JP2009129591A - Backlight device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a backlight device capable of driving a required number of light-emitting elements in matrix while restraining cost hike. <P>SOLUTION: The backlight device 120 includes a power source line 200 of an anode side, a power source line 210 of a cathode side, and a plurality of current paths 220, 222, 224 connected in parallel between the power source lines. The current paths 222, 224 serially connect four LEDs, and a current path 226 connects three LEDs in series. A total of VF of the four LEDs of the current paths 220, 222 and a total of VF of the three LEDs are equal, whereby, matrix driving of the required number of LEDs is enabled. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a backlight device serving as a light source of a liquid crystal display panel, and more particularly to a method of driving a light emitting element of a backlight device in a matrix manner.

  Since the liquid crystal display panel itself is not a light emitting element, it is desirable to attach a backlight device to the back surface thereof. Conventionally, a cold cathode fluorescent lamp (hereinafter referred to as CCFL) has been used for the backlight device, but recently, a low power consumption and long-life light emitting diode (hereinafter referred to as LED) has been used instead. ) Is increasing.

  The CCFL is a linear light source having an elongated cylindrical shape similar to a general fluorescent lamp, and has been widely used as a light source for a liquid crystal display panel in combination with a light guide plate utilizing the elongated shape. However, CCFLs are vulnerable to repeated switching on / off of the switch, and darkening occurs when the use period is further extended, the characteristics at low temperature are not good, and mercury is contained. Therefore, an LED is used as a new light source. Since the LED is a point light source, a plurality of LEDs are required for use as a backlight light source.

  There are a matrix driving method and a serial driving method for lighting a plurality of LEDs. FIG. 1A is an example of a 4 × 3 matrix drive circuit, and FIG. 1B is an example of a series drive circuit. The matrix drive circuit includes a common anode-side power supply line 3 connected to the light source drive circuit 4, and cathode-side power supply lines 6a, 6b, 6c individually connected to the light source drive circuit 4 for constant current control. A plurality of current paths 2 connected in parallel with each other. The current path 2 is composed of a plurality of LEDs 1 connected in series. In this specification, regarding the matrix arrangement of LEDs, the direction in which the current paths 2 are connected in parallel is referred to as “row”, and the direction in which the LEDs 1 are connected in series is referred to as “column”. Although FIG. 1A shows a matrix arrangement of 4 rows × 3 columns, it may be 3 rows × 4 columns, 6 rows × 2 columns, or 2 rows × 6 columns.

  As shown in FIG. 1B, the series drive circuit is a simple circuit in which all twelve LEDs are arranged in series on the power supply line 3 connected to the light source drive circuit 4. In the series drive circuit, the drive voltage increases in proportion to the number of LEDs, and when the part of the wiring connecting the LEDs is disconnected, the entire circuit is cut off and all the light sources to the liquid crystal display panel are lost. There are disadvantages. For this reason, when an LED is used for the backlight of a medium-sized to large-sized liquid crystal display panel, a matrix driving method is generally adopted.

  Since LEDs have variations in luminous intensity, the LEDs are classified into a plurality of ranks according to the variations. When it is desired to suppress variations in the in-plane luminous intensity distribution of the LEDs, the variation is minimized by using LEDs having the same luminous intensity rank. On the other hand, selecting LEDs having the same luminous intensity rank results in high costs.

  In Patent Document 1, in order to solve such a problem, LEDs classified into at least two different ranks are mounted on each small block partitioned so that the same number of LEDs are included, and the LEDs mounted on each small block The combination of ranks is matched.

JP 2007-65414 A

  In the conventional car navigation apparatus, as shown in FIG. 2A, for example, a 7-inch wide liquid crystal display panel is used, and 12 LEDs are linearly arranged in the longitudinal direction. The required number of LEDs is calculated based on factors required for the display, such as luminance, liquid crystal transmittance, color filter density, loss of optical members, and luminance increasing members. When the intervals at which the LEDs are arranged are equally spaced, the interval D is expressed by the following equation.

  If the length L of the long side of the effective display area of the 7-inch wide liquid crystal display panel is about 160 mm, and the number of LEDs is 12, the LEDs are arranged at a distance D of 12.3 mm, and the left and right LEDs are , Arranged at a distance of 6.15 mm, which is half the distance D from the edge.

  As described above, LEDs have variations in color tone and forward voltage drop (hereinafter abbreviated as VF) in addition to variations in light quantity, and LEDs are ranked according to color tone and VF. Since the light quantity of the LED is proportional to the drive current, there is a restriction that an equal drive current must be supplied to each column of the matrix drive circuit. For this reason, all the LEDs in each column need to have the same VF rank, or the combinations of VF ranks in each column need to be equal.

  FIG. 3 shows a matrix drive circuit in which the combinations of the VF ranks of 12 LEDs are matched. As shown in the figure, the LED in the first row is VF rank (A) 3.0V, the LED in the second row is 3.4V in VF rank (B), and the LED in the third row is VF rank. The VF (C) 3.8 V and the LEDs in the fourth row are set to VF rank VF (D) 4.2 V, and the total VF value of each column is equal to 14.4 V.

  By the way, the luminous efficiency of the LED has been improved by technological development and the like, and it is possible to output an equivalent luminance with a smaller number of LEDs than before. Moreover, the pseudo white LED used includes a sorting fee such as a unit price and a rank, a mounting cost, an inspection cost, and the like, which are reflected in the cost of the backlight device and the liquid crystal display device. For this reason, it is desired to reduce the number of LEDs as much as possible.

  When the number of LED lamps is reduced from 12 by the conventional matrix drive system, the number of LED lamps is 10 in order to equalize the drive current of each column. When the number of LEDs is 10, the distance D between the LEDs is 14.5 mm from the above formula (1) (see FIG. 2B), which is 14.5-12.3 compared to the case of 12 LEDs. = The distance is increased by 2.2 mm. If the number of lamps is reduced at a stroke in this way, the in-plane uniformity is deteriorated and light source spots appear strongly on the liquid crystal panel.

  On the other hand, it is possible to reduce the number of LEDs by 1 to 11 but in this case, as shown in FIG. 4, a light source driving circuit for driving a matrix of 4 rows × 2 columns. 4 and a light source driving circuit 4a for driving three LEDs connected in series must be provided separately, which complicates the circuit and causes an extra cost.

  An object of the present invention is to provide a backlight device that can drive a desired number of light-emitting elements in a matrix while suppressing an increase in cost while paying attention to such a conventional problem.

  The backlight device according to the present invention includes a first power line, a second power line, and a plurality of current paths connected in parallel between the first and second power lines, and the plurality of currents Each of the paths includes a plurality of light emitting elements connected in series, and the first current path included in the plurality of current paths includes a first number of light emitting elements and is included in the plurality of current paths. The second current path includes a second number of light emitting elements different from the first number, and the total of forward drop voltages of the light emitting elements included in the first current path and the light emitting elements included in the second current path. The difference in forward voltage drop is within a certain value.

  Preferably, the combination of the forward drop voltages of the light emitting elements included in the first current path is different from the combination of the forward drop voltages of the light emitting elements included in the second current path. Preferably, each light emitting element included in the first and second current paths has the same lightness rank classified based on light intensity and the same color rank classified based on color tone. Preferably, the light emitting elements included in the plurality of current paths are linearly arranged on the substrate at equal intervals. Preferably, the backlight device further includes a light guide plate that diffuses light from the light emitting element.

  According to the present invention, when the number of light emitting elements included in the first and second current paths is different, the current path controlled by the light source driving circuit is controlled by keeping the total of the forward drop voltages within a certain value. Thus, the optimal number of light emitting elements and the optimal interval between the light emitting elements can be determined without being restricted by the matrix drive circuit. Furthermore, according to the present invention, an existing circuit can be used without considering a change, addition, or another method of a driving circuit of a light emitting element, and as a result, the number of liquid crystal display devices can be reduced.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings. Here, an example will be described in which the backlight device has light emitting diodes arranged in a line and is converted into a surface light source by a light guide plate.

  FIG. 5 is a block diagram showing the configuration of the liquid crystal display device according to the embodiment of the present invention. The liquid crystal display device 100 includes a liquid crystal display panel 110 and a backlight device 120 as a light source of the liquid crystal display panel 110. The backlight device 120 includes a light source driving circuit 130 that drives LEDs, a light source unit 140 in which LEDs are arranged in a line, and a light guide plate 150 that converts light from the light source unit 140 into two dimensions. The light source unit 140 also includes a printed circuit board 142 on which a plurality of LEDs are arranged in a line. In the present embodiment, the number of LEDs is 11, and the 11 LEDs are driven by a matrix drive circuit as will be described later.

  Even if LEDs are manufactured by the same manufacturing method as described above, the quality and performance vary. The variations in LEDs include luminous intensity, chromaticity, and forward voltage drop (VF), and ranking is performed according to these. 6A and 6B are diagrams for explaining LED ranking, in which FIG. 6A shows an example of luminous intensity rank, FIG. 6B shows a tone rank, and FIG. 6C shows an example of VF rank.

  The luminous intensity rank classifies the brightness of the LED when a constant forward current is applied between the anode and the cathode of the LED. As shown to Fig.6 (a), it classify | categorizes into either rank 1 to rank 4 according to the luminous intensity of each LED when the electric current of 150 mA is sent, for example.

  The color tone rank of the LED is ranked according to the color tone emitted by the LED. For example, when the ideal white color coordinate is represented by x = 0.25 and y = 0.25, the white color emitted by each individual LED is represented as a coordinate having a predetermined range of variation around it. It will be. This relationship is shown in FIG. Here, the ranks are classified into nine ranks from a to i, and the e rank is the rank closest to white.

  As shown in FIG. 6C, the VF rank is classified into 4 ranks, for example. In order to drive the LED, a forward bias is applied to the LED, and the voltage drop at that time varies depending on the LED. In FIG. 6C, classification is made into four ranks VF (A) to VF (D) according to the value of the forward voltage drop.

  The printed circuit board 12 is configured by forming a wiring pattern such as a copper foil on an insulating plate such as an epoxy resin, and a plurality of LEDs are arranged in a line at equal intervals on the substrate. In the light guide plate 20, for example, a plurality of reflecting portions are formed on one surface of an acrylic plate having a thickness of 2 to 5 mm, and a line-shaped point light source incident from one side surface is reflected by the reflecting portion, and the liquid crystal display It becomes a surface light source of the panel 110.

  FIG. 7 shows a configuration of a matrix drive circuit that drives 11 LEDs in the backlight device according to the present embodiment. The matrix driving circuit includes an anode-side power line 200 connected to the light source driving circuit 130, cathode-side power lines 210a, 210b, and 210c, and a current path that is connected in parallel between the power lines 200 and 210a to 210c. 220, 222, 224. The current path 220 in the first column connects four LEDs in series, the current path 222 in the second column connects four LEDs in series, and the current path 224 in the third column has three Connect the LEDs in series.

  The number of LEDs in the first and second rows is equal, but the number of LEDs in the third row is different from the number of LEDs in the first and second rows. In this embodiment, when the number of LEDs in each column is different, the VF rank is selected so that the total value of VFs in each column is equal or the difference is minimized. In the current paths 220 and 222 in the first and second columns, the LED 250 in the first row has a rank VF (A) and VF is 2.9 V. The LED 252 in the second row has rank VF (A), and VF is 3.0V. The LED 254 in the third row has rank VF (B), and VF is 3.3V. The LED 256 in the fourth row has a rank VF (C), and VF is 3.7V. Therefore, the total value of VF of the LEDs in the first row and the second row is 12.9V. On the other hand, in the current path 224 in the third column, all three LEDs 258 are in rank VF (D) and VF is 4.3 V. Therefore, the total value of VF is 12.9V. Further, it is desirable that the light intensity rank and the color tone rank of the LEDs 250, 252, 254, 256, and 258 in the first to third columns are the same.

  By configuring the matrix driving circuit in this way, the constant current control is performed between the first and second power supply lines 200 and 210a to c from the light source driving circuit 130, so that the total forward voltage of each column is equal. Thus, each LED is lit with light intensity and color tone with little variation. Thus, even when the total number of LEDs is an indivisible number or a prime number, the LED can be optimally turned on by the matrix drive circuit.

  The rank VF shown in the matrix drive circuit shown in FIG. 7 is an example, and the present invention is not limited to this. Furthermore, the combination of VF ranks used for each column and the total VF value of each column can be changed as appropriate. Further, in the above example, the total VF of each column is all equal to 12.9 V, but the difference of the total VF of each column may be suppressed to a constant value (for example, within 0.1 V).

  Further, when eleven LEDs are driven, as shown in FIG. 8, the configuration may be 3 rows × 3 columns and two LEDs connected in series to the current path 226 in the fourth column. A combination as shown in the table of FIG. 8B can be used for the VF of each LED. The total of the VFs in the fourth column is 8.7 V, and the total of the VFs in the other columns is 8.8 V. It is. Of course, the total of VF in the fourth column is preferably 8.8V.

  FIG. 9A is a diagram illustrating an arrangement of 11 LEDs. The liquid crystal display panel 110 is a 7-inch wide size, and the length L of the long side of the effective display area is 160 mm. According to Equation 1 above, the LEDs are arranged at an interval of about 13.3 mm, and the LEDs at the left and right ends are arranged at 6.65 mm from the edge. The numbers inside the square in the figure indicate the number of lights.

  FIG. 9B shows the difference between the LED interval when the number of LEDs is 12, 11, and 10 and the interval when the number of LEDs is 12. When the number of lamps is reduced from 12 to 10, the distance between the LEDs is increased by 2.2 mm. However, when the number is reduced to 11 lamps, the distance is 1.0 mm, and the light source is not uneven. There are no spots.

  FIG. 10A is a schematic diagram of the liquid crystal display device 100 using the light guide plate 150. The light guide plate 150 is installed on the back surface of the liquid crystal display panel 110, and the light source unit 140 is installed along one side surface of the light guide plate 150. The point light source on the line irradiated from the light source unit 140 is reflected at a right angle by the light guide plate 150 and irradiates the liquid crystal display panel 110 as a surface light source. FIG. 10B shows an image in which the LEDs constituting the matrix drive circuit shown in FIG. 7 are arranged in a straight line. Moreover, as shown in FIG.10 (c), the planar light source of LED can also be obtained by making LED arranged in the line shape shown in FIG.10 (b) into a some line. In this case, a diffusion sheet for diffusing LED light may be interposed between the planar light source and the liquid crystal display panel 110.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments according to the present invention, and various modifications can be made within the scope of the gist of the present invention described in the claims. Deformation / change is possible.

FIG. 1A is an example of a conventional 4 × 3 matrix driving circuit, and FIG. 1B is an example of serial driving with 12 lamps. FIG. 2 (a) is a diagram for explaining a linear arrangement of 12 conventional LEDs and FIG. 2 (b) is a diagram for explaining a conventional linear arrangement of 10 LEDs. FIG. 3A is a diagram for explaining a combination of VF ranks in a conventional matrix drive circuit, and FIG. 3B is a table showing the VF rank of each LED. It is a figure explaining the subject of the conventional matrix drive circuit. FIG. 5 is a block diagram showing the configuration of the liquid crystal display device according to the embodiment of the present invention. FIG. 6A is a diagram illustrating the luminous intensity rank of the LED, FIG. 6B is a color tone rank, and FIG. 6C is a diagram illustrating the VF rank. FIG. 7A is a diagram showing a configuration example of a matrix drive circuit for 11 LEDs in the backlight device according to the present embodiment, and FIG. 7B is a table showing the VF of each LED. FIG. 8A is a diagram showing an example of another matrix drive circuit of 11 LEDs, and FIG. 8B is a table showing the VF of each LED. FIG. 9A is a diagram showing the intervals of 11 lamps arranged in a line, and FIG. 9B is a table for comparing the LED intervals of 12 lamps, 11 lamps, and 10 lamps. FIG. 10A is a schematic diagram of the liquid crystal display device of this example, FIG. 10A shows an example using a light guide plate, FIG. 10B shows an array example of matrix driven LEDs, and FIG. c) is a diagram showing a configuration example of an LED for obtaining a two-dimensional light source.

Explanation of symbols

100: liquid crystal display device 110: liquid crystal display panel 120: backlight device 130: light source driving circuit 140: light source unit 142: printed circuit board 150: light guide plates 200, 210a, 210b, 210c, 210d: power supply lines 220, 222, 224, 226: Current path 250, 252, 254, 256, 258: LED

Claims (6)

A first power line;
A second power line;
A plurality of current paths connected in parallel between the first and second power supply lines,
Each of the plurality of current paths includes a plurality of light emitting elements connected in series,
The first current path included in the plurality of current paths includes a first number of light emitting elements, and the second current path included in the plurality of current paths is a second number of light emission different from the first number. Including elements,
The backlight device, wherein a difference between a total forward voltage drop of the light emitting elements included in the first current path and a forward voltage drop of the light emitting elements included in the second current path is within a certain value. The backlight device according to claim 1, wherein a combination of forward drop voltages of the light emitting elements included in the first current path is different from a combination of forward drop voltages of the light emitting elements included in the second current path. 3. The backlight device according to claim 1, wherein each light emitting element included in at least the first and second current paths has the same light intensity rank classified based on light intensity and a color rank classified based on color tone. . 4. The backlight device according to claim 1, wherein the light emitting elements included in the plurality of current paths are linearly arranged on the substrate at equal intervals. 5. The backlight device according to claim 1, further comprising a light guide plate that diffuses light from the light emitting element. A liquid crystal display device comprising: the backlight device according to claim 1; and a liquid crystal display panel irradiated with light from the backlight device.

Images