JPH02234190A - Driving voltage setting method for liquid crystal display device and its evaluating device - Google Patents

Abstract

PURPOSE:To obtain an optimum driving condition containing reliability of a panel by setting a driving condition so that a fundamental frequency component of a light emission by the minimum unit picture element of a liquid crystal display device becomes minimum. CONSTITUTION:Magnification of a microscope 11 is set so that an optical signal sent to a photomultiplier tube 12 becomes only one picture element, and so that the optical signal which transmits through a liquid crystal cell of only one picture element is made incident on the photomultiplier tube 12. In this state, an output of the photomultiplier tube 12 is read by a DC voltmeter 15, and also, the output of the photomultiplier tube 12 is inputted to a spectrum analyzer 16 and a Fourier-transformation of a light emission waveform is executed, and a driving voltage is adjusted so that the minimum frequency component of this optical waveform becomes minimum. In such a way, even by a driving method by which a virtual flicker is invisible, a driving optimum point can be searched, and with respect to a liquid crystal 13, high reliability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a driving voltage setting method for an active matrix liquid crystal display device having a thin film transistor (hereinafter abbreviated as TPT) in a display element, and an evaluation device for the device. .

BACKGROUND OF THE INVENTION Liquid crystal display devices are highly anticipated as next-generation displays to replace CRTs due to their low power consumption and portability. Furthermore, with the advancement of OA, the area has become larger and the resolution has become higher. As a result, contrast needs to be improved, and active matrix systems, in which a switching element such as a TPT is added to each pixel, have been commercialized, ranging from a simple matrix system in which a liquid crystal is sandwiched between electrodes. This article describes the case where television images are received using this liquid crystal display device.

A method for driving an active matrix liquid crystal display panel will be explained with reference to FIG.

A television signal is applied to lines A1, Af&...A,.

This line includes scanning lines B+, Be...B. TFTs Q11, Q12...QIII, QI at the intersection with
Il% Q211...Q 2m, Q @1, Q @
The source (or drain) of t""Q am is connected. Furthermore, the scanning line has <TF as shown in the figure.
The gate electrode of T is connected. TPT drain (
The source) electrode is connected to a counter electrode T through a liquid crystal cell. Drive pulse Φ1, Φ snow...Φ. are sequentially applied to the gate electrodes, the TPT is turned on, and a television signal is written into each pixel electrode through the resource electrode. This state is maintained until a scanning pulse is applied to the gate electrode of the TPT in the next field. In this way, television images are displayed.

Generally, when displaying with a liquid crystal, alternating current driving must be performed to ensure the reliability of the liquid crystal cell.

That is, it is necessary to invert the polarity of the pixel signal every field. FIG. 3 is an equivalent circuit of a liquid crystal cell that can be placed in units of one pixel. 31 is TFT, 32 is a liquid crystal capacitor CLC1, and 33 is a capacitor Cs for holding charge at the drain. Generally, in addition to these, parasitic capacitance exists in a liquid crystal cell at a TPT or a crossing portion of a pass line. These parasitic capacitances include the cross capacitance C. between the gate line and the source line as shown in the figure. 34, TFT
The overlap capacitance of the gate and source at C, . 35
, the capacitance Csa3B at the overlap part between the gate and drain in the TFT section, and the capacitance between the source and drain 0.4
37 etc. child. The effects of these parasitic capacitances are as follows. C. One influence is crosstalk between each pixel. This is due to a change in the source signal, resulting in a drain due to capacitive coupling of 0.4. The potential of the pin changes. Furthermore, as described above, since a signal of opposite polarity is supplied for each field, this signal changes the drain potential due to capacitive coupling of 01, causing a shading phenomenon in which the lower side of the screen becomes darker. Furthermore, C. When the gate voltage changes from the ON state to the OFF state due to the effects of It causes deterioration (Unexamined Japanese Patent Publication No. 60-3
698, Japanese Patent Application Laid-Open No. 130-158095). Many academic conference presentations and patents have been reported regarding these problems, such as those listed below. Two scanning lines are paired, the combination of the two is changed for each field, and the two scanning lines are simultaneously driven with the same video signal (
,IOROZυSeki!゜Others, S.A.I.T゜｜(S10)'
84 DIGEST p31 (
1 (1984)). The time axis is manipulated using frame memory, etc., and the frame frequency is converted to a range where flicker does not become a problem (Yokotani et al., Kansai Branch Federation of Electrical Associations Conference G391 (198°4)). Supplying video signals of opposite polarity to each signal line (Japanese Patent Application Laid-Open No.
095 Publication). The polarity of the display signal is inverted for each signal line, and the polarity is also inverted for each selected horizontal line (Japanese Patent Application Laid-Open No. 3898-3898).

The display signal polarity is inverted for each scanning line (Japanese Unexamined Patent Publication No. 151815). The polarities of the display signals are reversed between adjacent signal lines, and the phases of odd and even lines are 90 degrees out of phase (T.
aglshl), etc.
88 DIGEST p. 28
5 (198B)), etc. The driving method described above is called a flickerless driving method because no flicker is visually visible.

Problems to be Solved by the Invention As mentioned above, liquid crystals need to be driven with alternating current. This is because if a DC voltage continues to be applied to the liquid crystal cell, the liquid crystal cell will deteriorate, causing reliability problems.

Therefore, even when AC driving is used, the effective voltages applied to the liquid crystal in even and odd fields must be equalized. Third
As shown in the figure, since these parasitic capacitances exist in the pixels, when AC driving is performed, the signals do not match in the positive or negative field. If it is driven in this state, the panel will flicker significantly and a DC voltage will always be applied to the liquid crystal, resulting in deterioration of the liquid crystal (
Figure 4(a)). To counter this, the method of reversing the polarity of the signal for each field was designed to minimize the flickering of the entire surface of the liquid crystal panel by lowering the center of the potential of the counter electrode relative to the signal potential. (Figure 4(b)). The point set in this way is the point where the effective voltage applied to the liquid crystal in even and odd fields is equal, and is the optimal point from the viewpoint of panel reliability. In other words, finding a driving condition that minimizes flickering will give the optimum point in terms of panel reliability. The difference between the center of the signal on the counter electrode and the center of the signal on the source electrode is called an offset voltage. The polarity of the signal is inverted, and the polarity is also inverted every time one horizontal line is selected, and the polarity of the display signal is inverted every scanning line, etc., which eliminates the apparent flicker component on the entire surface of the liquid crystal panel. However, it is not possible to find the minimum point of flicker by simply observing the emission waveform of the entire panel or a portion of it, and the image quality is poor. However, it is not possible to know the optimal driving conditions including panel reliability.

The present invention aims to solve the problems of the prior art.

Means for Solving the Problems The present invention is characterized in that driving conditions are set so that the fundamental frequency component of light emission in the smallest unit pixel of a liquid crystal display device is minimized.

Even in the normal driving method in which the signal is inverted in one field of action, or the above-mentioned flickerless driving method, by setting the fundamental frequency component of light emission in the smallest unit pixel to be the minimum, no apparent flickering force is visible. Even in the driving method, it is possible to find the optimum driving point, and high reliability can be obtained for the liquid crystal.

Examples Examples of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration diagram of a liquid crystal panel evaluation device used in the present invention. 11 is a transmission microscope, 12 is a photomultiplier tube which is a light receiving part, 13 is a liquid crystal panel, 14 is a liquid crystal panel driver, 15 is a voltmeter for monitoring the DC output of the signal from the photomultiplier tube, and 16 is a photomultiplier tube. A spectrum analyzer for frequency-analyzing the signal from the multiplier tube, 17 a high-voltage power supply for the photomultiplier tube, and 18 a computer for controlling these.

A method for setting the optimum drive point using the above measuring device will be described. First, by appropriately setting the magnification of the microscope, the optical signal sent to the photomultiplier tube is set to only one pixel. As a result, the optical signal transmitted through the liquid crystal cell of only one pixel enters the photomultiplier tube. Read the output of the photomultiplier tube with a DC voltmeter and calculate this value as A.
shall be.

This value corresponds to the average value of the light intensity that has passed through the liquid crystal four times. Furthermore, the output of the photomultiplier tube is input to a spectrum analyzer to perform Fourier transformation of the emission waveform. Normally, in the NTSC system, the field frequency is 80
Hz, the minimum frequency is 30H due to the deviation of the effective value applied to the liquid crystal cell due to even and odd fields.
The z component appears. The amplitude of this 30Hz component is assumed to be B, and the value of B/A is adjusted by adjusting the arbitrarily adjustable drive voltage parameters for each drive method, such as the offset of the opposing electrode, so that the value of B/A is minimized. It should be minimized. In FIG. 5, the horizontal axis represents the opposing offset voltage, and the vertical axis represents the value of B/A. Point C in the figure is the optimal driving point. The driving conditions set in this way are excellent in reliability. The driving method used in the examples may be normal driving or flickerless driving, and a photosensitive element such as a phototransistor may be used instead of a photomultiplier tube. A method may also be used in which the waveform is monitored and the voltage value is set at the point where the distortion of the waveform is minimized.

Effects of the Invention As described above, the polarity of the source signal can be reversed for each scanning line or source signal line by using the driving method and evaluation device set using the above method according to the present invention. Even with flickerless drive, the D applied to the liquid crystal
It becomes possible to set the C voltage to the minimum,
It is possible to know the driving conditions for highly reliable liquid crystal panels, which is of great technical significance.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a liquid crystal panel evaluation device according to the present invention, Fig. 2 is a wiring diagram explaining a conventional driving method of a liquid crystal display device, Fig. 3 is an equivalent circuit diagram of one pixel, and Fig. 4 is a circuit diagram to explain how to equalize the effective voltage applied to the liquid crystal in even and odd fields by adjusting the offset of the opposing voltage, and Figure 5 shows the optimal DC offset voltage by A/B. It is a graph. Name of agent: Patent attorney Shigetaka Kurino No. 1! I R electric motor stage ! ! I-machi Enban, and the final scanning pulse h 34 --1A-1n-11Bass Like Kate Snow Kate Otori Swords Ink O's Zero amount Source Cross Capacity Drain Cross Capacity Drain Dark Capacity 1a Envy? 3-Keru Nami-sho dead le exclusive circuit ＼ Sentence 1 ■ Sai Futzut t or 177 JO term i turn vs. 匍Oh 7t! ，
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Claims (8)

[Claims] (1) A switching element electrically connected to the image signal wiring and the scanning signal wiring is connected to the pixel electrode, and the driving voltage of the liquid crystal display device that performs display by the liquid crystal sandwiched between the pixel electrode and the counter electrode. A method for setting a driving voltage for a liquid crystal display device, the setting method comprising observing transmitted light or reflected light at a minimum unit pixel, and adjusting the driving voltage so that the minimum frequency component of this light waveform is minimized. (2) A method for setting a driving voltage for a liquid crystal display device according to claim 1, wherein signals of opposite polarity are supplied to each video signal line. (3) A method for setting a driving voltage for a liquid crystal display device according to claim 1, wherein signals of opposite polarity are supplied for each scanning line. (4) Claim 1, wherein the method for driving the liquid crystal display device is characterized in that signals of opposite polarity are supplied to each video signal line, and signals of opposite polarity are supplied to each scanning line. A driving voltage setting method for the liquid crystal display device described above. (5) An evaluation device for a liquid crystal display device in which a switching element electrically connected to an image signal wiring and a scanning signal wiring is connected to a pixel electrode, and display is performed by a liquid crystal sandwiched between the pixel electrode and a counter electrode. , a microscope for narrowing the field of view to a unit pixel for the purpose of observing light at the smallest unit pixel, a photosensitivity element for receiving transmitted light or reflected light from the unit pixel, and its peripheral equipment. An evaluation device for a liquid crystal display device, characterized in that: (6) The evaluation device for a liquid crystal display device according to claim 5, wherein the method for driving the liquid crystal display device is such that signals of opposite polarity are supplied to each video signal line. (7) The evaluation device for a liquid crystal display device according to claim 5, wherein the liquid crystal display device is driven in such a manner that signals of opposite polarity are supplied for each scanning line. (8) Claim 5, wherein the method for driving the liquid crystal display device is characterized in that signals of opposite polarity are supplied to each video signal line, and signals of opposite polarity are supplied to each scanning line. An evaluation device for the liquid crystal display device described above.