VGL and VGH of TFT LCD displays

VGL and VGH are key voltages used to control pixel switching in TFT LCD displays, directly affecting image display quality. VGH and VGL are two crucial voltage parameters in an LCD screen, playing a vital role in the driving process of LCD technology. VGH is the high-level voltage required to drive the liquid crystal molecules to orient themselves, while VGL is the corresponding low-level voltage. In practical applications, VGH and VGL often work in conjunction with VCOM (common electrode voltage) to optimize the displayed image effect.

VGH (Voltage Gate High)

Function: As a positive voltage (typically in the range of +15V to +20V), it is responsible for charging the gate capacitance of the TFT (thin-film transistor), turning on the TFT, thereby turning on the pixel and charging the liquid crystal capacitor to maintain the display for one field cycle.

VGL (Voltage Gate Low)

Function: As a negative voltage (typically ranging from -7.5V to -10V), it is used to turn off the TFT gate, ensuring that the TFT is completely cut off, preventing pixel capacitor charge leakage, maintaining image stability, and avoiding ghosting or crosstalk.

In LCD driving circuits, the stability of VGH and VGL voltages is crucial. The former maintains the TFT gate on (15-20V), while the latter is responsible for shutting it off (-7.5 to -10V). By designing bootstrap and negative voltage circuits, combined with capacitor and rectification technologies, these two voltages can be generated efficiently, avoiding screen flickering, white screen, and other malfunctions, while controlling cost and power consumption.

LCD screen with built-in boost circuit:

7-inch 2-channel MIPI LCD display

   MIPI 2 Lane 7-inch display

 

• Size: 7
• Resolution: 1024*600 dots
• Viewing angle: IPS
• Interface type: MIPI 2 Lane or 4 Lane
• Driver chip: 2 ICs
• Touch method: CTP
• Backlight brightness: 500 cd/m²
• Dimensions: 164.30 (W) * 97.00 (H) * 2.60 (T) mm

Features

In a liquid crystal display panel, each pixel is controlled by a thin-film transistor (TFT). The switching state of the TFT determines whether the liquid crystal molecules are oriented by an electric field, thus controlling the transmission and blocking of light to form an image. The VGH and VGL voltages provide the necessary switching voltages to the TFT. Specifically, when the TFT gate receives a VGH voltage, the TFT conducts, allowing data signals to pass through and driving the liquid crystal molecules to oriented; conversely, when the gate receives a VGL voltage, the TFT turns off, blocking data signals and allowing the liquid crystal molecules to remain in their original state.

The voltage values ​​of VGH and VGL directly affect the performance of an LCD screen. Excessively high or low VGH/VGL voltages can lead to incomplete or excessive orientation of liquid crystal molecules, thus affecting display quality, such as decreased contrast and color distortion. Therefore, during the design and manufacturing process of LCD screens, it is necessary to precisely control the voltage values ​​of VGH and VGL to ensure that the liquid crystal molecules can orient accurately and stably.

If the VGH or VGL voltage fluctuates or deviates from the design value (such as voltage loss or amplitude variation), it can lead to image malfunctions, such as screen flickering, white screen, reduced contrast, or bright spots. Therefore, circuit design must ensure voltage accuracy, typically achieved through a charge pump structure, while ensuring low current demand (generally below 10mA). The stability of VGH and VGL is also a crucial indicator of LCD screen quality. In practical applications, the voltage values ​​of VGH and VGL may change due to factors such as power supply fluctuations and temperature variations. If these changes exceed the LCD screen's tolerance range, it will cause display abnormalities. Therefore, high-quality LCD screens require stable VGH and VGL voltage outputs to cope with various environmental changes.

Measure VGL and VGH voltages (using a 7-inch display as an example).

If you encounter any problems during the measurement, debugging, or application of VGH/VGL in TFT LCD displays, please feel free to contact Hongcai Technology. We have an experienced team of engineers who can provide fast and reliable technical support.