How to solve the problem of LED heat dissipation

LED is called the fourth generation lighting 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 indication, display , decoration, backlight, general lighting 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 a problem that needs to be solved!

Research data shows that if the LED chip emits 100% light when the junction temperature is 25 degrees, then when the junction temperature rises to 60 degrees, the light emission is only 90%; when the junction temperature is 100 degrees, it drops to 80%; and at 140 degrees, it is only 70%. It can be seen that improving heat dissipation and controlling junction temperature are very important.

In addition, the heat generated by LEDs will also cause their spectrum to shift , color temperature to rise, forward current to increase (when powered by constant voltage), reverse current to increase, thermal stress to increase, and aging of phosphor epoxy resin to accelerate, etc. Therefore, heat dissipation of LEDs is the most important issue in the design of LED lamps .

How is the junction temperature of LED chips generated?

The reason why LEDs generate heat is that the added electrical energy is not completely converted into light energy, but part of it is converted into heat energy. The luminous efficiency of LEDs is currently only 100lm/W, and its electro-optical conversion efficiency is only about 20-30%. In other words, about 70% of the electrical energy is converted into heat energy.

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 100% generated , which is usually called "current leakage" and reduces the recombination rate of carriers in the PN region. The leakage current multiplied by the voltage is the power of this part , which is converted into heat energy, but this part does not account for the main component because the internal photon efficiency is now close to 90%.

2. The photons generated inside the chip cannot all be emitted to the outside of the chip and are finally converted into heat. This part is the main one, because currently this so-called external quantum efficiency is only about 30%, and most of it is converted into heat.

Although the luminous efficiency of incandescent lamps is very low, only about 15lm/W, they convert almost all electrical energy into light energy and radiate it out. Because most of the radiation energy is infrared, the luminous efficiency is very low, but it eliminates the problem of heat dissipation.

LED heat dissipation is now receiving more and more attention from people, because the light decay or life of LED is directly related to its junction temperature. If the heat dissipation is not good, the junction temperature will be high and the life will be short. High-power LED white light application and LED chip heat dissipation solution

Today, LED white light products are gradually being used in various fields. While people are experiencing the amazing pleasure brought by high-power LED white light, they are also worried about the various practical problems that exist!

First of all, from the perspective of the properties of high-power LED white light itself, high-power LEDs still have problems such as poor uniformity of light emission, short life of sealing materials, and especially the problem of heat dissipation of LED chips, which is difficult to be solved well, and cannot bring out the expected application advantages of white light LEDs .

Secondly, from the perspective of the market price of high-power LED white light, today's high-power LED is still a noble white light product, because the price of high-power products is still too high, and the technology is still to be improved, so high-power white light LED products are not available to everyone who wants to use them. The following is a breakdown of the related issues of high-power LED heat dissipation.

In recent years, with the efforts of industry experts, the following improvement solutions have been proposed for the heat dissipation problem of high-power LED chips:

1. Increase the amount of light emitted by increasing the LED chip area.

2. Use packaging of several small-area LED chips.

3. Change LED packaging materials and fluorescent materials.

So can the above three methods completely improve the heat dissipation problem of high-power LED white light products? In fact, it is very effective! First of all, although we increase the area of ​​the LED chip to obtain more luminous flux (the number of light beams passing through a unit area per unit time is the luminous flux, unit ml) and hope to achieve the desired white light effect, due to its actual large area, some counterproductive phenomena occur in the application process and structure.

So is it true that the heat dissipation problem of high-power LED white light cannot be solved? Of course not. In response to the negative problems caused by simply increasing the chip area, LED white light industry has improved the surface of high-power LED chips by improving the electrode structure and flip chip structure and packaging several small-area LED chips to achieve a luminous efficiency of 60lm/W with high luminous flux and low heat dissipation.

In fact, there is another way to effectively improve the heat dissipation problem of high-power LED chips. That is to replace the previous plastic or plexiglass with silicone resin for its white light packaging material. Replacing the packaging material can not only solve the heat dissipation problem of LED chips, but also increase the life of white light LEDs. It really kills two birds with one stone. What I want to say is that almost all high-power white light LED products such as high-power LED white light should use silicone resin as the packaging material. Why must silicone be used as the packaging material in high-power LEDs now? Because the absorption rate of silicone for light of the same wavelength is less than 1%. However, the absorption rate of epoxy resin for 400-459nm light is as high as 45%, and it is easy to cause serious light decay due to aging caused by long-term absorption of this short-wavelength light.

Of course, in actual production and life, there will be many problems such as heat dissipation of high-power LED white light chips, because the more widely people use high-power LED white light, the more in-depth and difficult problems will arise! The characteristic of LED chips is that they generate extremely high heat in a very small volume. The heat capacity of the LED itself is very small, so the heat must be conducted out at the fastest speed, otherwise it will generate a very high junction temperature. In order to lead the heat out of the chip as much as possible, people have made many improvements to 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 Cree 's LED is at least twice that of other companies because it uses silicon carbide as a substrate.

Even if the heat resistance between the chip and the packaging material can be solved, if the heat dissipation effect from the package to the PCB board is not good, it will also cause the temperature of the LED chip to rise and the luminous efficiency to decrease. Therefore, in order to solve this problem, Panasonic has designed round, linear, and flat white light LEDs with PCB substrates since 2005 to overcome the problem of heat dissipation interruption between the package and the PCB board.

Therefore, in the face of increasing current, how to increase heat resistance is also an urgent problem to be overcome at this stage. From various aspects, in addition to the problem of the material itself, it also includes the heat resistance from the chip to the packaging material, the thermal conductivity structure, the heat resistance from the packaging material to the PCB board, the thermal conductivity structure, and the heat dissipation structure of the PCB board, etc. These all need to be considered as a whole.

LED lighting heat dissipation problem solution

For common incandescent bulbs or fluorescent lamps, even if the product itself may generate heat during operation, the high heat of the components can still be effectively isolated, so that the light source and the power socket will not have unexpected problems due to heat. However, solid-state lighting is different. First, the high temperature of LED components concentrated at a single point must be treated with more active means to dissipate heat, and active and effective heat treatment mechanisms must be used to avoid problems with the lamps. The heat treatment of LED solid-state light sources is much more complicated than that of traditional lamps.

Traditional light sources or lamps often have the problem of generating high heat during operation, such as halogen bulbs or incandescent bulbs. In the case of incandescent bulbs, the tungsten filament is heated in a specially treated bulb to produce light.

In fact, high temperature is generated on the filament rather than the lamp holder. Even though the lamp holder will indirectly generate high temperature due to the radiant heat and heat conduction of the tungsten filament on the glass or metal of the lamp ball, the generated temperature is within an acceptable safety range. In addition, due to the non-direct contact conduction, the safety is relatively high.

However, if the lamps are replaced with LED solid-state light sources, their heat treatment may become a new application safety issue. Most people think that LED has the advantages of high energy conversion efficiency and low driving energy, so it is naturally safer to use. However, in fact, in order to achieve the application purpose of daily lighting, LED solid-state light sources must increase the power of a single set of components to enhance the output lumens of a single component . For example, lamp manufacturers will adopt the form of multi-LED component integration to enhance the output effect, and the simultaneous operation of multiple components can also improve the problem of LED solid-state light source light type biased towards point light source, so that LED solid-state light source technology lamps can produce a surface light source effect like a light bulb. If the output lumens of a single component are to be enhanced, a higher current is required to make the PN junction of the LED chip produce more lumens, but a higher current will also increase the temperature of a single-point LED component and make it more difficult to handle. Even in order to improve the light type performance and luminous efficiency of the lamp, the use of multiple components will also aggravate the high temperature problem of LED lamps and make the heat dissipation problem more difficult to handle.

Looking at the current development trend of the LED lighting market, most LED light source manufacturers will first take the market as the leading factor, because of the high unit price and high profit, they can also quickly enter the LED light source market with more advanced development technology through technological differences. For example, recessed lights, wall lights, and ceiling lights for interior decoration and situational lighting applications have become the more common design forms of LED light source lamps, and their energy-saving benefits after replacing traditional lamps are also the most concerned by relevant industry players.

Thermal management design, which must be focused on in LED light source lamps, poses a more stringent challenge for recessed lamps, wall lamps, and ceiling lamps that may be used in closed or semi-closed loops. Lamp developers must invest more resources in materials, product configuration, active/passive heat dissipation mechanisms, driver chip design, etc. to avoid product problems. In particular, LED recessed lamps are small in size and often use multiple components for integration, so the heat dissipation design of the module is more difficult. The housing of recessed lamps is designed with aluminum extrusion or heat sink, which can play a role in self-heating. But this is far from enough.

LED thermal management: NTC continuous operating temperature maintains LED lamp safety

If LED lamps are not equipped with adequate thermal management design, the life of the lamps may be drastically reduced due to frequent high-temperature operation during use, resulting in the need to frequently replace faulty LED lamps. In serious cases, it may even lead to accidents, such as the high temperature causing the wiring or surrounding decorations to catch fire!

During the product development phase, intelligent LED lighting control technology can be used to actively monitor the temperature performance of LED lamps and the overall light source module to simplify the thermal management of the device. At the same time, when the temperature of the lamp and the surrounding area rises to the range, the lamp must reduce the power consumption and reduce the LED brightness output, thereby improving the safety of LED solid-state light source lamps.

Considering a simpler design for the LED ceiling lamp housing, if the driver function used in the lamp itself is more focused on power conversion and LED component driving, and does not have an embedded temperature control microprocessor and heat dissipation processing module, in order to avoid increasing the cost of product raw materials, LED lamps can integrate NTC (Negative Temperature Coeffient) negative temperature coefficient Thermistor Sensors, which is a relatively cost-effective and safe design solution.

The purpose of the so-called NTC circuit is to monitor the temperature of the LED module lamp through electronics, and to shut down the LED solid-state light source module by default temperature warning or corresponding automatic processing driving conditions, so as to improve the safety of LED lamps. At the same time, the NTC circuit can also reduce the complexity of the design. Since the temperature coefficient of the NTC circuit is very large, it can detect the performance of small temperature changes and is widely used in related circuit designs that require measurement, control and compensation of temperature. In the design of LED light source modules, the NTC circuit is basically used to measure the temperature changes around the LED solid-state light source lamp. As for the measurement status, it will change with the voltage status of the NTC, and directly measure the corresponding relationship between the voltage and the temperature of the NTC circuit.

当NTC和周边电或整个模块温度提升时，NTC电的电阻随即降低，产品可依此相依关系进行相关安全控制机制反馈，例如减少LED发光组件的驱动电流或是直接强制关闭灯具照明，在灯具温度问题改善后自动回复照明状态，藉此获得灯具使用的安全性。

Monitoring the temperature of LED lamps can also be imported into the microcontroller using   the NTCTHERMISTOR component made in SMD form

If you want to achieve a better design for the aforementioned NTC circuit improvement, it is also a relatively pragmatic approach to combine it with an MCU for a more sophisticated safety design. In the development project, the status of the LED light source module can be divided into whether the light is normal or whether the light is turned off. Combined with the program logic judgment of temperature warning and temperature measurement, a more complete smart lighting management mechanism can be constructed.

For example, if a temperature warning appears on the lamp, temperature measurement shows that the module temperature is still within an acceptable range and can be maintained in a normal manner, with the operating temperature naturally dissipated through the heat sink. When the warning informs that the measured temperature has reached the benchmark for active cooling, the MCU must control the cooling fan to operate. Even when the temperature reaches the value, the system must directly shut down the driver power supply through the MCU, temporarily stopping the LED components from running and naturally dissipating the heat.

To determine whether the lamp is in use or turned off, a simple judgment bit can be used to make changes and understand the current usage status of the product. The more critical part is the temperature measurement. The measured temperature must be compared with the system's reference table in real time to confirm the normal or abnormal degree of the current module status. After calculating the temperature interval, temperature control management is automatically performed accordingly.

Similarly, when the temperature enters the zone, the control mechanism should immediately turn off the light source, and re-confirm the temperature 60 seconds or 180 seconds after the system is turned off. When the temperature of the LED solid-state light source module reaches the normal value, the LED light source is re-driven to continue to provide lighting. In short:

The public has always been concerned about the service life of lamps. If you only rely on using low thermal resistance LED components, you cannot build a good heat dissipation system for the lamp. Instead, you must effectively reduce the thermal resistance from the PN node to the surrounding environment to greatly reduce the PN node temperature of the LED, and successfully implement the goal of extending the service life of LED lamps and increasing the actual luminous flux. In addition, unlike general traditional lamps, the printed circuit board is both the power supply carrier of the LED and the heat dissipation carrier of the LED, so the heat dissipation design of the heat sink and the printed circuit board is very important. In addition, lamp manufacturers must also consider factors such as the quality, thickness and size of the heat dissipation material and the processing and connection of the heat dissipation interface.