JPH09269474A - Illuminator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce flickers and unevenness of the distribution of brightness on the display surface of a liquid crystal display by applying a specific phase difference between AC outputs of driving means of plural fluorescent tubes. SOLUTION: This illuminator is provided with plural fluorescent tubes 5a, 5b and transformers 3a, 3b, driving circuits 2a, 2b and an oscillator tube 1a for supplying AC outputs to the plural fluorescent tubes 5a, 5b and a phase shifting circuit 6 for applying a prescribed phase difference between AC outputs of the oscillator tube 1a. This phase shifting circuit 6 is constituted of an operational amplifier OP2, resistors R21-R23 and s capacitor C21 and delays the voltage phase of an AC signal having an inputted frequency by 90 degrees to output it. Consequently, since phases of voltages to be impressed on the plural fluorescent tubes 5a, 5b are shifted, the fluorescent tubes 5a, 5b are lighted by shifting phases of lighting waveforms so as to cancel brightness and darkness with each other. Since the phase difference is 90 degree, the difference between the maximum value and the minimum value of the sum of absolute values of respective AC voltages to be impressed on the tubes 5a, 5b becomes smallest and then brightness and darkness can effectively be canceled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lighting device suitable for use as a backlight of a liquid crystal display.

[0002]

2. Description of the Related Art A backlight of a liquid crystal display is a light source system for illuminating the display surface of a transmissive display from the back side, and is composed of a light source such as a fluorescent tube or EL (electroluminescence) and a driving power source for the light source. .
There are two types of fluorescent tubes, CCFT (cold-cathode fluorescent tube) and HCFT (hot-cathode fluorescent tube). The fluorescent tubes are arranged in one or a plurality of fluorescent tubes directly below the display surface. Directly arranged type and edge light type (or side light type) in which one or more fluorescent tubes are arranged on the side of the display surface.
There are two types. Further, as a drive power source, an inverter is used in order to reduce the flicker of the display of the liquid crystal display by turning on the light source of the backlight at a high frequency.

FIG. 5 is a block diagram showing an example of the configuration of an electric system of a conventional backlight. In FIG. 5, reference numeral 1 denotes an oscillator, which outputs an AC signal of several kHz to several tens of kHz. A drive circuit 2 converts a DC output supplied from a DC power supply (not shown) into an AC output according to the AC signal output from the oscillator 1 and outputs the AC output. Reference numeral 3 is a transformer, which boosts and outputs the AC output supplied from the drive circuit 2. Reference numerals 4 and 4 are capacitors. And 5,
Reference numeral 5 denotes two CCFTs, which are driven by the AC output from the transformer 3 via the capacitors 4 and 4 and lighted.

Since the lighting waveforms of the two CCFTs 5 and 5 shown in this figure are determined according to the oscillation waveform of the oscillator 1,
It is asynchronous with the scanning line signal and the data line signal of the liquid crystal display illuminated by the CCFTs 5, 5. Therefore, depending on the frequency relationship, the lighting waveforms of the CCFTs 5 and 5 and the scanning line signal and the data line signal of the liquid crystal display may cause a beat, and vertical stripes or horizontal stripes of brightness change may flow on the display surface.

As a countermeasure against this, the lighting waveform of the fluorescent tube is synchronized with the scanning line signal or data line signal of the liquid crystal display,
There is a backlight that prevents beats. However, when the lighting waveform and each signal are synchronized, a change in brightness occurs in a stationary state, and for example, the right edge and the bottom edge of the display surface are darker than other areas, and the brightness of the screen is reduced. There is a problem in that the distribution of P is uneven, or thin vertical stripes appear.

A conventional backlight for improving this problem is disclosed in Japanese Patent Laid-Open No. 6-222329.
In the backlight described in this publication, the lighting waveform of the fluorescent tube is synchronized with both the scanning line signal and the data line signal of the liquid crystal display, and the lighting waveform of the fluorescent tube is inverted every cycle of the data line signal. I am trying.
By doing so, the effect of vertical stripes generated when the lighting waveform is synchronized with the scanning line signal and the data line signal is reduced.

[0007]

As described above, in the conventional backlight, if the lighting waveform and the relationship between the scanning line and the data line signal are asynchronous, flicker occurs due to the beat, and the synchronization occurs. If it is removed, stationary vertical stripes will occur. Further, in the case of synchronization, although some improvement can be achieved by reversing the lighting waveform, the effect of stationary vertical stripes could not be completely eliminated.

The present invention has been made under such a background, and an object thereof is to provide an illuminating device capable of reducing flicker and nonuniform brightness distribution on a display surface of a liquid crystal display. .

[0009]

An illuminating device according to the present invention comprises a plurality of fluorescent tubes for illuminating a liquid crystal display device from the back side, and a plurality of driving means for supplying an AC output to the plurality of fluorescent tubes. And a phase shift means for giving a predetermined phase difference between the plurality of AC outputs of the driving means.

[0010]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a lighting device according to an embodiment of the present invention.
In this figure, 1a is an oscillator, 2a and 2b are drive circuits, 3a and 3b are transformers, 4a and 4b are capacitors, 5a and 5b are CCFTs, and the oscillator 1, the drive circuit 2 and the transformer 3 shown in FIG. , Capacitor 4 and CCFT 5. The oscillator 1a is
Operational amplifier OP1, resistors R11, R12, R13, R1
4. It is composed of capacitors C11 and C12 and outputs an AC signal with a frequency of 15 kHz. Reference numeral 6 denotes a phase shift circuit, which includes an operational amplifier OP2 and resistors R21, R22, R2.
3. The capacitor C21 is configured to output the voltage phase of the input AC signal having a frequency of 15 kHz by delaying 90 degrees.

The drive circuit 2a receives the AC signal output from the phase shift circuit 6 and lowers the output impedance by an internal amplifier circuit, or further amplifies the voltage and outputs it. The transformer 3a boosts and outputs the AC voltage output from the drive circuit 2a. The alternating current output from the transformer 3a passes through the capacitor 4a and the CCFT 5a.
Is applied to On the other hand, the drive circuit 2b amplifies the output of the oscillator 1a, the transformer 3b boosts the output of the drive circuit 2b, and the output of the transformer 3b is supplied to the CCFT 5b via the capacitor 4b.

With the above-described structure, in the lighting device of this embodiment, the CCFT 5 having two AC voltages as shown in FIG.
a, 5b. FIG. 2 shows changes over time in the terminal voltages V _5a and V _5b of the CCFTs _5a and _5b , and the voltage V _5a is delayed from the voltage V _5b by 90 degrees. Thus 2
Since the phases of the voltages applied to the CCFTs 5a and 5b of the book are out of phase, the CCFTs 5a and 5b are turned on by shifting the phases of the lighting waveforms so as to cancel the lightness and darkness of each other. In this embodiment, since the phase difference is 90 degrees, the difference between the maximum value and the minimum value of the sums of the absolute values of the AC voltages applied to the two CCFTs is the smallest, and the bright and dark can be effectively canceled. .

The configuration of the oscillator 1a and the phase shift circuit 6 shown in FIG. 1 is not limited to that shown in this figure. For example, it may be configured by logic so as to generate two series of pulse signals having a phase difference. You may In addition, the drive circuit 2a,
2b may be configured by an inverter. Oscillator 1a
The oscillation frequency is not limited to 15 kHz, and may be 50 Hz to several tens of kHz, several hundreds of kHz, or higher. Further, instead of the AC signal from the oscillator 1a, a signal synchronized with the scanning line signal of the liquid crystal display, the vertical line signal, or both can be used. In that case, the influence of flicker can be further reduced.

Next, another embodiment of the present invention will be described with reference to FIG. FIG. 3 is a block diagram showing a block configuration of an illumination device using three CCFTs. 3, the same components as those in FIG. 1 are designated by the same reference numerals. In the embodiment shown in this figure, the AC signal output from the oscillator 1a is input to a series circuit of two phase shift circuits 6a and 6a that generate a phase difference of 60 degrees, and 0 degree, 60 degrees,
And the drive circuit 2a after giving a phase difference of 120 degrees,
Input to 2a, 2a. This allows CCFT5
a, 5a, and 5a are 0 degree, 60 degree, and 12 degrees, respectively.
AC output with a phase difference of 0 degree is applied, and three CCs are applied.
The FT lights up with a lighting waveform having a phase difference corresponding to it.

According to this embodiment, three CCFT5s are used.
By shifting the phases of the lighting waveforms of a, 5a, and 5a, the change in brightness can be further reduced as compared with the case of using two CCFTs. Note that in the example shown in FIG.
The phase difference of 60 ° and 120 ° is generated by using 60 ° phase circuits. However, the 60 ° phase shift circuit and the 120 ° phase shift circuit are directly connected to the oscillator, respectively. It is also possible to generate a phase difference.

FIG. 4 is a block diagram showing an example of the configuration of a lighting device in the case of driving two CCFTs using a self-excited inverter. 4, the same components as those in FIGS. 1 and 3 are designated by the same reference numerals. In this figure, 7a is a drive circuit incorporating a switching element, a capacitor, etc., and 8a is a set of windings 8a-on the primary side.
1 and 8a-2 and a winding 8a-3 on the secondary side. The drive circuit 7a converts a direct current output supplied from a direct current power source (not shown) into an alternating current and turns the winding 8a-1.
The transformer 8a outputs the boosted AC output from the winding 8a-3 on the secondary side, and outputs the CC output via the capacitor 4a.
An AC voltage is applied to FT5a. The drive circuit 7a is
The AC signal output from the winding 8a-2 is used as the switching signal of the switching element having the built-in AC signal. A phase shift circuit 6 shifts the output signal of the winding 8a-2 by 90 degrees and outputs it. 7a is a drive circuit, which is a phase shift circuit 6
The switching element of the built-in inverter is driven according to the output of. 8a is a transformer and a drive circuit 7b
The AC output of is boosted and output, and is supplied to the CCFT 5 via the capacitor 4a.

With such a configuration, in the lighting device shown in FIG. 4, the phase shift of the AC waveform applied to the two CCFTs 5a, 5a is shifted by 90 degrees, and the lighting waveform has a phase difference corresponding thereto. . In this embodiment, one of the two drive circuits is a self-excited inverter, and a phase shift circuit gives a phase difference to the drive signal, so that the oscillator is omitted as compared with the embodiment shown in FIG. ing. It should be noted that according to the present embodiment as well, it is possible to reduce the difference in brightness between when the CCFT is turned on, as in the embodiment shown in FIG.

Although three embodiments of the present invention have been described above with reference to the drawings, the configuration of the illumination device according to the present invention is not limited to the one shown in the figure, and four or more fluorescent tubes are used. However, the method of giving the phase difference is not limited to 90 degrees in the case of two lines or 60 degrees in the case of three lines as a unit. The backlight system may be a direct type or an edge light type. Further, in the description of the above embodiment, the detailed description of the DC power supply of the drive circuit is omitted. However, as the DC power supply, a battery or a power supply whose output or the like is controlled by a DC-DC converter is used. it can. Further, according to the lighting device of the present invention, since the currents supplied to the plurality of fluorescent tubes have a phase difference, it is possible to reduce the fluctuation of the current output from the DC power supply, and to reduce the battery or DC- It is also possible to obtain the effect of reducing the capacity of the capacitor used in the DC converter.

[0019]

As described above, according to the present invention, since the AC outputs of the driving means for a plurality of fluorescent tubes have a phase difference, the lighting waveforms of the fluorescent tubes are out of phase.
It is possible to reduce flicker and uneven brightness distribution on the display surface of the liquid crystal display.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a lighting device according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing an applied voltage of a CCFT in the embodiment shown in FIG.

FIG. 3 is a block diagram showing a configuration of an illumination device according to another embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of an illumination device according to another embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration example of a conventional lighting device.

[Explanation of symbols]

 1a oscillator 2a, 2b drive circuit 3a transformer 4a capacitor 5a CCFT 6 phase shift circuit

Claims (1)

[Claims] 1. A plurality of fluorescent tubes for illuminating a liquid crystal display device from the back side, a plurality of driving means for supplying an AC output to the plurality of fluorescent tubes, and a plurality of AC outputs of the driving means. An illuminating device, comprising: a phase shift means for giving a predetermined phase difference.