At present, the most promising market for LEDs is undoubtedly a large-size LCD backlight in flat-panel LCD TVs, notebook computers and notebook monitors, but there is no doubt that general lighting is the ultimate development goal of LED. At present, LED bulb replacement lamps will become the most important growth area in the lighting market before the LED general lighting market has actually started. However, is there a large number of LED bulbs on the market that are equivalent to true solid-state lighting (SSL)? Is the LED really ready for general lighting?
LED general lighting faces four major challenges first
The US Department of Energy has said that solid-state lighting is the most devastating innovation in the field of lighting for 50 years. Many people in the industry agree with this and believe that LED is a disruptive product, and it is necessary to reform its original product. But in fact, LEDs still face four major challenges in the general lighting market: light quality, light performance, reliability, and simplification.
Zhou Xuejun, Marketing Director of Philips Asia, explained that LED light quality still has a gap compared with traditional lighting; some light effects can not be covered by LED applications; because LED lighting reliability is from chip, electricity, light, heat, machinery, The interconnection part and other factors determine, so it is impossible to maintain sufficient light output lumens to maintain the life; due to the lack of uniform standards, the LED lighting products on the market are now diverse, and the complexity also hinders it to be difficult to be accepted by more audiences in the short term. ''So, what are the current solutions for improving LED light quality and light efficiency?
Light quality
Generally speaking, the light quality we talk about mainly refers to light distribution, light angle and shadow, and color rendering. Here, Zhou Xuejun particularly emphasizes that designers should pay attention to: Is the color of light between different LED particles consistent? Are the changes in different particles consistent over time? Is there an chromatic aberration of an LED at different angles? What is the color rendering index?
The entire packaging process of LEDs consists of two important steps: bonding the chips and pins with gold wires, and performing phosphor coating. These two steps will cause color difference to the LED, especially the gold wire will block the light path. If improper operation may cause hidden cracks on the chip, thus forming a chip breakage hazard. To solve these problems, Philips Lumileds' latest LUXEON Rebel LED series uses the latest TFFC (film flip chip technology) and the unique Lumiramic fluorescence technology.
According to Zhou Xuejun, the electrode of TFFC technology is behind the board, and there is no vertical and horizontal wiring on the surface, so the physical structure is better. Lumiramic fluorescence technology uses ceramic fluorescent plates instead of phosphors to directly bond to the chip, so that the surface is completely flat without unevenness and unevenness, thus fundamentally avoiding the traditional packaging method caused by uneven phosphor coating. Most LEDs show color inconsistencies at angles that are off the center of the boresight. In addition, the use of Lumiramic technology can also reduce the range of white light bin (bin) to the original 1/4, Zhou Xuejun added.
At present, for the lighting industry, especially lighting design, the most troublesome problem is the sub-bin. For some experienced LED manufacturers, the usual practice is to use the entire output range of white LEDs in all gears. However, at a specific CCT (correlated color temperature), it is impossible to produce a white LED having high uniformity at a low cost, mainly because of the wavelength of the blue LED chip and the coating process of the phosphor. As a result, manufacturers may mix multiple LEDs in separate bins, but as a result, the range of applications is limited, increasing the complexity of the production process and generating more inventory. Cree's solution for this is EasyWhite bin, which uses Cree's multi-chip XLamp MC-E LED. The MC-E chip is packaged in a four-core package. Cree selects four white LED chips with different characteristics (overlying fluorescent). Powder) and packaged, the white light output after mixing can reach the expected color temperature, and is much smaller than the standard range specified by ANSI.
Light performance
Just like Moore's Law in the semiconductor industry, the LED industry also has a Haitz law, that is, the brightness of LEDs is doubled every 18-24 months, and in the next 10 years, the brightness is expected to increase by 20 times, and the cost will be reduced. Some 1/10. Today, the market can buy about 100-150 lumens for 1 dollar, and the US Department of Energy estimates that this number will reach 1000 lumens in 2020.
How is the LED luminous efficiency improved?
First look at the package. At present, the most common LED light sources on the market are in-line and patch. Zhu Xiaotian, deputy director of technology of Shenzhen Changguang Semiconductor Lighting Technology Co., Ltd., believes that the cooling outlets of these two packages have a small cross-sectional area, and the passage of a single light exit is narrow, and the utilization rate is relatively low. According to the light-emitting characteristics of LEDs, Changguang Semiconductor has designed a flat primary optical channel to form a planar light source array chip, which can minimize optical loss. In this regard, Zhu Xiaotian explained: "SMD's 3528 or 505 light source uses glass fiber or plastic as the substrate, and the bottom is not connected to any metal. The planar light source array chip uses a multi-chip integrated COB direct packaging technology, the light source is used. The aluminum substrate can dissipate more heat through the direct connection heat dissipation. It is reported that the company's LED surface light source technology with completely independent intellectual property rights has reached 130lm/w, and the overall lighting efficiency has been greater than 100lm/w.
In addition to packaging, chip structure, forward voltage and other factors are also critical. If the forward voltage can be controlled within a very small range, good light efficiency can be obtained. In this part, we must pay special attention to how to solve the droop phenomenon. It is well known that an increase in current density causes LEDs (mainly LEDs that use InGaN as a light-emitting layer, such as blue, green, etc.) to emit more light, but at the same time, the light effect is also reduced. Currently, chip design companies are working hard to find a solution that is as close as possible to the increase in light efficiency. That is to say, on a square millimeter area, more light is obtained on the premise that the light is not damaged, instead of increasing the die size. Of course, an increase in the thermal coefficient (LED output ratio at 100 ° C junction temperature and 25 ° C junction temperature) also helps to simplify the heat sink design, thereby reducing the cost per lumen in the final application.
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