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How to Solve the Problem of LED Heat Dissipation


Led is called the fourth generation of illumination source or green light source. It has the characteristics of energy saving, environmental protection, long life and small size. It can be widely used in various fields such as indication, display, decoration, backlight, general illumination and urban night scene. In recent years, some economically developed countries in the world have launched a fierce technical competition around the development of LED. Among them, LED heat dissipation has always been an urgent problem to be solved!

According to research data, if the illuminance of the LED chip is 50% when the junction temperature is 25 degrees, then the luminescence amount is only 90% when the junction temperature rises to 60 degrees, and drops to 80% when the junction temperature is 100 degrees; 140 The degree is only 70%. It can be seen that improving heat dissipation and controlling junction temperature are very important things.

In addition, the heat of the LED will also cause its spectral shift; the color temperature will increase; the forward current will increase (at constant voltage supply); the reverse current will also increase; the thermal stress will increase; the phosphor epoxy aging accelerates, etc. All kinds of problems, so the heat dissipation of LEDs is the most important issue in the design of LED lamps.

How is the junction temperature of the LED chip generated?

The reason why the LED heats up is because the added electric energy is not converted into light energy, but a part is converted into heat energy. The luminous efficacy of LEDs is currently only 100lm/W, and its electro-optical conversion efficiency is only about 20~30%. That is to say, about 70% of the electrical energy becomes heat.

Specifically, the LED junction temperature is caused by two factors.

  1. The internal quantum efficiency is not high, that is, when electrons and holes recombine, photons cannot be generated 100%, and it is generally called "current leakage" to lower the recombination rate of carriers in the PN region. Multiplying the leakage current by the voltage is the power of this part, that is, it is converted into heat, but this part is not the main component, because the internal photon efficiency is now close to 90%.

  2. The internally generated photons cannot be completely emitted outside the chip and finally converted into heat. This part is mainly because the current external quantum efficiency is only about 30%, and most of it is converted into heat.

Although the incandescent lamp has a very low luminous efficiency of only about 15 lm/W, it converts almost all of its electrical energy into light energy and radiates it. Because most of the radiant energy is infrared, the light efficiency is very low, but it is exempted. The problem of heat dissipation.

The heat dissipation of LEDs is now more and more important to people. This is because the light decay of LEDs or their lifetime is directly related to their junction temperature. The heat dissipation is not good, the junction temperature is high, and the lifetime is short.

High-power LED white light application and LED chip heat dissipation solution

Today's LED white light products are gradually being put into use in various fields, people are feeling the amazing pleasure brought by their high-power LED white light and are also worried about the practical problems of their existence!

First of all, from the nature of high-power LED white light itself. High-power LEDs still have poor uniformity of light emission, and the life of the sealing material is not long. Especially, the heat dissipation problem of the LED chip is difficult to be solved well, and the application advantages of white LEDs cannot be exerted.

Secondly, from the high-power LED white light market price. Today's high-power LEDs are still a kind of aristocratic white light products, because the price of high-power products is still too high, and the technology still needs to be improved, so that high-power white LED products are not used by anyone who wants to use them. The following OFweek semiconductor lighting network to break down the issue of heat dissipation of high-power LEDs.

In recent years, with the efforts of industry experts, several improvements have been proposed for the heat dissipation of high-power LED chips:

1. Increase the amount of luminescence by increasing the area of the LED chip.

2. Pack several small-area LED chips.

3. Change LED packaging materials and fluorescent materials.

So is it possible to completely improve the heat dissipation problem of high-power LED white light products through the above three methods? It’s awesome! First of all, although we increase the area of the LED chip, we can get more luminous flux (the amount of light passing through the unit area per unit time is the luminous flux, the unit ml), we hope to achieve the white light effect we want, but because of its actual The area is too large, which leads to some counterproductive phenomena in the application process and structure.

So is it true that the high-power LED white light cooling problem can't be solved? Of course it is not impossible to solve. In response to the negative problem of simply increasing the wafer area, LED white light manufacturers have improved from the surface of high-power LED wafers by improving the electrode structure and flip-chip structure by using several small-area LED chips. /W's high luminous flux has low luminous efficiency.

In fact, there is another way to effectively improve the heat dissipation of high-power LED chips. That is to replace the traditional plastic or plexiglass with its white packaging material with silicone. Replacing the packaging material can not only solve the heat dissipation problem of the LED chip, but also improve the life of the white LED. I want to say that almost all high-power white LED products like high-power LED white light should use silicone as the packaging material. Why do you have to use silica gel as a packaging material in high-power LEDs? Because silica gel absorbs light of the same wavelength less than 1%. However, the epoxy resin has a light absorption rate of 45% at 400-459 nm, and it is easy to cause serious light decay due to aging caused by long-term absorption of such short-wavelength light.

Of course, in the actual production and life, there will be many problems like the heat dissipation of high-power LED white light chips, because the more widely used applications of high-power LED white light, the more difficult problems will be solved!

LED chips are characterized by extremely high heat generation in a very small volume. The heat capacity of the LED itself is very small, so it is necessary to conduct this heat out at the fastest speed, otherwise it will produce a high junction temperature. In order to extract heat out of the chip as much as possible, people have made many improvements in the chip structure of the LED. In order to improve the heat dissipation of the LED chip itself, the most important improvement is to use a substrate material with better thermal conductivity. The thermal resistance of LEDs like Cree is at least twice as low as that of other companies because it uses silicon carbide as the substrate.

Even if the heat resistance from the wafer to the package material can be solved, the heat dissipation effect from the package to the PCB is not good, and the temperature of the LED wafer is also increased, and the luminous efficiency is lowered. So, just like Panasonic, in order to solve such problems, from 2005 onwards, white LEDs including circular, linear, and surface-shaped LEDs were integrated with the PCB substrate to overcome the possibility of appearing from the package to the PCB. Interruption of heat between the two.

Therefore, in the face of continuous improvement of current conditions, how to increase the heat resistance is also an urgent problem to be overcome at present. From all aspects, in addition to the problem of the material itself, it also includes from the wafer to the packaging material. The heat resistance, heat conduction structure, heat resistance of the package material to the PCB board, heat conduction structure, and heat dissipation structure of the PCB board all need to be considered as a whole.

LED lighting fixtures cooling questions

For the current common incandescent bulbs or fluorescent lamps, even if the product itself generates heat, the high heat of the components can be effectively isolated, so that the light source and the power supply socket do not cause unexpected problems due to heat. However, solid-state lighting is different. Once the LED components are concentrated in a single point of high temperature operation, more active means must be taken for heat treatment. At the same time, an active and effective heat treatment mechanism can be used to avoid problems with the lamps. The heat treatment of LED solid-state light sources is much more complicated than traditional lamps.

Conventional light sources or luminaires have many problems of high heat during operation, such as halogen bulbs or incandescent bulbs. In the case of incandescent lamps, the tungsten filaments are heated in specially treated bulbs to produce light.

In fact, the high temperature is generated on the filament instead of the lamp holder. Even if the lamp holder is indirectly generated by the radiant heat and heat conduction of the illuminating glass or the metal, the temperature generated is within an acceptable safety range. In addition to non-direct contact conduction, safety is relatively high.

However, the heat treatment in the form of a LED solid-state light source may become a new application safety issue. Most people think that LED has high energy conversion efficiency, low driving energy advantage, and high natural safety. However, in order to achieve the purpose of daily lighting, LED solid-state light source must strengthen the unit by increasing the power of single-group components. The output of the piece is clear. For example, the luminaire factory will adopt multiple LED components to enhance the output effect, and the multi-component simultaneous operation can also improve the problem of the LED solid-state light source bias point light source, so that the LED solid-state light sourcetechnology can produce a light bulb like a light bulb. The surface light effect.

If you want to enhance the output lumen of the cell, you must have a higher current to make the PN junction of the LED chip produce more lumens, but higher current will also make the temperature of the single-point LED component rise, more difficult to handle, even for Increasing the light-type performance and luminous efficiency of the luminaire and adopting the multi-component combination form will also aggravate the high temperature problem of the LED luminaire, making the heat dissipation problem more difficult to handle.

Looking at the current development trend of LED lighting market, most LED light source manufacturers will first take the market as the leading factor. Because of high unit price and high profit, they can also quickly enter the LED light source market with more advanced technology through technical differences. For example, The recessed lights, wall lamps and ceiling lamps used in interior decoration and situational lighting have become the common design forms of LED light source lamps. The power saving effect after replacing traditional lamps is also the most concerned by relevant industry.

The LED management of LED light source lamps must focus on the thermal management design. In the case of recessed lamps, wall lamps and ceiling lamps that may be used in closed or semi-dense cycles, more stringent challenges are encountered. The developers of lamps must be from materials, product configurations, and mains. / Passive heat dissipation mechanism, driver chip design and other aspects to invest more resources to avoid product problems. In particular, the LED embedded luminaires are small in size, and often adopt multi-component integration, and the heat dissipation design of the module is difficult. The embedded luminaire housing is designed with aluminum extrusion or heat sink to provide self-heat dissipation. But this is not enough.

LED thermal management: NTC continuous operating temperature to maintain safe use of LED lights

If the LED luminaire is not equipped with sufficient thermal management design, it may cause the luminaire to have a sharp decrease in life due to frequent high-heat operation during use. This may cause frequent troubles in replacing the faulty LED luminaire. In severe cases, it may even cause an accident due to high temperature operation. Cause the line or the surrounding decoration to burn with fire!

In the product development phase, intelligent LED lighting control technology can be used to monitor the thermal performance of the device through active monitoring of the temperature performance of the LED luminaire and the overall light source module, while the luminaire and ambient temperature rise to the segment. The luminaire must reduce the electric power and reduce the LED brightness output to improve the safety of the LED solid-state light source.

Like the LED ceiling lamp housing, considering the simpler design form, if the driver function of the lamp itself is focused on the power conversion and LED component drive, the temperature control microprocessor and the heat dissipation processing module are not embedded, in order to avoid increasing the raw material parts of the product. The cost of LED luminaires can be integrated with NTC (NegativeTemperatureCoeffient) negative temperature coefficient ThermistorSensors electricity, which is a relatively cost-effective safety design.

The so-called NTC power, its setting purpose is to improve the safety of LED lamps by monitoring the temperature of the module lamps through the electrons, through the default temperature warning or corresponding automatic processing of the driving conditions, and closing the LED solid-state light source module. At the same time, NTC can also reduce the complexity of the design.

Because the temperature coefficient of NTC is very large, it can detect the small temperature change performance, and is widely used in the related electrical design that needs measurement, control and compensation temperature. In the design of LED light source module, NTC is basically The measurement of the ambient temperature of the LED solid-state light source luminaire is measured. As for the measurement status, the voltage and the NTC temperature are directly measured according to the current voltage change of the NTC.

When the temperature of the NTC and the peripheral power or the whole module is increased, the resistance of the NTC power is reduced, and the product can perform feedback related to the safety control mechanism according to the dependency relationship, for example, reducing the driving current of the LED lighting component or directly forcibly turning off the lighting of the lamp. After the temperature problem is improved, the illumination state is automatically restored, thereby obtaining the safety of the lamp.

Monitoring LED lamp temperature can also be imported into the microcontroller. NTCTHERMISTOR components made in SMD form

In the above-mentioned improved form of NTC, if you want to achieve better design, it is a relatively pragmatic way to use MCU for more precise safety design. In the development project, the status of the LED light source module can be divided into whether the light is normal or not. Whether it is closed, with the logic of the temperature warning and temperature measurement program, to construct a more complete intelligent lighting management mechanism.

For example, if the temperature warning of the lamp occurs, the temperature of the module is still in the acceptable range after the temperature measurement, and the normal way can be maintained, and the operating temperature is naturally dissipated through the heat sink; and when the warning indicates that the measured temperature has reached the required heat dissipation The benchmark of the mechanism, at this time, the MCU must control the cooling fan to operate. Even when the temperature reaches the value, the system must directly turn off the power supply to the driver through the MCU, and the LED component temporarily stops running, and naturally performs heat dissipation processing.

To determine whether the luminaire is used or closed, you can use a simple judgment bit to make changes and understand the current state of use of the product. The key is the temperature measurement part. The measured temperature must be compared with the system reference table in real time to confirm the current The normal or abnormal degree of the module state, after the temperature interval is calculated, the temperature control is automatically performed correspondingly.

Similarly, when the temperature enters the section, the control mechanism should immediately turn off the light source, and then confirm the temperature again 60 seconds or 180 seconds after the system is turned off. After the temperature of the LED solid-state light source module reaches the normal value, the LED light source is re-driven. Continue to provide lighting.

In short:

Volkswagen has always been concerned about the service life of lamps. If only relying on low thermal resistance LED components can not build a good heat dissipation system for the luminaire device, and it is necessary to effectively reduce the thermal resistance from the PN node to the surrounding environment, the PN junction temperature of the LED can be greatly reduced, and the LED is successfully extended. The life of the luminaire and the goal of increasing the actual luminous flux. In addition, unlike traditional lighting, the printed circuit board is both the power supply carrier for the LED and the heat dissipation carrier for the LED. Therefore, the heat dissipation design of the heat sink and the printed circuit board is very important. In addition, luminaire manufacturers must consider the quality, thickness and size of the heat sink material as well as the handling and connection of the heat sink interface.

Yiwu Fuwei e-commerce firm (IACS Co., Limited) is a very professional Led manufacturer who produces high quality outdoor led light. Our popular products are solar led lights, UFO lights, underwater lights and so on. If you have any question about the led product, please feel free to contact us.