Most LEDs, in the traditional sense, do not seem to emit noticeable heat, unlike many other light sources, but this is not the case. In fact, proper temperature control is perhaps the most important aspect of LED system design. This is especially true for LED lighting, when the lamp is concentrated a large number of sufficiently powerful emitters. This article examines the role of heat in LED efficiency.
All light sources convert electrical energy into radiant energy and heat in varying proportions. Incandescent light bulbs emit primarily infrared (IR) radiation with a small amount of visible light. Fluorescent and metal halide sources convert most of the energy into visible light, but also emit in the infrared (IR), ultraviolet (UV), and thermal regions of the spectrum. LEDs produce little or no IR or UV energy, but only convert 20%-30% of the power into visible light. The rest of the power is converted into heat, which must be removed from the LED housing using the main printed circuit board and the radiator, housing, or frame elements of the lamp. The table below shows the approximate proportions in which power input energy is converted into heat and radiant energy, including visible light, for various polychrome (white) light sources.
Estimation of power conversion factor for “white” light sources
† From the Directory
‡OSRAM SYLVANIA
*Depends on LED efficiency. This range is for the current best technology in color temperatures from warm (150 lm/W) to cool (100 lm/W). The US Department of Energy's long-range plan (March 2009) calls for an efficiency increase of more than 50% by 2025.
Why is the issue of thermal conditions so important?
Excess heat directly affects both current efficiency and changes in efficiency over operating time. Short-term (reversible) effects are a shift in color and a decrease in light output, while the long-term effect is an accelerated decrease in light output and thereby a reduction in lifespan. beneficial use LED.
The light output of different color monochrome LEDs varies differently with temperature changes. Thus, amber and red LEDs are the most sensitive to temperature, and blue ones are the least sensitive (see graph). These individual temperature dependences may cause noticeable color shifts in RGB based systems if working temperature different from the recommended one. LED manufacturers test and grade (bind) their products for brightness and color based on photometric measurements under specific conditions - a 25 millisecond high-power pulse at a fixed temperature of 25°C. During the duration of the pulse, the temperature of the chip remains virtually unchanged. In operating mode, when DC At room temperature and applying technical measures to reduce the temperature, the LED chip temperature is generally 60°C or higher. Therefore, white LEDs will provide at least 10% less light, than specified by the manufacturer, and the reduction luminous flux for products with insufficient heat dissipation it can be significantly higher.
Continuous operation at elevated temperatures significantly accelerates the process of brightness reduction (degradation), which ultimately leads to a reduction in useful life. The graph below shows the lumen output over time (experimental data up to 10,000 hours and extrapolation beyond) for two identical LEDs at the same current, but with a chip temperature difference of 11°C. Design life (defined at 70% lumen reduction) decreased from approximately 37,000 hours to 16,000 hours (57% change) for a 11°C increase in temperature.
However, manufacturers continue to improve LED durability at higher operating temperatures. For example, manufacturers of high-power white LEDs typically estimate a lifespan of about 50,000 hours at 70% lumen reduction, at chip temperatures no higher than 100°C.
What determines the temperature of an LED chip?
Three reasons affect LED chip temperature primarily: drive current, heat sink efficiency, and temperature environment. In general, the higher the control current, the greater the heat dissipation. Heat must be dissipated from the chip to maintain expected lumen output, color, and lifespan. The amount of heat that can be removed from the system depends on the ambient temperature and the design of the heat sink.
A typical high power LED system consists of an emitter, a circuit board on metal base(MCPCB), as well as an external radiator. The emitter contains an LED chip, optics with an encapsulating compound, a thermally conductive substrate (used to remove heat from the chip), and is soldered to the MCPCB. MCPCB is a special type of printed circuit board with a thin dielectric layer on a metal substrate (usually aluminum). The MCPCB is mechanically attached to an external heatsink, which may be a device integrated into the luminaire design. In some cases, the role of a radiator is performed by the supporting body of the lamp. The size of the radiator depends on the amount of heat that must be dissipated and the thermophysical properties of the material.
Thermal design and environmental awareness are critical considerations in the design and application of LED lighting luminaires. The reliability of the product, and therefore its commercial value, will depend primarily on the design of the heat sink for heat dissipation and the ability to minimize the temperature of the emitter. Keeping the chip temperature in the lower range recommended by the manufacturer's specifications is necessary in order to maximize the performance potential of LEDs.
As of 2011, analysis of US Department of Energy bulletins.
Selection and translation - Lanskoy A.O., November 2011
Thermoregulation affects performance and service life
Today, LEDs are well known for their high performance and long lifespan. But few people know that these indicators directly depend on the operating temperature. The lower the temperature, the greater the light output and the longer the LED will work. Therefore, when designing a lamp, it is extremely important to provide for proper heat dissipation.
The colder it is, the more effective it is.
LED efficiency is highly dependent on operating and ambient temperatures. When indicating reference values, the manufacturer uses a room temperature of 25°C as a basis. Under normal operating conditions, the junction temperature is 80°C or higher*, experts note. At the same time, there is no concept of an optimal operating temperature for an LED, unlike a fluorescent lamp, for example, for which the ideal temperature is 35°C. If the temperature is higher or lower than stated, then the efficiency lighting fixture will decrease noticeably. For LEDs, the following applies: the lower the temperature, the better. This can be observed in deep-freezing devices, where the efficiency of LEDs significantly exceeds the reference values.
*Some manufacturers test LEDs at 85°C instead of 25°C. The results are indicated in so-called “hot lumens”. The colder the better - light output decreases as the temperature rises
Temperature determines service life
The service life of the LED also depends on the operating temperature. Today, a quality LED light should last about 50,000 hours. However, for this to happen, two important conditions must be met. Firstly, the indicated service life is determined taking into account an operating temperature of 80-85°C. More high temperatures significantly reduce the service life of the device. And vice versa more low temperatures will extend the service life. And, secondly, one should take into account the fact that LEDs gradually lose their illumination. On average, after 50,000 hours of operation, the luminous flux of LEDs drops by 70% compared to the original level. Operating temperature affects service life - LEDs operate up to 50,000 hours at a temperature of 80-85°C
Particular attention to cooling
Responsible manufacturers pay great attention to the thermal regulation of their LED lamps. To achieve their goals, they use cooling plates and special conductive foil. Thanks to these actions, the operating temperature of the LEDs varies within 60°C, and the service life of the device is thus significantly increased.
Copying is prohibited. The rights to the translation belong to the Moonlight Company in accordance with Art. 1260 of the Civil Code of the Russian Federation.
1. What is the average service life of LEDs?
It is impossible to lump all types of LEDs into one pile and treat them equally. The service life directly depends on the type of LED, the current supplied to it, cooling of the LED chip, the composition and quality of the crystal, layout and assembly as a whole.
LEDs are said to be extremely durable. But it is not so. The greater the current passed through the LED, the higher the temperature and the faster the aging (degradation) of the crystal occurs. And the more heat you transfer from the LED substrate to the radiator, the longer it will work. Therefore, LED assemblies with high-power LEDs require passive (mounting on an aluminum board and radiator) or active (cooler, fan) cooling.
With sufficient cooling, the LEDs can be “overclocked” by applying the maximum current recommended by the manufacturer.
Therefore, a priori, the service life of high-power LEDs is shorter than that of low-power indicator LEDs. Aging is expressed primarily in a decrease in brightness.
In LEDs with a power of 1W (operating current 0.350A) and more powerful, the heat generation is much more abundant than in 5mm. LEDs designed for a current of 0.02A. By light output 1 pc. A 1W LED replaces about 50 5mm LEDs. but it also heats up many times more.
On LED strips, the vast majority of manufacturers use ordinary resistors that limit the current as current-setting elements. For longer service life, manufacturers strive to provide reasonable current with possible variation in the output voltage of power supplies. When a section fails LED strip, repairing the backlight comes down to soldering out the burnt areas, usually multiples of 5 or 10 cm.
The service life of the LED strip is about 50 thousand hours.
2. Why do white LEDs have the shortest lifespan?
Unfortunately, no one has yet invented compounds that emit white light. The basis of the white LED is the InGaN structure, emitting at a wavelength of 470 nm (blue color) and a phosphor (special composition) applied on top of it, emitting at wide range visible spectrum and has a maximum in its yellow part. The human eye perceives a combination of this kind as White color. The phosphor degrades the thermal characteristics of the LED, so its service life is reduced. Now global manufacturers are inventing new and new options effective application phosphor.
Most high-power LEDs last between 50,000 and 80,000 hours. Is it a lot or a little?
50,000 hours is:
24 hours a day 5.7 years
18 hours a day 7.4 years
12 hours a day 11.4 years
8 hours a day 17.1 years
3. Do LEDs get hot?...
Everywhere they say that LEDs practically do not heat up. So why do LED devices need a heat sink and what happens if there is no heat sink?
The so-called p-n junction of the crystal glows in the LED. Roughly speaking, this is the place where one type of metal (-p) connects to another type (-n). The task is to find such a combination of different conductors so that as much light as possible comes out of this zone with minimal losses.
And this is where the problems begin. The ideal combination of -p and -n conductors has not yet been found, and it is unlikely that they will be found, and there will always be losses, whether we like it or not. Therefore, along with particles of visible light, a small amount of heat is also emitted. In the past, when LEDs were so dim that they were used only for display, no one counted this emitted heat - it was so negligible.
Now, with the advent of powerful and super-powerful LEDs, the ratio of light and heat emitted by the crystal has remained the same, but now it is more noticeable. For clarity, look at a regular ordinary microcircuit. Let's say this is a chip measuring 1 by 1 cm. The more tasks this chip performs, the hotter it heats up. But if it is a simple microcircuit, the chip body itself, as well as the metal pins-contacts with which it is soldered to the board, can serve as a heat sink. If we want to place millions of times more semiconductor elements inside the same microcircuit and force this microcircuit to perform millions of times more operations, the heat generation will increase many times and we will need to cool it forcibly. To avoid going far, look at any of the current computer processors - they are all equipped with an aluminum or copper radiator with forced airflow by a fan.
About the same thing happens in the LED. When we try to “squeeze” more light from the same chip area, the amount of heat generated inside the crystal itself increases proportionally. And to remove it, you need cooling.
Thus, powerful LEDs of the “piranha” type only need their own housing and a printed circuit board on which the LED is mounted as a heat sink. But for a super-powerful LED, additional cooling in the form of a radiator will be required. But where does this heat come from? In an LED, as already mentioned, there are losses during the conversion of electricity into light. But some of this light (photons) remains inside the crystal. For crystals where relatively a lot of light comes out and little remains inside, the definition of “high quantum yield” is applied. If the LED itself is not bright enough and there are relatively few “output” lumens per watt of applied voltage, then the definition of “low quantum output crystal” is applied here.
So for any average LED, the chip temperature always increases along with its power. The typical operating temperature of LEDs produced today ranges from 50°C to 120°C, and given the constant development of technology, it may reach 200°C in the near future.
If powerful LEDs combined into some kind of assembly, and even installed in a sealed housing, then the heating becomes significant. And if heat is not removed, the semiconductor junction overheats, which changes the characteristics of the crystal, and after some time the LED may fail. So it is very important to strictly control the amount of heat and ensure efficient heat dissipation. The thermal characteristics of devices are calculated already at the design stage, which eliminates any problems in operation.
How does the LED react to rising temperatures?
Speaking about the temperature of the LED, it is necessary to distinguish between the temperature on the surface of the crystal and in the region of the pn junction. Roughly speaking, this is the place where one type of metal (-p) connects to another type (-n). The service life depends on the first, the light output depends on the second. In general, as the temperature of the pn junction increases, the brightness of the LED decreases because the internal quantum efficiency decreases due to the influence of lattice vibrations. This is why good heat dissipation is so important.
For example, LEDs from Cree were placed on the printed circuit board. Figure 7 shows the results showing temperatures without a heatsink (left) and with a heatsink (right).
The decrease in brightness with increasing temperature is not the same for LEDs different colors. It is larger for AlGalnP and AeGaAs LEDs, that is, red and yellow, and smaller for InGaN, that is, green, blue and white.
4. What is the difference between a full-color RGB LED and a single-color one?
In a full-color LED, independent crystals of three emission colors (R+G+B) are installed on one substrate, and a monochrome LED contains crystal(s) of any one emission color.
5. How to adjust the brightness of the LED?
The brightness of LEDs can be adjusted very well, but not by reducing the supply voltage - this is something that cannot be done - but by the so-called method pulse width modulation(PWM), which requires a special control unit (in reality, it can be combined with a power supply and converter, as well as with an RGB matrix color control controller).
The PWM method consists in the fact that not a constant, but a pulse-modulated current is supplied to the LED, and the signal frequency should be from hundreds to thousands of hertz, and the width of the pulses and pauses between them can vary. The average brightness of the LED becomes controllable, while at the same time the LED does not go out.
6. What is the quantum efficiency of an LED?
Quantum yield is the number of light quanta emitted per recombined electron-hole pair. A distinction is made between internal and external quantum efficiency. Internal - in the pn junction itself, external - for the device as a whole (after all, light can be lost “along the way” - absorbed, scattered). The internal quantum efficiency for good crystals with good heat dissipation reaches almost 100%, the record external quantum efficiency for red LEDs is 55%, and for blue LEDs - 35%. External quantum efficiency is one of the main characteristics of LED efficiency.
7. What LED manufacturing technologies exist today?
In general, the technology looks like this: artificial sapphire plates are installed in a “reactor” (epitaxial growth installation). Then the gas mixture is fed. The base with the plates placed on it rotates in the reactor at a speed of 1000 rpm. During rotation, gas atoms “stick” to the surface of the crystalline substrate, forming dozens of layers. The result is an LED crystal hundreds of microns thick.
Next comes the planar processing of the films: etching them, creating contacts to the p- and p-layers, coating with metal films for the contact leads. The plates are then separated into individual crystals. A film grown on a single substrate can be cut into several thousand chips ranging in size from 0.24x0.24 to 1x1 mm2. How larger area crystal, the more light it is able to emit when current passes through it.
The technology is called metal-organic epitaxy. This process requires particularly pure gases. Modern reactors provide automation and control of the composition of gases, their separate flows, and precise control of the temperature of gases and substrates. The thicknesses of the grown layers are measured and controlled ranging from tens of angstroms to several microns. Different layers must be doped with impurities, donors or acceptors, to create a pn junction with a high concentration of electrons in the n region and holes in the p region.
The next step is to create LEDs from these chips. It is necessary to mount the crystal in the housing, make contact leads, and produce optical coatings that brighten the surface for outputting radiation or reflect it. If it is a white LED, then you need to apply the phosphor evenly. It is necessary to provide heat removal from the crystal and the case, to make a plastic dome that focuses the radiation into the desired solid angle. About half the cost of an LED is determined by these high technology steps.
It is very important to ensure and control the uniformity of structures on the surface of the substrates. The cost of installations for the epitaxial growth of semiconductor nitrides, developed in Europe (by Aixtron and Thomas Swan) and the USA (Emcore), reaches 1.5 - 2 million dollars. The experience of different companies has shown that it is possible to learn how to produce competitive structures with the necessary parameters using such an installation in a period of one to three years. This is a technology that requires a high production culture.
The need to increase power to increase luminous flux has led to the fact that the traditional form of a packaged LED is no longer satisfactory to manufacturers due to insufficient heat dissipation. It was necessary to bring the chip as close as possible to the heat-conducting surface. In this regard, the traditional technology and the somewhat more advanced SMD technology (surface montage details) are being replaced by the most advanced chip on board technology. An LED manufactured using SOV technology is shown schematically in the figure.
LEDs made using SMD and COB technology are mounted (glued) directly onto a common substrate, which can act as a radiator - in this case it is made of metal. This is how LED modules are created, which can have linear, rectangular or round shape, be rigid or flexible, in short, designed to satisfy any whim of the designer. Appear and LED bulbs with the same base as low-voltage halogen ones, designed to replace them. And for powerful lamps and spotlights, LED assemblies are made on a massive radiator.
LED boards used to have a lot of LEDs. Now, as power increases, there are fewer LEDs, but the optical system that directs the light flux to the desired solid angle plays an increasingly important role.
8. Where are LEDs used today and what are their prospects?
It is advisable to use LED lighting in cases where high reliability is required, where maintenance of a lighting installation is too expensive and requires special equipment or the work of climbers, where it is necessary to use color-dynamic solutions, where an energy-efficient solution is required, for example, when powered by a variety of generators.
The other side of the coin: LED lamps are ideal for dim but effective lighting. This specific example In terms of energy consumption, it is 90% more economical than the smallest 15W halogen light bulbs.
Every year, the light output and efficiency of LEDs increases by 30-50%. As of 2008, LED lamps are already more often used than lamps in architectural, decorative, landscape, underwater lighting, holiday illumination, show business, as well as in special applications - medicine and plant growing, for example.
In the foreseeable future, most likely, LEDs will replace lamps in emergency lighting of public places - entrances residential buildings, light indicators, etc. And also in transport - on planes, trains, cars. And as technology develops and production becomes cheaper, it comes to night lighting highways and streets. All this will provide significant savings in energy resources on a national scale.
9. Which global companies produce LEDs?
List of leading manufacturers in the world:
- “CREE” (USA);
- “Osram” (Germany);
- “Lumieleds Luxeon” (USA);
- “Seoul Semiconductor” (South Korea);
- “Nincha” (Japan);
- “Epistar” (Taiwan);
- “Edisson” (Taiwan);
- “Prolight Opto” (Taiwan).
Every year, the luminous flux of the most productive LED of each of the world brands increases steadily by 20-30%. The cost of 100 Lm of luminous flux falls by 10-15% per year, and hence the stable annual drop in prices for LED lighting devices.
Price LED device, of course, depends on the cost of the LEDs themselves. In the mass production of lighting products, LEDs constitute the largest line item in the budget for the production of LED devices.
10. Who invented the LED?
Back in 1907, a faint glow emitted by silicon carbide crystals due to then unknown electronic transformations was first noted. In 1923, our compatriot, an employee of the Nizhny Novgorod Radio Laboratory, Oleg Losev, noted this phenomenon during his radio engineering research with semiconductor detectors, but the intensity of the observed radiation was so insignificant that the Russian scientific community was not seriously interested in this phenomenon at that time.
Five years later, Losev specifically began researching this effect and continued it almost until the end of his life (O.V. Losev died in besieged Leningrad in January 1942, before reaching the age of 39). The discovery of "Losev Licht", as the effect was called in Germany, where Losev published in scientific journals, became a world sensation. And after the invention of the transistor (in 1948) and the creation of the theory of the p-n junction (the basis of all semiconductors), the nature of the glow became clear.
In 1962, American Nick Holonyak demonstrated the operation of the first LED, and soon after that he announced the start of semi-industrial production of LEDs.
A light emission diode (LED) is a semiconductor device; its active part, called a “crystal” or “chip,” like conventional diodes, consists of two types of semiconductor – electronic (n-type) and hole (p-type). -type) conductivity. Unlike a conventional diode, in an LED, at the interface of different types of semiconductors, there is a certain energy barrier that prevents the recombination of electron-hole pairs. Electric field, applied to the crystal, allows one to overcome this barrier and recombination (annihilation) of the pair occurs with the emission of a light quantum. The wavelength of the emitted light is determined by the magnitude of the energy barrier, which, in turn, depends on the material and structure of the semiconductor, as well as the presence of impurities.
This means that first of all you need a p-n junction, that is, contact of two semiconductors with different types conductivity. To do this, the near-contact layers of the semiconductor crystal are doped with different impurities: acceptor impurities on one side, donor impurities on the other.
But not every pn junction emits light. Why? Firstly, the band gap in the active region of the LED should be close to the energy of visible light quanta. Secondly, the probability of radiation during recombination of electron-hole pairs must be high, for which the semiconductor crystal must contain few defects, due to which recombination occurs without radiation. These conditions contradict each other to one degree or another.
In reality, in order to meet both conditions, one pn junction in the crystal is not enough, and it is necessary to manufacture multilayer semiconductor structures, the so-called heterostructures, for the study of which the Russian physicist academician Zhores Alferov received Nobel Prize 2000.
11. LED device.
The main modern materials used in LED crystals:
InGaN – high brightness blue, green and ultraviolet LEDs;
AlGaInP - high brightness yellow, orange and red LEDs;
AlGaAs - red and infrared LEDs;
GaP – yellow and green LEDs.
device of 5mm LEDs (left) and high-power LEDs (right)
LED device various types simplified in the figures. The light emitted by the semiconductor crystal enters a miniature optical system formed by a spherical reflector and the transparent lens-shaped diode body itself. By changing the configuration of the reflector and lens, installing secondary lenses, the required radiation direction is achieved.
three-crystal RGB LED under a microscope
In addition to lamp-type LEDs (3,5,10mm, their shape really resembles a miniature light bulb with two terminals), in Lately SMD LEDs are becoming more and more common. They are of a completely different design that meets the requirements of technology automatic installation onto the surface of the printed circuit board ( surface mounted devices – SMD).
Ultra-bright LEDs of this type are called emmiter (emitter, English "emitter").
SMD LEDs have more compact dimensions and allow automatic placement and soldering onto the board surface without hand assembled. Some LED manufacturers produce special SMD diodes containing three crystals in one package, emitting light of three primary colors - red, blue and green. This makes it possible to obtain, by mixing their radiation, the entire color scheme, including white, in ultra-compact sizes.
The brightness of an LED is characterized by luminous flux (Lumens) and axial luminous intensity (candelas), as well as by directional pattern. Existing LEDs different designs emit in a solid angle from 4 to 140 degrees.
Color, as usual, is determined by chromaticity coordinates, color temperature white light(Kelvin), as well as the radiation wavelength (nanometers).
To compare the efficiency of LEDs with each other and with other light sources, luminous efficiency is used: the amount of luminous flux per watt electrical power(characteristic "Lumen/Watt"). Also interesting characteristic turns out to be the price of one lumen ($/Lumen).
Based on luminous intensity, LEDs are divided into three main groups:
Ultra-high brightness LEDs, power from 1W (Ultra-high brightness LEDs) – hundreds of candelas;
High brightness LEDs, power up to 20 mW (High brightness LEDs) – hundreds and thousands of millicandelas;
Standard brightness LEDs – tens of millicandelas.
So, any LED consists of one or more crystals placed in a housing with contact leads and an optical system (lens) that forms the light flux. The wavelength of the crystal's emission (color) depends on the semiconductor material and doping impurities. Wavelength binning of crystals according to the wavelength of radiation occurs during their manufacture. In the delivery batch for modern production Crystals with similar radiation spectrum are selected.
A wide range of optical characteristics, miniature sizes and flexible discrete control capabilities have ensured the use of LEDs to create a wide variety of lighting devices and products. The LED emits in a narrow part of the spectrum; at a certain wavelength its color is pure, which is especially appreciated by designers.
The modern variety of LEDs and so-called LED assemblies often makes the task of choosing suitable components for the implementation of certain lighting applications non-trivial. The main parameters of LEDs are color, luminous intensity and viewing angle at half the radiation power. But for professional choice, assessing the quality and efficiency of products requires taking into account many other characteristics of LEDs. Below is a short list of them.
The value of characteristics and parameters in a particular case depends on the type of crystal, the design and size of the reflector, the structure and thickness of the phosphor coating, methods of forming the optical lens and other factors.
There is a “conspiracy theory” circulating on various sites on the Internet that the service life of an ordinary incandescent lamp is supposedly specially limited to 1000 hours due to a cartel agreement between the largest lamp manufacturers that took place in the 20s of the last century. Of course, any specialist will explain to you that the “Ilyich light bulb” has such a service life due to the principle of operation and no one specifically limits this period. And yet, this “conspiracy theory” has its supporters. They will probably be horrified to learn how long life LED lamp manufacturers promise their products. Small, little-known companies, without unnecessary curtsy, indicate the service life of the lamp as 100,000 hours. More reputable companies limit themselves to more modest numbers - only 35,000 hours. Can this data be trusted?
Typically, service life refers to the time a device operates before failure. Moreover, the moment of failure is not necessarily complete inoperability, but a drop in characteristics below a certain level. When assessing the service life of LEDs, the moment of their failure is defined as a decrease in the luminous flux below a certain percentage of the nominal value. And here discrepancies between different companies already begin. Some manufacturers consider this threshold to be a reduction in luminous flux by 30% of the nominal value, others – by 50%. The specified data, as a rule, is not reported in advertising materials, and often in the documentation for the lamps, which does not allow the buyer to make the right choice.
Masters of Extrapolation
Even if the situation with the threshold for reducing the luminous flux is clear, this does not mean that you have received reliable information about the operating time of the LEDs. The most common service life value, which is indicated in advertising materials, is 50,000 hours, i.e. 5 years and 8 months. Naturally, no one for so long new type the LED will not be tested. Events in the LED market are developing so quickly that within the specified time the LED will be discontinued and a new type will be launched in its place. Therefore, they test the LED and monitor its aging processes in extreme conditions(current strength and crystal temperature are at the limit of permissible values) for a relatively short period of time, and then extrapolate the dependence over a longer period of time for normal operating conditions.
Knowing the band gap of the semiconductor from which the crystal is made, as well as the failure rate at elevated temperatures, it is possible to determine the failure rate at normal temperatures using the Arrhenius model. Mean time between failures is the inverse of the failure rate.
Graph of lifetime versus temperature for various types of LEDs manufactured by Seoul Semiconductor, obtained by extrapolating test results at elevated temperatures
One international standard, which would describe testing LEDs under extreme conditions and then extrapolating the results, does not exist. However, in the United States there is an organization called JEDEC (Joint Electron Device Engineering Council), which develops JESD standards. Some LED manufacturers, such as Cree, use the JESD22 standard for LED testing (see table). LEDs are tested at the maximum permissible current, the duration of these tests is 1008 hours (42 days). The criteria for LED failure in all tests given in the table are: a change in bias voltage by more than 200 mV, a decrease in luminous flux by more than 15%, short circuit, circuit break. If at least one of these phenomena is observed, the LED is considered to be faulty.
Some tests Cree uses to determine LED reliability
| Type of test | Standard | Environmental parameters | Current supply |
| work at room temperature | JESD22 Method A108-C | temperature +45 C | constantly |
| work at elevated temperatures | JESD22 Method A108-C | temperature +85 C | constantly |
| work in conditions elevated temperature and high humidity | own methodology | temperature +60 C, relative humidity 90% | alternation: 1 hour served, 1 hour not served |
| operation at low temperatures | JESD22 Method A108-C | temperature -40 C | constantly |
Certainly, modern techniques allow with high accuracy predict the service life of the device, but no one can give a complete guarantee that theory and practice will agree.
The lifespan of an LED is affected by the following factors:
- crystal degradation;
- phosphor aging;
- mechanical deformations, internal stresses in the housing, etc.;
- clouding of the primary optics.
Crystal degradation
Let us recall that a white LED is usually a blue-emitting crystal coated with a phosphor. Due to the summation of the crystal's own radiation with the radiation of the phosphor induced by it, light is obtained that is perceived by vision as white.
In relation to LEDs, it is necessary to distinguish between the temperatures measured at different points: T B - circuit board, T S - substrate, T J - p-n junction, T A - environment
Crystal degradation leads to a decrease in radiation power. One of the reasons is the increase in the number of crystal lattice defects. The areas of the crystal where defects appear do not emit light, but they do generate heat.
Another reason is electrical migration of the material from which the electrodes are welded to the crystal. Atoms of the metals from which the electrodes are made penetrate into the crystal and disrupt the crystal structure.
As the crystal degrades, the leakage current increases, that is, a significant part of the current begins to pass through areas of the crystal that do not emit light. As a result, the voltage on the LED electrodes decreases, which means the power decreases.
The rate of crystal degradation increases significantly with increasing current above the nominal value, as well as with increasing temperature. Also, according to some experts, static electricity can cause defects in the crystal lattice. Therefore, it is recommended that LEDs be installed using standard ESD precautions.
Crystal degradation also manifests itself by a decrease in the voltage across the LED. This feature is used to automatically turn off a failed LED. If the luminaire has a built-in intelligent system control, it will turn off the failed LED and redistribute the currents in the remaining LEDs so that they do not exceed the maximum permissible level. Along with a system for automatically turning off faulty LEDs, the lamp can also use an adaptive system for controlling the current supplied to the LED, a discussion of which is beyond the scope of this article.
While it is important to take into account the process of crystal degradation when determining the life of an LED, LED degradation is also determined by many other factors.
Phosphor degradation
Like fluorescent lamps, LEDs have a phosphor. This allows some specialists who are skeptical about the use of LEDs for lighting to express the thesis that the service life of an LED cannot be longer than the service life of a fluorescent lamp, i.e. 10000 hours. However, such a comparison is incorrect. Firstly, a significant contribution to phosphor degradation in fluorescent lamps plays the phenomenon of spraying paste covering the electrodes. There is no such phenomenon in LEDs. Secondly, LEDs use completely different, more expensive phosphors. For example, one widely used phosphor option is cerium-doped gallium gadolinium garnet. Such phosphors have a long service life.
LEDs and fluorescent lamps use completely different phosphors
In an LED, phosphor degradation is determined mainly by temperature. After all, the phosphor is usually applied directly to the crystal, which heats up quite strongly. Other factors influencing the phosphor are not so significant.
Degradation of the phosphor leads not only to a decrease in the brightness of the LED, but also to a change in the shade of its glow. With severe degradation of the phosphor, a blue tint of the glow is clearly visible. This is due both to a change in the properties of the phosphor and to the fact that the crystal’s own radiation begins to dominate in the spectrum.
Mechanical damage
During the production of LEDs, internal stresses may arise in them, which manifest themselves later.
The LED crystal with leads soldered to it and a heat-dissipating substrate is filled with transparent plastic. Poor solder joints can deteriorate over time. If the solder joints of the electrodes are destroyed, the circuit will break. If the soldered connection of the crystal with the heat-sinking substrate is destroyed or even the contact area is reduced, this leads to accelerated degradation of the crystal. The cause of the destruction of the connection, as well as the rupture of thin conductors leading to the crystal, can be internal stresses in the plastic. They arise both as a result of violations of production technology and during operation of the LED at a temperature exceeding the maximum permissible value.
Clouding of the primary optics
The primary LED optics (i.e., the optical system directly integrated into the LED) is made of plastic or silicone. The clouding of these materials may be due to exposure to ultraviolet radiation. In white LEDs, built on the basis of ultraviolet LEDs coated with a three-color phosphor, this problem actually exists. But such LEDs have not yet become widespread.
Many modern types LEDs have no primary optics at all
In white LEDs based on blue-emitting crystals, clouding of the primary optics can again be caused by severe overheating. It should be noted that many modern types of LEDs do not have primary optics at all.
Lamp service life
So, we found out that the main problem causing a decrease in the working life of LEDs is a violation of the temperature regime during operation. In turn, the temperature regime is determined by the design of the lamp. Therefore, it would be more correct to talk not about the service life of LEDs, but about the service life of the lamp. Unfortunately, in advertising materials, manufacturers often indicate the service life of LEDs, when in a lamp, due to overheating, the LEDs may work less than the lifespan stated by the manufacturer (but under normal operating conditions, the service life of the LEDs and the entire lamp may be the same).
Ensuring the temperature of the LEDs (and ultimately the temperature p-n junction) within specified limits by removing heat is called thermal management. Thermal management issues include not only the design of the heat sink, but also the design of the entire case. Unfortunately, many manufacturers install LEDs in existing housings that were originally designed for HPS lamps. As a result, the required temperature conditions are not provided.
Along with ensuring heat dissipation, automation that controls the power supply of LEDs is important. If it is discovered that the LEDs are overheated and the cooling system can no longer cope, the power supplied to the LEDs must be reduced.
IN street lamp EcoWay LL-DKU-02-095-XXXX-65D manufactured by LeaderLight, the radiator is covered with a special casing that protects it from dirt, but does not interfere with air circulation
Beyond management temperature p-n transition, there are two more components in the lamp that have an important impact on the service life of the entire device: the driver and secondary optics.
Modern element base allows you to create drivers with a service life of 50,000 hours or more. Also important are the stability of the supply voltage and current provided by the driver, as well as its resistance to surges in mains voltage.
Secondary optics lenses in LED lamps usually made of plastic, which becomes cloudy over time. Reflectors are often made of plastic coated thin layer metal This is where the problem of tarnishing the metal surface may arise. These problems are solved by using modern materials, as well as sealing the lamp body.
"Overclocking" LEDs
You can “squeeze” more out of an LED if you make it work in a mode not provided for in the documentation. In some ways, this is reminiscent of “overclocking” computers, with the only difference being that they increase not the clock frequency, but the current flowing through the LED. Not only enthusiasts trying to understand the limits of the capabilities of electronic components, but also some little-known companies from Southeast Asia are engaged in “overclocking” LEDs.
It is necessary to distinguish between two types of “overclocking” of LEDs. The first is associated with an overestimation of the operating current in the characteristics of the LED. There are small companies that install a phosphor-coated crystal into a housing. The resulting product is marketed under its own brand. But the same chip manufacturer supplies dozens of LED manufacturers with its products. How then to overtake competitors? Unscrupulous companies follow the path of indicating in the documentation a higher nominal value of the current flowing through the LED than is recommended by crystal manufacturers. As a result, the brightness of the LED increases, but its operating time decreases.
The second method is associated with the lamp manufacturer knowingly exceeding the rated LED supply current. As a result, the same goal is achieved - increasing the brightness, this time of the lamp. But the operating time is also reduced.
When “overclocking” LEDs, you can increase the service life by cooling the crystal more strongly than during normal operation. However, one must understand that even if normal thermal regime The service life of LEDs during overclocking is still reduced, since one of the reasons for crystal degradation is the current exceeding the maximum permissible value.
conclusions
The service life of the lamp is determined not only by the quality of the LEDs used, but also by the parameters of other structural components. The use of modern materials and electronic components, as well as a properly designed driver and cooling system, makes it possible to increase the service life of the luminaire to the service life of the LEDs declared by the manufacturer. But this requires significant investments in research and production, which not all companies can afford. Particular vigilance should be exercised in cases where the promises of manufacturers of final products are not confirmed by anything other than data on the operating time of LEDs under ideal conditions. What can be considered confirmation? Best option, quite natural for leading firms - when guarantee period matches or is close to the declared resource, i.e. is 3 – 5 years. If the warranty period is 1 - 2 years, be guided by the service life of the lamp given in the official documentation for it, and not in advertising brochures. Otherwise, you can only rely on the reputation of the lamp manufacturer.
