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TFT LCD Display Module Working Principle

What is TFT?

TFT - Thin Film Transistor (Thin Film Transistor)

TFT LCD- thin - film transistor liquid crystal display

Working principle of TFT LCD

In tft-lcd, the TFT functions as a switch tube. The commonly used TFT is a three-terminal device. Use the voltage applied to the gate to control the current between the source and the leakage electrode. For display screens, each pixel is structurally visible as a layer of liquid crystal sandwiched between the pixel electrode and the common electrode. More importantly, from an electrical point of view you can think of it as capacitance.

To charge the pixel P (I, j) in row j and column I, switch T (I, j) on and apply the target voltage to the signal line D (I).When the pixel electrode is fully charged, the charge in the capacitor is preserved even if the switch is disconnected, and the liquid crystal molecules between the electrodes continue to have an electric field. The data (column) driver ACTS to apply the target voltage to the signal line, while the gate (row) driver ACTS to switch on and off. The key to display the image is also the molecular orientation of the liquid crystal under the action of an electric field. In general, the arrangement of LCD molecules can be deformed by the orientation on the inside of the substrate to realize different display modes. Under the action of electric field, the orientation of liquid crystal molecules changes, and the intensity of incident light changes after passing through the liquid crystal layer through the coordination with the polarizer. To achieve image display.

Tft-lcd colorization is generally achieved by adding a layer of color filter on the front panel of the display. It requires a black matrix of red, green, blue and shading on each pixel.

TFT LCD manufacturing process tft-lcd manufacturing process has the following parts:

  • TFT array is formed on TFT substrate; Color filter pattern and ITO conductive layer are formed on the color filter substrate. Using two substrates to form a liquid crystal box; Install the peripheral circuit, assemble the module assembly of backlight, etc.
  • The function of TFT LCD DRIVER generates various voltages required for liquid crystal display, and processes the instructions sent by the external CPU.
  • Structure relationship of LCD, IC, FPC, POL.

1. Analysis of common undesirable phenomena of LCD+IC+FPC+POL

  1. Principle error, LAYOUT error. IM0,IM3 connect wrong, left and right reverse, one of the leads is wrong, the power line is connected wrong, the ground line is not connected, etc.100% of poor.
  2. Incorrect wiring of test rack. IM0,IM3 are connected wrong, data line is connected wrong, power line is connected wrong, ground line is not connected, etc.100% of poor.
  3. Program error.8/16 wrong selection, high, low 8-bit selection error (P0,P2 port), initialization code error, insufficient reset time in the program, some function is not called.100% of poor. Poor goldfinger contact. Welding, there is clamped tin, virtual welding.
  4. Poor contact with connectors.
  5. Welded FPC, broken goldfinger.
  6. FPC components virtual welding (double voltage capacitor virtual welding, crystal resistance virtual welding), components damage (diode broken), crystal resistance is too small.
  7. Plug and unplug connector, the metal wire is broken at the reinforcement point (some wire breaking can be observed under the microscope).
  8. Poor compression of FPC. There's a foreign body somewhere in the small end of the goldfinger, ACF conducting particles, compression temperature, compression pressure, etc. Cause one or more goldfingers to break or contact resistance too high.
  9. IC poor compression. On IC PAD, conductive particles on ACF, pressing temperature, pressure, etc. Cause one or more critical pads to have poor contact (circuit break) or excessive contact resistance. Summary: the causes of white screen are divided into two aspects: software and hardware. The software needs to find the analysis program, and the hardware part is a broken line or short connection. VGH,VGL has no boost.

2. Flower screen - displays irregular flower spots

There is boost voltage (VGH,VGL voltage is normal), can not normally display red, green, blue and pictures. It is usually for software reasons and is often seen when the solution company adjusts the program.

  1. incorrect number of data.16/18.
  2. No display instruction was sent before the display data was sent. General IC: 22H instruction.
  3. Wrong setting window instruction.
  4. the picture CODE of the picture is wrong.240 * 320-241 * 320.

3. Show light colors

  1. Initialization code. Partial light, partial not light, common GAMMA code wrong.The GAMMA code shall be provided by the LCD supplier. The initialization code does not change the GAMMA.
  2. the components are falsely welded on the PC (voltage doubling capacitor, wave capacitor), resulting in too low VGH and VHL voltages.
  3. The resistance of crystal vibration is too small. The selected crystal oscillator resistance is too small.
  4. Generally independent of diodes.
  5. Reasons for FPC compression. There is a foreign body somewhere in the small end of the goldfinger, an ACF conducting particle, the conductivity of the conducting particle on the ACF, the temperature at which it is pressed, the pressure at which it is pressed, and so on. Causing excessive contact resistance somewhere. 
  6. Reasons for IC compression. On IC PAD, conductive particles on ACF, conductive properties of conductive particles on ACF, compression temperature, pressure, etc. Causing excessive contact resistance somewhere. 
  7. The polarizer is cut wrong and pasted backward. For wide Angle polaroid (for products 2.4 "and above, 0.22 thick), common Angle polaroid (for products 2.2 and below, 0.12 thick) does not have this problem. 

Principle: the orientation of polarizer's suction axis is the same as that on the LCD drawing. Workshop: cut small pieces and draw lines according to the polaroid drawing, and paste them according to the polaroid drawing (top and bottom)

4. Screen flash (the reason is similar to screen light, the polarizer will not cause screen flash)

  1. Initialization code. Partial flashing, partial non-flashing, common GAMMA code wrong. The GAMMA code shall be provided by the LCD supplier. The initialization code does not change the GAMMA. The frame rate setting is too small. 
  2. the components are falsely welded on the PC (voltage doubling capacitor, wave capacitor), resulting in too low VGH and VHL voltages.
  3. Excessive resistance of crystal vibration. The selected crystal oscillator resistance is too large.
  4. generally independent of diodes.
  5. FPC pressing reason. There is a foreign body somewhere in the small end of the goldfinger, an ACF conducting particle, the conductivity of the conducting particle on the ACF, the temperature at which it is pressed, the pressure at which it is pressed, and so on. Causing excessive contact resistance somewhere. 

  6. Reasons for IC compression. On IC PAD, conductive particles on ACF, conductive properties of conductive particles on ACF, compression temperature, pressure, etc. Causing excessive contact resistance somewhere.

5. CROSSTALK (often seen in white characters on a black background, with thin stripes on both sides of white characters.

  1. The resistance of crystal vibration is too small.
  2. initial code problems. The frame rate setting is too high.
  3. Components (voltage doubling capacitance) are falsely welded, and VGH and VGL are not boosted enough.
  4. Pc pressing problem.
  5. Ce pressing problem.

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