Liquid Crystal Capacitor (Clc)
The Liquid Crystal Capacitor (Clc in short) is a parallel plate capacitor formed by the upper glass common electrode and the lower glass display electrode of the TFT LCD.
When the TFT is powered, the voltage across Clc drives the liquid crystal to deflect.
The equivalent circuit of Clc of a Pixle is shown in Figure 1.
Storage Capacitor (Cs)
Because the capacitor value of Clc is too small, to keep the images on screen til the next refresh, it is necessary to increase the Storage Capacitor (Cs) to increase the effective capacitance value of the pixel.
The Cs architecture is mainly divided into two types: Cs on gate and Cs on common.
They are respectively the plate capacitance formed by the display electrode and the gate line or the common line.
Considering the Cs on gate only uses the next gate line while Cs on common needs to be rerouted on lower glass, the architecture of Cs on gate is more common.
The equivalent circuits of a Pixel of both architectures are shown in Figure 2. below.
How does LCD change brightness or color?
The change in brightness
By changing the voltage across the capacitor Clc, the state of the liquid crystal is adjusted to control how much light penetrates the LCD.
Take TN LCD in 90°4V as an example, the relationship between voltage and transmittance is shown in Figure 3.
The change in colors
The upper glass of the TFT LCD, CF panel, is composed of small dots in three colors: red (R), green (G), and blue (B).
By changing the intensity of the three primary colors, the mixture of them can generate various colors.
Taking a 6-bit LCD as an example, it can display colors of 2⁶ x 2⁶ x 2⁶ =262,144, and the color mixing effect is shown in Figure 4.
Correlation between LCD voltage and light transmittance
The level of liquid crystal operating voltage controls the amount of transmitted light, thereby controlling the brightness and darkness of the lights from red, green, and blue.
Taking TN LCD in 4V as an example, the curve shows the correlation between liquid crystal operating voltage and light transmittance in Figure 5.
LCD imaging process
Equivalent circuit of TFT
TFT is used as a voltage control switch to control the writing and holding of the voltage of each pixel so that the corresponding position of the LCD can be written with the correct voltage.
The working model of the N-MOS tube is shown in Figure 6, and the equivalent circuit of signal writing of the pixel is shown in Figure 7.
Write the first line of data in TFT
Turn on the first row while turning off the rest with the previous voltage.
Update the first line to the desired voltage set previously, as shown in Figure 8.
Write the second line of data in TFT
Turn off the first line, the voltage has been fixed, so the display color has also been fixed.
Turn on the second row, keep the rest off, and update the second row to the preset voltage.
By analogy, the image writing of the entire screen can be completed.
Liquid crystal polarity inversion
DC drive impacts the liquid crystal molecules
Long-term DC driving may damage the liquid crystal molecules.
The abnormality is mainly manifested in that the rotation angle of the liquid crystal is inconsistent with the preset value, resulting in afterimages.
Afterimages caused by DC drive
The afterimages, caused by the DC signal driving, can be divided into plane residue and line residue.
Common abnormalities are shown in Figure 11.
Causes of afterimages
- Residual DC-driven liquid crystal ions;
- When the voltage is turned off, the ions in the LCD cell are adsorbed on the surface of PI (polyimide);
- Ions left in the LCD built in the electric field.
Polarity inversion method of LCD
The polarity direction of the electric field applied to the liquid crystal molecules must constantly change.
Because the liquid crystal molecules may be damaged after they maintain a certain polarity for a long time.
There are four main LCD polarity inversion methods: frame inversion, row inversion, column inversion, and dot inversion. The polarity transformations are shown as following figure 13.
Polarity inversion characteristics
Comparison of characteristics of different polarity inversion types as the following table:
|Common AC Power||Low||High||N/A||N/A|
Vcom drive mode
Vcom electrode driving modes are separated into DC (direct current) Vcom and AC (alternating current) Vcom.
Given the flicker of Dot inversion is the best, DC Vcom is often used for large-size LCDs.
DC drive schematic diagram, as shown in Figure 14.
Because AC Vcom does not support dot inversion, it is often used in small-size LCDs.
AC Vcom drive schematic diagram, as shown in Figure 15.
The flicker phenomenon happens because the voltage asymmetry between the positive and negative polarities of the liquid crystal causes the brightness of the positive and negative polarities to be unbalanced. The V-T curve is shown in Figure 16.
Improvement: we can adjust Vcom to reduce flickers. However, since Vcom of the whole panel is not the same.
We have been in LCD manufacturing for over 15 years, working with customers for their projects to provide appropriate LCD solutions, together bringing the idea to market.