For LED lighting applications, in order to achieve the brightness requirement of having an alternative light source, it usually adopts the "quantity" design method, that is, designing a large number of LED light-emitting elements per unit area, or adopting a single element to improve the luminous efficiency of the design. But in this way, the temperature of the surface or point-like components needs to be dealt with urgently, and the heat dissipation methods can be carried out by active or passive design, especially the design of active heat dissipation is more complicated. It is an important key to develop high-brightness LED lighting to avoid the light decay of LED components affecting life. . .
LED (Light EmitTIng Diode) solid-state lighting is a future industry that has been considered to have great potential in recent years, because consumers and industry are looking forward to using LED solid-state lighting to solve the problem of a large amount of energy wasted on inefficient light source lighting. With the advantages of small size, high luminous efficiency, and power saving, most people also have high hopes for the future development of LED solid-state lighting. LED solid-state lighting is different from traditional light sources, so it has many advantages in terms of environmental protection and energy saving. Let's observe the common daily lighting sources first, except incandescent lamps and fluorescent lamps. Incandescent lamps basically perform in luminous efficiency. That is, it tends to be a disadvantage. Even with the advantages of low cost and established usage habits, it has become a non-environmental and inefficient lighting product under the social atmosphere in which environmental protection concepts are rising. In terms of fluorescent lamps, although the photoelectric technology of high-frequency gas discharge is used to achieve power saving benefits, in fact, the fluorescent lamp control process cannot avoid mercury that is harmful to the environment, and it is not the best light source choice in terms of environmental protection.
Back to the development of LED solid-state light sources, early LEDs were mostly used for indicating light sources, that is, medium / low brightness and low power driven light sources such as signal lamps and indicator lights. Therefore, there is no consideration for heat dissipation. On the one hand, they are indicating light sources. It is only used to identify the current use status of the device and the prompt of the switch status, not for lighting purposes, because the driving power is not high and there is no obvious heat dissipation problem to be solved. But the problem is coming. The purpose of using high-brightness LEDs is mostly for the development of alternative environmentally friendly light sources. Such a design method will cause many impacts.
When the LED solid-state light source is thinking in the direction of daily lighting applications, there will be a problem of insufficient brightness. It is necessary to try to increase the luminous efficiency by using increased power on the LED element, or use more high-brightness LEDs for modular design to let The light source has the requirement of "high brightness" for lighting applications.
Cooling design with high efficiency and environmental protection is a key
The core design of the LED component is that a piece of LED die uses an applied voltage to produce a light-emitting result. Similar to a general silicon chip, the LED chip will also produce light decay due to long-term use. Most design solutions are designed to enhance the device For the brightness of the light, more use is made to increase the bias voltage of the crystal, that is, to increase the power of the electrical energy applied to the LED, so that the chip can excite higher brightness. In this way, strengthening the LED power will also accelerate the light decay and life problems of the crystal Appearance, even the high temperature generated by the component itself due to enhanced brightness, will also cause a shortened product life.
When the brightness of a single LED die increases, the power consumption wattage of a single LED will also increase from 0.1W to 1, 3, or even 5W, and most LED light source modules will also show the results of packaging modules. The thermal impedance is increased by increasing the luminous efficacy, and it generally increases continuously from 250K / W to 350K / W.
Examination of the test results reveals that LEDs also have a phenomenon of increasing "power" and decreasing "lifetime", which will cause LED light source components that may have a lifespan of 20,000 hours to be reduced to Only 1,000 hours of life left. Especially when the device is operating at a temperature of 50 degrees Celsius, it can maintain an optimal life of 20,000 hours. However, when the LED device is operated at a temperature of 70 degrees Celsius, the average life is reduced to 10,000 hours. Operating at 100 degrees Celsius will have only 5,000 hours of life left.
Status of thermal impedance of LED module design
In addition to the critical component LED, which is susceptible to temperature, most of the light source design is also developed with a modular concept. Even to replace the traditional light source, the light emitting element and the electronic circuit can only be integrated in a very small space, because LED is a DC drive Most of the lamps are connected to AC power. To simplify the implementation complexity of LED light sources, the current mainstream method is to directly integrate the power rectifier, transformer module and LED light-emitting components, but the problem comes because The available circuit space is relatively small, and when the convection space in the device is relatively small, it is naturally impossible to obtain a better heat dissipation effect, and the heat dissipation of the module can only be performed through the relevant measures of active forced heat dissipation. .
If the thermal flow model is observed by the thermal impedance module to predict the temperature of the junction of the LED die, the junction means the pn junction of the semiconductor, and the thermal impedance R is defined as the temperature difference and the corresponding power dissipation ratio, and the thermal impedance There are many formation factors, but through the inspection method of the heat flow model, it can be more clearly confirmed that the heat dissipation treatment is due to which key issues reduce its efficiency, and can be dissipated from components, assembly methods, substrate materials, and structures project Improvement. The heat flow model of a general LED solid-state light source can be viewed from several key points.
For example, the LED light-emitting element can be disassembled into LED die, wire bonding between die and pin, and encapsulated plastic, and then the observation will be extended to the LED light source module, that is, there will be LED elements, bonded metal pins, Metal Core PCB (MCPCB) circuit board, aluminum extruded fins for heat dissipation, etc., and the heat flow model can observe several series of heat flow impedances, such as junctions, metal sheets carrying grains, circuit boards and environment, etc. Then examine the thermal circuit of the series impedance and try to find the crux of the problem of low heat dissipation efficiency.
From a deeper observation of the model, we can find that the heat dissipation process from the junction of the die to the entire external environment is actually summed up by several heat dissipation paths, for example, the material properties of the die and the loading metal sheet, The optical resin contact encapsulating the LED die material and the thermal resistance characteristics of the circuit board material, the surface contact of the LED element, or the aluminum extruded heat dissipation fin adhesive between the heat dissipation, and the combination between the cooling device and the air, etc., constitute the whole Heat flow cooling process. How to effectively dissipate the operating temperature of the LED solid-state light source will affect the lighting performance, energy utilization efficiency, device life and other important keys of the entire light source application, and the way to improve heat dissipation can be derived from chip-level technology, LED die packaging technology, circuit board Level of technology to improve.
In terms of wafer-level heat dissipation treatment methods, due to traditional wafer manufacturing methods, sapphire is often used as the substrate for design, and the thermal conductivity of the sapphire substrate is close to 20W / mK. In fact, it is difficult to quickly dissipate the heat generated by the LED epitaxy. The current mainstream In the practice of LED chip-level heat dissipation treatment, especially for high-power, high-brightness LED components, the use of flip chip (Flip-Chip) form, effectively use flip chip to conduct epitaxial heat conduction.
There is also a way to use "vertical" electrodes to make LED elements. Since the LED elements are equipped with metal electrodes at both the upper and lower ends, this can get more help in heat dissipation. For example, the GaN substrate is used as a material. Since the GaN substrate is a conductive material, the electrodes can be directly connected under the substrate to obtain the benefit of rapid dissipation of the epitaxial temperature, but this method will also Compared with the traditional sapphire substrate, the cost is much higher, which will increase the manufacturing cost of the device.
As for the package-level heat dissipation enhancement methods, there are quite a few. Here are a few common practices. Generally speaking, the LED manufacturing process will use optical grade epoxy resin to cover the entire LED, thereby making the LED component perform better in terms of mechanical strength, and even protect the related circuits in the component, but the ring Although the method of oxygen resin can improve the strength of the device, it also limits the temperature operating range of the device. Because the optical grade epoxy resin is used at high temperatures, the optical properties of the epoxy resin will deteriorate due to high temperature or strong light. , Even the material itself will cause deterioration.
At present, the common way to improve packaging is to use the traditional shell-type packaging technology only for most medium / low-power LED components. For high-brightness and high-power LED components, most of them use the Lumileds Luxeon series packaging method to dissipate heat. The path is concentrated on the metal underneath. The internal package is replaced by a silicone resin with excellent optical properties and high temperature resistance and strong light resistance. This packaging method can obtain better mechanical strength performance. Irradiated, high-intensity blue LEDs have a stronger tolerance.
In terms of circuit board-level heat dissipation improvement, the more common method is to use FR4 (PCB) production. The heat conduction performance will be performed on metal-based PCBs, such as MCPCB, Integrated Metal Substrate (IMS) processing, advanced high-performance heat conduction Will use ceramic substrate (Ceramic) to make.
Generally FR4 (PCB) has the advantage of low cost, but the thermal conductivity is relatively poor, and it is mostly used for low-power LED loading. Due to the high operating temperature of metal-based PCBs (MCPCB, IMS), for example, the MCPCB structure is composed of a copper foil layer, an insulating (dielectric) layer, and an aluminum substrate. The general copper foil layer (circuit) is 1.0 to 4.0 ounces, and the insulating (dielectric) The layer is 7.5um ~ 150um, and the thickness of the aluminum substrate (metal core) layer is about 1mm ~ 3.2mm. It can be used in a 140 degree Celsius environment, but the production cost is medium to high. The unit price and cost of ceramic substrates are higher, because ceramics have a good thermal expansion coefficient, which can make the loaded wafers more suitable, but they cannot be used in large-area circuits. For LED light source applications, most are only used to carry The block circuit of the LED element is used to improve the heat conduction efficiency.
In addition to the aforementioned common circuit boards, there are actually a variety of substrate technologies that have relatively good thermal conductivity technologies, such as ceramic substrates (alumina), aluminum-magnesium alloys, flexible printed circuit boards, and direct steel bonded substrates (DBC) , Metal matrix composite substrates and other technologies, but some technologies still have process, loading or cost considerations, which must be determined by whether the actual heat flow model limitations and improvement of the final product are worth replacing the carrier board.
Appearance chassis, configuration restrictions and modular circuit design
LED solid-state light sources meet the needs of practical applications, because there will be no DC power supply at the installation site, and most of the installation environments that replace traditional light sources are only AC AC power. In order to allow LED solid-state light sources to achieve convenient replacement installation methods, related The design must be toward integrated power conversion circuits or the development of AC LEDs. However, in fact, the development cost of AC LEDs is still high, and the current status of related products remains to be observed. Therefore, at this stage, the design method for integrated power conversion is more feasible.
Most installation environments, if they are designed to replace the high-incandescent lamps, there will be considerable technical challenges! Because of the small size of incandescent lamps, LED solid-state light sources must integrate the driving circuit, power conversion circuit, temperature sensing circuit and active heat dissipation circuit. In this way, the product configuration with limited circuit space and limited space, the first to face It is the heat dissipation design.
At present, the design of bulb-type LED solid-state light source, the circuit adopts modular design, in order to simplify the circuit design, there are also related power chip manufacturers to introduce the solution of integrated power supply, temperature control, power conversion, active cooling drive circuit dedicated to integrating LED bulb circuit design There is currently no single-chip solution, but complex circuits and various discrete digital / analog components have been integrated into several integrated circuits to solve the problem, so that the design of the bulb-type LED solid-state light source will not be limited by space and must be compromised or Let the product lose the design limitation of replacing the traditional light source.
Taking the design of the bulb type as an example, a large number of aluminum extrusion configuration mechanisms are used in the bulb socket. This allows the heat generated by the internal circuit and the LED to be dissipated through the aluminum extrusion mechanism of the bulb body, and adopts modular matching The chip solution greatly simplifies the internal circuit and reduces the thermal resistance of internal temperature conduction. With the active heat dissipation mechanism, a miniaturized fan is used to force air-cooled heat dissipation to solve the design and development needs of miniaturized LED bulbs.
Another common type of recessed light fixture is also a product line that LED solid-state light sources are actively striving for. Because recessed light fixtures (recessed lights), the common design is to use halogen bulbs as the light source, which is inefficient for high heat, low efficiency, and high cost Light source, but in order to meet the needs of decorating and beautifying the environment, it is also a very common application light source in many interior designs. Although there are also fluorescent lamp-type recessed lamp designs, the fluorescent lamp type will have a large volume problem. Some indoor environments The light source created by the atmosphere will not use this type of light source. Back to the design and application of LED solid-state light sources to replace these indoor recessed lights, like bulb-type LED solid-state light sources, the design challenges of recessed lights are more stringent, because most of the recessed lights require soft light, point light source LEDs For the light-emitting method, the optical lens must be used to improve the characteristics of the light source, which will cause an increase in volume. Although some products use packaging technology to improve the light type, in general, the degree of modification of the light type is limited.
In addition, the volume of the recessed lamp is more limited, and most of it is installed in the decorative ceiling, mezzanine, and woodwork. The recessed lamp has higher heat dissipation requirements, so that it can get better application security. The design of LED light source of embedded lamps, because the installation of lamps and lanterns is mainly matched with the decoration, in the design aspect, it can achieve decentralized functional design. For example, the power circuit and the embedded lamp body are developed separately, which can make the power conversion circuit not It will become a part of the thermal resistance of the heat flow model in the embedded lamp module, so that the light source itself only needs to set the driving circuit and the active heat dissipation related circuit, which can effectively reduce the product volume, or increase the installation space of the heat dissipation cabinet and heat dissipation component, and enhance the entire light source The heat dissipation efficiency, or the space of the optical lens of the dressing light type is increased, and the use satisfaction of the product is improved.
Since the advent of semiconductor elements such as LEDs, most of them have been used as indicators and display panels. At present, in order to develop daily lighting applications, high-power and high-brightness LED component technologies have also been gradually developed, accompanied by improved brightness and energy application efficiency The demand for peripheral technology has also continued to improve. For example, high-performance AC-DC conversion, LED drive circuits, temperature control circuits, etc., and assembly configurations and designs that improve overall heat dissipation efficiency have pushed LED solid-state light sources toward Can replace the technical level of traditional light sources! LED can be used as a light source at present. Not only can it achieve high efficiency and directly convert electrical energy into light energy, and it has a service life of tens of thousands of hours, relatively low maintenance costs, but also has strong characteristics that exceed the fragility of traditional bulbs. At the same time, it has the advantages of environmental protection, mercury-free, small size and rich color gamut.
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