The development of trend for COB light source

In the field of LED packaging, COB is the abbreviation of Chip on Board, which is an integrated surface light source technology that directly attaches LED chips to the substrate. The LED light source using COB technology, the chip is directly dissipated to the substrate, and the thermal power density is high; the chips are densely arranged, and the optical power density is high; the size of a single illuminator is small, which is very suitable for the commercial lighting field that has strict requirements on the size of the light source. Compared with single-chip packaged LED devices, COB packaging with multiple chips, multiple solder joints, and high power density also leads to an exponential increase in the difficulty of the packaging process. It can be said that COB light source is on the one hand the leader in commercial lighting applications, and on the other hand, it is also the crown jewel in the field of LED packaging.

Light effect is king

The main function of the light source is lighting. For LED, there is a limitation on the premise of lighting – energy saving. Therefore, the first stage of COB light source development is also in pursuit of light efficiency. Because the metal substrate circuit board or ceramic substrate used by COB light source reflects The rate is not as good as that of the silver-plated bracket, the lower reflectivity superimposes the mutual absorption of light between multiple chips, and the luminous efficiency of the COB light source is more than 30% lower than that of the SMD device. Later, the introduction of the mirror aluminum substrate solved the reflectivity problem, but the multi-chip light source Mutual absorption still makes COB light sources less efficient than discrete devices. With the improvement of off-chip quantum efficiency and phosphor light conversion efficiency, the luminous efficiency of COB light source has reached the industry-accepted “sweet spot” of 100lm/W, which is 2-3 years later than the light source assembled with discrete devices.

Improve light quality

In the era of pursuing light efficiency, white LED are mainly composed of blue light chips + YAG yellow powder spectrum, the color rendering index is generally around 70, and the color temperature is also mainly 5000-6000K. With the development of chip and phosphor technology, LED light efficiency has a “margin” greater than the sweet spot, which provides the possibility to improve the light quality of COB light sources. By adding red powder, cyan powder, short-wave yellow powder, or even using a multi-wavelength blue light chip, the color rendering index of the COB light source has increased from 70 to 80, 85, 90, 95, 98, and even the current COB light source with a color rendering index of 100 is nothing. difficulty. In this process, 0 (low) blue light source for eye protection, pork lamp, vegetable lamp, jewelry lamp and many other “special color point” light sources that meet special needs are also produced.

Adjustable color temperature

In different scenarios, changing the color temperature of the light source can bring a better lighting experience. When there is no space limit, it is very easy to realize the color change temperature, as long as the devices with high and low color temperature are mixed and packaged and driven separately. The color temperature change of the COB light source is also the same idea. The chips in the COB light source are divided into two groups, which are respectively coated with mixed phosphors of different types and concentrations, so that the two groups of chips can generate white light with different color temperatures. Changing the current of the two groups of chips can make the light source emit white light with different color temperatures. The size of the COB light source is small. After lighting, there are no light-emitting points with two different color temperatures, but a uniform light-emitting surface. This is the advantage of the COB light source. Puzzled. After years of exploration, for different chip structures, people have invented and improved several technologies such as printing, spraying, and fluorescent sheets, and realized the localized and quantitative coating of different types of phosphors in COB light sources.

By optimizing the types and ratios of the two color temperature fluorescent glues in the toning COB light source, high CRI and high light efficiency can be achieved in the entire color changing range. Figure 2 is the spectral curve of a current color-changing COB light source with high color temperature and low color temperature provided by Guangsheng Semiconductor. The phosphor of this light source has been optimized. In the process of changing color from 3000-6000K, at any color temperature, color rendering The index can reach more than 95, and the index that people are very concerned about is also greater than 80.

The change is up to you

Adjustable color temperature expands the application field of COB light source, but the obtained color can only appear on the line connecting the color coordinate points of high and low color temperature in the color gamut diagram. In the field of commercial lighting, the use of lighting sources with different color coordinates can more highlight the visual effects of different colors of goods and meet people’s aesthetic needs. From this perspective, only color temperature adjustment is not enough. The COB light source with the ability to adjust the domain came into being.

A COB light source with adjustable color gamut and the spectral curve of each primary color

Figure 3 is a color gamut adjustable COB light source developed by Semiconductors. In this light source, in addition to white light with high and low color temperature (the central area in the figure), red, green and blue are also packaged around the periphery. A monochromatic LED chip, the COB light source has 5 light-emitting units that can individually adjust the brightness: warm white, cool white, red, green, and blue. The full color gamut adjustment of the COB light source can be realized by changing the driving current or duty ratio of the five groups of light-emitting units.