KR20010053693A - A liquid crystal display having different common voltages - Google Patents

Abstract

PURPOSE: LCD having a different common voltage is provided to prevent a flicker caused by a signal delay of a gate voltage, by applying a different common voltage to both ends of a gate line. CONSTITUTION: Different common voltages are applied on a common electrode formed on right and left sides of a substrate(120). A first common voltage applied on the common electrode formed on the left side of the substrate(120) is higher than a second common voltage applied on the right side of the substrate(120). The second common voltage is higher than the first common voltage by a magnitude for compensating a kickback voltage. The second common voltage is applied to a center part of a common electrode positioned on upper and lower ends among many common electrodes formed on the right side of the substrate, so that a potential difference between the common electrodes becomes 0 or a value near to 0.

Description

A liquid crystal display having different common voltages

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display (LCD), and more particularly, to a thin film transistor (TFT) liquid crystal display having different common voltages.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor liquid crystal display (hereinafter referred to as a TFT LCD), and more particularly, to a potential difference of a common voltage applied to the right side of a liquid crystal panel to prevent generation of flicker. The present invention relates to a liquid crystal display device having different common voltages to be equal to.

A plurality of gate lines parallel to each other and a plurality of data lines insulated from and intersecting the gate lines are formed on the substrate of the TFT-LCD, and the area surrounded by these gate lines and the data lines defines one pixel. TFTs are formed at portions where the gate lines and the data lines of each pixel cross each other.

1 is an equivalent circuit for a unit pixel in a typical TFT-LCD. As shown in Fig. 1, the gate electrode g, the source electrode s, and the drain electrode d of the TFT 10 are connected to the gate line Gn, the data line Dm, and the pixel electrode P, respectively. The liquid crystal material is formed between the pixel electrode P and the common electrode Com, which is equivalently represented as the liquid crystal capacitor Clc. The storage capacitor Cst is formed between the pixel electrode and the front gate line Gn-1, and the parasitic capacitance Cgd is generated between the gate electrode and the drain electrode due to misalignment. The liquid crystal capacitor Ccl and the storage capacitor Cst act as loads that the TFT-LCD should drive.

The operation of the TFT-LCD will be described as follows.

First, the TFT 10 is conducted by applying a gate-on voltage to the gate electrode connected to the gate line Gn to be displayed, and then a data voltage representing an image signal is applied to the source electrode s to drain the data voltage. It is applied to the electrode (d). Then, the data voltage is applied to the liquid crystal capacitor Clc and the storage capacitor Cst through the pixel electrode P, respectively, and an electric field is formed by the voltage difference between the pixel electrode Cp and the common electrode Com. At this time, since the liquid crystal deteriorates when an electric field in the same direction is continuously applied to the liquid crystal material, the LCD panel drives the image signal to be repeated positively and negatively with respect to the common electrode in order to prevent deterioration of the liquid crystal. This is called an inversion driving method.

On the other hand, when the TFT is turned on, the voltage applied to the liquid crystal capacitor Clc and the storage capacitor Cst must continue to be maintained even after the TFT is turned off, but the parasitic capacitance Cgd between the gate electrode and the drain electrode is maintained. Due to this, the voltage applied to the pixel electrode causes distortion. The distorted voltage is called a kickback voltage, and the kickback voltage ΔV is obtained by the following equation.

Here, Vg denotes a change amount of the gate voltage, that is, a difference between the gate on voltage Von and the gate off voltage Voff.

This voltage distortion always acts in the direction of lowering the voltage of the pixel electrode regardless of the polarity of the data voltage, which is shown in FIG.

In Fig. 2, Vg, Vd, and Vp represent the gate voltage, the data voltage, and the voltage of the pixel electrode, respectively, and Vcom and ΔV represent the common electrode voltage (common voltage) and kickback voltage, respectively. As shown by the dotted line in Fig. 2, in the ideal TFT-LCD, when the gate voltage Vg is on, the data voltage Vd is applied to the pixel electrode, and the data voltage is maintained even when the gate voltage is turned off. In the LCD, as shown by the solid line of FIG. 2, in the portion where the gate voltage is changed, the pixel voltage Vp is lowered by the kickback voltage under the influence of the kickback voltage ΔV.

On the other hand, the effective value of the voltage applied to the liquid crystal is determined by the area between the pixel voltage Vp and the common voltage Vcom. When the liquid crystal display is driven in an inverted driving method, the area of the pixel voltage centered on the common voltage is used. It is necessary to adjust the common voltage level so as to be symmetrical. For this purpose, a constant common voltage in which the area of the pixel voltage is symmetric is applied to the common electrode.

When the area of the pixel voltage Vp centered on the common voltage Vcom is not symmetrical, the amount of pixel voltage charged in each pixel is different from frame to frame, and the screen flickers when the pixel voltage is reversed. This is because flicker occurs. However, even when a constant common voltage is applied to the common electrode in order to prevent the flicker phenomenon, the flicker phenomenon still occurs for the following reason.

On the other hand, in general, the gate line has a resistance component and a parasitic capacitance component, and as a result, there is a delay of the gate voltage by the time constant determined by the product of these two values. This signal delay becomes larger as the size of the liquid crystal panel increases.

Further, the farther the gate voltage is from the input terminal, that is, the larger the delay of the gate signal is, the smaller the change amount ΔVg of the gate voltage is. As a result, the kickback voltage ΔV becomes smaller.

Therefore, when the common voltage is constantly applied, since the voltage is not maintained as the center value of the pixel voltage, the flicker phenomenon occurs because the value of the voltage charged in the pixel on a frame basis is changed. This phenomenon becomes even more problematic as the screen of the liquid crystal display becomes larger and the gate lines become longer.

Therefore, the present invention is to prevent the flicker phenomenon caused by the signal delay of the gate voltage.

1 is a diagram illustrating an equivalent circuit for a unit pixel of a general thin film transistor liquid crystal display.

2 is a diagram illustrating voltage distortion caused by a kickback voltage.

3 is a diagram schematically illustrating the present invention.

4 illustrates a TFT-LCD according to an embodiment of the present invention.

5 is a view showing the structure of a TFT-LCD panel according to an embodiment of the present invention.

The liquid crystal display device according to the present invention for achieving the above object is

A plurality of thin film transistors having a plurality of gate lines, a plurality of data lines, a gate electrode connected to the gate line and a source electrode connected to the data line, and a pixel electrode connected to a drain electrode of the thin film transistor. A first substrate;

A second substrate having a common electrode opposed to the pixel electrode;

A gate driver for applying a gate signal for turning on and off the thin film transistor to the gate line;

A data driver for applying a data voltage representing an image signal to the data line;

A first common voltage is applied to a common electrode at a first point located close to the gate driver, and a first point is applied at a second point between a second common electrode and a third common electrode located far from a point applied to the gate driver. It includes a common voltage generator for applying a second common voltage greater than the common voltage.

Here, the second common electrode and the third common electrode is preferably an intermediate portion of the wiring for electrically connecting. The second point may be a point at which the potential difference between the second common electrode and the second point is equal to the potential difference between the second common electrode and the second point.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention.

3 is a diagram schematically illustrating the present invention.

In Fig. 3, A and B are close to the portion of the gate driver (not shown) for applying the gate voltage, and C and D are for the portion farther from the point of application to the gate driver. The gate lines are the first gate line G1 and the last gate line Gn, respectively.

In the present invention, different common voltages Vom1 and Vcom2 are applied to both ends A, B, C, and D of the gate lines G1 and Gn, and point E is point C when applying the common voltage Vcom. The common voltage is applied to the and D points. Herein, application of different common voltages Vcom1 and Vcom2 will be described. Flickering occurs because the kickback voltage at points A and B is smaller than the kickback voltage at points C and D due to the signal delay caused by the gate line. In order to prevent this, the present invention applies the common voltage Vcom2 to the C and D points of the panel 100 and the common voltage Vcom1 to the A and B points.

At this time, the common voltage Vcom2 is greater than the common voltage Vcom2. This will be described in detail as follows.

If the kickback voltage at the point A located adjacent to the gate driver is ΔV (0) and the kickback voltage at the point C located distant is ΔV (L), these kickback voltages may be represented by Equation 2 below.

Here, Von (0) and Von (L) represent gate-on voltages at points A and C, respectively, and Cgd, Cst, and Clc represent parasitic capacitance, storage capacitance, and liquid crystal capacitance between the gate electrode and the drain electrode.

In Equation 2 above, since Von (0)> Vom (L) due to the signal delay of the gate line, ΔV (0)> ΔV (L). In other words, the farther away from the gate driver, the lower the kickback voltage. Accordingly, the farther away from the gate driver, the lower the pixel voltage is lowered by the kickback voltage. Therefore, if the common voltage Vcom (L) at point C is larger than the common voltage Vcom (0) at point A, the flicker phenomenon due to the gate delay can be prevented. Equation 3 between the kickback voltage and the common voltage to prevent the equation.

As can be seen from Equation 3, the difference between the common voltages at points C and A (Vcom (L) -Vcom (0)) is the difference between the kickback voltages at points A and C (Von (0) -Vom ( L)), the flicker phenomenon due to the signal delay of the gate line can be prevented.

Meanwhile, the present invention applies the common voltage Vcom2 to the point E so that the common voltage at the point C and the D is the same. The reason for doing this is to prevent the common voltage Vcom2 of the C and D points from being changed by the resistance of the wiring connecting the C and D points. That is, when the common voltage Vcom2 is applied to the C point as in the related art, the voltage at the C point becomes Vcom2 due to the wiring resistance. However, if the voltage at the D point is the wiring resistance R1 and the current A, Vcom2-(R1). XA) = Vcom2 '. Therefore, the potential difference between the point C and the point D occurs as much as R1 × A. The potential difference at these two points C and D adversely affects flicker and afterimages on the upper and lower sides of the LCD screen.

Accordingly, in the present invention, the E point is positioned in the middle of the two points C and D, and the common voltage Vcom2 is applied to the C and D points through the E point. As a result, the present invention ensures that there is no potential difference at point C and point D.

4 illustrates a TFT-LCD according to an embodiment of the present invention. As shown in FIG. 4, the TFT-LCD according to the embodiment of the present invention includes a TFT-LCD panel 100, a gate driver 200, a data driver 300, and a common voltage generator 400.

The data driver 300 applies a data voltage representing an image signal to each data line D of the TFT-LCD panel 100, and the gate driver 200 applies a data voltage applied to each data line D to the pixel. A gate voltage for turning on the TFT of each pixel is output so that it can be applied to the electrode.

The TFT-LCD panel 100 is formed by crossing the data line D1 transferring the data voltage from the data driver 300 and the gate lines G1 and Gn transferring the gate voltage from the gate driver 200. do. The source electrode and the gate electrode of the TFT of each pixel are connected to this data line and the gate line. In FIG. 4, for convenience of description, only the equivalent circuit for one pixel is illustrated in the TFT-LCD panel 100, where the voltage charged in the pixel electrode is represented by Vp and the voltage charged in the storage capacitor electrode is represented by Vst.

The common voltage generator 400 applies different common voltages Vcom1 to one end points A and B of the gate lines G1 and Gn, and the other end points C and G of the gate lines G1 and Gn. Apply Vcom2 to the middle part (point E) of D). At this time, Vcom2 is applied with a value larger than Vcom1, and specifically, a value satisfying Equation 3 is applied.

Here, point E may be the middle portion of point C and point D, or may be biased to one of two points. However, even if the E point is in the middle or one side of the two points, the potential difference between the two points is zero or close to zero.

5 is a view showing the structure of a TFT-LCD panel according to an embodiment of the present invention.

As shown in Fig. 5, the TFT-LCD panel according to the embodiment of the present invention is composed of a first substrate 110 and a second substrate 120 facing the first substrate. Here, typically, the first substrate 110 includes a thin film transistor and a pixel electrode, which is also called a thin film transistor substrate. In addition, the second substrate 120 includes a color filter and a common electrode, and is called an opposing substrate.

In the center of the two substrates 110 and 120 is a display area 130 composed of a plurality of pixels for displaying an image signal. An upper portion of the exposed first substrate 110 is connected to a plurality of data lines (not shown), and the data pads 140 transferring data voltages from the outside are clustered. In addition, the left side of the exposed first substrate is connected to a plurality of gate lines (not shown), and gate pads 150 for transferring the gate voltage from the outside are clustered. Connection portions 141 and 151 connecting the gate lines and the data lines and the respective pads 140 and 150 are formed between the display area 130 and the pads 140 and 150, respectively.

Four common electrode contact points 161, 162, 163, and 164 are distributed at four corners outside the display area 130, and the contact points 161, 162, 163, and 164 are redundant beside the pads 140 and 150. The common voltages Vcom1 and Vcom2 are applied from the outside through the dummy pads 171, 172, and 173.

At this time, the common voltage Vcom2 applied from the dummy pad 173 to the contact points 163 and 164 is applied to the contact points 163 and 164 as follows.

First, the contact points 163 and 164 are electrically connected by the common connection line 180, and are wired to be electrically connected to the dummy pad 173 in the middle portion of the common connection line 180. The common voltage Vcom2 is applied to the dummy pad 173. Here, the common connection line 180 is formed of a plurality of wires of a low resistance metal film to minimize the voltage drop caused by the resistance between the two contact points 163 and 164 formed on the right side of the two substrates 110 and 120. do.

However, the metal film wiring has a resistance component. Therefore, when a common voltage is applied at one end of the common connection line 180, the common voltage at the other end may have a potential difference with the common voltage at one end by the resistance component.

Therefore, the intermediate portion to which the common voltage Vcom2 is applied is a portion where the potential difference between the voltage at one end and the other end does not occur. As a result, the voltage at the contact point 163 and the voltage at the contact point 164 become substantially the same.

Meanwhile, according to the exemplary embodiment of the present invention, different common voltages Vcom1 and Vcom2 are applied to the common contact points 161, 162, 163, and 164. In this case, the distance between the common contact points 163 and 164 is far from each other. A potential difference with the vicinity of the point E of FIG. 3 to which the voltage Vcom2 is applied may occur.

Therefore, those skilled in the art will form four common contact points on the right side of the LCD panel 100, that is, the opposite side of the gate driver 200, connect the common contact points with the common connection lines described above, and common voltage in the middle portion of the common connection lines. It is easy to apply Vcom2. In this case, the middle part is the same as the middle part described above.

As mentioned above, although the Example of this invention was described, this invention is not limited to the said Example, Of course, many other changes and a deformation | transformation are possible.

As described above, according to the present invention, by applying different common voltages to the gate lines at both ends, it is possible to prevent the flicker phenomenon caused by the signal delay of the gate voltage.

In addition, as the applied common voltage is disproportionated due to the resistance of the common connection line, it is possible to prevent flicker occurring on and below the LCD panel.

Claims (3)

A plurality of thin film transistors having a plurality of gate lines, a plurality of data lines, a gate electrode connected to the gate line and a source electrode connected to the data line, and a pixel electrode connected to a drain electrode of the thin film transistor. A first substrate; A second substrate having a common electrode opposed to the pixel electrode; A gate driver for applying a gate signal for turning on and off the thin film transistor to the gate line; A data driver for applying a data voltage representing an image signal to the data line; A first common voltage is applied to a common electrode at a first point located close to the gate driver, and a first point is applied at a second point between a second common electrode and a third common electrode located far from a point applied to the gate driver. A liquid crystal display having different common voltages including a common voltage generator configured to apply a second common voltage greater than the common voltage. In claim 1, The second point is, And a common middle portion of a wiring electrically connecting the second common electrode and the third common electrode. In claim 1, And the second point is a point at which the potential difference between the second common electrode and the second point and the potential difference between the second common electrode and the second point are the same.