In recent years, due to the advancement of technology and efficiency, the application of LEDs has become more and more extensive. With the upgrade of LED applications, the market demand for LEDs has also developed toward higher power and higher brightness, which is commonly known as high-power LEDs. .
For the design of high-power LEDs, the major manufacturers currently focus on large-size single low-voltage DC LEDs. There are two approaches, one is a traditional horizontal structure, and the other is a vertical conductive structure. As far as the first method is concerned, the manufacturing process is almost the same as that of general small-sized die. In other words, the cross-sectional structure of the two is the same, but unlike small-sized die, high-power LEDs often need to operate at high current Below, a little unbalanced P and N electrode design will lead to severe current crowding effect. As a result, in addition to making the LED chip less than the brightness required by the design, it will also damage the reliability of the chip (Reliability).
Of course, for upstream wafer makers / fabs, this method has high process compatibility (CompaTIbility), and there is no need to purchase new or special machines. On the other hand, for downstream system manufacturers, the surrounding coordination, Such as the design of the power supply, etc., the difference is not big. However, as mentioned above, it is not easy to spread the current evenly on large-size LEDs. The larger the size, the more difficult. At the same time, due to the geometric effect, the light extraction efficiency of large-size LEDs is often lower than that of smaller sizes. .
The second method is much more complicated than the first method. Since the commercial blue LEDs are almost all grown on the sapphire substrate, to change to the vertical conductive structure, the conductive substrate must be joined before the non-conductive The sapphire substrate is removed, and then the subsequent process is completed; as far as the current distribution is concerned, in the vertical structure, it is less necessary to consider lateral conduction, so the current uniformity is better than the traditional horizontal structure; in addition, the basic In terms of physical theory, materials with good electrical conductivity also have the characteristics of high thermal conductivity. By replacing the substrate, we have also improved heat dissipation and lowered the junction temperature, which indirectly improves the luminous efficiency. But the biggest disadvantage of this approach is that due to the increased complexity of the process, the yield is lower than the traditional horizontal structure, and the manufacturing cost is much higher.
Basic structure and key technology of high voltage light emitting diode (HV LED)
Epistar is the first in the world to propose a high-voltage light-emitting diode (HV LED) as a solution for high-power LEDs. Its basic structure is the same as that of AC LEDs, but it is formed by dividing the chip area into multiple cells and connecting them in series. Its characteristic is that the chip can determine the number and size of its cells according to the needs of different input voltages, which is equivalent to providing customized services. Since it can be optimized for each cell, a better current distribution can be obtained, thereby improving luminous efficiency.
There are three major technical differences between high-voltage light-emitting diodes and general low-voltage diodes. The first is the trench. The purpose of the trench is to separate a plurality of unit cells. Therefore, the substrate under the trench needs to reach an insulating substrate. Its depth varies according to different epitaxial structures, generally about 4 ~ 8um, and the width of the trench is not certain. However, if the trench is too wide, it means that the effective light-emitting area is reduced, which will affect the luminous efficiency performance of the HV LED. Therefore, it is necessary to develop a high aspect ratio process technology and reduce the process line width to increase the luminous efficiency.
The second is the insulating layer (IsolaTIon). If the insulating layer does not have good insulating properties, the entire design will fail. The difficulty is that the trench with a high aspect ratio must be coated with good coverage, tight film quality and insulation The film layer with good performance is also the key to the single crystal AC LED manufacturing process.
The third is the interconnection wire between chips (Interconnect). Generally speaking, to achieve good connection, the wire needs a relatively flat surface when it is bridged. A deep stepped structure will make the wire structure weak, and it is easy to be damaged under high voltage and high current drive, resulting in chip Failure, so the development of the planarization process becomes important. The ideal state is to be able to flatten the deep trenches while making the insulating layer, so that the interconnecting wires can be smoothly connected.
In addition, the main difference between the application of the high-voltage light-emitting diode and the general low-voltage diode is that it can not only be used in Constant DC, as long as it is connected to a bridge rectifier, it can also be used in the AC environment. , Very flexible. In high-voltage light-emitting diodes, the external rectifier abandons the practice of adopting homogenous gallium nitride for AC LEDs and adopts silicon rectifiers, which not only reduces energy consumption, but also prevents the impact of excessive reverse bias on the chip; finally Because high-voltage light-emitting diodes have less internally bridged light-emitting areas than AC LEDs, the light-emitting efficiency is relatively high and the durability is also better.
As a solution for large-size, high-power LEDs
The efficiency of high-voltage light-emitting diodes is better than that of conventional low-voltage light-emitting diodes. The main reason is that the small current and multi-cell design can spread the current evenly, thereby improving the light extraction efficiency. In some applications, in addition to the need to consider the efficiency of the chip itself, the price of the final product is also an important indicator; for example, in the current lighting field, LED light sources are still not regarded as mainstream products, the key point is that the price is still On the high side. Due to the high price of LED light sources, in addition to the price of the chip itself, the overall bill of material (BOM) needs to be considered. For example, because the light-emitting diode is essentially a polar component, it must be supplied It can only be lit by the forward bias voltage, so the general LED lighting source must be attached to the AC to DC (AC / DC) power conversion system, which is a cost that must be paid.
And because the LED itself is small, the heat source is easy to concentrate, which causes the so-called hot spot phenomenon, which shortens the life of the light emitting element itself. In order to solve the hot spot problem, the heat dissipation design on the LED light source is also indispensable. At present, the metal heat sink is the most common in heat dissipation design, but the metal heat sink not only increases the weight of the light source, but also increases the cost of the light source. Due to the high efficiency of the high-voltage light-emitting diode itself, it will reduce the waste heat and the need for heat dissipation, thereby reducing costs; from the perspective of power conversion, high-voltage small-wattage power converters such as the back-to-back servant circuit, in addition to the size Xiaowai, because of the use of fewer components, the cost is lower. Therefore, the advantage of the high-voltage light-emitting diode is not only the chip itself, it can directly or indirectly further improve the efficiency of the overall module.
In summary, in application and design, single-chip high-voltage light-emitting diodes have the following benefits:
1. Save the loss of transformer energy conversion and reduce the cost.
2. In addition to high-voltage DC applications, external bridge rectifier circuits can also be designed to operate under AC.
3. The small size does not occupy space, and has excellent flexibility for packaging and optical design.
4. In addition to red phosphors, blue and red HV LEDs can be used with appropriate yellow and green phosphors to make more efficient high-CRI warm white LEDs.
At present, in Epistar Optoelectronics, we will first perform basic inspections of design criteria based on the customer's various parameter requirements; further perform simulations based on relevant optical, electrical, and thermal models to determine the size, number, and final product presentation of unit cells After the form, it will be verified by practice; and based on the data collected in practice, the original design will be verified, or modified to achieve the optimized result. At present, Epistar R & D Center has begun to establish the simulation of light, electricity and heat models related to high-voltage light-emitting diodes.
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