Efficiency of LED lamps. How many years will it take for an office LED lamp to pay for itself?

Recently, on one website I saw a payback calculator for LED lamps. I immediately became interested in how many years it would take to pay for itself. led lamp, because on at the moment Not every customer wants to install LED lamps.

According to the calculator, an office LED lamp should pay for itself in just 3.68 years. Now let’s check whether we actually get such a figure.

For the office, a CERTAIN manufacturer of LED lamps produces a recessed lamp with a power of 42 W, with a luminous flux of 3500 lm, efficiency = 94%, color rendering index 80. Such a lamp costs $ 175. This lamp completely replaces the lamp with fluorescent lamps LVO 4×18, which costs only $25. As you can see, an LED lamp for office space is 7 times more expensive than a lamp with fluorescent lamps.

First, let's compare the two lamps.

Now, based on these data, we will calculate the annual energy consumption and how many years will it take for the LED lamp to pay for itself. We have 2000 working hours a year (for an office worker). We will change fluorescent lamps after 10,000 hours, because... the luminous flux will begin to decrease.

The result was quite disastrous.

Why then did this happen?

It's very simple. The payback time of an LED lamp depends on the price of electricity and operating time. The higher the cost of kWh and the number of operating hours, the shorter the payback period.

After doing some reverse calculations, I realized that the manufacturers of LED lamps do not spare us at all (including designers, since we are also office workers), they force us to work seven days a week and set us the maximum estimated tariff for electricity. In general, they charged everything to the maximum to get the minimum payback period.

In this case we will have the following result.

To be honest, I'm a little upset. I expected the payback period for the LED lamp to be at least 5 years. The manufacturer promises 3.68 years, but in reality it is about 10 years. Moreover, for 10 years, provided that the office operates seven days a week and at the maximum calculated rate.

The declared 70,000 hours for an LED lamp is just a theory, but in practice, who knows how it will behave in 5-10 years.

I think that by the time it pays for itself, and according to my calculations this is 10 years, this lamp will already be obsolete, although it will be in working condition.

In the current conditions, manufacturers of LED lamps will only be FOR an increase in electricity prices, since the use of LED lamps directly depends on the price of electricity.

It is advantageous to install LED lamps in areas where the cost of electricity is high. I think this is more relevant for European countries.

Maybe I didn’t take everything into account or do you have more accurate information on this topic?

P.S. I'm not against LED lights at all. I just love numbers. In my opinion, the cost of an LED lamp needs to be further reduced so that it can be used everywhere. An LED lamp has many advantages compared to a fluorescent lamp, but it also has one big drawback - the price.

LED lampsModern have sufficient brightness, which could not be said about the LEDs of the previous generation, whose low brightness significantly limited their use. Currently,..

Efficiency. The efficiency of modern LED lamps is 22%. In addition to high efficiency, LED lamps also boast great durability, up to 50,000 hours, which in turn is equivalent to 17 years of operation, 8 hours a day. Modern have sufficient brightness, which could not be said about the LEDs of the previous generation, whose low brightness significantly limited their use. Currently, after the issue of LED brightness, their popularity has increased dramatically. Despite the high cost, but thanks to their high efficiency, service life and significant savings on electricity and installation work, LEDs are gaining more and more popularity. In addition, long service life LED lamps allows you to install them in hard-to-reach places, this is especially true when using LEDs V . For more than 130 years of history, incandescent lamps, which have dominated the world of lighting technology all this time, have had a large number disadvantages: this is a fragile thread that can fail during shaking, and a large percentage of heat output, which significantly reduces the ratio useful power to the luminous flux. The efficiency of conventional incandescent lamps is only 2.6%. A more technologically advanced fluorescent lamp has a slightly higher efficiency of 8.7% and has also made a significant contribution to energy savings. The use of fluorescent lamps has revealed several significant disadvantages: a short service life in real conditions, possible flickering, and possible refusal to turn on at low temperatures, as well as flashing when there is a lack of voltage. In addition, burnt out fluorescent lamps require special disposal. Fluorescent lamps They have an extremely negative attitude towards the intermittent cycle of operation, on and off.

have high efficiency, low power consumption and long service life, bright light, excellent illumination and no flicker. Due to their high performance characteristics they are becoming more and more widespread, they are especially often used in. Company Professional Light and Sound offers you a wide range of modernLED lamps And high quality at an affordable price, based on qualityLED lamps(Cm:) .

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How efficient are LEDs really and how can you extend their lifespan?

How to measure their efficiency at home and increase the efficiency, as well as increase the durability of LED lamps?

To answer all these questions, it is enough to conduct several visual experiments, without using any complex laboratory instruments.
LED is one of the most efficient and easiest to use light sources. However, at the same time, it still wastes most of the energy consumed, converting it not into light, but into heat.

Of course, there is no need to compare LEDs with a regular light bulb; here they have run far ahead. But how high do you think their real efficiency is?

How to measure LED efficiency

Let's check this live, not by the labels on the packages and data from tables on the Internet, but by the colorimetric method at home.

If you lower an LED into water and measure the temperature difference before it turns on and some time after, you can find out how much energy from it will turn into heat.

Knowing the total amount of energy expended and energy lost into heat, you can really find out how much benefit there is from this source light turned into light.

The container in which measurements will be made must be insulated from temperature fluctuations outside and inside. A regular thermos flask is suitable for this.

With some modification, you will have a completely usable homemade colorimeter.

To isolate and prevent current leakage, all wires and terminals on the LED should be coated with a thick layer of electrical insulating varnish.

Before the experiment, pour 250 ml of distilled water into the flask.

Place the LED in water until it completely covers it. In this case, the light should come out freely.

Turn on the power and start counting down the time.

After 10 minutes, turn off the voltage and measure the water temperature again.

At the same time, do not forget to mix it well.

Now you need to repeat the experiment, but this time, tightly seal the matrix with some opaque material. This is necessary so that energy cannot leave the system in the form of light.

The experiment with the sealed specimen is repeated again in the same sequence:

• 250ml distilled water

• initial temperature measurement

• 10 minutes of “glow”

• final temperature measurement

1 of 4

After all measurements and experiments, you can proceed to calculations.

Efficiency calculation

Let's say for this model the average consumption of the light source is 47.8 W. Operating time – 10 minutes.

If we substitute this data into the formula, we find that over a period of 600 seconds, 28,320 J were spent on lighting the LED.

In the case of the sealed model, the water heated from 27 to 50 degrees. The heat capacity of water is 4200 J, and the mass is 0.25 kg.

Another 130 J per degree was spent heating the bulb, plus you need to add energy to heat the LED itself. It weighs 27 grams and is mainly composed of copper. The result is a figure of 27377 J.

The ratio of energy released and energy expended will be 96.7%. That is, more than 3% is missing. This is precisely heat loss.

In the case of an open LED, the water heated from 28 to 45 degrees. All other variables remained the same. The calculation here would look like this:

What conclusion can be drawn from all these experiments and calculations?

As can be seen from this small experiment, about 28% of the energy left the system directly in the form of light. And if we take into account 3% of heat losses, then only 25% remains.

As you can see, before ideal sources light, as many sellers present them, LEDs are still very far away.

Even worse, there are often models on the market that are extremely low quality with even lower efficiency.

Brightness and power

Let's now compare the brightness different models and let's see what it depends on and whether we can somehow influence it. To make a reliable comparison, use a regular piece of pipe and a lux meter.

Let’s say a previously tested high-quality sample provides illumination of 1100 lux. And this is with a power consumption of 50 W.

And if you take more cheap model? The data may turn out to be two times lower - less than 5500 Lux.

And this is with the same power! It turns out that you will pay the same amount for light as in the first case, but you will receive it 50% less.

Is it possible to get 3 times more light while spending as little energy as possible?

It is possible, but for this you will need an LED operating in a slightly different mode. To understand how to do this, you need to take some more measurements.

First of all, you should be interested in the dependence of brightness on power consumption. Gradually increase the power and monitor the lux meter readings.

As a result, you will reach such a nonlinear relationship.

If it were linear, you'd get something like this.

It will turn out even more interesting if you calculate the relative efficiency of the LED, taking the power value of 50W as 100%.

You can see how its effectiveness is deteriorating. This deterioration with increasing power is inherent in all LEDs. And there are several reasons for this.

Why LED Efficiency Deteriorates

One of them is, of course, heating. With increasing temperature, the probability of photon formation in the p-n junction decreases.

In addition, the energy of these photons decreases. Even with good cooling housing, temperature p-n junction may be tens of degrees higher, since it is separated from the metal by a sapphire substrate.

And it doesn't conduct heat very well. The temperature difference can be calculated by knowing the dimensions of the crystal and the heat generated on it.

With a heat release of 1 W, taking into account the thickness and area of ​​the substrate, the junction temperature will be 11.5 degrees higher.

In the case of a cheap LED, everything is much worse. Here the result is more than 25 degrees.

High junction temperatures lead to rapid degradation of the crystal, shortening its service life. This is where blinking, blinking, etc. occurs.

I wonder if manufacturers are unaware of this difference in temperature or are they deliberately creating doomed devices?

Often, components that seem to be in normal, expensive lamps operate in extreme conditions, at maximum temperatures, without any safety margin.

As long as the current is small, it is not noticeable. But due to the quadratic relationship, as the current increases, more and more of the energy turns into useless heat.

How to increase efficiency

That is, connect another LED in parallel, thereby halving the resistance losses. And this method certainly works.

By connecting two LEDs in parallel to the lamp instead of one, you will get more light with less energy and, accordingly, less heat.

Of course, this also extends the life of the LED.

You don’t have to stop and connect 3.4 diodes instead of one, it won’t be any worse.

And if there is not enough space for several LEDs, then you can install an LED originally designed for high power. For example, a 100-watt, 50-watt lamp.

It is in this way that the efficiency of the lamp can be increased several times, with the same energy consumption as the original source, but with less power, and operating at the limit of its capabilities.

Moreover, using no more than a third of the maximum power, you will forever forget what it means to replace burnt-out LEDs.

At the same time, their operating efficiency and efficiency will increase noticeably.

Therefore, when purchasing LEDs, always be interested in the crystal size. After all, their cooling and internal resistance depend on this.

The rule here is that the more, the better.

After writing the previous article, I myself still had an unanswered question - what exactly is more profitable to buy and how much you can win in the long and short term. Plus, there are still some uncertainties about the efficiency of LEDs. And the question encourages me to search for an answer to it, so I continued to develop this direction. I won’t say that the material turned out to be a full-fledged article, but as a supplement to the previous information, it contains essential data that will be useful.

First, let's figure out exactly what the efficiency of the LEDs discussed in the last part is. Previously, I took the data mainly from the iva2000 article, without checking it, because... there they considered more the issue of the efficiency of photosynthesis when illuminated with light of different spectrums. Now I decided to look into the overall efficiency.

We will consider LEDs from CREE, because... on the one hand, they are today the most advanced in technology and, accordingly, light output per unit of power, and on the other, all their indicators are stable and well documented (unlike no-name manufacturers). Here the specified company should pay me for advertising, but alas, I am not writing on their behalf, but simply because it is easier and more accessible.

So, what kind of LEDs will we study? I will not post here the entire process of studying and selecting specific series, so as not to flood the material with “water”. In short, I will say that I absorbed the most powerful and at the same time the most efficient chips, subject to free availability and favorable price. According to these criteria, two types are suitable: white ones will be from the XM-L series.

These are 10-watt chips with an efficiency of 158 lm/W (but not at maximum power, and only at 1 W). Cool white (6000-6500K), neutral white (4000-4500K) and warm white (3000-3500K).
And red ones from the XP-E series, High Efficiency Photo Red 650-670nM.
Links to LED documentation at the end of the article.

Let's deal with the whites. Last time, the difference in efficiency of white LEDs was not taken into account and the efficiency was assessed only in relation to the McCree photosynthetic activity curve.

This time I decided to clarify this issue more thoroughly. Unfortunately, the documentation for LEDs never gives the efficiency, but only lumens per watt, so I had to do a reverse calculation. Based on the spectrum of the LED and the photopic curve, it is calculated how many lumens the LED would have if its efficiency were 100%, and then the number of real lumens taken from the documentation for the LED is divided by this number. And this is what we got for three types of white LEDs:

It is noteworthy that despite the increase in lumens during the transition from cold-white to warm-white spectrum (at the same power radiation), table values lm/W and the overall efficiency of the LED drops very significantly - from 40 to 23%. The thing is that the phosphor, of which there is much more of a warm-white LED in a warm-white glow, does not itself have 100% efficiency, and even, apparently, when there is a large amount of it, it has a shading effect (the rays emitted by the lower layers are absorbed by those lying above and disappear ). At the same time, the lumen per watt indicator is used at a current of 2A (out of a maximum of three) - it can be seen that it drops from 140 at 350mA to 108 (for cool white). There is no such table in the Cree document - absolute lumens are given there at a given current, and the power must be calculated using data from the current-voltage characteristic graph. Here is the relevant data from the datasheet:

With them everything is a little simpler, because... The luminous flux is indicated not in luminas but in milliwatts. It is enough to divide the milliwatts of radiation by the watts of consumption and we get the efficiency with high accuracy! If only the LEDs would provide this data, 2/3 of the work wouldn’t have to be done!

They're already looming interesting options the use of individual LEDs and their combinations. Now let's recalculate the efficiency table taking into account the newly obtained data.

It can be seen that the red Photo-red are ahead of everyone by a large margin. But you can’t illuminate with pure red, so you need to combine it, and here there are options with white and blue. Let’s immediately note (I considered everything, but threw out what didn’t turn out promising) the combination of warm white and red. The low efficiency of warm white LEDs negates all the advantages of red ones. But cool whites are very good in this combination! They themselves have good efficiency, further enhanced by red LEDs, and the lack of the red spectrum is also covered by them. The combination of red and blue also looks good. Then there are just cold whites and HPS 1000, and the rest don’t really hold up. Well, let's see how it will look complete - with drivers.

Further, the logic of the calculations was based on the assumption that we want to get more photosynthetically active radiation for the same money, so all figures, including prices for LEDs and drivers, are given to the total value of phytoactive radiation of the lamp 100 µmol/s.

Color coding as in the previous table - to make it easier to understand where which LEDs are and not take up space with repeating headings.

But this is only the starting price - how much money you need to invest to get a 100 µmol/s light bulb. This is not enough - you need to see how much it will cost to operate. And if you also take into account the energy costs over time, then you get a complete picture, which I present for everyone to see!

Saved for history, updated below

Thanks to the close attention of commentators, it turned out that not all LEDs that are sold on Aliexpress under the name CREE are actually LEDs. The cheapest of them, about $1.50 for a 10-watt diode or less, are most likely counterfeits with manufactured chips Chinese company LatticeBright, which are several times cheaper than the original ones and, unfortunately, have approximately 2 times worse performance. In this regard, I searched for prices of the corresponding LEDs in the company Compel, which is the official distributor of cree in the Russian Federation. Prices there are much higher than in China, but small wholesale is quite profitable, including compared to foreign suppliers.
And along the way, I corrected two points - I added lamp replacement once a year for the HPS curve. And I corrected an error (my oversight), due to which the price of all lamps was calculated at the same power (100W), whereas the original idea was per unit of photoactive radiation. In the new chart, these prices are for a lamp emitting 100 μmol/s, not 100 W. I apologize for the oversight.

How to make sense of this bundle of twigs?

On the left is the price of the lamp at the start. Let me remind you that in this case they will all emit the same amount of phytoactive radiation, but have a different spectrum. The lower the bar starts, the cheaper the set. On the X axis we have months. It is assumed that the lamp operates 12 hours a day, 7 days a week, for a total of 36 months, i.e. 3 years. This is only a little more than 13 thousand hours, and for LEDs 50 thousand are stated. And if everything is done correctly with cooling, and the LEDs are also supplied with a current of 0.7 of the maximum (this means more efficiency by a whole third), then they will work even more , i.e. more than 10 years with virtually no degradation.

The more horizontal the line is, the greater the efficiency of the lamp. We see that many lines start higher (more expensive chips), but over time they turn out to be cheaper than cheaper analogues. The line for photo red LEDs is indicative of this - it has the smallest slope.

The most surprising thing is that the cheapest ones are now... The most expensive photo red LEDs! This is because they have the most high efficiency and the most “easily digestible” spectrum - they need the least amount at the beginning and they consume the least amount of electricity in the future! The combinations “Cold white + red photo red” are of great interest. On this chart The curve is shown with a ratio of white: red as 2:1 in power. And just “cold white”. These three lines fan out, where the outer ones are white and red LEDs, and the middle one is a combination of them. To grow plants, all components of the spectrum are needed, but in different combinations. It turns out that all options for combinations of spectra are most effectively covered by just one combination - cold white and red LEDs (but in different numerical ratios).
It is worth noting that the blue + red combination, although it has a lower slope than white + red, gives a significantly worse price/luminous flux indicator, so it does not catch up with the white + red combination even in 3 years. In a 10-year perspective it may be preferable, but this is an exceptional case.
The phytolamp turns out to be not so cheap. If you take into account its efficiency, it is more expensive than even cool-white LEDs, and in the long term... Money for electricity is a waste...
DNAT is not very cheap at first (I was surprised how much electronic ballasts for them cost, but Em It’s not worth taking ballasts - they have low efficiency, the lamp too due to flickering, they also hum and heat up like a stove) and over time they do not catch up - especially taking into account the replacement of lamps - which will have to be done at least once a year, which is displayed as steps on the graph. So off to the garden.

Here is the spectrum of a combination of white and red LEDs, superimposed on the MkCree curve (4:1 in power, did not change it to 2:1):

Of course, it is wrong to judge such things based on the beauty of the graphs, but given the numbers that say the same thing, in my opinion the graph is almost ideal in terms of covering the spectrum of the photosynthetically active range.

The conclusion remains the same - buy cool white LEDs and red CREE Photo red and you will have a lot of light for your plants and savings for your wallet!
It is also possible to illuminate with pure red LEDs; one of the commentators wrote about such an experience. This will be most appropriate if the plants are partially illuminated by natural light (vegetable garden on a windowsill, balcony, loggia, when direct sunlight does not hit at all or for a couple of hours a day - then the plants receive mainly blue rays from the sky, and they are sorely lacking in red ones, just like total intensity Sveta. Here red LEDs will fill the existing gap perfectly. Only these should be highly efficient LEDs with a radiation wavelength of 660 nM and it is better if they are CREE Photo red. That's it, I'm off to order diodes!

By appropriately selecting the semiconductor material and additive, it is possible to specifically influence the characteristics of the light emission of the LED crystal, primarily the spectral region of the emission and the efficiency of converting the input energy into light:

• GaALAs- aluminum gallium arsenide; It is based on red and infrared LEDs.
• GaAsP- gallium arsenide phosphide; AlInGaP - aluminum-indium-gallium phosphide; red, orange and yellow LEDs.
• GaP- gallium phosphide; green LEDs.
• SiC- silicon carbide; The first commercially available blue LED with low luminous efficiency.
• InGaN- indium gallium nitride; GaN - gallium nitride; UV blue and green LEDs.

To obtain white radiation with a particular color temperature, there are three fundamental possibilities:

1. Conversion of blue LED radiation by yellow phosphor (Figure 1a).

2. Conversion of UV LED radiation by three phosphors (similar to fluorescent lamps with the so-called three-band spectrum) (Figure 1b).

3.Additive mixing of red, green and blue LEDs (RGB principle, similar to color TV technology). The color hue of white LEDs can be characterized by the value of the correlated color temperature.

Most types of modern white LEDs are produced on the basis of blue ones in combination with conversion phosphors, which make it possible to obtain white radiation with a wide range color temperature- from 3000 K (warm white light) to 6000 K (cold daylight).

Operation of LEDs in power circuits

An LED crystal begins to emit light when current flows in it in the forward direction. LEDs have an exponentially increasing current-voltage characteristic. They are usually powered by constant stabilized current or constant voltage with pre-connected limiting resistance. This prevents unwanted changes in rated current that affect stability luminous flux, and in the worst case may even damage the LED.
At low powers, analog linear regulators are used; to power high-power diodes - network blocks with stabilized current or voltage output. Typically, LEDs are connected in series, parallel, or in series-parallel circuits (see Figure 2).

A smooth decrease in brightness (dimming) of LEDs is carried out by regulators with pulse-width modulation (PWM) or a decrease in forward current. Using stochastic PWM, it is possible to minimize the interference spectrum (problem electromagnetic compatibility). But in in this case with PWM, interfering pulsation of the LED radiation may be observed.
The amount of forward current varies depending on the model: for example, 2 mA for miniaturized panel-mount LEDs (SMD-LEDs), 20 mA for LEDs with a diameter of 5 mm with two external current leads, 1 A for high-power LEDs for lighting purposes. The forward voltage UF usually ranges from 1.3 V (IR diodes) to 4 V (indium gallium nitride LEDs - white, blue, green, UV).
Meanwhile, power circuits have already been created that make it possible to connect LEDs directly to a 230 V AC network. To do this, two branches of the LEDs are switched on anti-parallel and connected to standard network through ohmic resistance. In 2008, Professor P. Marx received a patent for a circuit for dimming LEDs powered by a stabilized alternating current(see Figure 3).
The South Korean company Seoul Semiconductors has integrated a circuit (Figure 3) with two anti-parallel circuits, (in each of which large number LEDs) directly in one chip (Acriche-LED). The forward current of the LEDs (20 mA) is limited by an ohmic resistor connected in series to the anti-parallel circuit. The forward voltage across each LED is 3.5 V.

Energy efficiency

Energy efficiency of LEDs (efficiency) is the ratio of radiation power (in Watts) to electrical power consumption (in lighting terminology, this is the energy output of radiation - i.e.).
In thermal emitters, which include classic incandescent lamps, to generate visible radiation (light), the coil must be heated to a certain temperature. Moreover, the main share of the supplied energy is converted into thermal (infrared radiation), and only ?e = 3% for conventional ones is transformed into visible radiation, and 7% for conventional ones. halogen lamps incandescent

LEDs for use in applied lighting convert the supplied electrical energy into visible radiation in a very narrow spectral region, and thermal losses occur in the crystal. This heat must be removed from the LED using special design methods in order to ensure the necessary light and color parameters and maximum service life.
LEDs for lighting and signaling purposes have virtually no IR and UV components in the emission spectrum, and such LEDs have significantly higher energy efficiency than thermal emitters. With favorable thermal conditions, LEDs convert 25% of the supplied energy into light. Therefore, for example, for a white LED with a power of 1 W, approximately 0.75 W is due to thermal losses, which requires the presence of heat-dissipating elements or even forced cooling in the design of the lamp. Such management of the thermal regime of LEDs is of particular importance. It is desirable that manufacturers of LEDs and LED modules provide energy efficiency values ​​in the list of characteristics of their products

Thermal mode control
Let us remember that almost 3/4 of the electricity consumed by an LED is converted into heat and only 1/4 into light. Therefore, when designing LED lamps, a decisive role in ensuring their maximum efficiency is played by optimization of the thermal regime of LEDs, in other words, intensive cooling.

As is known, heat transfer from a heated body is carried out due to three physical processes:

1. Radiation

Ф = W? =5.669?10-8?(W/m2?K4)??A?(Ts4 – Ta5)
where: W? – thermal radiation flux, W
? – emissivity
Тs – surface temperature of a heated body, K
Ta – temperature of the surfaces enclosing the room, K
A is the area of ​​the heat-emitting surface, m?

2. Convection

F = ?? Huh? (Ts-Ta)
where: Ф – heat flow, W
A is the surface area of ​​the heated body, m?
? – heat transfer coefficient,
Тs – temperature of the boundary heat-removing medium, K
Ta – surface temperature of a heated body, K
[for unpolished surfaces? = 6...8 W / (m? K)].

3. Thermal conductivity

Ф = ?T?(А/l) (Тs-Та) =(?T/Rth)
where: Rth= (l / ?T?A) – thermal resistance, K/W,
Ф – thermal power, W
A – cross section
l-length - ?T – thermal conductivity coefficient, W/(m?K)
for ceramic cooling elements?T=180 W/(m?K),
for aluminum – 237 W/(m?K),
for copper – 380 W/(m?K),
for diamond – 2300 W/(m?K),
for carbon fibers – 6000 W/(m?K)]

4. Thermal resistance

The total thermal resistance is calculated as:

Rth par.total=1/[(1/ Rth,1)+ (1/ Rth, 2)+ (1/ Rth,3)+ (1/ Rth,n)]

Rth afterword = Rth,1 + Rth, 2 + Rth,3 +....+ Rth,n

Resume
When designing LED luminaires, every possible measure must be taken to alleviate the thermal behavior of the LEDs through conduction, convection and radiation. Therefore, the primary task when designing LED lamps is to ensure heat removal due to the thermal conductivity of special cooling elements or the housing design. Then these elements will remove heat by radiation and convection.
The materials of the heat sink elements should, if possible, have minimal thermal resistance.
Good results were obtained with heat-removing units of the “Heatpipes” type, which have extremely high heat-conducting properties.
One of the best heat sink options is ceramic substrates with pre-applied current-carrying paths, directly to which the LEDs are soldered. Cooling structures based on ceramics remove approximately 2 times more heat compared to usual options metal cooling elements.
The relationship between the electrical and thermal parameters of the LED is illustrated in Fig. 4.
In Fig. 5 shows a typical design powerful LED with an aluminum cooling element and a circuit of thermal resistances, and in Fig. 6-8 – various methods cooling.

Radiation

The surface of the lighting fixture on which the LED or module with several LEDs is mounted should not be metal, since metals have a very low emissivity. The surfaces of luminaires in contact with LEDs should, if possible, have a high spectral emissivity?.

Convection

It is desirable to have a sufficiently large surface area of ​​the lamp body for unhindered contact with ambient air flows (special cooling fins, rough structure, etc.). Additional heat removal can be provided by compulsory measures: minifans or vibrating membranes.

Thermal conductivity

Due to the very small surface area and volume of LEDs, the necessary cooling by radiation and convection is not achieved.

Example of calculating thermal resistance for a white LED

UF= 3.8 V
IF = 350 mA
PLED = 3.8 V? 0.35 A = 1.33 W
Since the optical efficiency of the LED is 25%, only 0.33 W is converted into light, and the remaining 75% (Pv=1 W) is converted into heat. (Often in the literature, when calculating the thermal resistance RthJA, they make the mistake of assuming that Pv = UF ? IF = 1.33 W - this is incorrect!)

Maximum permissible temperature active layer (p-n junction – Junction) TJ = 125°C (398 K).

Maximum ambient temperature TA = 50°C (323 K).

Maximum thermal resistance between barrier layer and surroundings:

RthJA= (TJ – TA)/ Pv = (398 K – 323K)/1 W = 75 K/W

According to the manufacturer, the thermal resistance of the LED

RthJS = 15 K/W

Required thermal resistance of additional heat-dissipating elements (cooling fins, heat-conducting pastes, adhesive compounds, board):

RthSA= RthJA – RthJS = 75-15 = 60 K/W

In Fig. 9 explains the thermal resistances for the diode on the board.
The relationship between the temperature of the active layer and the thermal resistance between the blocking (active) layer and the solder point of the crystal leads is determined by the formula:

TJ= UF ? IF? ?e? RthJS + TS

where TS is the temperature measured at the solder point of the crystal leads (in this case it is equal to 105°C)

Then, for the example under consideration with a white LED with a power of 1.33 W, the temperature of the active layer will be determined as
TJ = 1.33 W? 0.75? 15 K/W + 105°C = 120°C.

Degradation of emissive characteristics due to temperature load on the active (blocking) layer.
Knowing the actual temperature at the solder point and having data provided by the manufacturer, it is possible to determine the thermal load on the active layer (TJ) and its effect on radiation degradation. Degradation refers to the decrease in luminous flux over the life of the LED chip.

Effect of barrier layer temperature
Fundamental requirement: the maximum permissible temperature of the blocking layer should not be exceeded, as this can lead to irreversible defects of the LEDs or spontaneous failures.
Due to the specific physical processes occurring during the operation of LEDs, changing the temperature of the blocking layer TJ within the range of permissible values ​​affects many LED parameters, including forward voltage, luminous flux, chromaticity coordinates and service life.