JPH07248483A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To fix a potential difference between a gate and a source of a TFT (thin film transistor) and to attain the long life of a liquid crystal by impressing a voltage taking different potential corresponding to polarity inversion of pixel electrode potential and the polarity inversion of counter voltage as a non- selective voltage of a scanning line. CONSTITUTION:The voltage taking different potential corresponding to the polarity inversion of the pixel electrode potential and the polarity inversion of the counter voltage is impressed as the non-selective voltage of a scanning voltage. Thus, the potential difference between the scanning voltage and a pixel voltage, that is, the potential difference (Vgs) between the gate and source of the thin film transistor is fixed roughly. That is, Vgs (F1a), Vgs (F1b), Vgs (F2a) and Vgs (F2b) are fixed roughly respectively. Thus, a leakage current of the thin film transistor is fixed roughly, and the voltage impressed to the liquid crystal becomes symmetrical. Thus, liquid crystal AC drive is completed roughly, and the long life of the liquid crystal is realized, and further, a flicker and burning are reduced, and high picture quality is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device using a switching element composed of a thin film transistor (hereinafter abbreviated as TFT).

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been widely used as display devices for televisions, graphic displays, etc., due to their thinness and low power consumption. Above all, TFT
An active matrix liquid crystal display device using a switching element made of is excellent in high-speed response and suitable for increasing the number of pixels, and is expected to realize high image quality, large size, and color screen of a display screen. Some have been researched and developed, and some have already been put to practical use. At the same time, it is desired to improve the reliability of the liquid crystal display device.

The display element portion of the active matrix type display device has an active element substrate provided with an active element such as a TFT and a pixel electrode, and a counter substrate provided with a counter electrode arranged to face the active element substrate. And a liquid crystal composition sandwiched between these substrates, and a polarizing plate attached to the outer surface of each substrate. An equivalent circuit of one pixel of such a liquid crystal display device is shown in FIG.
Shown in. In FIG. 3, a switch composed of a transistor 5 is formed at each intersection of a signal line 2 connected to the signal line driving circuit 1 and a scanning line 4 connected to the scanning line driving circuit 3, and a gate electrode 6 of the transistor is scanned. The line 4 is connected to the drain electrode 7, and the signal line 2 is connected to the drain electrode 7. In addition,
Reference numeral 8 denotes a source electrode of the transistor and a pixel electrode connected to the source electrode, and a liquid crystal capacitor 9 made of a liquid crystal composition is formed between the source electrode and the pixel electrode connected to the source electrode.

FIG. 4 is a waveform diagram for explaining driving of one pixel shown in FIG. In FIG. 4, the waveform of the solid line is the scanning voltage (V _y ) applied to the scanning line, and the waveform of the dotted line is 1
The signal line voltage (V _x ) applied to the signal line from the signal line drive circuit, whose polarity is inverted for each frame, and the waveform of the dashed line indicate the counter electrode voltage (V _c ) applied to the counter electrode, respectively. There is. An ideal potential of the pixel electrode (V _s ) when such a scanning voltage, a signal voltage and a counter electrode voltage are applied is shown by a broken line in the same figure.

[0005]

However, the TFT
Is not a perfect switching element, so leakage current will occur during the holding period. It is known that this leak current depends on the voltage applied to each electrode of the transistor, particularly on the potential difference (V _gs ) between the gate potential and the source potential. From the equivalent circuit of FIG. 3, the gate potential is the potential of the scanning line in FIG. 4, and the source potential is the pixel electrode potential in FIG. Therefore, if the potential difference (V _gs ) between the gate potential and the source potential changes, the leak current also changes. Particularly, in the liquid crystal drive having the signal line voltage (V _x ) and the common electrode voltage (V _c ) whose polarities are inverted every frame, the potential difference (V _gs ) between the gate voltage and the source voltage changes the _common electrode voltage (V _gs ). _Since it fluctuates with _c ), the fluctuation of leakage current also becomes large. For example, in the conventional liquid crystal driving shown in FIG. 4, before and after writing in the positive polarity frame (F1) and before and after writing in the negative polarity frame (F2), the potential difference (V _gs ) is also different in 4 levels. That is, V _gs (F1a) ≠ V _gs (F1b), V _gs (F1a)
≠ V _gs (F2a), V _gs (F1a) ≠ V _gs (F2b),
V _gs (F1b) ≠ V _gs (F2a), V _gs (F1b) ≠ V
_gs (F2b) and V _gs (F2a) ≠ V _gs (F2b). As a result, before and after writing in the frame of positive polarity, and before and after writing in the frame of negative polarity, the leakage current differs by 4 levels, and the positive voltage and the negative voltage applied to the liquid crystal are symmetrical. There is a problem that the liquid crystal cannot be AC driven.

If the liquid crystal cannot be driven by an alternating current, the life of the liquid crystal is shortened and reliability is deteriorated. Further, there is a problem that flicker and image sticking occur, which adversely affects the image quality.

The present invention has been made in order to solve such a problem, and since the life of the liquid crystal can be extended, the liquid crystal display is highly reliable, and since flicker and burn-in do not occur, a liquid crystal display excellent in image quality is obtained. The purpose is to provide a device.

[0008]

The liquid crystal display device of the present invention is an active device in which a plurality of scanning lines and a plurality of signal lines are wired in a matrix and pixel electrodes are arranged at the intersections thereof through transistor switching elements. In a liquid crystal display device having an element substrate, a counter substrate having a counter electrode facing the pixel electrode, and a liquid crystal composition sandwiched between the active element substrate and the counter substrate, the polarities of the signal lines are inverted at a predetermined cycle. Means for applying a signal voltage to the counter electrode, a means for applying a counter voltage that inverts the polarity in synchronization with the polarity inversion of the signal voltage to the counter electrode, and a polarity inversion of the pixel potential for the non-selection voltage of the scanning line, Means for applying a voltage having a different potential corresponding to the polarity reversal of the opposite voltage, and a means for applying a signal to the selection voltage of the scanning line so that the potential change amount from the selection voltage to the non-selection voltage is substantially constant. Characterized by including and.

[0009]

The potential difference between the gate and the source of the TFT is substantially reduced by applying a potential that takes a different potential to the non-selection voltage of the scanning line corresponding to the polarity reversal of the pixel electrode potential and the polarity reversal of the counter voltage. Since it can be kept constant, the liquid crystal can be driven by alternating current. Therefore, the life of the liquid crystal can be extended, the problem of image quality such as flicker image sticking can be solved, and a liquid crystal display device with high image quality and high reliability can be obtained.

[0010]

An embodiment of a liquid crystal display device according to the present invention will be described below with reference to the drawings. FIG. 1 shows drive waveforms of an active matrix type liquid crystal display device according to the present invention. Further, FIG. 2 shows a circuit diagram for obtaining the driving waveform of FIG.

As shown in FIG. 1, in the liquid crystal display device according to the present invention, the scanning line voltage applied to the scanning line is different from that of the conventional active matrix type liquid crystal display device. That is, as the non-selection voltage of the scanning voltage, a voltage having a different potential corresponding to the polarity reversal of the pixel electrode potential and the polarity reversal of the counter voltage is applied.

As a result, the potential difference between the scanning voltage and the pixel voltage, that is, the potential difference (V _gs ) between the gate and the source of the transistor becomes substantially constant. That is, V _gs (F1a),
V _gs (F1b), V _gs (F2a) and V _gs (F2b)
Are approximately constant. Therefore, the leak current of the transistor becomes substantially constant, and the voltage applied to the liquid crystal becomes symmetrical. As a result, the liquid crystal AC drive becomes almost complete, the life of the liquid crystal can be extended, and flicker and burn-in can be reduced, so that high image quality can be obtained. In the above-described embodiment, the case where the signal voltage or the like is inverted at the frame period has been described, but the same effect can be obtained even when the signal voltage is inverted at a period that is an integral multiple of one horizontal scanning period.

On the other hand, if there are multiple levels of the non-selection voltage of the scanning voltage, there arises a problem that the amplitude of the scanning voltage varies depending on the frame. Therefore, the value of the pixel electrode potential level shift, which is proportional to the amplitude of the scan voltage generated at the rise of the scan voltage, differs for each frame, and the conventional compensation method of applying a bias voltage to the counter electrode voltage becomes incomplete. Becomes a new cause of asymmetry. However, in the present embodiment, a signal is applied to the selection voltage of the scan electrode so that the potential change amount from the selection voltage to the non-selection voltage is substantially constant. That is, FIG.
In, the problem can be solved by setting the scanning voltage so that V _g (F1) = V _g (F2).

[0014]

According to the liquid crystal display device of the present invention, a means for applying a signal voltage whose polarity is inverted in a predetermined cycle to a signal line, and an opposing voltage which is inverted in polarity in synchronization with the polarity inversion of the signal voltage are applied to the opposing electrode. The means for applying, the non-selection voltage of the scanning line, the means for applying a voltage having different potentials corresponding to the polarity reversal of the pixel potential and the polarity reversal of the counter voltage, and the selection voltage of the scanning line, Since the liquid crystal can be AC-driven by the means for applying a signal that makes the potential change amount from the selection voltage to the non-selection voltage substantially constant, the life of the liquid crystal can be extended. Further, flicker and image sticking can be reduced. As a result, a liquid crystal display device with high reliability and high image quality can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing drive waveforms of an active matrix type liquid crystal display device according to the present invention.

FIG. 2 is a diagram showing a circuit diagram for obtaining the drive waveform of FIG.

FIG. 3 is a diagram showing an equivalent circuit in one pixel of the liquid crystal display device.

FIG. 4 is a diagram showing driving waveforms for explaining driving of one pixel shown in FIG.

[Explanation of symbols]

1 ... Signal line drive circuit, 2 ... Signal line, 3 ... Scan line drive circuit, 4 ... Scan line, 5 ... Transistor, 6 ... Gate electrode, 7 ... Electrodes, 8 ...
...... Source electrode and pixel electrode connected to the source electrode, 9 ... Liquid crystal capacitance, 10 ... Counter electrode.

Claims (1)

[Claims] 1. An active element substrate in which a plurality of scanning lines and a plurality of signal lines are wired in a matrix form and pixel electrodes are arranged at intersections thereof through a transistor switching element, and a counter electrode facing the pixel electrodes. A liquid crystal display device comprising a counter substrate having a liquid crystal composition sandwiched between the active element substrate and the counter substrate, and means for applying a signal voltage whose polarity is inverted at a predetermined cycle to the signal line, Means for applying a counter voltage to the counter electrode, the polarity of which is inverted in synchronization with the polarity reversal of the signal voltage, and the polarity of the pixel voltage and the polarity of the counter voltage, to the non-selection voltage of the scanning line. Means for applying a voltage having a different potential corresponding to inversion, and a signal for making the potential change amount from the selection voltage to the non-selection voltage substantially constant are applied to the selection voltage of the scanning line. And a liquid crystal display device.