JP2012033514A - Planar light source device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a planar light source device and a liquid crystal display with color unevenness and brightness unevenness restrained.SOLUTION: The planar light source device is provided with a plurality of light-emitting diodes 1 arrayed in a row, a first circuit 9 connecting in series the light-emitting diodes 1 arranged at either end in an arraying direction out of the plurality of light-emitting diodes 1, and second circuits 8, 10 different from the first 9, connected to the rest of light-emitting diodes other than those 1 arrayed at either end. Further, each light-emitting diode 1 at either end is given a current different from that of the rest of light-emitting diodes.

Description

  The present invention relates to a planar light source device, and more particularly to a planar light source device using an LED as a light source.

  In a liquid crystal display device, a liquid crystal panel including two substrates sandwiched between liquid crystal layers and a planar light source device as a backlight are provided on the back side of the liquid crystal panel. As a light source of a planar light source device, a linear cold cathode tube has been conventionally used as the light source. However, R, G, and B three-color light emitting diodes (hereinafter referred to as LEDs) are used because of their long life and good light emission. Some use a light source in which a plurality of each is arranged.

  A configuration of a liquid crystal display device using this LED will be described with reference to FIG. FIG. 1 is a diagram showing a configuration of an edge light type (also called side light type) liquid crystal display device in which a light source is provided at an end portion. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view. Reference numeral 1 denotes an LED, 2 denotes a light guide plate, 3 denotes a reflector, 4 denotes an optical sheet, 5 denotes a reflection sheet, and 6 denotes a liquid crystal panel. A plurality of R, G, and B LEDs 1 are arranged to emit white light. The light emitted from the LEDs 1 of the respective colors is reflected directly or by the reflector 2 and enters the side surface of the light guide plate 3. The light incident on the light guide plate 3 propagates while repeating total reflection in the light guide plate. The light in the light guide plate is emitted to the entire surface (the entire light emitting surface) on which the optical sheet 4 is provided. The light emitted to the side opposite to the light emitting surface is reflected by the reflection sheet 5 and enters the light guide plate 4 again. The light emitted from the light emitting surface side of the light guide plate 3 passes through the optical sheet 4 composed of a diffusion sheet, a protective sheet, a lens sheet, a prism sheet, etc., and enters the liquid crystal panel 6. The light that reaches the liquid crystal panel 6 is modulated in accordance with the video signal, and displays R, G, and B colors.

  In addition to the edge light type liquid crystal display device shown in the figure, a direct type with a light source provided on the back side of the liquid crystal panel, or a light source plate provided with a light source on the back side of the light guide plate in order to promote heat dissipation of the LED, through a reflector Some light is incident on.

  For example, a total of about 40 R, G, B LEDs 1 are provided in a 15-inch liquid crystal display device. The arrangement of the LEDs 1 is provided so that the entire display area becomes uniform white light, and the number thereof is adjusted so that the luminance of light of each color is equal. In a conventional liquid crystal display device, the LEDs 1 are connected in series, in parallel, or a combination thereof, and supplied with voltage and current. The circuit of this LED is shown in FIG. Here, 1 is an LED. A plurality of LEDs 1 are provided and connected in a series and parallel combination.

  In order to keep the variation and fluctuation of the LED 1 constant, in the conventional liquid crystal display device, as shown in Patent Document 1, constant current control is performed for the entire circuit to which the LED is connected. As a result, even if the LED drive voltage varies or fluctuates, or the LED itself varies in forward voltage, a constant current always flows through the LED circuit. Therefore, it is possible to obtain a planar light source device with no variation in backlight luminance. In addition, a planar light source device is used in which LEDs for each color of R, G, and B are connected in series or in parallel, voltage and current are supplied to each color circuit, and the luminance (light emission color) of each color is adjusted. There is also a liquid crystal display device.

  Such conventional liquid crystal display devices and planar light source devices have the following problems. Even if the same current or the same voltage is supplied to the LEDs 1, there are variations, fluctuations and temperature characteristics differences among the products of the LEDs 1, and the respective LEDs 1 do not have the same luminance and light amount. The arrangement and the number of LEDs 1 of the planar light source device are designed so that each color LED 1 has the same luminance. Accordingly, there is a problem in that variations in the luminance and light quantity of the LED 1 affect the luminance of the surface light source device, resulting in display unevenness and color unevenness. There is also a problem that the display characteristics of the planar light source device vary from product to product.

  Further, when a plurality of LEDs 1 are arranged as the planar light source device, a difference in temperature occurs depending on the location. For example, the LED 1 provided near the center of the planar light source device has a high temperature. The temperature characteristics of the LEDs 1 are different, and the brightness and light quantity of the LEDs 1 are different depending on the temperature of the usage environment. Therefore, when a plurality of LEDs 1 are arranged as a planar light source device or a liquid crystal display device, there is a problem in that the temperature varies depending on the location, and the luminance and light quantity of each LED 1 vary. In the case where the LED 1 having such a variation in luminance is used, a difference in luminance occurs depending on the display area. The luminance varies depending on the position, and display unevenness occurs. In addition, when the LEDs 1 of the respective colors are connected for each color, the luminance varies depending on the respective colors, and color unevenness may occur. As described above, the conventional planar light source device has a problem that display unevenness occurs. Furthermore, the change in the temperature of the planar light source device varies from product to product, resulting in a problem that display characteristics vary from product to product. In the liquid crystal display device using such a planar light source device, display unevenness and the like occur, and further, display characteristics vary from product to product.

Japanese Patent Laid-Open No. 11-305198

  As described above, the conventional planar light source device and the liquid crystal display device have a problem that color unevenness, luminance unevenness, or display unevenness occurs and display characteristics vary.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a planar light source device that suppresses uneven color or uneven display.

  A planar light source device according to the present invention includes a plurality of light emitting diodes arranged in a row, and a first circuit in which the light emitting diodes arranged at both ends in the arrangement direction among the plurality of light emitting diodes are connected in series. A planar light source device comprising: a second circuit different from the first circuit connected to the remaining light emitting diodes other than the light emitting diodes arranged at both ends.

  ADVANTAGE OF THE INVENTION According to this invention, the planar light source device and liquid crystal display device which suppressed color nonuniformity or display nonuniformity can be provided.

FIG. 1A is a plan view showing a configuration of a liquid crystal display device. FIG. 1B is a cross-sectional view illustrating a configuration of the liquid crystal display device. It is a circuit diagram of LED of the planar light source device concerning Embodiment 1 of this invention. It is another circuit diagram of LED of the planar light source device concerning Embodiment 1 of this invention. It is a circuit diagram of LED of the planar light source device concerning Embodiment 2 of this invention. It is another circuit diagram of LED of the planar light source device concerning Embodiment 2 of this invention. It is a circuit diagram of LED of the planar light source device concerning Embodiment 3 of this invention. FIG. 7A is a plan view showing a configuration of a different liquid crystal display device. FIG. 7B is a plan view showing a configuration of a different liquid crystal display device. FIG. 8A is a plan view showing a configuration of a different liquid crystal display device. FIG. 8B is a plan view showing a configuration of a different liquid crystal display device. It is a circuit diagram of LED of the conventional planar light source device.

Embodiment 1 of the Invention
A configuration of a liquid crystal display device using the planar light source device according to the present invention will be described with reference to FIG. FIG. 1A is a plan view of a liquid crystal display device, and FIG. 1B is a cross-sectional view. Reference numeral 1 denotes an LED (light emitting diode), 2 denotes a reflector, 3 denotes a light guide plate, 4 denotes an optical sheet, 5 denotes a reflection sheet, and 6 denotes a liquid crystal panel. A plurality of R, G, and B LEDs 1 are arranged to emit white light. The light emitted from the LEDs 1 of the respective colors is reflected directly or by the reflector 2 and enters the side surface of the light guide plate 3. The light incident on the light guide plate 3 propagates while repeating total reflection in the light guide plate. The light in the light guide plate is emitted to the entire surface (the entire light emitting surface) on which the optical sheet 4 is provided. The light emitted to the side opposite to the light emitting surface is reflected by the reflection sheet 5 and enters the light guide plate 3 again. The light emitted from the light emitting surface side of the light guide plate 3 passes through the optical sheet 4 composed of a diffusion sheet, a protective sheet, a lens sheet, a prism sheet, etc., and enters the liquid crystal panel 6. The light that reaches the liquid crystal panel 6 is modulated in accordance with the video signal, and displays R, G, and B colors.

  For example, a total of about 40 R, G, B LEDs 1 are provided in a 15-inch liquid crystal display device. The array of the LEDs 1 is provided so that the entire display area becomes uniform white light, and the number thereof is adjusted so that the luminance of the light of each color becomes equal. A method for adjusting the luminance of the LED 1 serving as the light source of the planar light source device according to the present invention will be described with reference to FIG. FIG. 2 is a circuit diagram of the LED 1. 7 is a variable resistor. Here, for explanation, only three LEDs 1 are shown and are connected in series. A power source is connected to both ends, and constant current control is performed. A variable resistor 7 is connected to each LED 1 in parallel. When the resistance value of the variable resistor 7 corresponding to the LED 1 having low luminance is increased, the current flowing through the variable resistor 7 is decreased and the current value flowing through the LED 1 is increased. Therefore, the brightness of the LED 1 is increased. Conversely, when the resistance value of the variable resistor 7 corresponding to the LED 1 having high luminance is lowered, the current flowing through the variable resistor 7 increases and the current value flowing through the LED 1 decreases. Therefore, the brightness of the LED 1 becomes weak. Thereby, the brightness | luminance of each LED1 can be adjusted. Even if the same current is supplied to the LEDs 1, the brightness of the individual LEDs 1 varies depending on the variation of the LEDs themselves and the difference in temperature characteristics, and when the display unevenness occurs, the brightness of the individual LEDs 1 is adjusted. As a result, the occurrence of display unevenness can be suppressed.

  For example, when the emission color of red (R) is weak, the resistance value of the variable resistor corresponding to the red LED 1 is increased to increase the luminance of R. Conversely, when the R emission color is strong, the resistance value of the variable resistor corresponding to the red LED 1 is lowered to increase the R luminance. Similar adjustments can be made for green (G) and blue (B) LEDs 1. As a result, the luminance of each color can be adjusted, and the emission color can be controlled. Further, since it is only necessary to connect the variable resistor 7 such as a volume meter to the LED 1, it becomes possible to adjust the luminance unevenness etc. with a simple configuration. Furthermore, the dispersion | variation for every product of a planar light source device by the dispersion | variation in LED1 can be suppressed.

  Further, when the planar light source device is assembled and turned on using the LED 1 as a light source, the temperature of the LED 1 varies depending on where the LED 1 is disposed. Due to this temperature difference, the luminance of the LED 1 varies, and display unevenness may occur. In addition, there may be variations in the temperature characteristics of the LEDs 1 here, or there may be differences in temperature characteristics depending on the LEDs 1 of the respective colors. In such a case, the planar light source device is turned on, and the luminance is detected and measured for each position or for each color (for each wavelength). Then, the resistance value of the variable resistor 7 of the LED 1 corresponding to the place where the luminance is weak and the color (wavelength) is increased. Conversely, the resistance value of the variable resistor 7 of the LED 1 corresponding to the location and color (wavelength) where the luminance is strong is lowered. Thus, by changing the resistance value of the variable resistor 7 corresponding to each LED 1, the brightness of the planar light source device is adjusted to be uniform. Thereby, even when the temperature characteristics of the LED 1 vary, the occurrence of display unevenness and color unevenness can be suppressed. By using such a planar light source device as a backlight, a liquid crystal display device free from display unevenness and color unevenness can be obtained.

  As described above, the LEDs 1 have individual variations. Therefore, even if the same current or voltage is supplied, the same luminance may not be obtained. A method for making the brightness of each LED 1 the same will be described below. First, the LEDs 1 are individually turned on, and the brightness of each LED 1 is detected and measured. The resistance value of the variable resistor 7 corresponding to the LED 1 having low luminance is increased to increase the luminance. On the contrary, the resistance value of the variable resistor 7 corresponding to the LED 1 having a high luminance is lowered to weaken the luminance. Thereby, the brightness | luminance of LED1 can be made the same. The same effect can be obtained by providing a resistor having an appropriate value instead of the variable resistor 7. The above-described method can also be used for controlling the luminance of the LED 1.

  In FIG. 2, the LEDs 1 are connected in series and the variable resistors 7 are connected in parallel to the LEDs 1. However, as shown in FIG. 3, the LEDs 1 are connected in parallel and the variable resistors 7 are connected in series to the respective LEDs 1. You may connect to. Here, as in FIG. 1, only three LEDs 1 are shown for explanation. When the LEDs 1 are connected in parallel, it is desirable to perform constant voltage control, contrary to the case where they are connected in series. By reducing the resistance value of the variable resistor 7 corresponding to the LED 1 having low luminance, the luminance of the LED 1 can be increased. On the contrary, the luminance of the LED 1 can be reduced by increasing the resistance value of the variable resistor 7 corresponding to the LED 1 having a high luminance. Thereby, a planar light source device in which display unevenness and color unevenness are suppressed can be obtained.

  As shown in FIGS. 2 and 3, the embodiment of the present invention is not limited to the LED 1 connected in series or in parallel, and may be a combination of series connection and parallel connection. The variable resistor 7 may not correspond to all the LEDs 1 provided in the planar light source device, and the same effect can be obtained if a plurality of variable resistors 7 are provided in some of the LEDs 1. . Furthermore, the variable resistor 7 may not correspond to each LED 1, and for example, one variable resistor 7 may be provided corresponding to two or more LEDs 1. Of course, two or more variable resistors 7 may be provided corresponding to one LED 1. Furthermore, the variable resistor 7 may be provided corresponding to only some of the LEDs 1.

  Moreover, even when the LED 1 is divided into two or more circuits and supplies current and voltage separately, the occurrence of display unevenness, luminance unevenness, and color unevenness can be suppressed by providing the plurality of variable resistors 7 as described above. Can do. For example, R, G, and B LEDs 1 may be divided into three circuits, and current and voltage may be supplied separately. By providing a plurality of variable resistors 7 in each circuit, it is possible to suppress the occurrence of display unevenness for each color (wavelength). The embodiment of the present invention is not limited to the illustrated one, and a plurality of variable resistors 7 may be provided for a plurality of LEDs 1. Thereby, display unevenness, luminance unevenness, and color unevenness of the planar light source device can be suppressed. Therefore, the luminance of the backlight light source can be made uniform. Moreover, it is suitable to use for a liquid crystal display device.

Embodiment 2 of the Invention
The light source of the planar light source device according to the second embodiment of the present invention will be described with reference to FIG. An LED is used as the light source, and FIG. 4 is a circuit diagram of the LED. 1 is an LED, and 8 is a switching element. Since the configuration of the planar light source device is the same as that described in the first embodiment, the description thereof is omitted. The LEDs 1 are connected in series, and the switching elements 8 corresponding to the respective LEDs 1 are connected in parallel. The switching element 8 is operated by a switching signal from a microcomputer or the like.

  As described above, the LEDs 1 have variations, fluctuations, and temperature characteristics, and there may be differences in luminance. A method of suppressing this luminance difference and suppressing display unevenness and color unevenness in the planar light source device will be described. Each LED 1 is turned on once, and each luminance is detected and measured. Alternatively, after the planar light source device is assembled, the luminance of the planar light source device is detected for each position or for each color. The switching element 8 corresponding to the LED 1 whose luminance is weak is turned off. As a result, since all of the current flowing through the circuit flows to the LED 1, the luminance increases. On the contrary, the switching element 8 corresponding to the LED 1 having high luminance is turned ON. Since a resistor having an appropriate resistance value is connected to the switching element 8, a part of the current flowing in the circuit flows to the switching element. Therefore, the current flowing through the LED 1 is reduced, and the luminance of the LED 1 is lowered. By adjusting the individual LEDs 1 in this manner, display unevenness, color unevenness, and brightness unevenness of the planar light source device can be suppressed.

  Further, as shown in FIG. 5, by connecting a plurality of switching elements 8 in parallel with LED 1, the current flowing through LED 1 can be adjusted stepwise. Thereby, the brightness | luminance of LED1 can be adjusted more finely. If the switching element 8 has an appropriate load resistance, the same effect can be obtained without providing a resistance. In the present embodiment, since the switching element 8 may be connected to the LED 1, it is possible to adjust the luminance unevenness and the like with a simple configuration. Furthermore, the dispersion | variation for every product of a planar light source device by the dispersion | variation in LED1 can be suppressed. The switching element 8 may not correspond to all the LEDs 1 provided in the planar light source device, and the same effect can be obtained if a plurality of switching elements 8 are provided. Furthermore, the switching element 8 may not correspond to each LED 1, and for example, one switching element 8 may be provided corresponding to two or more LEDs 1. Furthermore, the switching element 8 may be provided corresponding to only some of the LEDs 1.

Embodiment 3 of the Invention
A method for suppressing display unevenness, color unevenness, and luminance unevenness of the planar light source device according to the third embodiment will be described with reference to FIG. An LED is used as the light source, and FIG. 6 is a circuit diagram of the LED. Reference numeral 1 denotes an LED, 9 denotes a first circuit, 10 denotes a second circuit, and 11 denotes a third circuit. The planar light source device has the same configuration as that shown in FIG.

  First, the planar light source device is turned on by connecting to the same circuit. And the brightness | luminance for every position and every color is detected in the display area. The LEDs 1 are divided into a plurality of groups based on the detected brightness level. Here, as shown in FIG. 6, the LED 1 is divided into a group of LED 1 having a low luminance (hereinafter referred to as a first group), a group of LEDs 1 having a high luminance (hereinafter referred to as a third group), and a luminance between the first group and the second group. The LED 1 is divided into three groups (hereinafter referred to as a second group). Divide each group into separate circuits. Here, a circuit corresponding to the first group is referred to as a first circuit 9, a circuit corresponding to the second group is referred to as a second circuit 10, and a circuit corresponding to the third group is referred to as a third circuit 11. Here, two LEDs 1 are connected in series to the first circuit 9 and the third circuit 11, and three LEDs 1 are connected in series to the second circuit 10, and constant current control is performed.

  Since the first circuit wants to increase the luminance, it is controlled with a higher current than the other circuits. On the other hand, the third circuit wants to lower the luminance, so it is controlled with a lower current than the other circuits. The second circuit is controlled by a current between the first circuit and the third circuit. Thereby, the brightness | luminance of LED1 can be adjusted with each circuit, and the display nonuniformity of a planar light source device, color nonuniformity, and luminance nonuniformity can be suppressed. Of course, the present invention is not limited to the circuit configuration shown in the figure, and the same effect can be obtained if the LEDs 1 are divided into a plurality of groups for each current to be used and different circuits are used. The same effect can be obtained if the number of LEDs 1 connected to each circuit is one or more. Further, the LEDs 1 may be connected in parallel or may be a combination thereof. The control of the LED 1 is not limited to the current control, and the same effect can be obtained even if the LED 1 is provided in parallel and the voltage is controlled. Furthermore, these embodiments may be combined. Further, the LED 1 may be divided and connected to a plurality of circuits from the beginning, and the above adjustment method may be repeated several times to make the luminance more uniform. In the planar light source device, the present invention divides a plurality of LEDs 1 into different circuits and individually controls currents and voltages supplied to the respective circuits. Therefore, the present invention is not limited to the above-described embodiment. In the present embodiment, the LEDs 1 may be connected to different circuits and the current or voltage may be individually controlled, so that it is possible to adjust luminance unevenness and the like with a simple configuration. Furthermore, the dispersion | variation for every product of a planar light source device by the dispersion | variation in LED1 can be suppressed.

Other embodiments.
The above-described embodiments may be used in combination. For example, taking Embodiments 1 and 3 as an example of combination, LEDs 1 of different colors of R, G, and B may be divided into three different circuits. In each circuit, the LEDs 1 may be connected in series, and the variable resistors 7 may be provided in parallel to the individual LEDs 1. Furthermore, each color LED 1 may be further divided into a plurality of different circuits. Thereby, the brightness unevenness and display unevenness of each emission color of the planar light source device can be adjusted more finely.

  In the present invention, the brightness of each LED 1 may be adjusted only when the planar light source device or the liquid crystal display device is assembled. If it is a planar light source device or a liquid crystal display device in which the luminance of the LED 1 is adjusted when assembling, luminance unevenness, display unevenness, and color unevenness are suppressed. In this case, an adjustment means for making the luminance uniform after commercialization becomes unnecessary, and thus a planar light source device or a liquid crystal display device can be manufactured with a simpler configuration. If the surface light source device or the liquid crystal display device is turned on for a certain period of time and the adjustment is made after the temperature has become almost constant and the change in temperature has been reduced, a surface shape in which uneven brightness is suppressed during continuous use. A light source device or a liquid crystal display device can be obtained. Of course, an adjustment unit that adjusts the resistance value of the variable resistor 7 and an adjustment unit that operates the switching element may be provided in the planar light source device or the liquid crystal display device as a product. As a result, it is possible to adjust the luminance as needed, and it is possible to obtain a planar light source device and a liquid crystal display device having uniform luminance regardless of the usage environment and changes over time.

  In addition, the variable resistor 7 or the switching element 8 may be provided only in the LED 1 where the temperature change is large to adjust the luminance. Thereby, the dispersion | variation in the temperature characteristic of LED1 can be suppressed, and the planar light source device with uniform brightness | luminance can be obtained. Accordingly, it is possible to obtain a planar light source device with little change in display characteristics even in different environments.

  The planar light source device according to the present invention is preferably used for a liquid crystal display device. Further, the liquid crystal display device is not limited to the liquid crystal display device illustrated in FIG. FIG. 7A is a plan view showing the configuration of the liquid crystal display device, and FIG. 7B is a cross-sectional view. Since the same reference numerals as those in FIG. 1 indicate the same configuration, the description thereof is omitted. In the liquid crystal display device shown in FIG. 7, the LED 1 is provided in a planar shape. Moreover, the light guide plate is not provided. The light from the LED 1 provided in a planar shape is diffused by the optical sheet 4 made of a diffusion sheet or the like and is incident on the liquid crystal panel 6. In this case, since each LED 1 corresponds to the display area, the occurrence of display unevenness can be further suppressed.

  Further, it can be used for a liquid crystal display device having a structure as shown in FIG. FIG. 8A is a plan view showing the configuration of the liquid crystal display device, and FIG. 8B is a cross-sectional view. Since the same reference numerals as those in FIG. 1 indicate the same configuration, the description thereof is omitted. In the liquid crystal display device shown in FIG. 8, the LED 1 is provided on the back surface of the light guide plate 3 with the reflection sheet 5 interposed therebetween. The light from the LED 1 reflected by the reflector 2 enters the side surface of the light guide plate 3. In the liquid crystal display device having such a configuration, since the heat of the LED 1 is radiated, the temperature rise is small. Therefore, the influence on the luminance due to the variation in the temperature characteristics of the LED 1 is small. The configuration of the liquid crystal display device according to the present invention is not limited to the illustrated configuration.

  The planar light source device according to the present invention is not limited to use in a liquid crystal display device, but can also be used as a light source device, and can also be used in other display devices. The effect of the present invention can be obtained by providing the above-described luminance adjusting means (variable resistor, switching element or connection to different circuits) for a plurality of light sources. Accordingly, the light source is not limited to the LED, and the same effect can be obtained if there are a plurality of light sources such as a light emitting element, an electroluminescence element (EL), and a cold cathode tube.

1 LED
2 reflector 3 light guide plate 4 optical sheet 5 reflective sheet 6 liquid crystal panel 7 variable resistor 8 switching element 9 first circuit 10 second circuit 11 third circuit

Claims (3)

A plurality of light emitting diodes arranged in rows;
A first circuit in which the light emitting diodes arranged at both ends in the arrangement direction among the plurality of light emitting diodes are connected in series;
A second circuit different from the first circuit, connected to the remaining light emitting diodes other than the light emitting diodes arranged at both ends;
A planar light source device comprising:   The planar light source device according to claim 1, wherein a current different from that of the remaining light emitting diodes is caused to flow through each of the light emitting diodes at both ends. A light guide plate for guiding light from the plurality of light emitting diodes;
The planar light source device according to claim 1, wherein the plurality of light emitting diodes are arranged such that light emitted from the light emitting diodes is incident on a side surface of the light guide plate.

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