JPH01167734A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain an excellent image of high picture quality at all times by invariably optimum driving even in case of variation in temperature, in-use time, etc., by detecting a flicker of a liquid crystal cell at all times and adjusting a voltage applied to a counter electrode. CONSTITUTION:This device consists of a photosensor 401 for detecting the flicker, a multiplexer circuit 402 which divides a light transmission signal voltage outputted by the photosensor 401 into a (+) field and a (-) field, a mean value detecting circuit 403 which detects the mean value of the light transmission signal voltage, a comparator 404 which compares two mean values, and a voltage varying circuit 405 which varies the offset of a voltage according to whether the output of the comparator is plus or minus. Then the flicker of the liquid crystal cell is detected all the times to adjust the voltage, and consequently the light transmission signal voltage Vf of liquid crystal becomes a flicker whose cycles is 60Hz or one vertical scanning period (1V) at all times as shown in a figure, so that it is not sensed with the human eye. Consequently, even when the temperature, in-use time, etc., vary, the adjustment is so made as to enable the best driving at all times and excellent images of high quality are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is applied to a liquid crystal display device that displays a general television signal, such as an NTSC signal, as an image.

BACKGROUND ART FIG. 1θ is a block diagram showing the configuration of a conventional liquid crystal display device. In FIG. 10, 1001 is a switching transistor (- thin film transistor made of amorphous silicon, etc. for boat fishing) that switches each picture element, hereinafter referred to as TPT.
1002 is a liquid crystal cell, 1003 is T
A picture element electrode connected to the drain electrode d of FTlool,
1004 is a counter electrode common to all picture elements, 1005 is one picture element, and 1006 is a gate electrode g of TFTloool.
1007 is a signal electrode connected to the source electrode S of TFTlooI for applying a signal to the pixel electrode 1003;
008 is 1 vertical scanning period (IV
), 1009 is a clock Φ input terminal for inputting a clock φ for sampling the video signal, and 1010 is a horizontal synchronization signal input for inputting a horizontal synchronization signal f)l. terminal, 10
11 is a video signal sample and hold circuit that samples and holds a video signal with a clock φ and drives it line-sequentially during a horizontal scanning period (lH) @; 1012 is a vertical synchronization signal input terminal to which a vertical synchronization signal fV is input; and 1013 is each picture element TFTl of
In order to drive ooI line-sequentially, vertical scanning circuits are shown that sequentially output signals with a pulse width of one horizontal scanning period (IH) from xl to Xi in one vertical scanning period (IV). The output of the video signal sample and hold circuit 1011 is column j from Yl to Yj, and the output of the vertical scanning circuit is Xl.
There are i rows from Xi to Xi, and the screen is composed of i rows and j columns.

Figures 11(a) and 11(b) are operational waveform diagrams of this conventional liquid crystal display device. Figure 11(a) is a voltage application waveform diagram at the top of the screen, and Figure 11(b) is a voltage application waveform diagram at the bottom of the screen. 3A and 3B respectively show applied waveform diagrams. Here, in order to simplify the diagram, a white raster signal will be used as the video signal. 1st
1 (a) and (b), Vsig is the signal output from the video signal sample and hold circuit 1011, and Vcente is the signal electrode voltage signal applied to the signal electrode 1007.
r is the center voltage of the signal electrode voltage signal Vsig, Vg is a scanning electrode voltage signal output from the vertical scanning circuit 1013 and has a pulse width of one horizontal scanning period (IH) and is applied to the scanning electrode 1006, and V com is the counter electrode 1004 This is the counter electrode voltage signal applied to. Also, FIG. 11(C),
(d) shows a light transmission waveform diagram in the operating state at the top of the screen.

In the conventional liquid crystal display device configured as described above,
The operation will be explained below with reference to FIG. 10 and FIGS. 11(a) to (d).

In FIG. 10, the video signal input from the video signal input terminal 1008 is sent to the video signal sample hold circuit 1011.
Then, a signal electrode voltage signal Vsig as shown in FIGS. 11(a) and 11(b) is applied to the signal electrode 1007. Then, the scanning electrode 1006 shown in FIG. 10 is attached to the scanning electrode 1006 shown in FIG.
), (b) are applied to the counter electrode 1004 shown in FIG. 11 (a), (b).
) is applied to the counter electrode voltage signal VcoI11 shown in FIG. Then, in a certain row of scan electrodes 1006, from the scan electrode voltage signal Vg, l horizontal scan period (1) [) TFTloo
I is turned on, a signal electrode voltage signal Vsig is applied to the picture element electrode 1003, and TPTlo is applied to the liquid crystal cell 1002.
Charge is charged through the on-resistance Ron of oI. After that, the TFT 1001 is turned off, and the liquid crystal cell 1002
The charge charged in is the off resistance Roff of TPTIOol.
The voltage decreases due to off-leakage discharged throughout the period and is maintained for the vertical scanning period (1V). These things are done in all picture elements, and the image is displayed on the entire screen.2 The voltage applied to the picture element electrode 1003 at this time is as shown in FIGS. 11(a) and 11(b). The electrode voltage signal becomes Vd. Here, when the picture element electrode voltage signal Vd is higher than the counter electrode voltage signal vCOI11, it is called a (+) field, and when it is lower, it is called a (-) field. This picture element electrode voltage signal Vd is TPTlooI as generally known.
Since the pulsed signal of the scanning electrode voltage signal Vg leaks into the picture element electrode 1003 through the gate-drain overlap capacitance Cgd, the voltage becomes smaller by ΔV shown in the first equation.

ΔV = V g - Cgd (Cgd + CIc)...
...=”(L) However, Cgd: Gate-drain overlap capacitance CIc: Equivalent capacitance of liquid crystal cell v8: Gate voltage Because of this ΔV, the effective voltage of the (+) field V rm
If s is large (-) field effective voltage V rms is small, light transmission signal voltage V as shown in FIG. 11(C)
fl, conversely, if the effective voltage Vrms under the (+) field is small and the effective voltage vrIIIS under the (-) field is large, the light transmission signal voltage Vf2 as shown in FIG. 11(d)
becomes. These represent the magnitude of the amount of light transmission per vertical scanning period (IV), so-called flicker, and the period is 2.
Since the vertical scanning phase jump (2v) is 30H2, it is felt by the human eye. Therefore, the effective voltage V r in the (+) field shown in the shaded areas in FIGS. 11(a) and (b)
It becomes necessary to adjust the counter electrode voltage signal V com so that the effective voltage V rms in the (-) field becomes equal to the effective voltage V rms in the (-) field.

Problems to be Solved by the Invention However, in the above configuration, FIG. 11(a),
The effective voltage v rms in the (+) field and the effective voltage V in the (-) field shown in the shaded area in (b)
The counter electrode voltage signal V com so that the rms is equal
Even if we adjust the
If the temperature, operating time, etc. change, the TFT characteristics and liquid crystal characteristics will also change significantly, so the effective voltage V rms in the (+) field and the effective voltage V rms in the (=) field.
The rms balance collapses, and the counter electrode voltage signal V c
There was a problem in that the om had to be readjusted. As a practical matter, the counter electrode voltage signal Vc varies with respect to temperature, usage time, etc.

It was nearly impossible to finely adjust m.

Moreover, if left in that state, it will not only cause flickering, which will make visitors uncomfortable, but will also disrupt the balance between the effective voltage V rms in the (+) field and the effective voltage V rms in the (-) field, causing the liquid crystal to deteriorate. This also had a fatal drawback in that a direct current component was applied, resulting in an extremely shortened lifespan.

Also, in the (+) field at the top of the screen, the source
The drain voltage is small and the gate voltage is sufficiently low relative to the source and drain, so there is little off-leakage, but in the (+) field at the bottom of the screen, the source-drain voltage is large and the gate voltage is not low enough relative to the source, so off-leakage occurs. will increase. For this reason, FIG. 11(a),
The effective voltage V rms in the (+) field and the effective voltage V r in the (-) field shown in the shaded area in (b)
The counter electrode voltage signal Vcorti with equal ms is:
The lower the screen becomes, the lower the counter electrode voltage signal v
Even if you adjust coI11, the flicker does not completely disappear on the entire screen, and you have to constantly see images with poor display quality that flicker on a part of the screen.Moreover, in that part, a DC component is applied to the liquid crystal. This also led to the problem that the service life was extremely short.

In view of the above, an object of the present invention is to provide a liquid crystal display device that can always provide good high-quality images in any environment and has a long life.

Means for Solving the Problems The present invention provides an image display area in which a liquid crystal display panel equipped with a counter electrode displays an image signal on a display portion, and a signal for detecting flickering of transmitted light of a liquid crystal cell. a flicker detection area for displaying a flicker, a flicker detection means for detecting flicker displayed in the flicker detection area, and a voltage variable circuit for changing a voltage applied to a counter electrode based on the output of the flicker detection means. It is a liquid crystal display device.

Effects The present invention has the above-described configuration, and by constantly detecting flicker in the liquid crystal cell and adjusting the voltage applied to the counter electrode, even when the temperature, operating time, etc. change, the present invention always maintains optimal drive.
A good high-quality image can always be obtained, and the life of the liquid crystal can be extended.

Embodiment FIG. 1 is a perspective view of a panel showing the panel structure of a liquid crystal display device in a first embodiment of the present invention. In FIG. 1, reference numeral 101 indicates seven flicker detection areas for detecting flicker, 102 an image display area for displaying images, 103 a photosensor for detecting flicker, and 104 a liquid crystal panel.

FIG. 2 is a block diagram showing the configuration of a liquid crystal display device according to a first embodiment of the present invention. In Figure 2, 20
1 is TFT, 202 is liquid crystal cell, 203 is TFT201
204 is a counter electrode common to all picture elements, 205 is one picture element, 20
6 is connected to the gate electrode g of TFT201 and TPT20
1tt operation; 207a is a signal electrode connected to the source electrode S of the TFT 201 in the flicker detection region 101; and 207a is a signal electrode for applying a signal to the picture element electrode 203;
07b is a signal electrode that is connected to the source electrode S of the TFT 201 in the image display detection area 102 and applies a signal to the picture element electrode 203; 208 is a signal electrode whose polarity is reversed every vertical scanning period (1V) in order to drive the liquid crystal with alternating current; Video signal Vsigl
209 is a clock φλ that inputs a clock φ for sampling the video signal.
210 is a horizontal synchronizing signal input terminal into which the horizontal synchronizing signal fH is input; 211 is a video signal sample and hold circuit that samples and holds the video signal V 51g1 with a clock Φ and drives line-sequentially every horizontal scanning period (IH); 212
is a vertical synchronization signal input terminal that inputs the vertical synchronization signal fV,
213 is 1 to drive the TFT 201 of each picture element line-sequentially.
A vertical scanning circuit that sequentially outputs a signal with a pulse width of one horizontal scanning period (IH) from XI to Xi during a vertical scanning period (1v), and 214 is an intermediate voltage input that inputs an intermediate voltage V mid to be applied to the flicker detection area 101. A terminal 215 widens the pulse width of the horizontal synchronizing signal fH and outputs an H blanking signal V.
One-shot circuit that outputs fH, 216 is intermediate voltage V
mid and video signal Vsigl as H blanking signal Vf
A multiplexer circuit that outputs the switching video signal Vsig2 at H is shown. The outputs of the video signal sample and hold circuit 211 are in j columns from Yl to Yj, the outputs of the vertical scanning circuit are in i rows from xl to Xi, and the screen is composed of i rows and j columns.

FIG. 3 shows a signal waveform diagram when a signal for detecting flicker is synthesized with the video signal Vsi31 of the liquid crystal display device in the first embodiment of the present invention.

In Figure 3, (a) is for AC driving the liquid crystal]
Video signal Vsi whose polarity is inverted every vertical scanning period (1■)
gl, (b) is H with widened pulse width of horizontal synchronization signal fH
Blanking signal VfH1 (C) shows intermediate voltage Vmid, which is the voltage at which flicker is most noticeable, and (d) shows video signal Vsig2, which is obtained by switching intermediate voltage Vmid and video signal Vsigl with H blanking signal VfH. . , here, talking about the properties of flicker. In general, the higher the brightness, the easier it is to feel flicker, but if a voltage that saturates the liquid crystal is applied, the brightness will be high, but the liquid crystal molecules will stand up completely, causing flicker. I no longer feel it.

For this reason, the state where the liquid crystal molecules are about half erect, that is, the voltage at about 172, which is the voltage at which the liquid crystal is saturated, is best felt, and that voltage is the intermediate voltage V mid shown in Figure 3(c).
It is said that

FIG. 4 is a flicker detection circuit diagram for detecting flicker in a liquid crystal display device according to the first embodiment of the present invention.

In FIG. 4, 401 is a photosensor such as a phototransistor for detecting flicker; 402 is a multiplexer circuit that divides the light transmission signal voltage output from the photosensor 401 into a (+) field and a (-) field;
403 is an average value detection circuit that detects the average value of the light transmission signal voltage, 404 is a comparator that compares the average value of two light transmission signal voltages, and 405 is a voltage variable that varies the voltage offset depending on the positive/negative of the comparator output. Each circuit is shown.

FIGS. 5(a) and 5(b) are voltage signal waveform diagrams applied to the signal electrodes of the liquid crystal display device in the first embodiment of the present invention,
Figures (c) to (e) respectively show light transmission signal voltage waveform diagrams.

The operation of the liquid crystal display device of the first embodiment configured as described above will be explained below.

In FIG. 2, the video signal V 51g1 in FIG. For only one THiJ question, the multiplexer circuit 216 changes the intermediate voltage V mid
is selected, and the video signal Vsi as shown in FIG. 3(d) is generated.
32 is input to the video signal sample and hold circuit 211. Then, the video signal sample and hold circuit 211 applies a video signal Vsig2b as shown in FIG. A video signal Vsig2a as shown in FIG. 5(a) is applied to the signal electrodes 207b of the image display area 102, respectively.

Then, as in the conventional case, the scanning electrodes 206 are shown in FIG.
A scanning electrode voltage signal Vg shown in FIG. 11(b) is applied, and a counter electrode voltage signal vcoI11 shown in FIGS. 11(a) and 11(b) is applied to the counter electrode 204. Then, in the flicker detection region 101, the light transmission signal voltage obtained from the photosensor 401 shown in FIG.
If rms is small, the optical transmission signal voltage V fl as shown in FIG. 5(C), and conversely, the effective voltage V of the (+) field
rms is small (-) field effective voltage V rm
If s is large, the light transmission signal voltage Vf2 becomes as shown in FIG. 5(d). Here, we will explain the operation of the flicker detection circuit when, as an initial state, the counter electrode voltage signal V com is such that the effective voltage V rms of the (+) field is large and the effective voltage V rms of the (-) field is small. do.

The light transmission signal voltage Vfl shown in FIG. 5(C) obtained from the photosensor 401 is divided into (+) field and (-) field light transmission signal voltages by a multiplexer circuit 402, and each is divided by an average value detection circuit 403. The averaged values are compared by a comparator 404. Then, since the input on the (+) side is large, the output of the comparator 404 is (+), and the voltage variable circuit causes the counter electrode voltage signal ■coll+ to rise. As a result, the (+) field effective voltage VrwS is smaller than before (-) field effective voltage V
rms increases. While repeating these feedback loops, at some point the effective voltage V rms of the (+) field and the effective voltage V rms of the (-) field
s become equal, and the light transmission signal voltage V shown in FIG. 5<e)
f, the flicker period is one vertical scanning period (tV)
This results in a 60Hz flicker that is not noticeable to the human eye.

As described above, according to this embodiment, by constantly detecting the flicker of the liquid crystal cell, the light transmission signal voltage Vf of the liquid crystal always has a period of one vertical scanning period rm as shown in FIG. 5(e).
(IV), which is 80 Hz flicker, which is not perceptible to the human eye. Even if the temperature, usage time, etc. change, the counter electrode voltage signal Vcow is automatically adjusted so that the drive is always optimal, so that good high-quality images can always be obtained.

FIG. 6 is a panel perspective view showing the panel structure of a liquid crystal display device according to a second embodiment of the present invention.

In FIG. 6, 601 is a flicker detection area, 602 is an image display area, 603a is a photo sensor (1), 603
b is 7. r-t! The sensor (2) and 604 C each show a liquid crystal panel, and the above configuration is the same as that in Figure 1, but the difference from the configuration in Figure 1 is that photosensors are provided in two places, the top and bottom. .

FIG. 7 is a flicker detection circuit diagram for detecting flicker in a liquid crystal display device according to a second embodiment of the present invention.

In FIG. 7, 701a is a photosensor (1), 70
1b is a photosensor (2), 702a and b are multiplexer circuits, 703a and b are average value detection circuits, 704
705a and 705b indicate comparators, and voltage variable circuits 705a and 705b, respectively, and the above structure is the same as that of FIG. 4.

What is different from the configuration shown in FIG. 4 is that independent flicker detection circuits are provided for the upper and lower photosensors, respectively, and a counter electrode voltage signal Vcoml and a counter electrode voltage signal Vc are provided.

This is the point where m2 is obtained.

FIGS. 8(a) and 8(b) show the structure of a counter electrode of a liquid crystal display device in a second embodiment of the present invention, where (a) is a structural diagram and FIG. 8(b) is an equivalent circuit of the counter electrode, respectively. ing.

In FIG. 8(a), 204 is a counter electrode which is common to all picture elements as in the first embodiment, but a conductor made of a low resistance material such as aluminum is placed on each of the upper and lower sides. A counter electrode voltage signal Vcoml is provided on the upper conductor 801a.
A counter electrode voltage signal V com is applied to the lower conductor 801b.
This embodiment differs from the first embodiment in that 2 is input respectively. In FIG. 8(b), when there is a voltage difference between the upper conductor 801a and the lower conductor 801b, in the central part, the voltage difference is from the upper conductor 801a to the lower conductor 801b.
Although the current distribution is uniform to 01b, the current distribution is slightly different at both the left and right end portions, but here it is assumed that the counter electrode is sufficiently large and the current distribution is uniform in the area where the image is displayed. Then, for each picture element in i row and j column, m row and n
The counter electrode voltage signal in the column is Vco (a+, n),
Let r be the counter electrode resistance for one picture element from the upper conductor 801a to the lower conductor 801b. The counter electrode resistance r at this time is set to a value that is very small compared to the impedance of the liquid crystal.

The operation of the liquid crystal display device of the second embodiment configured as described above will be explained below. The operation is similar to the first embodiment, and the counter electrode voltage signals Vcoa+1 and V com2 are the effective voltage V rn of the (+) field.
It is assumed that the effective voltage V rrns of the +s and (-) fields are equal to each other, and a description thereof will be omitted here.

In FIG. 8(a), a counter electrode voltage signal Vcoa+1 is applied to the upper conductor 801a, and a counter electrode voltage signal Vcom2 is applied to the lower conductor 801b.

At that time, from the equivalent circuit shown in FIG.
becomes like the second equation.

Vcoa+(n+, n)=(i-m)/(i-r)・(
Vcon+l-Vcom2)=(2) As can be seen from the second equation, the counter electrode voltage signal Vc.

m(n+, n) is a partial voltage of the counter electrode voltage signal Vcoml and the counter electrode voltage signal Vcow2, and the voltage changes linearly from the 1st row to the i column. This causes the counter electrode voltage signal to linearly change from Vcoml to Vc at the top and bottom of the screen.

It can be adjusted up to m2, and if the flicker detection circuits on the top and bottom of the screen are used to create a 60Hz flicker for each, it is possible to similarly create a 60Hz flicker from the 1st row to the i column, and the screen Overall, flicker is no longer perceptible to the human eye.

As described above, according to this embodiment, the flicker of the liquid crystal cell is constantly detected at the top and bottom of the screen, and the counter electrode voltage signal Vcon+1 and the counter electrode voltage signal V coa+2 obtained from the respective flicker detection circuits are used to detect the flicker of the liquid crystal cell at the top and bottom of the screen. By applying to the extraction electrode, the effective voltage V rms in the (+) field and the effective voltage V rms in the (-) field are
It is possible to adjust the counter electrode voltage signal that makes the voltages equal to each other from the 1st row to the i column of the screen, and it is possible to always obtain a good, high-quality image without flickering on the entire screen, and no DC component is applied to the liquid crystal on the entire screen. This dramatically extends the lifespan of the LCD.

In addition, in the above embodiment, the counter electrode 20 shown in FIG.
4 is common to all picture elements in the flicker detection area 101 and the image display area 102 shown in FIG. 1, and only one photosensor 103 is used, but as shown in FIG. It is also possible to divide the screen into a plurality of parts in the horizontal direction and provide as many photosensors as the number of divided opposing electrodes.In this case, it becomes possible to more finely and accurately adjust the flicker over the entire screen.

Furthermore, here, the extraction electrode and flicker detection circuit were provided only at both ends of the counter electrode, but another extraction electrode and flicker detection circuit may be provided between them, which would allow for more fine and accurate flicker adjustment over the entire screen. becomes possible. Furthermore, although the counter electrode 204 shown in FIG. 8 is used as a common electrode for all picture elements, it may be divided into multiple columns as shown in FIG. 9(a).

As described in detail, according to the present invention, it is possible to realize a liquid crystal display device that always performs optimum driving even when the temperature or usage time changes. As a result, a good, high-quality image without flickering can be obtained at all times, and since no direct current component is applied to the liquid crystal, the life of the liquid crystal is dramatically extended, which has a great practical effect.

[Brief explanation of the drawing]

FIG. 1 is a panel perspective view showing the panel structure of a liquid crystal display device according to a first embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of the same embodiment, and FIG. 3 is an image of the same embodiment. A signal waveform diagram when a signal for detecting flicker is combined with a signal, FIG. 4 is a flicker detection circuit diagram for detecting flicker in the same embodiment, and FIG. Figure 6 is a panel perspective view showing the panel structure of a liquid crystal display device according to a second embodiment of the present invention; Flicker detection circuit diagram for
i (a) shows the crystal display device of the counter electrode of the same example, and D configuration. Figures 2 and 3) are operating waveform diagrams of the same prior art. 101.601... Flicker detection area, 102.60
4... Image display area, 103.603... Photo sensor, 104.604... Liquid crystal panel, 204.90
1... Counter electrode, 801... Electric conductor. Agent's name: Patent attorney Toshio Nakao Idiot No. 1 @ 104 LCD Panel F! 07-TPT n2 - liquid crystal cell 2oy -! & Cumulative electrode 204 - Single open electrode 205 - Ml l'20&-- One run 1 10'la, To-M t l Jilii-Ig number input way #1 child 211-・I* image signal Sample bold circuit 211'-
-1 Vertical Interperiod Signal Input Figure 3 Figure 4 Figure 5 Figure 6 Figure 7/L1. , fD Fig. 9 70m2 com 2 Fig. 10 (α) (b) lθ01- Ding FT Fig. 12

Claims (4)

[Claims] (1) A liquid crystal display panel including a counter electrode has an image display area for displaying an image signal on a display portion and a flicker detection area for displaying a signal for detecting flicker of the liquid crystal cell, and the flicker detection area 1. A liquid crystal display device comprising: flicker detection means for detecting flicker displayed on a display; and a voltage variable circuit for changing a voltage applied to a counter electrode based on the output of the flicker detection means. (2) The liquid crystal display device according to claim 1, wherein one or more flicker detection means are provided in the flicker detection area. (3) the counter electrode has a busbar-shaped conductor electrode made of a conductor on two sides facing each other, and two flicker detection means are located near the two conductor electrodes, respectively;
Two flicker detection means are respectively connected to two voltage variable means, the output of one of the voltage variable means is connected to the conductive electrode near the input flicker detection means, and the output of the other voltage variable means is connected to the other voltage variable means. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is connected to the conductive electrode. (4) The liquid crystal display device according to claim 1, 2 or 3, wherein the counter electrode is divided into a plurality of regions.