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Highlights Of The Key Technologies Of Efficient Metal Radiatorless LED Lighting
Nov 07, 2017

Highlights of the key technologies of efficient metal radiatorless LED lighting

The LED lighting project looks very clear, that is, under the premise of ensuring light quality, the need for a substantial increase in light efficiency, for example, more than double the energy-saving lamps; prices need to drop significantly, preferably close to energy-saving lamps; make it significantly better than the cost Energy-saving lamps.

The current LED general lighting mostly by the power LED plus metal heat sink and constant current drive circuit. The bulky metal heat sink not only increases the cost and weight of the lamp, but also consumes a large amount of aluminum resources, contrary to environmental protection. A LED lamp is like a metal ball, not conducive to safety, especially high-power LED lights. Therefore, many consumers still choose to buy energy-saving lamps.

This article will be combined with my company's R & D work to introduce a LED 4π light extraction of high photon extraction rate, high efficiency, no metal radiator LED universal lighting (direct replacement of incandescent and comparable luminous flux of fluorescent energy-saving LED lighting )technology.

The LED general lighting lamp luminous efficiency than fluorescent energy-saving lamps more than doubled; color rendering index up to 96; can produce luminous flux of dozens to 1600 lm and higher luminous flux of LED general lighting, L70 life of up to 30000 hour. Can be directly replaced by 10 ~ 100W and more high-power incandescent and luminous flux of fluorescent energy-saving lamps.

First, the LED chip 4π light, improve PN junction extraction rate and the actual light efficiency

White LED light-emitting process efficiency η:

Η = ηI × ηO × ηC × K

ΗI: internal quantum efficiency; ηO: external quantum efficiency; ηC: photon down conversion loss; K: luminescent powder absorption.

Some people have analyzed that in the ideal case , ηI = 0.95; ηO = 0.5; ηC = 0.875; K = 0.95. Therefore, the highest ideal energy efficiency η = 39.5%. Here the external quantum efficiency ηO refers to the photon in the exit process by the chip, window material, phosphors and lenses along the way to absorb or reflect at different refractive index medium interface back to the internal absorption and so on the result, that is, the LED light Extraction rate. If 3500K warm white light optical equivalent of 320 lm / W calculation, the maximum luminous efficiency of 320 × 0.395 = 126 lm / W. This is apparently underestimated. But from this we can see that one of the important and great potential factors to improve the luminous efficiency of LED is to improve the light extraction rate.

LED light from the LED chip PN junction, the original 4π solid angle of its original light is emitted in all directions uniform natural light, but at present almost all of the LED components are ≤ 2π out of the light.

LED applications from the early lights to the digital display and the current color large-screen display, LCD backlighting, etc., in these applications, the original need is 4π exit light with reflective bowls and lenses gathered forward, that is Converted to ≤2π emitted light, including in-line, straw hat, surface mount (SMD) and COB, etc .; such transformation is needed for these applications, but also correct.

However, such a transformation allows the chip originally retrofitted to the front of the light gathering, will significantly reduce the PN junction emission of light extraction rate, which reduces the effective LED light effect, which does not necessarily require ≤ 2π light LED Lighting is not necessary. If the LED chip 4π light, will significantly improve the LED PN junction photon extraction rate, that is to improve the actual LED light efficiency.

A portion of the 2π solid angle light (indicated in blue) emitted by the PN junction of the chip can be directly emitted from the light exit window, and the other part of the light is totally reflected by the transparent medium surface and then reflected by the reflector bowl or directly reflected by the reflector bowl. Among them, the direct exit light is about 2π [1-cos (sin-1 (1 / 1.5)] / 2π = 25%, where we set the refractive index of the transparent medium to 1.5 and the reflected light from the reflector reflects 75% The reflectivity of the reflector is 0.75. The total light extraction rate is (25 + 75 × 0.75)% = 81%, excluding the multiple reflections of the reflector and the absorption loss of the transparent medium.

LED chip down 2π light (red), to go through the back of the chip coated reflective film, reflecting the bottom of the bowl, reflecting the reflection of the wall of the bowl, multiple reflections, bowl and wall absorption several times, it is estimated that the emissivity of about 60% (Depends on the reflectance of the reflective bowl wall and bottom, electrode surface, dielectric face between electrodes, solid crystal glue, etc.).

Therefore, the total extraction rate of LED chip light = (0.81 + 0.6) 2π / 4π = 71%, that is, about 30% of the light is absorbed by the LED element and becomes thermal energy.

Figure 2 is a schematic diagram of the LED chip 4π light. The LED chip is a transparent chip of the chip substrate, and at least one string of chips connected in series or in series and parallel with each other is fixed on a transparent substrate of an LED light-emitting element by a transparent glue, and the chip is covered with a transparent medium layer or a light-emitting glue layer.

If the chip substrate is sapphire, the epitaxial layer on the sapphire and the PN junction is GaN, the P electrode is ITO, the transparent substrate of the LED element is glass and the transparent medium is silica gel, and their refractive indices are respectively 1.77, 2.4, 1.8, 1.45, 1.5, Can be seen from Figure 2, leaving the LED chip PN junction up and down the hemisphere 2π light emission can be successfully exits, basically no multiple internal absorption of the sapphire substrate absorption, regardless of medium absorption, LED chip light can be almost 100% Exit.

That is, the actual luminous efficiency of the LED element with 4π light output is about 100% higher than that of the SMD LED with 71% = 71%. Our experimental results are basically consistent with this.

Visible, so that the LED chip 4π light can improve the actual luminous efficiency of LED components about 40%, while reducing the LED heat. Taking into account the different structure of the existing LED components, 4π out of the light should be less than or equal to 2π out of the light efficiency of more than 30%.

In fact, this concept has been known to almost all LED workers, but not practical, the key is not able to solve the thermal issues LED chips.

Second, gas cooling analysis

To make the LED chip 4π out of the light, the chip must be surrounded by a high transmittance and can be transparent heat medium. It is easy to think of the first thing to do with liquid heat, because liquids in transparent media generally have a much higher thermal conductivity than gas. For example, water has a thermal conductivity of 0.5 W / (m · K), which is 20 times the thermal conductivity of air of 0.025.

For more than a decade, people have been studying the use of liquid heat to achieve the LED chip 4π light, but the liquid heat still has some insurmountable difficulties, for example, the liquid viscosity coefficient is much larger than the gas, the viscosity of water viscous coefficient of 8937μP, Is 10 times that of air and is 77 times that of helium. The high viscosity causes the LED chips to easily vaporize due to the heat generated by the chips, and the resulting gas is hard to run away because of the high viscosity of the liquid. Surrounded by a static gas, and any gas is still static heat insulation, so easy to overheat the chip burned. In addition, there is no liquid, easy electrolysis, erosion of chips and light-emitting materials, blisters and other pollution after the collapse, so far there is no good practical products.

Gas and liquid, although the thermal conductivity is low, but the viscosity is much smaller than the liquid, easy to form a gas convection, LED work effectively when the heat dissipated away, so as to obtain good heat dissipation.

Initially, people put the LED chip mounted on a strip or flat transparent substrate, work in the air, the use of air cooling. However, due to low thermal conductivity of the air, high viscosity, it is difficult to effectively heat. If the LED chip is mounted on a flat panel, then the heat concentration is more detrimental to heat dissipation, making it difficult to produce a high luminous efficiency, but also sufficient output luminous flux of LED lights. For example, Ushio's LED filament lamps have an output flux of only 36 lm and an efficacy of only 60 lm / W . Another example is the Panasonic LED chip mounted on a transparent flat panel air-cooled LED light bulbs, the output luminous flux of 210 lm, luminous efficiency of 47 lm / W . The LED chip 4π LED light bulb out of the light efficiency but less than the existing ≤ 2π light LED components made of LED lights, the existing LED chip light angle ≤ 2π A19-shaped bulb efficiency of 40 ~ 90 lm / W, the reason is that there is no solution to the effective heat dissipation LED chips, resulting in increased PN junction LED chip temperature, low luminous efficiency, the output flux is small.

The author's company effectively solves the problem of heat dissipation of the 4π light-emitting chip . The solution is that at least one LED chip of the same or different light-emitting color is dispersedly and fixedly mounted on a transparent substrate strip with a transparent adhesive, and the chip and the transparent At least one layer of transparent adhesive layer or luminous powder layer is arranged around the substrate strip; an electric outlet line is formed at the two ends of the transparent substrate to make LED light-emitting strips (also called LED filaments); the LED light-emitting strips are mounted on a vacuum- Blister, the blister filled with high thermal conductivity, low viscosity coefficient of heat transfer and protection of the LED gas; LED electrode by the vacuum sealed blister stem led out by the LED driver and an electrical connector Connectors, electrical connectors are used to connect external power supplies to produce an LED filament lamp with a similar appearance to incandescent bulbs, high efficacy and no metal heat sink, as a direct replacement for incandescent and energy saving lamps.

A19 LED bulbs with a full-lamp efficacy of up to 170 lm / W have been manufactured; the output luminous flux is up to 760 lm; and the color rendering index (CRI) is up to 96%. Recently, the company laboratory has made A19 lamp with color temperature 5000K, CRI 71 and luminous efficiency of 193 lm / W. Its light efficiency more than double the energy-saving lamps. So that the LED 4π light, LED lamps without metal heat sink into the actual use of the era. Figure 3 is a schematic of the LED filament lamp in series with four LED light bars.

The high thermal conductivity low viscosity gas is preferably a helium or helium-hydrogen mixture. Helium has a thermal conductivity of 0.14 W / (m · K), six times that of air, a viscosity factor of only 194 μP , which is 1/8 that of air, a thermal conductance of hydrogen of 0.15, a viscosity coefficient of 87.6 and Low cost, but the use of less safety; to reduce costs, available helium-hydrogen mixture. High thermal conductivity Low viscosity coefficient of gas easy to form an effective convective cooling, LED chips can work quickly away heat generated, passed to the bulbs of the bulb, and then blown out to the surrounding air through the blisters.

Secondly, a light-emitting bar transparent substrate with high thermal conductivity is used, for example, hard glass, quartz glass, sapphire, transparent ceramic, AlN and the like. At the same time to adopt high thermal conductivity, high translucent solid crystal and luminous glue, and minimize their thickness, but also try to increase the transparent substrate and luminous powder and heat the gas contact area to reduce the LED PN junction to the thermal resistance of the cooling gas. The at least one luminescent powder layer may be coated on the periphery of the transparent substrate and the LED chip, for example, the LED chip and the chip-less side of the light-emitting bar may be coated or only on one side of the chip, or may be coated on the transparent substrate first Cover a layer of luminous powder layer, LED chip is fixed on the luminous powder layer, the chip is electrically connected and then coated with a layer of luminous powder layer.

In addition, the light-emitting powder can also be coated on the inner wall of the bulb shell. The LED chip of the light-emitting strip has only one layer of transparent plastic. The light-emitting powder is far away from the chip, which is beneficial to reduce the light failure and increase the service life of the lamp.

We can use blue plus red or orange LED chip to improve CRI, but also can be mixed with RBG trichromatic or multi-color LED chips to make white LED light bar, without the use of luminescent powder. The chip may also be a chip-coated or opaque chip on the chip substrate, and the LED light-emitting bar is still 4π out of light, but its light efficiency will be lower than that of the chip substrate. The chip can also be made by flip-chip LED chip, flip chip made of light-emitting bar on a transparent substrate printed with electrical connection lines, a light-emitting bar can also be made by using a high-voltage LED chip (HVLED) Reduce the electrical connection between the chip, improve yield and production efficiency.

The luminous efficiency of such LED filament lamp than the existing bulb with ≤ 2π LED components made of light more than 30%, and no metal radiator, can save a lot of aluminum, more environmentally friendly, light weight, has now begun to be Market acceptance, mass production.

However, there are also concerns that it is difficult to achieve a lifetime of 30,000 hours or more and it is difficult to produce a high-power LED lamp with an output luminous flux higher than 800 lm. The following will be described separately.

Third, life analysis

LED lamp life depends mainly on the operating temperature of the LED PN junction and luminescence powder light fades.

Figure 4 shows the attenuation of the luminous flux of different PN junction temperatures of the commonly used GaN LEDs . The L70 lifetime is shown with different junction temperatures. Can be seen from the figure, if the junction temperature <85 ℃, L70 life of up to 30,000 hours or more. LED PN junction temperature is not easy to measure, with the main peak displacement, junction voltage changes, infrared imager, luminous efficiency changes can be estimated PN junction temperature.

The results of our life test are shown in Figure 6, which is an average (Lr) of 400 lm LED filament lamp lifetimes for 14 stable and initial efficiencies> 0.9. The dashed line in the figure shows the light decay curve for the energy star's 35,000 hour lifespan, of which 1000 hours is defined as the initial value (100%).

Now let's take a look at the importance of high viscosity and low thermal conductivity gas bulbs. Fig. 7 shows the same 3.9W LED filament lamp with the change of luminous efficacy over time under the same input power after the helium gas filled at room temperature and the broken exhaust pipe is put into the air.

However, once the air is put in, not only is its stable luminous efficiency reduced by 19%, but its ratio of stable luminous efficiency to initial luminous efficiency is reduced to <0.75,Its PN junction junction temperature> 150 ℃! Obviously it has been difficult to work properly. Here you can clearly see the importance of high thermal conductivity, low viscosity gas, but also shows the air-filled LED filament light low luminous efficiency, low luminous flux reasons.

In addition, assuming that the LED bulb life of up to 30,000 hours, if working 3 hours a day, as long as 20 years of work to maintain the purity of the gas inside the bulb, the bulb must be vacuum sealed, sealed with existing organic or inorganic glue It is impossible to maintain the purity of its gas for a long time. Vacuum sealing also completely isolates the effects of the surrounding environment on the LED components. The LED can operate under the complete absence of moisture, acids, sulphides, oxygen, PM2.5 and the like in the surrounding air and is more likely to have a useful life of up to 20 years the above.

Fourth, ceramic tube LED lamp analysis

Earlier, it was predicted that LED filament lamps can only produce low-power lamps below 500 lm. Such a prophecy is not unreasonable because the filament lamp assembled by the LED filament (light emitting strip) is difficult to be output by the single lamp because of the small contact area between the LED filament and the radiating gas, small heat radiation area and large thermal resistance Achieved> 800 lm.

The goal of semiconductor lighting is to replace 10 ~ 150W common incandescent lamp and its considerable luminous flux of fluorescent energy-saving lamps. In fact, the only way semiconductor lighting can become the mainstream of general lighting. With reference to the US Energy Star, 40W, 60W, 75W, 100W, 150W incandescent lamp initial output luminous flux were 450,800,1100,1600 and 2600 lm.

How to make a luminous flux of 800 ~ 2600 lm high-power metal-free radiator LED lighting? The key is: to maintain the LED 4π light, high efficiency, low fever, based on further improve the LED cooling area, reduce the LED PN The heat resistance of the heat-dissipating air around the lamp and the further improvement of the heat-dissipating capacity of the bulb.

Rutgers solution to this problem is the technical solution : the LED light bar or LED chip directly on the outer wall of a transparent tube of high thermal conductivity, such as transparent ceramic tube, quartz tube, sapphire Tubes, etc. Transparent ceramic tubes have a thermal conductivity of up to 23 W / (m · K) and a total light transmittance of more than 95%. The thermal conductivity of the transparent ceramic tube is close to that of the sapphire chip substrate. Both the inner and outer surfaces of the transparent ceramic tube are in contact with the cooling gas and Cooling, greatly increased the LED cooling area, reducing the thermal resistance of LED chips PN junction to the cooling gas.

The LED light bar is fixed on the outer surface of a transparent ceramic tube. The upper end of the glass column on the bulb column has a spring or bracket for fixing the upper end of the ceramic tube. The lower end of the ceramic tube is connected and fixed to the lead wire of the column. The lead wire is connected to the driver output of the lamp. The input of the driver is connected with the lamp cap. The lamp cap is used to connect the external power supply, and the external power supply can turn on the LED lamp.

Fifth, high-power multi-tube lamp analysis

Ruidisheng multi-tube lamp program broke through the bottleneck , so that 800 ~ 1600lm and higher lumen without metal radiator LED lighting can be achieved. This multi-tube lamp is equivalent to the single lamp bulb split, divided into several, there is a free flow of air between the lamp gap, easy to form air convection, so that the lamp can effectively heat, greatly increased Lamp heat exchange with the surrounding air cooling capacity, which can increase the lamp power and output flux.

Multi-tube lamp cooling solution can be called tube convection heat dissipation and air convection heat pipe between the two convection cooling technology, can be made of small size, the output luminous flux higher without metal radiator LED general lighting. Has been developed into a luminous flux of 800 ~ 1600 lm of multi-tube ceramic tube LED lamps and luminous flux up to 4000 lm of the experimental sample light.

Each of which is vacuum sealed and filled with a high thermal conductivity, low viscosity coefficient of thermal protection gas, each lamp has a transparent ceramic tube LED light column; each LED lamp constant current or current-limited drive , To avoid the lamp due to LED current and temperature feedback caused by inconsistent light fades, to ensure that the lamp light failure consistent and long service life.

Has been made into 2 tubes, 3 tubes and 4 tubes t5 high-power LED lights. The 5000K color temperature of the typical parameters of the multi-tube lamp:

2 tube lamp: 850lm, 6.4W, 133lm / W, CRI: 81; lamp height: 110mm, maximum diameter: 40mm, weight: 58g;

3 Lamp: 1250lm, 9.3W, 134lm / W, CRI: 82; Lamp height: 110mm, Max diameter: 48mm, Weight: 64g;

4 tube lamp: 1630lm, 12.2W, 134lm / W, CRI: 81; lamp height: 110mm, maximum diameter: 52mm, weight: 70g;

Can be seen, 6.4W 2-tube lamp is equivalent to 60W incandescent lamp; 12.2W 4-tube lamp is equivalent to 100W incandescent lamp. The entire light efficiency is above 130lm / W, no metal radiator, small size, light weight. Multi-tube lamps from Ruidisen can be easily made into high-power LED lamps with different power by changing the number of tubes, the size of tubes and ceramic tubes.

Six, summary

The luminous efficiency of the LED chip 4π is higher than that of the existing LED with the wavelength of ≤2π by more than 30%, and can be made into a high-efficiency low-power LED filament lamp by radiating the gas with a high thermal conductivity and a low viscosity coefficient. LED light column can further improve the cooling capacity, LED light to produce greater output ceramic tube lamp; dual gas convection heat pipe technology can be multi-LED 800-1600 lm and higher luminous flux of LED general lighting. The entire lighting efficiency than double the energy-saving lamps, no metal radiator, low cost, light weight, long life, alternative 10-100W and higher power incandescent and considerable luminous flux of energy-saving lamps. Is a new generation of LED general lighting. Because of its high luminous efficiency, simple structure and low cost, it will help to promote semiconductor lighting to replace incandescent and energy-saving lamps and become the mainstream of universal lighting.