain constant brightness of an EL lamp.
The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.

## Claims

1. A tangible computer-readable medium having computer-executable instructions stored thereon, the instructions comprising:
instructions for measuring input voltage (Vin) to a ballast;
instructions for measuring output voltage (Vout) from the ballast; and
instructions for adjusting Vin based at least in part on measured values of Vout and Vin relative to one another to thereby adjust brightness of an EL lamp, wherein the ballast is configured to power the EL lamp.

2. The medium of claim 1, wherein the instructions further comprise:
instructions for establishing a relationship between an age of the EL lamp and a quality value; and
instructions for calculating the quality value.

3. The medium of claim 2, wherein the instructions for adjusting Vin comprise:
instructions for adjusting Vin based on the relationship between the quality value and the age of the EL lamp.

4. The medium of claim 1, wherein the instructions further comprise:
instructions for accelerating a decay of the brightness of the EL lamp based on an age of the EL lamp.

5. The medium of claim 1, wherein the instructions further comprise:
instructions for determining a remaining lamp life value corresponding to a set brightness level.

6. A tangible system for providing illumination, comprising:
means for measuring an input voltage (Vin) to a ballast;
means for measuring an output voltage (Vout) from the ballast; and
means for adjusting Vin based at least in part on measured values of Vout and Vin relative to one another to thereby adjust brightness of an EL lamp, wherein the ballast is configured to power the EL lamp.

7. The system of claim 6, wherein the means for measuring an input voltage includes an input voltage detector.

8. The system of claim 6, wherein the means for measuring an output voltage includes an output voltage detector.

9. The system of claim 6, wherein the means for adjusting Vin includes a processor.