LED chip technology and application design knowledge

Compared with traditional light sources such as incandescent lamps and compact fluorescent lamps, light-emitting diodes (LEDs) have many advantages such as high luminous efficiency, long life, and high directivity. They are increasingly favored by the industry and are used in the general lighting market. In order to accelerate the popularization of LED lighting applications, there are still problems from the aspects of cost, technology, and standards that must be overcome in the short term. Technical aspects, including color temperature, color rendering, and efficiency improvement, still need to be further improved. The application of LED in the general lighting market involves many requirements, which must be considered from a system perspective, such as LED light source, power conversion, drive control, heat dissipation, and optics.

Thin-film chip technology is emerging

At present, the key to the development of LED chip technology lies in the substrate material and wafer growth technology. In addition to the traditional sapphire materials, silicon (Si), silicon carbide (SiC), zinc oxide (ZnO) and gallium nitride (GaN) are also the focus of current research. Whether it is a high-power chip for key lighting and overall lighting, or a low-power chip for decorative lighting and some simple auxiliary lighting, the key to technological improvement is how to develop more efficient and stable chips. Therefore, improving the efficiency of LED chips has become the key to improving the overall technical indicators of LED lighting. In just a few years, with the help of a series of technical improvements such as chip structure, surface roughening, and multi-quantum well structure design, LED has made a major breakthrough in luminous efficiency. The development of LED chip structure is shown in Figure 1. It is believed that as the technology continues to mature, the quantum efficiency of LED will be further improved, and the luminous efficiency of LED chips will also rise accordingly.

Figure 1 The development history of LED chip structure

Thin film chip technology (Thinfilm) is a key technology for producing ultra-bright LED chips. It can reduce lateral light loss and enable more than 97% of the light to be output from the front through the bottom reflective surface (Figure 2). This not only greatly improves the LED luminous efficiency, but also simplifies the design of the lens.

Figure 2 Comparison of front light output of common LED and thin-film LED

High-power LED packaging technology can be divided into three categories: single chip, multi-chip integration and chip-on-board packaging, which will be explained below.

Luminous efficiency, heat dissipation and reliability are the advantages of single chip packaging

Single chip packaging is the most widely used packaging technology. Its main technical bottlenecks are the chip yield, color temperature control and phosphor coating technology. The Golden DRAGON Plus LED of OSRAM Opto Semiconductors uses silicone packaging. Its packaging appearance and internal structure are shown in Figure 3. The LED has a beam angle of 170 degrees and can ideally match the secondary optical lens or reflector cup. Its silicone lens has the characteristics of high temperature resistance and low attenuation. The unique packaging design further improves the heat dissipation performance of the LED, so that the thermal resistance of the product is controlled at about 6.5℃ per watt, which helps to reduce thermal resistance. In addition, the specific formulation of phosphor makes the color temperature of the LED cover the range of cold white, neutral white and warm white. The advantages of single chip packaging are high light efficiency, easy heat dissipation, easy light matching and reliability.

Figure 3: OSRAM Opto Semiconductors Golden DRAGON Plus LED package appearance and internal structure

Multi-chip integrated packaging in a small volume can achieve high luminous flux

Multi-chip integrated components are another common packaging form of high-power LED components. They can be divided into low-power and high-power chip integrated components. The former is most typical of 1-watt high-power LED components integrated with six low-power chips. The advantage of this type of component is its low cost, which is the main production method for many high-power components. The high-power chip combination is represented by the OSTAR SMT series, and its package appearance is shown in Figure 4. Through optimized design, the thermal resistance of the final product can be controlled at 3.1℃ per watt, and it can drive high power up to 15 watts. The advantage of this package is that it can achieve high luminous flux in a very small space.

Figure 4: Package appearance of OSRAM Opto Semiconductors OSTAR SMT LED

COB effectively improves heat dissipation defects

COB technology follows the traditional semiconductor technology, that is, directly fixing the LED chip on the printed circuit board (PCB). Using this technology, there are currently LEDs with a thickness of only 0.3 mm or less. Since the LED chip is in direct contact with the PCB board, the heat conduction area is increased, and the heat dissipation problem is improved. This packaging form is mainly based on low-power chips.

Improve heat dissipation efficiency and extend lamp life

The life of lamps has always been one of the main issues that everyone is concerned about. To build a good lamp heat dissipation system, it is not enough to simply choose LED components with low thermal resistance. The thermal resistance from the PN junction to the environment must be effectively reduced to minimize the PN junction temperature of the LED and increase the life of the LED lamp and the actual luminous flux. Unlike traditional light sources, PCB is both the power supply carrier of the LED and the heat dissipation carrier. Therefore, the heat dissipation design of the PCB and the heat sink is also particularly important. In addition, the material, thickness, area size of the heat dissipation material, the processing of the heat dissipation interface, and the connection method are all factors that lamp manufacturers need to consider.

Optical design should properly utilize LED standards

The directivity and point light source of LED are one of the most typical features that are different from traditional light sources. How to use these two characteristics of LED is the key to the optical design of lamps. Through the secondary optical design of LED, LED lamps can achieve a relatively ideal light distribution curve. For example, in the overall lighting of the room, the brightness of the lamp is required to be high, and a lampshade with a higher transmittance can be used to improve the light output efficiency; in addition, there are lamps that add light guide plate technology to make the LED point light source become a surface light source, improve its uniformity and prevent glare; in addition, some auxiliary lighting and key lighting require a certain focusing effect to highlight the illuminated object, so you can choose to match some focusing lenses or reflective cups to meet the optical requirements.

The drive design must ensure constant current output

LED requires the driver circuit to ensure constant current output. When the LED is working in the forward direction, the relative change area of ​​the LED forward voltage is very small. To ensure the constant LED drive current is to ensure the constant LED output power. In addition, dimming design is also one of the mainstream designs of the current driver circuit. This is widely used in some scene lighting. Different brightness is adjusted according to different environments to fully achieve energy-saving effects. At present, the main design direction of the driver is centered around improving the power factor of the power supply, reducing power consumption, improving control accuracy and accelerating response speed. In addition to the design of the drive power supply, PCB wiring and series-parallel methods are also design considerations.

Standard setting is indispensable

As a new field, LED lighting needs to formulate product standards, measurement standards, control and interface standards, etc. In addition, the LED lighting products on the market are of varying quality, and many product information is incomplete, which can easily mislead consumers. At the same time, there is competition from other high-efficiency light sources such as organic light-emitting diodes (OLEDs) and traditional low-cost light sources. The LED lighting industry is in urgent need of a complete standard system to maintain and promote the healthy and sustainable development of the industry. Currently, the U.S. Department of Energy (DOE) is actively promoting relevant standards for semiconductor lighting, and mainland China, Taiwan, South Korea, Japan, etc. are also actively developing LED standards.

High-power LED technology for lighting faces many challenges

Although LEDs have found room for application in accent lighting and decorative lighting in indoor lighting, there are still many challenges for LEDs to become true general lighting or ambient lighting, such as initial cost, luminous efficiency at low color temperature, color rendering index, and system reliability.

Reduce initial costs through overall system optimization

Indoor lighting, especially home lighting, is relatively sensitive to cost. Although the styles of LED lamps are increasing and the luminous efficiency is getting higher and higher, the problem of high prices still exists. This requires further reduction of the price of LED light sources, and at the same time, it is necessary to optimize the design from the overall system level to reduce the total cost. From the initial price of compact fluorescent lamps of around US$15 when they first entered the market to the current price of less than US$1.5, it can be seen that as the market continues to develop, the price of LED lamps will be more acceptable to the general public in the near future.

Get rid of the limitation of phosphors and reduce the luminous efficiency at low color temperature

Indoor home lighting tends to prefer a lower color temperature below 4,000K. Warm white light makes the whole environment warmer and more relaxing, while cool white light gives people a clean, efficient and bright feeling, which is suitable for office lighting and outdoor lighting. Due to the influence of phosphors, the luminous efficiency of LEDs at low color temperatures is often about 30% lower than that at high color temperatures.

Hybrid red LED takes into account both light efficiency and color rendering index

The higher the light efficiency of LED, the lower its color rendering index is. Indoor lighting requires the ability to objectively display the brightness and color of objects, so as to obtain the real effect of the human eye directly observing the external scenery. Therefore, a higher color rendering index is usually required. This requires that the color rendering index of LED be further improved while improving the light efficiency. However, it is also possible to obtain an effect of a display index greater than 90 by mixing some red light LEDs at the lamp level.

Improve high current drive efficiency and reduce LED costs

At present, the driving current of a 1-watt LED can reach 350 to 1,000 mA. However, under high-current driving conditions, although the luminous flux is increased, the overall efficiency decreases significantly. Therefore, a balance must be found between the overall cost and the system light efficiency. If the luminous efficiency of LEDs under high-current driving can be improved, the number of LEDs required can be greatly reduced while ensuring higher system luminous efficiency, thereby significantly reducing costs.

Reducing LED package size helps increase design flexibility

The development of LED indoor lamps will also develop in the direction of art, miniaturization and personalization under the premise of energy saving, environmental protection and health. Therefore, reducing the package size of LEDs can increase the flexibility and innovation space in lamp design. In some occasions where mixed light is required to improve color rendering, a smaller package size will be beneficial to the design of the mixed light lens and the effect of mixed light.

Key components are essential to improving system life and reliability

For the application of LED in general lighting, the overall efficiency, life and reliability must be improved from a system perspective. The system composition of traditional lighting products is relatively simple, while the LED lighting system involves multiple components (Figure 5).

Figure 5 Components of LED lighting system (LED lamp = ED + power supply + driver + heat dissipation + optics)

LED light sources are compact and efficient, offering a wide range of colors and output power.

Power conversion efficiently converts AC power, batteries and other power sources into safe low-voltage constant current power.

The control and drive uses electronic circuits to drive and control the LED with constant current.

Thermal Management In order to achieve longer working life, LED junction temperature control is very important and heat dissipation needs to be analyzed.

Optical components focus light where it is needed, requiring the use of lenses, reflectors or light-guiding materials.