JP2007250986A - Led backlight apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the luminance and chromaticity of light radiated from LEDs from varying while a backlight apparatus is lighted. <P>SOLUTION: Not only green and blue LED light emitting elements but also a red LED light emitting element is formed of a nitride semiconductor to reduce shifting of a light emitting wavelength with temperature variation. Thus, the light receiving sensitivity characteristics of a color sensor is made to be nearly constant to reduce the error of detected values of the luminance and chromaticity of the light emitted from the LEDs. By using a control circuit, a driving current value or a driving pulse value of the LED light emitting elements are controlled so as to keep the set desirable luminance and chromaticity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technology of a backlight device using LEDs.

  In recent years, cold cathode discharge lamps that have been used as backlight sources for liquid crystal display devices are being replaced by LEDs (Light Emitting Diodes). Unlike cold cathode discharge lamps, the advantages of using LEDs are that there is no need to use mercury and that it is environmentally friendly, that it emits light as soon as a current is passed, so it has good responsiveness, and it has low voltage and low current. Since it emits light, the light emission efficiency is good.

  Up until now, backlights using LEDs as light sources have been mainly used for small-sized applications such as mobile phones and mobile devices, but nowadays they are used for medium-sized applications such as monitors of 20-inch and above and large-scale applications such as liquid crystal televisions of 40-inch and above. Also tend to be adopted. In particular, in applications such as large liquid crystal display televisions, in order to take advantage of the improvement in color reproducibility of liquid crystals, LEDs having light emission wavelengths of three primary colors of red, green, and blue (RGB) are suitable as light sources. Note that Patent Documents 1 and 2 are known as techniques related to the present application.

  The LED has a characteristic of increasing the amount of light emission in proportion to the current. However, the light emission efficiency depends on the LED temperature, and when the LED temperature rises due to the increase of the current, the light emission efficiency rapidly decreases. Therefore, when an LED is incorporated in a backlight device, the heat generated by the LED itself causes a change in luminous efficiency immediately after lighting and after a certain period of time, resulting in a change in luminance as a backlight device. Become. In addition, as the element material of the LED, an InGaN system is mainly used for blue and green LEDs, and an AlInGaP system is used for red LEDs. That is, since different material systems are used depending on the color of the LED, the amount of change in the luminous efficiency with respect to the temperature is also different for each LED, which affects the chromaticity change in addition to the luminance change described above.

Therefore, by applying the technique disclosed in Patent Document 2, the light intensity emitted from the RGB LEDs is detected using a color sensor provided in the backlight device, and the detected values are used to flow to the LEDs. By controlling the current, it is possible to maintain the desired luminance and white chromaticity by adjusting the luminance change and the chromaticity change.
JP 2004-333583 A JP 2004-21147 A

  However, in particular, AlInGaP, which is a red LED element material, has a problem that the emission wavelength shifts with a temperature change in addition to the above-described characteristic change. Furthermore, since the light receiving sensitivity of the color sensor has wavelength dependency, there is a problem that a difference occurs in the detection value of the color sensor before and after the shift of the emission wavelength. Therefore, even if the detection value satisfies a desired value, it differs from the actual luminance and chromaticity obtained from the backlight device, and it is difficult and inaccurate to precisely control the luminance and chromaticity. There was a problem of being.

  This invention is made | formed in view of the above, The place made into the subject is preventing the change of the brightness | luminance and chromaticity of the light radiated | emitted from LED during lighting of a backlight apparatus.

  The LED backlight device according to the first aspect of the present invention includes a red, green, and blue LED light emitting element formed of a nitride semiconductor, a color sensor that detects light emitted from the LED light emitting element, and luminance or chromaticity. And a control circuit for controlling a drive current value or a drive pulse width of the LED light emitting element so that a detection value detected by the color sensor becomes a set value.

  In the present invention, not only the green and blue LED light-emitting elements but also the red LED light-emitting elements are formed of nitride semiconductors, so that the shift of the emission wavelength due to temperature change can be reduced. This also makes the light-receiving sensitivity characteristic of the color sensor almost constant and reduces the detection value error, so that the drive current of the LED light-emitting element is maintained so as to maintain the desired luminance and chromaticity set by the control circuit. By controlling the value or the driving pulse width, it is possible to prevent changes in luminance and chromaticity of the LED backlight device.

  In the LED backlight device, the red, green, and blue LED light-emitting elements each include an InGaN active layer.

In the LED backlight device, the red LED light emitting element is formed of a nitride semiconductor that emits ultraviolet light, and a phosphor in which Eu ³⁺ that emits red light by excitation with the ultraviolet light is added to the periphery of the red LED light emitting element. It is characterized by arranging.

In the present invention, the red LED light emitting element emits ultraviolet light, and a phosphor added with Eu ³⁺ that emits red light when excited by the ultraviolet light is disposed around the red LED light emitting element. It is possible to reduce the shift of the emission wavelength due to the phosphor due to temperature change. This also makes the light-receiving sensitivity characteristic of the color sensor almost constant and reduces the detection value error, so that the drive current of the LED light-emitting element is maintained so as to maintain the desired luminance and chromaticity set by the control circuit. By controlling the value or the driving pulse width, it is possible to prevent changes in luminance and chromaticity of the LED backlight device.

  In the LED backlight device, the red LED light emitting element has an AlInGaN active layer, and the green and blue LED light emitting elements have an InGaN active layer.

  ADVANTAGE OF THE INVENTION According to this invention, the change of the brightness | luminance and chromaticity of the light radiated | emitted from LED can be prevented during the lighting of a backlight apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a perspective view showing an exploded configuration of a direct-type LED backlight device according to the first embodiment. The configuration of the present embodiment is fixed by an LED 1 that emits monochromatic light in red (R), green (G), and blue (B), an LED mounting board 2 on which a plurality of LEDs 1 are mounted linearly, and a front frame 3. A plurality of LED mounting boards 2 are arranged in parallel with a pair of diffusion sheets 5a and 5b arranged opposite to each other with the lens sheet 4 interposed therebetween, a diffusion plate 6 fixed by the front frame 3 and arranged between the diffusion sheet 5b and the LED 1. A unit case 7 that is disposed in the front frame 3, accommodates the lens sheet 4, the diffusion sheets 5 a and 5 b, and the diffusion plate 6, a color sensor 8 that is disposed on the side surface or the back surface of the unit case 7, and the outside of the unit case 7. The control circuit 9 is connected to the color sensor 8 and all the LEDs 1.

  In addition, a reflection member such as a white reflection sheet is provided at the bottom of the unit case 7 except for the LED 1 and the LED mounting substrate 2, and this bottom is used as a reflection region. The radiated light emitted from each of the RGB LEDs 1 is synthesized in white inside the unit case 7, and the outside of the unit case 7 through the lens sheet 4, the diffusion sheets 5 a and 5 b, and the diffusion plate 6 acting as a surface light emitting unit. Is emitted. The diffusion plate 6 has an optical diffusion effect of diffusing light, and the lens sheet 4 and the diffusion sheets 5a and 5b stacked on the diffusion plate 6 increase the brightness of the diffused light more effectively. . Note that the RGB mixing ratio, that is, the quantity and location of the RGB LEDs 1 arranged in the unit case 7 is determined in advance so that a desired white chromaticity can be obtained from the surface light emitting unit.

  Next, the configuration and operation of the LED 1, the color sensor 8, and the control circuit 9 in the present embodiment will be described more specifically.

The LED 1 is used as a light source as an LED backlight device. The light emitting elements of the blue and green LEDs ₁ are made of a nitride semiconductor having In _x Ga _1-x N that has been generally used in the past as an active layer (light emitting layer) composition. The light emitting element of the red LED ₁ is also made of a nitride semiconductor having In _x Ga _1-x N as the active layer composition, like the light emitting elements of the green and blue LED ₁ . Here, to obtain blue light emission, the value of x is set to 0.25 to 0.35. To obtain green light emission, the value of x is set to 0.45 to 0.55, and to obtain red light emission. The value of x is set to 0.75 to 0.85. The blue LED 1 has an emission spectrum peak at a wavelength of 440 to 470 nm, the green LED 1 has an emission spectrum peak at a wavelength of 515 to 535 nm, and the red LED 1 has an emission spectrum peak at a visible wavelength longer than the wavelength of 610 nm. A certain LED 1 is used. In addition, the value of x mentioned above and the wavelength of the emission spectrum peak of LED1 in each color are not restricted to these, What is necessary is just to be able to obtain each light emission of red, green, and blue from LED1. Further, instead of packaging all the RGB LEDs 1, it is possible to provide an LED backlight device that emits monochromatic light by packaging each LED 1.

  The color sensor 8 is composed of a photodiode such as silicon, and can be formed by integrating RGB photodiodes having RGB wavelength sensitivity characteristics as shown in FIG. In addition, the present invention is not limited to this, and it is also possible to make the respective RGB photodiodes close to each other. Photodiodes corresponding to each of RGB can eliminate incident light of other colors by forming a color filter in the light receiving portion of the photodiode. For example, in the case of a photodiode that receives red light, a color filter that excludes blue light and green light is formed in the light receiving portion of the photodiode. The color sensor 8 is not limited to a photodiode, and a light receiving element such as a photomultiplier tube or a CCD (charge coupled device) can be used as a sensor.

  As shown in FIG. 3, the control circuit 9 includes a color sensor data processing unit, a comparison calculation processing unit, and an LED driving unit. The brightness and chromaticity of the light emitted from each LED 1 is received by the color sensor 8, and the detected value is transmitted to the control circuit 9 connected to the color sensor 8. The transmitted detection value is converted from an analog value to a digital value by the color sensor data processing unit and transmitted to the comparison calculation processing unit. The comparison calculation processing unit compares the received luminance and chromaticity detection values with predetermined desired luminance and chromaticity values, and transmits the comparison result to the LED driving unit. The LED drive unit controls the drive current value or drive pulse width of each LED 1 based on the received comparison result. The comparison calculation processing unit also has a function of correcting luminance and chromaticity, dimming, and the like when necessary. The functions of the color sensor data processing unit, the comparison calculation processing unit, and the LED driving unit can be combined in a microcomputer, an IC, or the like.

  Subsequently, the conventional case using AlInGaP as the light emitting element of the red LED (hereinafter referred to as “conventional example”) and the case of this embodiment using InGaN as the light emitting element of the red LED 1 (hereinafter referred to as “present implementation”). Comparison with “form” will be described.

  FIG. 4 is a comparison diagram showing a comparison of RGB emission spectra at the initial lighting (solid line) and after 30 minutes (broken line) in a state where adjustment by the control circuit 9 is not performed. FIG. 4A is a comparison diagram showing a comparison of conventional examples, and FIG. 4B is a comparison diagram showing a comparison of this embodiment. In the case of the conventional example, as shown in FIG. 4 (a), it is possible to grasp that the red emission wavelength after 30 minutes has shifted greatly unlike the blue and green emission wavelengths. it can. On the other hand, in the case of this embodiment, as shown in FIG. 4B, the red emission wavelength shift hardly occurs after 30 minutes as compared with the conventional example. That is, by forming not only the green and blue LEDs 1 but also the red LEDs 1 from a nitride semiconductor, it is possible to reduce the shift of the emission wavelength accompanying the temperature change after a lapse of a certain time. In other words, nitride semiconductors generally have strong bonds between atoms, are chemically stable and have high phonon energy, and therefore have high thermal conductivity. When a current is passed through LED 1, a red LED is conventionally used. The temperature rise can be reduced as compared with AlInGaP mainly used. Thereby, the shift | offset | difference of a light emission wavelength can be reduced by utilizing as a red LED1 the nitride semiconductor with favorable thermal conductivity and a small temperature change at the time of current injection.

  FIG. 5 is a comparison diagram showing changes in luminance and chromaticity adjusted by the control circuit 9. FIG. 5A is a diagram showing a case of a conventional example, and FIG. 5B is a diagram showing a case of this embodiment. In the case of the conventional example, as shown in FIG. 5A, as the lighting time elapses (the displacement from the left side to the right side is taken as the time axis), the luminance value and the chromaticity value (in the xyz color system) It can be understood that both the x value and the y value have changed greatly. On the other hand, as shown in FIG. 5B, even when the lighting time has elapsed, changes in luminance and chromaticity hardly occur compared to the conventional example. That is, in the case of the conventional example, as shown in FIG. 4A, a red emission wavelength shifts with the passage of lighting time, and the light receiving sensitivity of the color sensor 8 is as shown in FIG. Therefore, there is a difference in the detection value of the color sensor 8 before and after the emission wavelength shift. As a result, even when the detection value of the color sensor 8 is controlled to be a desired value by the control circuit 9, as shown in FIG. A change in the degree will occur. On the other hand, in the present embodiment, as shown in FIG. 4B, the shift of the red emission wavelength can be reduced, so that even if the lighting time has elapsed, the control circuit 9 By adjusting, desired luminance and chromaticity can be maintained.

  According to the present embodiment, not only the green and blue LEDs but also the red LEDs are formed of the nitride semiconductor, so that the shift of the emission wavelength accompanying the temperature change can be reduced. This also makes the light-receiving sensitivity characteristic of the color sensor almost constant and the detection value error can be reduced. Therefore, the drive current value or drive pulse width of the LED is controlled by the control circuit so as to maintain a desired value. Thus, changes in luminance and chromaticity can be prevented.

[Second Embodiment]
The configuration of the red LED 1 in the second embodiment will be described. The light-emitting element of the red LED 1 is composed of a nitride semiconductor that emits ultraviolet light having a wavelength shorter than purple (for example, 430 nm or less) having an active layer (light-emitting layer) composition of Al _x In _y Ga _z N. Is used. Here, the value of x is 0.01 to 0.06, the value of y is 0.05 to 0.19, and the value of z is 0.75 to 0.95. In addition, the value of x, y, z mentioned above is not restricted to these, What is necessary is just to be able to obtain light emission of ultraviolet light from red LED1. Further, a phosphor added with Eu ³⁺ that emits red light by excitation with ultraviolet light and has a constant emission wavelength with respect to a temperature change is arranged dispersed or fixed around the light emitting element of the red LED 1. . As an example of this phosphor, Y ₂ O ₃ : Eu ³⁺ or Y ₂ O ₂ S: Eu ³⁺ that enables highly efficient red light emission by excitation with ultraviolet light can be used. The emission of Eu ³⁺ is a line spectrum peculiar to rare earth ions, and the emission wavelength is constant with changes in temperature. Since other configurations are the same as those in the first embodiment, description thereof is omitted here.

Next, in the conventional case where AlInGaP is used as the light emitting element of the red LED (hereinafter referred to as “conventional example”), AlInGaN that emits ultraviolet light as the light emitting element of the red LED 1 and red light emission by excitation with the ultraviolet light Comparison with the case of the present embodiment including a phosphor added with Eu ³⁺ (hereinafter referred to as “this embodiment”) will be described.

  FIG. 6 is a comparative diagram showing a comparison of RGB emission spectra at the initial lighting (solid line) and after 30 minutes (broken line) in a state where adjustment by the control circuit 9 is not performed. FIG. 6A is a comparison diagram showing a comparison of conventional examples, and FIG. 6B is a comparison diagram showing a comparison of this embodiment. In the case of the conventional example, as shown in FIG. 6A, it is possible to grasp that the red emission wavelength after 30 minutes has shifted greatly unlike the blue and green emission wavelengths. it can. On the other hand, in the case of the present embodiment, as shown in FIG. 6B, the red emission wavelength shift hardly occurs after 30 minutes as compared with the conventional example. That is, the temperature change after a lapse of a certain time is caused by the phosphor that is excited by the ultraviolet light emitted from the light emitting element of the red LED 1 to emit red light and further makes the emission wavelength of the red light constant with respect to the temperature change. The shift of the emission wavelength associated with can be reduced.

  Further, in the present embodiment, changes in luminance and chromaticity after a lapse of a fixed time after adjustment by the control circuit 9 are the same as those described with reference to FIG. 5 in the first embodiment. Then, explanation is omitted.

According to the present embodiment, the red LED emits ultraviolet light, and a phosphor added with Eu ³⁺ that emits red light when excited by the ultraviolet light is disposed around the red LED light emitting element to change the temperature. As a result, the shift of the emission wavelength due to the phosphor can be reduced. This also makes the light-receiving sensitivity characteristic of the color sensor almost constant and reduces the detection value error, so that the drive current of the LED light-emitting element is maintained so as to maintain the desired luminance and chromaticity set by the control circuit. By controlling the value or the driving pulse width, it is possible to prevent changes in luminance and chromaticity of the LED backlight device.

It is a perspective view which shows the decomposition | disassembly structure of the direct-type LED backlight apparatus in 1st Embodiment. It is a wavelength characteristic figure which shows the light reception sensitivity of a color sensor. It is a schematic diagram which shows the structure of a control circuit. In 1st Embodiment, it is a comparison figure which shows the comparison of the light emission spectrum of RGB in the lighting initial stage when not controlling by a control circuit, and after 30-minute progress. It is a comparison figure which shows the change of the brightness | luminance at the time of fixed time passage when control by a control circuit is performed. In 2nd Embodiment, it is a comparison figure which shows the comparison of the emission spectrum of RGB in the lighting initial stage when not controlling by a control circuit, and after 30-minute progress.

Explanation of symbols

1 ... LED
2 ... LED mounting substrate 3 ... Front frame 4 ... Lens sheet 5a, 5b ... Diffusion sheet 6 ... Diffusion plate 7 ... Unit case 8 ... Color sensor 9 ... Control circuit

Claims (4)

Red, green and blue LED light-emitting elements formed of nitride semiconductor;
A color sensor for detecting light emitted by the LED light emitting element;
A control circuit for controlling a drive current value or a drive pulse width of the LED light emitting element so that a detection value detected by the color sensor becomes a set value in order to adjust luminance or chromaticity;
An LED backlight device comprising:   The LED backlight device according to claim 1, wherein the red, green, and blue LED light-emitting elements have an InGaN active layer. The red LED light emitting element is formed of a nitride semiconductor that emits ultraviolet light, and a phosphor added with Eu ³⁺ that emits red light by the ultraviolet light excitation is disposed around the red LED light emitting element. The LED backlight device according to claim 1. The red LED light emitting element has an active layer of AlInGaN,
The LED backlight device according to claim 3, wherein the green and blue LED light emitting elements have an InGaN active layer.

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