LED is known as the fourth generation of lighting or green light source, with energy saving, environmental protection, long life, small size and other characteristics, can be widely used in a variety of instructions, display, decoration, backlight, general lighting and urban night and other fields. In recent years, some of the world's economically developed countries around the development of LED launched a fierce technical competition. LED heat dissipation has been a problem to be solved! Research data show that if the LED chip junction temperature of 25 degrees when the light is 100%, then the junction temperature rose to 60 degrees, the amount of light only 90%; junction temperature For 100 degrees to 80%; 140 degrees only 70%. Can be seen to improve heat, control junction temperature is very important thing. In addition to this LED heat will make its spectral movement; color temperature rise; forward current increases (constant pressure power supply); reverse current also increased; increased thermal stress; phosphor epoxy aging accelerated All kinds of problems, so that the LED cooling is the most important LED lighting design of a problem.
LED chip junction temperature is how to produce
The reason LED heat is because the added power is not all converted into light energy, but part of the conversion into heat. LED light effect is currently only 100lm / W, the electro-optical conversion efficiency of only about 20 to 30%. That is, about 70% of the electricity has become heat.
Specifically, the LED junction temperature is due to two factors caused.
1. The internal quantum efficiency is not high, that is, in the electron and hole recombination, and can not produce 100% of the photon, commonly known as the "current leakage" leaving the PN zone carrier recombination rate reduced. Leakage current multiplied by the voltage is part of the power, that is, into heat, but this part does not account for the main component, because now the internal photon efficiency is close to 90%.
2. The internally generated photons can not be all emitted to the outside of the chip and finally converted to heat, this part is the main, because the current known as the external quantum efficiency is only about 30%, most of them are converted into heat. Although incandescent light efficiency is very low, only 15lm / W or so, but it will almost all of the energy into light energy and radiation, because most of the radiation is infrared, so the light effect is very low, but exempt from the The problem of heat dissipation. LED cooling is now more and more people pay attention, it is because the LED light failure or its life is directly related to its junction temperature, heat dissipation is very good junction temperature, life is short.
High power LED white light application and LED chip cooling solution
Today's LED white light products are gradually used in various fields into use, people feel the high power LED white light brings amazing pleasure while also worrying about its existence of the practical problems! First of all from the nature of high-power LED white light itself. High-power LED is still there is poor luminous uniformity, closed material life is not long, especially its LED chip cooling problem is difficult to get a good solution, and can not play the white LED is expected to application advantages. Followed by high power LED white market price. Today's high-power LED or an aristocratic white products, because the price of high-power products is still too high, and technically still need to be improved, so that high-power white LED products are not used to be able to use. Below to break down the issue of high-power LED heat dissipation.
In recent years in the industry experts under the efforts of high-power LED chip cooling problem made a few improvements to the program:
Ⅰ, by increasing the LED chip area to increase the amount of light.
Ⅱ, the use of several small area LED chip package.
Ⅲ, change the LED packaging materials and fluorescent materials.
Then it is not through the above three methods can completely improve the high-power LED white products, the problem of heat dissipation! In fact, we have the LED chip area increases, in order to obtain more luminous flux (light unit time through The number of beams per unit area is the luminous flux, unit ml) hope to achieve the white effect we want, but because of its actual area is too large, resulting in the application process and structure of the emergence of some counterproductive phenomenon. Then it is not high-power LED white light problem really can not solve it? Of course not can not solve. In view of the negative problem of simply increasing the area of the wafer, the LED white light industry has improved from the surface of the high power LED chip to achieve 60lm according to the improved structure of the electrode structure and the structure of the flip chip and the use of several small area LED chips / W high luminous flux low heat dissipation efficiency.
In fact, there is a way to effectively improve the high-power LED chip cooling problem. That is, its white packaging materials with silicone instead of the previous plastic or plexiglass. Replacement of packaging materials can not only solve the problem of LED chip cooling can improve the life of white LED, really double-edged sword ah. I would like to say that almost all high-power white LED such as high-power white LED products should be used as a package of silicone materials. Why now high-power LED must be used as a packaging material for silicone? Because silica gel on the same wavelength of light absorption rate of less than 1%. But the epoxy resin on the 400-459nm light absorption rate of up to 45%, it is easy due to long-term absorption of such short wavelength light after the aging of the light failure caused by serious.
Of course, in the actual production and life there will be a lot of high-power LED white chip cooling such a problem, because people on the application of high-power LED white light will appear more and more difficult to understand the various problems! LED chip is characterized by Very small volume produces very high calories. The LED itself, the heat capacity is very small, it must be the fastest speed of these heat conduction out, otherwise it will produce a high junction temperature. In order to lead the heat out of the chip as much as possible, people in the LED chip structure on a lot of improvements. In order to improve the LED chip itself, the heat, the main improvement is the use of better thermal conductivity of the substrate material. The thermal resistance of the LEDs of the company is at least twice as much as the thermal resistance of other companies because of the use of silicon carbide as the substrate. Even if it is possible to solve the heat resistance from the wafer to the packaging material, the heat dissipation efficiency of the LED chip is also increased due to the poor heat dissipation effect from the package to the PCB. So, like Panasonic in order to solve this problem, from 2005 onwards, put the circular, linear, surface type of white LED, and PCB substrate design into one, to overcome may appear in the package from the PCB The problem of heat disruption. Therefore, in the face of continuous improvement of the current situation at the same time, how to increase the heat resistance, but also the urgent need to be overcome at this stage, in all respects, in addition to the material itself, but also from the chip to the packaging materials Of the heat resistance, thermal structure, packaging materials to the PCB between the heat resistance, thermal structure, and the PCB board cooling structure, which need to be considered as a whole.
For the current incandescent bulbs or fluorescent lamps, even if the product itself may produce heat, but the components of the high heat can still be effectively isolated, so that the light source and the power connector will not be caused by heat and unexpected problems. But the solid-state lighting is different, one to focus on a single LED single-point operation of high temperature, must take more positive means for heat treatment, and with active and effective heat treatment mechanism in order to avoid problems with the lamp. LED solid-state light source heat treatment problems are much more complex than traditional lamps. Traditional light or lamps have a high heat problem, such as halogen bulbs or incandescent bulbs, if the incandescent form, that is, in the special treatment of light bulbs within the tungsten wire to produce light. In fact, the high temperature is generated on the filament rather than the lampholder, and even if the lamp holder is heated by the bulb glass or the heat radiation of the tungsten wire, the heat conduction indirectly generates a high temperature, but the temperature is in an acceptable safety range. Coupled with non-direct contact conduction, safety is relatively high. But replaced by LED solid-state light source form of the lamp, the heat treatment may become a new application security issues. Most people will think that LED with high energy conversion efficiency, low driving energy advantages, natural use of high security, but in fact the LED solid-state light source in order to achieve the purpose of daily lighting applications, must increase the power of a single group of components to strengthen the unit The output of lumens, such as the lamp factory will take the form of multi-LED integrated components to enhance the output effect, and multi-component operation can also improve the LED solid-state light source light source point light source problem, so that LED solid-state light source technology can produce light- The effect of the surface light source. If you want to strengthen the output lumen of a single component, you must have a higher current to make the LED chip PN junction to produce more lumens, but higher current will make the single point LED component temperature rise, more difficult to deal with, even for the Improve the light of the light performance, luminous efficiency and take the multi-component and form, LED lamps will also increase the high temperature problem, so that heat problems more difficult to deal with. Looking at the current trend of LED lighting market trends, most of the LED light source manufacturers will be the first market-oriented, because the high unit price, high profits, but also by technical differences quickly into the development of technology more avant-garde LED light source market, for example, Interior decoration, lighting applications, embedded lights, wall lamps, ceiling lamps become LED light source lamps more common form of design, the replacement of traditional lamps after the power efficiency is also the most concerned about the industry concerned.
LED light source lamps must be focused on the thermal management design, in the use of closed or semi-mortal use of downlights, wall lamps, ceiling products, the formation of more stringent challenges, lighting developers must from the material, product configuration, the main / Passive cooling mechanism, drive chip design and other aspects into more resources to avoid the problem of product safety. In particular, LED embedded lighting small size, and often adopt more components integration, the module's thermal design is more difficult. Embedded lamp shell aluminum extrusion or heat sink design, can play their own heat dissipation. But this is far from enough.
LED thermal management: NTC continuous operation temperature to maintain the use of LED lights safe
If the LED lamps are not equipped with enough thermal management design, in the course of the use of the lamp may be due to frequent high heat caused by a sharp drop in life, resulting in frequent replacement of fault LED lamps, and even may lead to accidents, In the product development stage, you can use intelligent LED lighting control technology, through the active monitoring of LED lamps and the overall light source module temperature performance, simplify the thermal management of the device work, while the lights when the lighting And the surrounding temperature rose to the section, the lamps must reduce the power, reduce the LED brightness output, in order to enhance the use of LED solid-state light fixtures security. Like LED ceiling lamp shell to consider the simpler design form, if the lamp itself is used in the drive function than the focus on the power conversion and LED components driven, not embedded temperature control microprocessor and heat treatment module, in order to avoid increased product raw materials The cost of LED lamps can be integrated with NTC (NegativeTemperatureCoeffient) negative temperature coefficient ThermistorSensors electricity, which is a cost-effective relatively high safety design. The so-called NTC power, its purpose is to set up through the electronic back to monitor the LED module lamp temperature, through the default temperature warning or corresponding to the automatic processing drive conditions, mining closed LED solid state light source module way to enhance the use of LED lighting safety , While NTC can also reduce the complexity of the design. As the NTC electrical temperature coefficient is very large, it can be detected that the small temperature changes in performance, is widely used in the need to measure, control and compensation of temperature-related electrical design, and NTC in the LED light source module design, the basic On the measurement of LED solid-state light source of the product around the temperature changes, as measured with the NTC will change the status of the voltage, directly measured voltage and NTC electrical temperature corresponding relationship.
When the NTC and peripheral power or the entire module temperature increases, NTC electrical resistance is reduced, the product can be dependent on the relevant safety control mechanism feedback, such as reducing the LED light-emitting components of the drive current or directly forced to close the lighting, After the temperature problem is improved, it will automatically return to the lighting state, so as to obtain the safety of the lamp.
Monitor the temperature of the LED lamps can also be imported into the SMD form produced by the NTCTHERMISTOR components
The improvement of the aforementioned NTC form, if you want to achieve a better design, with the MCU for more sophisticated security design is also a relatively pragmatic approach in the development project, the LED light source module can be divided into the status of light, light Whether it is closed, with the temperature warning and temperature measurement program logic to determine the construction of a more perfect intelligent lighting management mechanism.
For example, if the lamp temperature warning, the temperature measurement that the module temperature is still acceptable range, can maintain the normal way through the heat dissipation of the heat dissipation running temperature; and when the warning that the measured temperature has been required to perform active cooling Mechanism, the MCU must control the cooling fan action, even when the temperature reaches the value, the system must be directly through the MCU to close the drive power supply, LED components temporarily stop running, the natural heat treatment. To determine whether the use of lighting or off, you can use a simple bit of judgment to understand and understand the current state of use of the product, the more critical is the temperature measurement part of the measured temperature must be real-time and the system reference table to compare to confirm the current Module status of the normal or abnormal degree, calculate the temperature spacing, the corresponding automatic temperature control management. Similarly, when the temperature into the sector, the control mechanism should immediately turn off the light source, while the system shut down after 60 seconds or 180 seconds after the temperature again to be confirmed until the LED solid-state light source module temperature reaches normal, and then re-drive the LED light source, Continue to provide lighting.
In short: the public has been concerned about the lamp life. If you only rely on the use of low thermal resistance of the LED components can not be built for the lamp device a good cooling system, and must effectively reduce the PN node from the surrounding environment to the thermal resistance, in order to greatly reduce the LED PN node temperature, and the successful implementation of extended LED Lamp life and improve the actual luminous flux target. In addition; different from the traditional traditional lighting, printed circuit board is the LED power supply carrier, but also LED heat carrier, so the heat sink and printed circuit board thermal design is very important. In addition, the lamp manufacturer must also consider the quality of the heat dissipation material, thickness and size, as well as the cooling interface processing and connection and other factors.