Design and implementation of LED backlight module in ultra-thin LCD TV

The LCD screen itself does not emit light. In order to clearly see the content of the LED display, a certain amount of white light backlight is required. The backlight is an optical component inside the LCD display, which is composed of a light source and necessary optical auxiliary components. The traditional backlight source uses a cold cathode fluorescent lamp (CCFL), which has poor color reproduction and contains mercury vapor that is harmful to the human body. The LED backlight has good color reproduction, long life, does not contain mercury, and is beneficial to environmental protection. In terms of the drive circuit, the traditional CCFL backlight drive circuit is very complicated, requiring a drive voltage of thousands of volts and can only be driven by a special inverter. LED can work at low voltage, has a fast response speed, and is relatively easy to control. In recent years, LED has been gradually introduced into the existing LCD display technology, and the backlight source is very likely to become the next industry monopolized by LED. If the LED backlight source is improved by increasing contrast and regional control, the performance is far better than that of the CCFL backlight source. At present, LED backlight sources have been popularized in small and medium-sized panels, such as mobile phones, digital photo frames, etc. With the development of LED technology and the continuous maturity of LED chips, LED LCD TVs may gradually replace traditional CCFL LCD TVs.

Backlight sources are divided into direct-lit backlight sources and edge-lit backlight sources according to the position of the light source. In direct-lit backlight sources, the LED light source is placed directly below the light-emitting surface. The light emitted by the light source is diffused and mixed by the diffusion plate over a certain distance and then emitted as a surface light source. Direct-lit backlight sources require a certain light mixing distance. The edge-lit backlight structure is more advantageous for ultra-thin module design. With the rise of the "ultra-thin trend", large-size ultra-thin edge-lit LED backlight sources have become a research hotspot for major TV manufacturers and upstream companies.

1 Structural design

The LED backlight designed in this paper is an ultra-thin side-lit structure. The entire structure includes: LED light strip, driver board, film material, light guide plate, heat sink, upper frame, and backboard. The backlight uses white light LED. The entire structural design takes the center point of the Active Area as the design center of all components, and designs other dimensions based on the size of the LCD screen used in LCD TVs. The structure is designed by comprehensively considering the requirements of circuit design and optical design. The structural design starts with the UⅣOUT layout diagram to express the overall mechanism and the assembly relationship between each component, and then proceeds to the parts drawing structure design. The upper frame adopts a segmented design, which is segmented in the length direction to avoid excessive length. The outlet ends of the light strip are set at the four corners of the backboard to reduce the winding length.

2 Optical Design

The function of the backlight module is to transform the light emitted by a point light source into a surface light source through diffuse reflection. In order to obtain a qualified surface light source, the first thing to do is to select a suitable LED. This design uses a white light dual-chip LED. By presetting the white field photometric index and combining the research and analysis of influencing factors such as the LCD screen and optical film, the calculation of the luminous flux required for the entire backlight source is completed. Based on the calculated luminous flux, combined with the optical characteristics of the LED, the number of LEDs required is calculated. Table 1 shows the electrical and optical characteristics of the selected LEDs.

Table 1 LED electrical and optical characteristics

In order to improve the brightness of the backlight source, the film material structure is matched as follows: a diffuser layer + a BEF layer + a DBEF layer. The diffuser is used to atomize the light by refraction and reflection of the diffuser material, making the emitted light more uniform: the BEF is used to gather the light so that it enters the liquid crystal module vertically to increase the brightness: DBEF uses 50% of the light originally absorbed by the traditional absorption polarizer to increase the brightness. The diameter of the light guide plate dots is 0.54 to 1.55 mm. Figure 1 shows the distribution of some dots on the light guide plate.

Figure 1 Light guide plate dot distribution diagram

3 Circuit Design

The LED backlight circuit design mainly includes the light bar design and the drive control circuit design. Figure 2 shows the structural block diagram of the entire circuit part.

Figure 2 Circuit structure diagram

3.1 Light bar design

The light source uses a dual-chip white light LED, and the light strips are distributed around the backlight source. In order to achieve better current uniformity, the light strips are connected in series and parallel, and two parallels are connected in multiple strings. In order to achieve better heat dissipation, the LED light strips are made of aluminum substrate. The input voltage of the entire system is 24V, which is provided by an external power converter.

3.2 Drive control circuit design

The driver chip is a three-way peak current mode PWM controller that provides high-precision LED current by closed-loop control of the output current. The chip contains three peak current mode controllers that provide feedback to the IC to ensure higher efficiency and higher accuracy. The gate driver on the chip is optimized to drive a logic-level FET with a source current of 0.25A or a sink current of 0.5A, and each output current can be dimmed individually by linear or PWM dimming methods.

The closed-loop system of the chip can dynamically adjust its output voltage to adapt to the line and load adjustment of the LED current. The three-way driver integrated in a single package ensures better current matching between each string, while reducing the number of chips in the entire system. It has a 40V linear regulator to provide 5V power to power the IC. The switching frequency of the three converters is controlled by an internal oscillator, and the three channels have a phase difference of 1200 to reduce the input current ripple. High-voltage drive, good current matching. The current accuracy between LED strings is 4-2%, and linear and PWM dimming are supported.

Figure 3 PW_M_ waveform (D=75%)

3.3 Heat dissipation design

Since LED has low light efficiency, it will generate a lot of heat when working. If the heat dissipation problem is not solved, the brightness of LED will be attenuated and the service life will be shortened. In order to suppress the heat generated by LED from affecting the brightness and color of the backlight module, it is necessary to carry out heat dissipation design for LED.

In order to achieve better heat dissipation, the LED array is welded on an aluminum substrate (MCPCB), which has many times better heat dissipation than ordinary PCB (FR4 PCB). Under the aluminum substrate, a long heat sink is designed. A heat-conducting double-sided adhesive is applied between the heat sink and the light bar, and the aluminum substrate and the heat sink are fixed with screws. A heat-conducting sheet with a protective film is applied to the heat accumulation area around the backlight source. In order to reduce thermal resistance, thermal vias are added to the LED pad. In this way, the cross-sectional area of ​​the PCB board perpendicular to the heat flow direction is reduced, thereby reducing the thermal resistance between the top and bottom layers and enhancing the heat conduction of the LED lamp. Finally, the heat is dissipated by air convection in the TV case.

4 Test Results

The ultra-thin LED backlight for LCD TV designed in this paper is a side-lit structure, and the light source uses white LED. Figure 4 shows the assembled LED backlight, which is 9.9mm thick. Using BM-7 for a nine-point test, the center brightness is 5,500nit, the brightness uniformity is 82%, and the color reproduction reaches 95% @CIE 1976. The overall power of the backlight is 150W, of which the LED power consumption is 135W, and the drive circuit efficiency is 90%. The drive circuit of the backlight system is a high-voltage drive, with a simple circuit and good current consistency.

Figure 4 Assembled backlight source

5 Conclusion

This paper designs a large-size ultra-thin LED backlight for LCD TVs, with a center brightness of 5,500nit, an overall backlight power of 150W, and a brightness uniformity of 82%. The thickness of the whole machine is 9.9mm, reaching the industry-leading level. The backlight designed in this paper has passed the reliability test, laying the foundation for mass production.