White LED driver IC design and technology development

White LED driver IC design and technology development

Introduction: In pursuit of a wide color gamut, global LCD TV manufacturers have begun to introduce LED backlight structures. Of course, this is not a simple matter. In addition to cost considerations, the most important issue is power consumption.

Sony's Qualia 005 LED backlight module for 40-inch and 46-inch LCDs released in 2004 used 450 3-primary-color LEDs, which resulted in a power consumption of over 450W. However, with the advancement of technology, the power consumption of Sony's 70-inch LED-backlit LCD TV sold in 2007 has been greatly reduced to 200W, and the number of 3-primary-color LEDs used is still 450. Such figures are quite amazing, because in about 2 years, the power consumption can be reduced by half, and the supported panel size can be nearly doubled.

Caption: The Qualia 005 backlight module uses 450 three-primary-color LEDs, and its power consumption is as high as over 450W.

LCD TVs move towards lower power consumption

Regardless of how Sony achieved this technical goal, the fact is that the technical research and development of reducing TV backlight power consumption has been continuously carried out among global industry players. In early 2006, according to Japanese media reports, the total power consumption of 32-inch LCD TVs was about 180W. It is expected that by the second half of 2006, the total power consumption of 32-inch LCD TVs, which was then known as the mainstream size, may be reduced to below 100W.

However, if we convert it to the current mainstream 40-inch LCD TV, the detailed approach is that, based on the fact that LCD TVs reduce power consumption by about 15% every year, by the end of 2006, the total power consumption will be reduced to about 230W (270W X 15%). If the backlight module technology can be improved, there is a chance to reduce the power consumption of the backlight module by 40%, so the total power consumption will be nearly 159W. At this rate of improvement, the total power consumption can reach 100W by 2010. Of course, the backlight under this goal is still a cold cathode tube.

LCD LED backlight has been expected by the market

But no matter what, LED backlight module is already one of the technologies expected by the market. In other words, the issue of high power consumption has to be solved. Because for large-size LCD TVs, nearly 60% of the electricity is consumed by the backlight module. However, reducing the total power consumption is not just about LED backlight. It also includes changing the driving circuit technology, enhancing the light guiding efficiency, reducing the thermal effect, etc., which are also technologies that assist in reducing the total power consumption.

Currently, the lighting methods of LEDs vary depending on the application. Most LCD TVs use direct backlights, while monitors and small-size application products are mainly edge-lit. This is due to the differences in the brightness and color rendering requirements of both parties. Generally speaking, monitors mostly use the side-lit RGB3 primary color lighting method. For smaller applications, such as car navigation panels, mobile phone panels, PDA panels, etc., white light LED side-lit methods are mostly used. However, the side-lit white light LED backlight method has gradually developed towards large sizes. Currently, some notebook computers have also begun to use side-lit white light LED backlight modules. Therefore, in terms of appearance design, the goals of thinness and high flexibility can be achieved. Although white light LEDs have insufficient color rendering in the red part, because notebook computers are mostly used for document and other static image display, there are not many users who expect to achieve the same quality as LCD TVs. Therefore, basically, insufficient color rendering is not a big problem. However, there are of course ways to overcome it, such as adding red light LEDs to the white light LED array, etc. These methods have also been overcome by backlight module manufacturers one by one.

Caption: Currently, the lighting methods of LEDs vary according to different applications, and most LCD TVs use direct backlighting. (Samsung Electronics)

Looking forward to high-efficiency and high-brightness LEDs

However, if we expect to popularize the 3-primary color LED backlight, we believe there are still many problems to be solved, and it may even take several years. In fact, the key is still the cost, because for LCD TVs, what is needed is high brightness, so high-efficiency and high-brightness LEDs of more than 1W must be used. In addition, because it is a direct-light input, the number of 3-primary color LEDs used may increase due to the increase in panel size. However, this approach will bring another problem, that is, the thermal effect will increase due to the increase in the number of 3-primary color LEDs used. In this way, more heat sink fins, fans, heat pipes, etc. must be used to maintain the temperature in the module. In addition, a large number of LEDs are used, and in order to make the color tone uniform, in addition to strictly selecting LEDs with the same luminous wavelength, a color sensor must be used to adjust the RGB color difference, but the impact caused is the increase in cost, so it is impossible to effectively make the price meet the expectations of consumers. However, this is not absolute. Other technologies can also be used to increase the panel size without using too many 3-primary color LEDs.

However, the desire to reduce the power consumption of LED backlight modules is not just something that backlight module and LCD TV system manufacturers need to work on. In fact, LED chip manufacturers also play a very important role. Because if the efforts of chip manufacturers can enhance the luminous efficiency of LEDs, then the demand for electricity by LEDs will be greatly reduced, and the resulting benefits will be directly reflected in the LED backlight modules.

Caption: Reducing total power consumption is not just about LED backlight alone. Changing the drive circuit technology, enhancing light-guiding efficiency, reducing thermal effects, etc. are all technologies that help reduce total power consumption.

Thermal effect is a common problem but difficult to solve

In recent years, LED chip manufacturers have been actively developing high-brightness LED chips. The most practical approach is to make the LED support a larger current to produce a higher brightness. According to current specifications, the maximum current that an LED with an area of ​​30um2 can withstand is about 30mA. This result still cannot meet the application demand of "making the LED produce a higher brightness" because the market expects to be able to introduce 350mA of current into an LED chip with an area of ​​1mm2, so that a single chip can produce a higher internal quantum efficiency. If the driving voltage is 3V, then after conversion, the power flowing into the LED is about 1W.

However, it is not enough to simply increase the current, because according to experience, about 3/4 of the power flowing in will produce thermal effects, that is, only 1/4 (0.25W) will be converted into light, and about 0.75W of power will become thermal effects. As for the troubles caused by thermal effects, it is well known that LEDs will change the wavelength of light due to temperature, reduce luminous efficiency, and cause color deviation of image quality.

The development of white light LEDs for LCD backlights is not just about increasing the brightness of the LEDs, but also about addressing challenges such as brightness uniformity, high color rendering, and increased longevity. Therefore, after completing high-current LED chips, how to package them on materials with high thermal conductivity and heat capacity has become a very important issue for the industry.

Improving optical output power through packaging

At present, the Lighting Promotion Association predicts that 100 lm/W white light LEDs will reach the practical stage of 1 yen per lumen. To achieve the improvement of the average color rendering rating of LEDs, the development of phosphors plays a very important role. Because the simulated white light LED obtained by using phosphors with blue light LEDs has a weaker ability to render red and green at wavelengths, some companies have begun to use ultraviolet LEDs with RGB phosphors. However, this approach is still complicated because it is not possible to simply mix the phosphors of different colors, because how to achieve an output close to natural white light is a difficult problem. However, regarding the uniformity of luminous characteristics, it is generally believed that only the uniformity of the concentration of the phosphor materials of white light LEDs and the production technology of phosphors should be improved to overcome the above problems.

Technically, if the light output efficiency of blue LED chips reaches 360mW, combined with advanced packaging technology, it is not difficult to obtain 100lm/W white light output. It is not a difficult issue with today's technology. For example, companies including Cree and Nichia have developed high-brightness blue LED chips in 2006. The next step is how to reduce the loss of external quantum efficiency, which is a test for the packaging industry's ability.

However, when increasing the current input, the thermal effects brought about must be faced rigorously, because we must try to reduce thermal impedance, improve heat dissipation and so on. Current practices include: reducing the thermal impedance of the chip, the thermal impedance of the control module and the printed circuit board, improving the heat dissipation of the chip, etc.