JPH06222721A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To overcome the differences in the luminous fluxes emitted by respective fluorescent lamps themselves so as to uniformalize display brightness over the entire surface by varying the capacities of output capacitors to the respective fluorescent lamps to regulate the currents to the respective fluorescent lamps. CONSTITUTION:The capacities of the output capacitors to the respective fluorescent lamps of an inverter system power source circuit for lighting the fluorescent lamps which uses plural pieces of the fluorescent lamp as the back light source of a transmission type liquid crystal display element are respectively varied to have uniformalize the brightness of the back light source over the entire surface. Namely, the cold cathode discharge lamps (fluorescent lamps) 4A, 4B, 4C can be formed to a small outside diameter but variations are generated in the brightness as well with each of the tubes by the variations, etc., in the mercury vapor quantity in the tubes, the film thicknesses of the fluorescent films, etc., by each tube. The lamp currents flowing to the respective cold cathode discharge lamps 4A, 4B, 4C are respectively regulated by regulating the capacity values of the capacity varying type ballast capacitors 5A, 5B, 5C, to regulate the respective cold cathode discharge lamps 4A, 4B, 4C to the uniform brightness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a plurality of fluorescent lamps as a light source of a backlight for illuminating a transmissive liquid crystal display element from the back, and these are turned on by an inverter type power source. TECHNICAL FIELD The present invention relates to a liquid crystal display device in which the brightness of the light emitting surface of a backlight is uniform over the entire surface and display quality is improved.

[0002]

2. Description of the Related Art Liquid crystal display devices are increasingly using a backlight in order to make the display easy to see or to make a color display look bright. These backlight units include a system directly below the fluorescent lamp (a system in which multiple fluorescent lamps are arranged directly below the light emitting surface of the backlight according to the size of the screen) and a sidelight system (light emitting surface of the fluorescent lamp). There is a method in which the light is refracted and reflected behind the light emitting surface by using a light guide plate or the like, which is arranged at a position hidden from the normal light emitting surface at the end), but in either method, the brightness on the light emitting surface is different. Various techniques have been developed so as to be uniform over the entire surface.

[0003]

When the area of the display surface is large, if the luminance of the light emitting surface of the backlight is increased and a bright display is desired, the total amount of luminous flux can be freely increased. However, on the back surface of the light emitting surface of this method, there are places where a large amount of direct light from the lamp is incident and places where mainly only reflected light is incident, and the brightness of the light emitting surface may become uneven. In order to equalize the brightness of the light emitting surface of the backlight of this system, conventionally, for example, in Japanese Patent Laid-Open No. 1-1117677, the size of a large number of meshes of a net that allows light from a fluorescent lamp to pass through is determined by these sizes. The amount of passing light passing through the mesh is set to be substantially equal on the output side of the passing light, and the passing light of the shield net is almost equalized on the output side. Curtain technology has been disclosed.
However, even with this technique, there is a problem in that the brightness of the light emitting surface of the backlight is not uniform when there is a difference in the amount of light emitted from each fluorescent lamp itself, which is a light source. Such a problem occurs not only in the direct type, but also in the sidelight type in which fluorescent lamps are provided at both ends of the display surface.

The present invention is a liquid crystal display device equipped with a backlight unit using a plurality of fluorescent lamps as described above, and the display brightness is made uniform over the entire surface by overcoming the difference in the luminous flux of the fluorescent lamps themselves. An object is to provide a liquid crystal display device.

[0005]

In order to solve the above problems, in the present invention, the capacity of the output capacitor to each fluorescent lamp of the inverter type power supply circuit for lighting the fluorescent lamp is made variable and each fluorescent lamp is changed. The current to the back light source was adjusted so that the brightness of the back light source could be made uniform over the entire surface.

[0006]

In recent years, liquid crystal display devices have come to be widely used for portable personal computers, word processors and the like, taking advantage of the fact that the depth is short even when the screen is large. In such applications, a liquid crystal display device equipped with a backlight is predominantly used in order to make it easy to see anywhere without being affected by the surrounding environment, particularly in the case of color display. In such an application, a battery power source is also preferably used for lighting the backlight so that it can be used as a portable device and can be used anywhere. Further, in order to shorten the depth, cold cathode discharge lamps having a narrow tube diameter are widely used as fluorescent lamps. However, in a cold cathode discharge tube, it is necessary to apply a high voltage of several thousand V at the start of discharge. There are some that have. Due to such a trend, the backlight unit of the liquid crystal display device for the above-mentioned application uses the inverter type power supply. In addition, since such a backlight also needs to be lightweight, instead of using an inductor using an iron core connected in series as in a hot cathode fluorescent lamp for normal lighting and using it as a ballast, a capacitor is used. Is used as a ballast. In the present invention, the capacity of the output capacitor (ballast capacitor) that supplies the lamp current to each fluorescent lamp of this inverter type power supply circuit is made variable so that the lamp current can be arbitrarily adjusted for each fluorescent lamp. The brightness of each of the light lamps was adjusted so that the brightness of the display surface of the liquid crystal display device including the backlight unit using a plurality of fluorescent lamps was made uniform over the entire surface.

[0007]

1 is a schematic circuit diagram of an embodiment of the present invention.
As the light source of the backlight, a cold cathode discharge lamp having an outer diameter of 3 mm and requiring a high voltage of V _PP of about 8 kV to start discharge was used. In the figure, 1A and 1B are transistors, 2 is a capacitor for obtaining a high frequency current of about 40 kHz from DC in cooperation with a transformer primary winding inductance, and 3 is a step-up transformer for obtaining the voltage of several thousand V. 4A, 4B, 4C, etc. are cold cathode discharge lamps, 5A, 5
B, 5C, etc. are variable-capacity output capacitors (ballast capacitors). The cold cathode discharge lamp can make the outer diameter of the tube extremely thin as described above, but the brightness varies depending on the tube-to-tube variations such as the amount of mercury vapor in the tube, the gas pressure of the inert gas, and the thickness of the fluorescent film. Also varies from pipe to pipe. The brightness of each cold cathode discharge lamp is adjusted uniformly by adjusting the capacitance value of the variable capacity ballast capacitor and adjusting the lamp current flowing in each cold cathode discharge lamp. However, in practice, it is not necessary to significantly change the capacity of the output capacitor, so a variable-capacity trimmer capacitor is connected in series or in parallel to a certain fixed-capacity ballast capacitor to make the entire ballast capacitor as a whole. The capacity of can be adjusted.

[0008]

As described above, according to the present invention, when a plurality of fluorescent lamps are used as the light source of the backlight, the uneven brightness is suppressed due to the uneven characteristics of each fluorescent lamp. As a result, a liquid crystal display device having good display quality in which the brightness of the display surface is made uniform over the entire surface can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic circuit diagram of an embodiment of the present invention.

[Explanation of symbols]

1A, 1B ... Transistor, 2 ... High frequency generating capacitor, 3 ... Step-up transformer, 4A, 4B, 4C ... Cold cathode discharge lamp, 5A, 5B, 5C ... Variable capacity type output capacitor.

Claims (1)

[Claims] 1. A liquid crystal display device using a plurality of fluorescent lamps as a light source behind a transmissive liquid crystal display device, wherein an output capacitor for each fluorescent lamp of an inverter type power supply circuit for lighting the fluorescent lamps. The liquid crystal display device is characterized in that the brightness of the back light source can be made uniform over the entire surface by making the respective capacities variable.