JPH0830798B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a thin film transistor (hereinafter abbreviated as TFT).
The present invention relates to an active matrix type liquid crystal display device provided with a non-linear element array such as the above, and more particularly to an active matrix type liquid crystal display device provided with a mechanism for minimizing the occurrence of flicker during display.

[Prior Art] Among display devices using liquid crystal, non-linear elements, especially TFTs
A color display device that combines an array and a color filter is a color TV because of its high display quality, thinness, and lightness.
It has been commercialized by being applied to receivers and computer terminals.

FIG. 4A is a cross-sectional perspective view of a liquid crystal display device using the TFT array as described above, FIG. 4B is an arrow view of a portion indicated by IV B-IV B in FIG. 4A, and FIG. 4C is IVC-IVC in Figure 4B
It is a top view of the part shown by.

Referring to FIG. 4A, the active matrix type liquid crystal display device includes a TFT side substrate on which a TFT is formed and a TFT side substrate 1.
Counter electrode side substrate 2 provided at a position facing the TFT, and the TFT
Liquid crystal layer 48 sandwiched between the side substrate 1 and the counter electrode side substrate 2
And the outer peripheral portion of the liquid crystal layer is sealed with a sealing resin 50. On the TFT-side substrate 1, a plurality of gate buses 41 for transmitting signals to the gate electrodes of the TFT and a plurality of gate buses 41 connected in a direction intersecting with the gate buses 41 for transmitting video signals to the sort electrodes of the TFT. A source bus 42 is formed. The plurality of gate buses 42 are connected to the plurality of gate electrode terminals 3 by TF.
It is connected at one end of the T-side substrate, and is connected to the plurality of source bus terminals 42 at the end of the TFT-side substrate to the plurality of source electrode terminals 4. A counter electrode is formed on the counter electrode side substrate 2, and the voltage applied to the counter electrode is supplied from the counter electrode terminal 5 provided at the end of the TFT side substrate 1 via the counter electrode and the connection electrode 47 of the terminal. Supplied. The TFT 43 is provided at each intersection of the plurality of gate buses 41 and the plurality of source buses 42.

Referring to FIG. 4B, a plurality of display electrodes 44 formed on the TFT side substrate and a counter electrode 45 provided on the counter electrode side 2.
Are opposed to each other with the alignment film 49 and the liquid crystal layer 48 sandwiched therebetween.
Referring to FIG. 4C, a TFT 43 is formed at the intersection of the gate bus 41 and the source bus 42. The gate electrode of the TFT 43 is connected to the gate bus 41, the source electrode is connected to the source bus 42, and the display electrode 44 is connected to the drain electrode.

Next, the operation will be described. The gate ON signal is applied to the gate bus 41, and the TF connected to the gate bus 41 is connected.
All T43s are turned on. A voltage based on a video signal synchronized with the ON signal of the gate is applied to each display electrode 44 via the source bus 42. Even if the TFT gate OFF signal is applied and the TFT is turned off, the charge accumulated in the display electrode is held for a time constant determined by the TFT OFF resistance and the liquid crystal cell capacitance. In this way, by scanning the gate electrodes one after another, an image can be displayed on the screen. FIG. 5 is an equivalent circuit diagram of one TFT provided at the intersection of the gate bus 41 and the source bus 42 and the display electrode 44 connected to the drain thereof. Referring to FIG. 5, TFT4
The gate electrode 51 of FIG. 3 and the drain electrode formed on the TFT side substrate have a portion overlapping each other, and as a result, the parasitic capacitance C _9d as shown in the figure is provided between the gate electrode 51 and the drain electrode 53. Is formed. Now, assuming that the liquid crystal capacitance between the display electrode 44 and the counter electrode 54 is Cl _c , the potential of the drain 53 of the TFT shifts as shown by the following equation.

Here, V shift: level shift amount of drain potential C _9d : capacitance between gate / drain Cl _c : capacitance of liquid crystal V gate: potential difference between ON signal and OFF signal of gate As described above, the TFT is between the gate and the drain. Since there is a parasitic capacitance (C _9d ) on the gate signal, when the gate signal changes from the ON signal to the OFF signal, the potential difference between the ON signal and the OFF signal is divided by the ratio of C _9d and the liquid crystal cell capacitance (Cl _c ). The potential of the drain, that is, the potential of the display electrode shifts by the amount indicated by V shift in the above formula.

FIG. 6 is a diagram showing temporal changes in the video signal and the drain potential. Referring to FIGS. 5 and 6, the drain 53
Potential is shifted by an amount indicated by an arrow in the figure. Further, in order to display an image on the liquid crystal, an image signal which is an AC voltage signal is applied to the source electrode 52 and the counter electrode 54. As described above, when the video signal having the same polarity as the OFF signal of the gate signal is input due to the shift of the drain potential (the portion indicated by A in FIG. 6), the gate potential of the TFT and the drain 53 The potential difference of the drain potential becomes small (the portion indicated by c in FIG. 6). As a result, the off characteristics of the TFT also deteriorate, and the holding voltage applied to the liquid crystal cell 60 decreases.
Therefore, the waveform of the drain potential is asymmetric with positive and negative polarities. Since the drain potential shift due to C _d9 described above and the polarity of the drain potential waveform are asymmetrical, as shown in FIG. 6, the video signals externally applied to the source electrode 52 and the counter electrode 54 of the TFT are symmetrical. Even if it is an AC signal, the signal applied to the drain 53 of the liquid crystal cell 60 becomes a signal to which a DC bias is applied (the signal shown by the dotted line in FIG. 6) and the AC voltage of the video signal applied to the counter electrode 54. When viewed with the center value as a reference, the video signal applied to the drain electrode of the TFT is an asymmetrical AC signal.

As described above, in the liquid crystal display device driven by TFT, the voltage waveform applied to the liquid crystal cell is generally an asymmetric waveform including a DC component. Therefore, even if the potential of the counter electrode 54 is set to the center value of the AC voltage of the video signal, DC bias is applied to the liquid crystal cell, so that flicker occurs in the display and the display quality is impaired. Therefore,
A method is used in which the DC potential is set to the drain potential waveform, that is, the center value of the display potential waveform, so that the voltage waveform applied to the liquid crystal cell 60 approaches a symmetrical AC voltage to reduce the occurrence of flicker. (Hereinafter, the DC potential of the counter electrode that minimizes the flicker will be referred to as "optimum counter DC potential".)

[Problems to be Solved by the Invention] A plurality of design values such as the switching characteristics of the TFT, C _9d , or the dielectric constant of the liquid crystal act on the asymmetry of the drain potential waveform described above. Since such a design value shows a change with temperature or a change with time, the drain potential waveform also shows a change with temperature and time. Therefore, even if the optimum opposing DC potential is initially set, there is a problem that flicker occurs in the display due to changes in the drain potential waveform due to temperature and time.

In order to alleviate this drawback, there was a method to alleviate the asymmetry of the drain potential waveform by adding an auxiliary capacitance (C _s ) as shown in Fig. 7 and making the liquid crystal capacitance apparently large. It was extremely difficult to solve the above problems.

Further, there is a problem that a great amount of labor and time are required because the optimum counter DC potential is initially set while observing the flicker of the display.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an active matrix type liquid crystal display device having high display quality and capable of coping with variations in characteristics of thin film transistors due to temperature and time.

[Means for Solving the Problems] A liquid crystal display device according to the present invention corresponds to a plurality of gate bus wirings and a plurality of source bus wirings arranged in a direction intersecting with each other, and each intersection of the gate bus wirings and the source bus wirings. An active matrix substrate having a gate electrode on a gate bus line, a source electrode connected to the source bus line, and a display electrode arranged in a matrix connected to a drain electrode of the switching element. , Facing the active matrix substrate,
The counter substrate includes a counter electrode facing the display electrode, and a liquid crystal layer interposed between the counter substrate and the active matrix substrate. The liquid crystal display device includes a display electrode potential detection unit that sequentially detects the potential of the display electrode in synchronization with the switching of the polarity of the alternating voltage corresponding to the display operation, and a counter electrode potential detection unit that detects the potential of the counter electrode. Comparing the counter electrode potential obtained by the calculating means and the counter electrode potential detecting means with the average potential obtained by the calculating means, the potential applied to the counter electrode is calculated as the average value potential. And a counter electrode drive control circuit including counter electrode potential setting means for substantially matching the above.

[Operation] FIG. 3A shows the drain potential waveform and the opposing DC potential when the opposing DC potential is optimized. In this case, the voltage applied to the liquid crystal cell has the same positive and negative polarities. However, as shown in FIG.
When the DC potential deviates from the optimum value, that is, the average value of the drain potential waveform, the voltage applied to the liquid crystal cell becomes asymmetric due to the polarity. Therefore, in order to optimize the opposite DC potential that deviates from the optimum value, the opposite DC potential should be adjusted so that the voltage waveform that is asymmetric with respect to the applied voltage polarity becomes symmetrical.
The electric potential may be adjusted.

Therefore, in the present invention, the potential of the display electrode is sampled by the display electrode potential (drain potential) detecting means, an average value thereof is obtained, and this value is controlled so as to coincide with the counter electrode potential.

Embodiment of the Invention FIG. 1 shows an embodiment of a liquid crystal display device according to the present invention.
It is a block diagram which shows the circuit structure of a liquid crystal display device.

Referring to FIG. 1, a liquid crystal display device using a TFT includes a video signal generation circuit 10 for generating a video signal, a driver connected to the video signal generation circuit, and a driver for displaying an image on the liquid crystal. Driver controller 9 for controlling different TFTs, and a gate driver 7, a source driver 8 connected to the driver controller 9 for individually driving the TFTs on the liquid crystal, a gate driver 7, a source driver 8
Connected to the liquid crystal display unit for displaying an image, a potential detection means for detecting the potential of the drain electrode of the TFT and the potential of the counter electrode provided in the liquid crystal display unit, and the detected potential of the drain of the TFT And a calculating means for calculating the average value of the potential of the counter electrode, a comparing means for comparing the average value of the potential of the drain electrode of the TFT and the average value of the potential of the counter electrode, and the comparison result. And a control means for controlling the potential of the counter electrode to the average value of the potential of the drain electrode of the TFT. The comparison means and the control means constitute a counter electrode potential setting means. FIG. 2 is a diagram showing a method for detecting the potential of the drain electrode of the TFT 43 of the liquid crystal display device according to the present invention, that is, the potential of the display electrode 23. With reference to FIG. 2, the lead wire terminal 6 is provided on the display electrode 44 through the lead wire 21. This lead terminal 6 is provided at one end of the TFT side substrate.

Referring to FIG. 1, the liquid crystal display unit includes a TFT side substrate 1 and a TFT side substrate 1.
A gate electrode terminal 3 connected to the gate driver 7, a source electrode terminal 4 connected to the source driver 8, and a lead wire terminal 6 for detecting the potential of the display electrode 44, which are provided on the FT substrate 1. , And a counter electrode terminal 5 for transmitting a video signal to the counter electrode provided on the counter electrode side substrate 2.

The potential detecting means includes a voltmeter 11 for detecting the potential of the display electrode and the potential of the counter electrode. The calculation means is a memory 13 for storing the potential of the display electrode and the potential of the counter electrode.
And a memory controller 12 for controlling the memory 13, and an arithmetic unit 14 for actually performing an arithmetic operation. The comparison means includes a comparator 15. The control means includes a signal generation circuit 17 for generating a signal for setting the potential of the display electrode and the potential of the counter electrode to 0, and a signal generation circuit controller 16 for controlling the signal generation circuit 17.

The operation of the liquid crystal display device according to the present invention will be described with reference to FIG. The potential difference between the drain potential (S1) and the counter electrode potential (S2) is detected by the voltmeter 11. (Hereinafter, this potential difference is referred to as a drain voltage.) Using the value of the potential difference thus detected, the opposite DC level is controlled as follows.

The memory controller 12 operates based on the timing signal (S11) synchronized with the switching of the voltage polarity of the AC voltage by the driver controller 9, samples the drain voltage value (S3) detected by the voltmeter 11, and samples the sampled value (S4). ) Is stored in memory. The calculator 14 refers to the contents (S5) of this memory and refers to the average value (S5) of the drain voltage waveform.
6) is calculated and output to the comparator 15. Comparator 1
5 is the average value (S6) of this drain voltage waveform and the current counter D
The C potential (S9) is compared, and the comparison result (S7) is output to the signal generation circuit controller 16. The signal generation circuit controller outputs a signal (S8) for controlling the opposite signal generation circuit based on the comparison result (S7) by the comparator, and shifts the opposite DC potential so as to match the average value of the drain voltage waveform (S10). ) Is output. By causing the counter signal generating circuit 17 to perform the above-described operation by using the counter electrode drive control circuit configured as described above, it is possible to automatically adjust the counter DC potential.

In addition, the above-mentioned automatic adjustment mechanism of the opposite DC potential does not always need to be operated at all times, and it is operated at fixed intervals by a trigger from the clock or an external trigger, for example, an automatic adjustment switch is provided. It may be configured to be operated each time it is turned on.

Further, in the above-described embodiment, the lead wire is provided in a part of the display electrode in the screen and is used for detecting the drain voltage waveform, but the present invention is not limited to this, and a separate pseudo display electrode or
A TFT may be provided and the same detection may be performed. in addition,
The lead line for detection does not necessarily have to pass through the display electrode, and the lead line may be directly connected to the drain electrode of the TFT.

As a method of sampling the drain voltage by the voltmeter, there are a method of sampling only one cycle of the drain voltage waveform, a method of sampling over multiple cycles, and any method may be used.

As described above, according to the present invention, the potential waveform of one electrode such as the drain of a switching element such as a TFT for transmitting a video signal to the display electrode is detected, and the average value of this waveform is detected. By finding, the optimum counter DC potential was found, and the DC potential of the counter electrode of the liquid crystal display element was positioned at the optimum counter DC potential. Therefore, the DC potential of the counter electrode and the optimum DC potential of the counter always match, so that flicker does not occur. As a result, it is possible to provide an active matrix type liquid crystal display device having high display quality, which can cope with variations in the characteristics of thin film transistors due to temperature and time.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a liquid crystal display device according to the present invention, FIG. 2 is a diagram showing a method for detecting a potential of a display electrode, and FIG. 3 is a diagram for explaining an effect of the present invention. FIG. 4A is a cross-sectional perspective view of a conventional liquid crystal display device, FIG. 4B is an arrow view of a portion indicated by IV B-IV B in FIG. 4A, and FIG. FIG. 4B is a plan view of a portion indicated by IV C-IV C in FIG. 4B, FIG. 5 is a diagram for explaining the shift of the drain voltage of the TFT, and FIGS. 6 and 7 show a conventional liquid crystal display device. It is a figure for demonstrating a problem. 1 is a TFT side substrate, 2 is a counter electrode side substrate, 3 is a gate electrode terminal, 4 is a source electrode terminal, 5 is a counter electrode terminal, 6 is a lead wire terminal, 7 is a gate driver, 8 is a source driver, and 9 is A driver controller, 10 is a video signal generation circuit, 11 is a voltmeter, 12 is a memory controller, 13 is a memory, 14 is a calculator, 15 is a comparator, 16 is a signal generation circuit controller, and 17 is a signal generation circuit. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A plurality of gate bus wirings and a plurality of source bus wirings are arranged in a direction intersecting with each other, formed corresponding to respective intersections of the gate bus wirings and the source bus wirings, and a gate electrode is formed on the gate buss. A wiring is provided with an active matrix substrate on which a switching element having a source electrode connected to the source bus wiring and a display electrode connected to a drain electrode of the switching element are arranged in a matrix, and facing the active matrix substrate. A liquid crystal display device comprising: a counter substrate having a counter electrode facing the display electrode; and a liquid crystal layer interposed between the counter substrate and the active matrix substrate. Display electrode potential detecting means for sequentially detecting the potential of the display electrode in synchronization with the switching of the polarity of the alternating voltage, and the counter electrode. A counter electrode potential detecting means for detecting a potential of a pole, a calculating means for obtaining an average value of the detected potentials of the display electrodes, and a counter electrode potential obtained by the counter electrode potential detecting means and the calculating means. And a counter electrode drive control circuit comprising counter electrode potential setting means for comparing the average value potentials and making the potential applied to the counter electrode substantially match the average value potential. apparatus.