As always, I'll start by saying that I'm a techno-fetishist. Those. I love gadgets and unusual solutions. Therefore, some of my decisions do not have significant fundamental reasons for occurring in my system. And, perhaps, this is one of such decisions.
At the same time, I still want to note the fact that significant fundamental reasons for such a decision may well exist in other systems. For example, cold seas, large farms where energy consumption for lighting is significant, conceptual design solutions where you need to hide everything unnecessary, etc. But more on that later. Therefore, I think that it is also not worth treating this decision only as a toy.
Well, I think I’ve cleared away the fog of mystery and we can get to the point. So we will discuss light. Or rather, the light is based on powerful LEDs. Moreover, not his “light” side, but the “dark” side, the one to which the attention of the average person, and even professionals, is least devoted when it comes to him. Namely, cooling. Those. we will only talk about the released thermal energy, which is a by-product of lighting and must be recycled.
Efficient heat dissipation from LEDs is essential. Their overheating first leads to a decrease in efficiency, and then to destruction, since destructive processes, harmful electron migration, etc., are activated in a warm semiconductor. Moreover, the effect can be collapsing. Those. The hotter the LED, the greater its resistance, the more it heats up.
This forces manufacturers to supply LED lamps massive radiators. If the workmanship is poor, the diode overheats and may fail within a year or two. Also, rapid failure is caused by poor quality of the diode itself, since internal inhomogeneity and the presence of impurities lead to the formation of hot spots with all the ensuing consequences. Typically, an LED loses its properties gradually, year after year the brightness deteriorates and heat dissipation increases.
From all of the above, a simple conclusion is drawn: if you want light with stable characteristics and a durable lamp, firstly, you need good cooling.
But even having achieved effective cooling, the question still remains - heat recovery. Typically, heat is removed by air flow from radiators and simply dissipated in the room. The benefits of such recycling are, frankly speaking, not obvious. On the other hand, we spend money on heating the aquarium. For example, my assemblies, and there are four of them per area of 600x600mm (~200l), generate about 80W of heat. Agree, if I can effectively transfer this heat to the jar, I can save about 30% of the energy needed to heat it (taking into account the fact that at night I have to rely only on the heater). Or even 50%, if I want the fish to simulate a temperature difference between day and night. Not a bad addition to your pension. And even more so it becomes noticeable where we are talking about kilowatts!
I will also touch on another problem of heat dissipation using classical methods - the radiator must be in direct contact with the LED, which is in close proximity to the aquarium. Those. we can say that heat dissipation occurs directly above the aquarium. Why is this bad? In a simple case, in the summer, we must additionally spend energy on removing this excess heat, which in one form or another is transferred to the aquarium. The problem is even more noticeable in cold systems, where the temperature must be maintained at 4 degrees Celsius. There a real struggle unfolds and there is a significant overconsumption of electricity for cooling.
Now about the beautiful things. Integrating an aquarium with powerful lighting into the design is often a big challenge. The aquarium has clear requirements for its support, severely limiting design ideas. One such limitation, of course, is light. After all, it requires cooling, and therefore effective air convection. Simply put, the designer must take into account that it will be necessary to leave some space above the can to accommodate the impressive dimensions of the lamp.
Well, and perhaps the most important thing: the light power under classical lighting is severely limited. And this imposes restrictions on the depth and width of the aquarium, forcing certain proportions to be observed. And even though this is a trifle for apartment systems, for species aquariums, and even those burdened with design, this can be a real problem.
How can all the above problems be solved? Don’t even hope, we won’t reinvent the wheel. Man has long come up with effective means of heat removal for high-energy systems. This liquid cooling. Or water cooling systems, if water is used as a coolant. Briefly - SVO. You can meet such systems in everyday life quite often. They are, for example, used in cars to cool the engine. Heating radiators are, in principle, the same system that allows heat to be transferred from the boiler room to our house through a liquid coolant.
Therefore, we can safely say that such systems have proven themselves. The experience of their use goes back hundreds of years.
Moreover, such systems are successfully used to cool heavy-duty computers, including at home. This is where we come to the essence of the idea. The fact is that it is the components of such cooling systems that are best suited to implement a similar system in an aquarium for cooling LEDs.
Let's figure out what such a system consists of?
The heart of the system is the pump. Almost literally. It forces the fluid inside the system - the coolant - to move.
Radiator. There's nothing tricky here. It dissipates the heat that the coolant has accumulated. The dimensions of the radiator can be truly impressive! Of course, when implementing small systems, their compactness is called into question, but when creating large ones, the mere fact that the radiator can be moved outside the viewing area already makes the system ultra-compact for the observer.
A water block is precisely that element of the system that is capable of removing thermal energy from the source and transferring it to the coolant. Perhaps the most technologically advanced thing in such systems. It comes to the point that some amateurs make water blocks from silver, polishing the base to a mirror, and all in order to not leave a single watt of thermal energy at the source.
Thus, the heat received by the water block is transferred to the coolant, the pump pumps it into the radiator, where the heat is dissipated.
It goes without saying that the radiator, pump and water block can be located at a significant distance from each other. And this solves all the problems we mentioned above.
Let us separately return to the problem of useful heat recovery. By replacing the radiator in this system with a heat exchanger, which we immerse in the water of the aquarium, we will be able to transfer thermal energy specifically to the aquarium. Titanium heat exchangers are relevant for marine aquariums. This metal does not corrode in salt water. It is unacceptable to use other metals for this purpose!
Fortunately, there are solutions for this case too. As an example, we can take a heat exchanger designed for freon cooling systems. Well, for example, like the one in the picture. At the time of writing this article, you could buy it on the website http://www.fish-street.com/
For starters, light. My lights are based on DNK assemblies. Here they are, in the picture, in all their glory.
You can get acquainted with their characteristics on the manufacturer’s website.
The diode field size is about 40mm. Those. we are not talking about the plate itself, but about the area occupied by the diodes themselves. The plate actually plays the role of a heat distributor and, as planned by the manufacturer, should be attached to the radiator with four screws. Yes, in general, you can read all this on the manufacturer’s website. There is even a film about how to assemble a lamp. I won't dwell on it.
Those. in fact, I needed a water block that was 40x40mm or larger. After searching the Internet, I realized that in Russia the cost of water blocks is unacceptable for me. And I went to ebay. Just enter the word “waterblock” in the line and you will get a lot of options. Personally, I chose the cheapest and, accordingly, the most ineffective - water blocks made of anodized aluminum. At the same time, their performance is quite sufficient for my task. The cost of one block is approximately $4.
I needed four of them, but I ordered five. One in reserve, because Even the photo shows that the quality of welding leaves much to be desired. Suddenly it will leak...
The advantage of these water blocks is that they have a simple shape, as well as side fittings, which will make the structure thinner.
Their size exactly matches the needs - 40x40x12mm. 8mm fitting.
In fact, water blocks are the first and most important step for creating a water cooling system. Foundation. It is here that you understand how much heat will need to be removed, whether the water block can handle it, and the requirements for the cross-sectional diameter of the hoses are also formed. In this case, the outer diameter of the fitting is 8mm. And I had to select other components based on this diameter.
The next step is choosing a radiator. You need to understand how much heat will be collected by the water blocks to determine the heat transfer requirements for the radiator. Accordingly, take the one that can dispel it or more. I chose this one for myself.
It has a large reserve of heat dissipation. But more is better than less. And the main thing is that with such a radiator size, you can, if desired, abandon active cooling. Those. do not use coolers to blow it out. Of course, you need to take into account that with passive cooling, the radiator must be installed horizontally, and also that air must pass through it unhindered.
The radiator fittings are 8mm in diameter. The cost is approximately $25.
Now, knowing the number of water blocks, the size of the radiator and, in general terms, the length of the coolant line, you can choose a pump. It’s difficult to say what to rely on first when choosing a pump. Moreover, the characteristics indicate such indirect things as lifting height and pumping volume per hour. Requirements for them arise empirically. But the larger the radiator, the more water blocks, the longer the hoses, the more powerful the pump should be. For such a radiator, I recommend using a pump with a lift of at least 3 meters and a flow rate of at least 300 l/h. Personally, I chose this one.
Its characteristics:
Consumption: 500 l./h.
Lifting height: 3 m.
Power: 12 V.
Power: 10 W.
Noise: 16 dB.
Expansion tank: 250 ml.
Fittings: 8 mm.
I would like to especially note the presence of an expansion tank in this pump. Like any liquid, coolant tends to expand when heated. And he needs a place for this. If you take a pump without a tank, you will need to implement it in a different form. To do this, you can buy a separate expansion container or make one out of plastic containers. But there must be a container, otherwise you may encounter the fact that when heated, the increased volume of the liquid will rip the hoses from the fittings, and a flood will result. An unpleasant thing in every way.
You also need to look at the noise level. It is very small for this pump. Less than a cooler. Remember that sellers often underestimate this figure. Therefore, try to pay attention to the quality of the pump in order to verify the veracity of such indicators. In my case, the pump visually has a good engineering design. There is a silicone mount and a grid to prevent uncontrolled water fluctuation. Those. It is clear that they have worked on the noise issue. Cost matters here too. This pump cost me about $30.
Now it was necessary to solve the issue of combining all elements into a single system. I highly recommend silicone hoses for this. You can buy them from companies that supply medical products (see links at the end of the article). It cost me about 300 rubles for five meters. Or about $5 at the exchange rate at that time.
I ordered all this in November 2014. and waited until about mid-December of the same year. While the wait dragged on, I began working on the engineering design.
For myself, I set a goal - to place a lamp in the lid of the aquarium. The height of the lamp should not exceed 50mm. Plus, I wanted to have easy access to the entire serviceable area of the aquarium without having to drag the light around.
While experimenting in AutoCAD, I came up with the following concept design.
The design was simple to manufacture. Minimum elements:
1. Guides, for which I decided to use aluminum tubes in heat shrink. Heat shrinkage is needed to prevent metal corrosion when exposed to moisture. Jumpers, as planned, serve two functions at once: they are used as an organizer for wires and hoses; fasten the structure. Made from acrylic.
2. Driver housings. They are at the center of the structure. This is where I planned to place the controller and drivers. They also had to be acrylic. Only one wire had to go to the lamp. Unfortunately, life decreed otherwise, but more on that later.
3. Four light modules, the same acrylic. Their design is complex, consisting of several layers. In the drawing (top row) you can see their layer-by-layer design, as well as the dimensions of other parts of the lamp and the required number of blanks.
This entire structure is “floating”. Those. modules can move along guides, and the entire lamp can be “rolled up” like a caterpillar. This solves the problem of both easy access to the aquarium and adjusting the location of light sources for optimal illumination of corals.
I ordered the production of blanks from Laser Center. A week later they were given to me. All this cost me about $50. Here's what happened:
A couple of weeks later the components arrived at SVO and I began putting the lamp together.
To my great regret, I did not demonstrate outstanding gluing skills and the gluing itself turned out to be “dirty.” I glued it with acrylic glue (acrylic shavings dissolved in dichloroethane).
The design is collapsible. The upper part is attached with screws. I cut the threads directly into acrylic. In principle, to simplify things, you can make through holes and fasten them with a pin.
After preparing the lamp module for assembly, we could begin the assembly itself. First, we needed to apply thermal paste to the water blocks and LED assemblies.
This is a very important and necessary stage. For effective heat removal, it is necessary that the areas of the assemblies and water blocks are in maximum contact with each other. In case of poor-quality contact, local overheating will form in such places, which will negatively affect the achievement of set goals.
If possible, distribute the thermal paste as evenly as possible over both surfaces. I used some leftover pasta from my supplies. It turned out to be dried out and because of this it was difficult to apply it perfectly. But fortunately, the assembly you see in the photo is from an old lamp. It already had a layer of thermal paste applied. This made my task somewhat easier. I recommend leveling the thermal paste with a plastic card, using it as a spatula.
Next, the two surfaces need to be pressed against each other with force and slightly twisting the water block left and right to achieve uniform distribution of the thermal paste between the parts. Evidence of this will be the protrusion of thermal paste along the edges of the water block, as well as very noticeable gluing of the parts.
Now, it was necessary to place the resulting “sandwich” into the body. Although the body had very adjusted dimensions, there was still some play. It was also necessary to achieve compression of the “sandwich” from above and below when assembling the module.
I decided to make silicone cushions that would press the structure inside. To do this, I applied silicone in five places on the water block mounting side and in four on the assembly side.
When assembling, I left a gap of a couple of millimeters.
After waiting for the silicone to harden, I tightened the screws all the way. This is what I got after four hours of waiting.
Everything was great and wonderful and I started the first tests. As you probably remember, in the first version I wanted to place the drivers in acrylic housings. Unfortunately, practice has shown that this was not the best idea. They overheated there. In general, it was predictable, because... I did not provide for heat dissipation in any way. I didn’t take a photo of this disgrace, because... To be honest, I was upset. I had no time for them. Later you will see the result on one of the drivers.
Time was pressing for me, because... I assembled the lamp for the launch of a new aquarium. I decided to radically change the concept of driver placement, placing them in a separate aluminum housing. That's how it happened.
The objectives are solved due to the fact that in a liquid-cooled LED lamp containing a housing made of heat-conducting material, made in the form of a hollow container filled with a dielectric liquid, with ribs installed outside along the perimeter, hermetically sealed with a lid and a hole for filling liquid, a light source, a diffuser made of glassy material, heat-generating light sources are mounted in the center of the outer part of the housing base, the internal volume of the housing is divided into compartments by two longitudinally oriented strips made of material with low thermal conductivity, installed with gaps relative to the end walls of the housing, the lid is equipped with ribs located at an acute angle to the longitudinal axis products The use of liquid as a coolant allows for reliable thermal contact with all heat-generating components of the lamp, regardless of their geometric shape and location relative to the body, which contributes to the durability of its operation.
The utility model relates to lighting technology, in particular to lighting devices based on high-power LEDs with a cooling device, intended for street, industrial, household and architectural design lighting.
LED lighting devices have a long service life, a high level of safety, compactness, and a number of other positive properties; but they have serious problems in organizing heat removal from LEDs. When using high-power LEDs, there is a risk of overheating of the LEDs during operation, and this leads to a decrease in the service life and reliability of the lamps.
There are various ways to dissipate heat in LED lighting devices.
LED lamps are known (PM 80156, 85982, 110816), in which the LEDs are placed in a metal housing to ensure thermal contact with it, while the housing simultaneously serves as a heat sink. In addition, to increase the dissipated thermal power of the LEDs, the cooling radiator can have forced surface cooling, for example, using a fan (I 2313199, N05V 33/02)/
A cooling device (RU, 104412 U1, N05K 7/20) is known, designed to prevent overheating of heat-producing components (transistors, diodes, capacitors, chokes, transformers, etc.) during their operation. The cooling device contains a housing with a lid filled with a coolant, which is used as transformer oil with regulated electrical insulating properties, or silicone liquid with electrical insulating properties.
Fuel-generating elements are components of electronic ballasts for gas-discharge or LED lamps.
There is a known light device with a passive cooling device (RU, 113555 U1, F21S 8/00), in which the housing is made in the form of a hollow sealed container filled with a dielectric liquid, closed at the bottom with glass for light output; a volumetric heat insulator is installed inside the container with gaps relative to its walls with a central channel formed by a tube made of a material with high thermal conductivity, and the LEDs and power supply are placed inside the container with gaps relative to the plane of the glass and the lower surface of the heat insulator.
The closest to the proposed utility model is the LED lighting device (RU, 103596 U1, F21S 10/00), taken as a prototype. The lighting device contains a heat transfer system made in the form of a sealed chamber, partially filled with a coolant (for example, water, alcohol, acetone, etc.). A heat-removing metal substrate with LEDs is mounted on the lower outer surface of the chamber. The side surface of the chamber is equipped with ribs and serves as a radiator. To circulate the coolant, either gravitational forces or the capillary structure of the inner surface of the chamber are used, depending on the location of the metal substrate with LEDs.
The disadvantage of the described designs is the possibility of overheating under certain weather conditions, and, as a result, failure of the lamp. And in the latter version, in addition, there is a complex design of the internal part of the housing, namely, the capillary structure.
The objectives of the proposed utility model are:
Simplifying the design of LED lighting device,
Increasing the efficiency of heat removal from LEDs,
Increasing the service life of the lighting device,
Improving the performance properties of the lamp when used for street lighting due to the possibility of using high-power LEDs.
The objectives are solved due to the fact that in a liquid-cooled LED lamp containing a housing made of heat-conducting material, made in the form of a hollow container filled with a dielectric liquid, with ribs installed outside along the perimeter, with a hermetically sealed lid and filler hole,
heat-generating light sources are mounted in the center of the outer part of the housing base and covered with a protective cap made of glassy material, the internal volume of the housing is divided into compartments by two longitudinally oriented strips made of a material with low thermal conductivity, installed with gaps relative to the end walls of the housing, the lid is equipped with ribs located at an acute angle to the longitudinal axis of the product
A distinctive feature of the proposed utility model is the constructive integration of the base of the heat-generating elements, the radiator housing, and the heat-removing liquid into a single cooling system in such a way that heat from the LEDs is removed through the aluminum base (or additionally through a printed circuit board), and then to the radiator housing through the heat-removing liquid .
The body is monolithic made of heat-dissipating material with fin plates installed on three sides along its perimeter to increase the area of the cooled surface.
It is possible to increase the power of the lamp by increasing the number of housings by connecting using side joining elements.
The cover is designed to be hermetically installed on the body, and, just like the side walls of the body, it is equipped with finning plates, which are located vertically at an acute angle to the longitudinal axis of the product.
The location of the fin plates in a vertical plane allows any air flow to participate in cooling, and the location at an angle to the surface ensures the possibility of self-cleaning of the plates from various types of sediment. It is known that air removes heat better from a clean surface.
If the light source is mounted not just on the aluminum base of the housing, but through a printed circuit board, then the printed circuit board is also made of an aluminum plate, is rigidly mounted in the center of the outer part of the housing base, and is oriented in the longitudinal direction (i.e., located opposite the middle compartment of the internal cavity of the housing).
The light source is represented by an LED module, in which light-emitting elements are combined into a line of series-connected LEDs. Optical lenses can be installed on top of each LED, depending on the required characteristics.
The light source is protected by an optically transparent diffuser made of glassy material. The light diffuser is a very important element of the lamp - it determines the quantitative and qualitative light flux, and, accordingly, the quality of lighting.
The use of liquid as a coolant allows for reliable thermal contact with all heat-generating components of the lamp, regardless of their geometric shape and location relative to the body, while there is no mechanical load on the printed circuit board, which contributes to the longevity of its operation. The heat capacity of liquid is higher than the heat capacity of air. The heat-conducting liquid provides convection heat transfer, which is absent in the case of heat removal with solid gaskets or compounds.
Water, alcohol, acetone, etc. can be used as a heat-removing liquid.
In addition, in the winter season, antifreeze can be used as a heat-conducting liquid.
The utility model is illustrated by the following drawings:
Figure 1 is a general diagram of a liquid-cooled LED lamp;
Figure 2 - outer surface of the base of the LED lamp;
Fig.3 - LED lamp cover (top view)
Liquid Cooled LED Light contains a housing (1) made of heat-conducting material, equipped with heat-dissipating fins-heat exchangers (2) installed on three sides along its perimeter, a printed circuit board (3) with LEDs (4), mounted in the center of the outer part of the base of the housing (5) with orientation in the longitudinal direction. The internal volume of the housing is divided into 3 compartments (6) by two longitudinally oriented strips (7) made of material with low thermal conductivity, installed with gaps relative to the end walls of the housing, and filled with heat-conducting liquid (8). The housing is hermetically sealed with a lid (9) and equipped with a hermetically sealed hole (10) for pouring liquid. The cover is equipped with ribs located at an acute angle to the longitudinal axis of the product (11). From the outside, the light source is protected by an optically transparent diffuser made of glassy material (not shown in the drawings).
A liquid-cooled LED lamp works as follows.
When voltage is applied, LEDs emit light energy accompanied by heat.
Heat transfer from operating LEDs occurs to the board and then through the base of the case (or directly through the base of the case) and the heat-removing liquid, which circulates freely inside the case. The liquid heats up faster in the middle compartment, since this is where the fuel-generating elements are located. In accordance with the laws of the process of convective movement of liquid and due to the inclined position of the lamp in the working position, its more heated layers move along the central compartment from the base to the opposite end, where they are evenly distributed among the side compartments. Here the flow speed decreases, which leads to intense heat transfer from the housing to the air flow.
The heat-dissipating liquid effectively and evenly removes heat from the printed circuit board and other heat-producing components of the lamp and transfers it to the walls and cover of the housing. The vertical arrangement of heat-dissipating plates-fins on the body and cover of the lamp enhances heat dissipation, as this facilitates the participation of all air flows in the cooling process. The arrangement of the heat-dissipating fins of the cover at an angle to the longitudinal axis of the product promotes self-cleaning from precipitation and dirt, and facilitates the care of the product during operation.
Thus, the implementation of the utility model solves all the problems set by the authors.
1. LED lamp with liquid cooling, containing a housing made of heat-conducting material, equipped with fins installed on three sides along its perimeter, and filled with heat-removing liquid, a light source represented by an LED line, protected by an optically transparent diffuser made of glassy material, a hermetically sealed lid and a hole for filling liquid, characterized in that the internal volume of the housing is divided into three compartments by two longitudinally oriented strips made of material with low thermal conductivity, installed with gaps relative to the end walls of the housing, the light source is mounted in the center of the outer part of the base of the housing directly or through a printed circuit board with oriented in the longitudinal direction, the cover is equipped with ribs located at an acute angle to its longitudinal axis.
2. Liquid-cooled LED lamp according to claim 1, characterized in that antifreeze is used as a heat-removing liquid.
Thanks to their high luminous flux and long lifespan (on the order of tens of thousands, or even hundreds of thousands of hours), LED lamps are a very competitive solution. However, many suppliers and manufacturers of LED luminaires have difficulties when working with new high-power LEDs (from 20 W). And a particularly common problem is the design of correct and reliable heat dissipation. An incorrectly selected thermal operating mode of the LED can lead to undesirable consequences. First of all, overheating can lead to LED failure. All CREE LEDs have a critical transition temperature of 150°C; exceeding this threshold will lead to burnout of the LED crystal and a long repair process.
Secondly, operation at elevated temperatures significantly reduces the service life of LEDs (Figure 1). The graph shows the dependences for three temperatures at the “solder point” of the LED: 55, 85 and 105°C. The graphs marked LM-80 show the time during which the tests were performed. Graphs marked TM-21 show the decline in luminous flux from the original level as a function of time. As can be seen from the graphs, at elevated operating temperatures, the service life of LEDs is significantly reduced: at 105°C, the service life of LEDs is 200 thousand hours less than at a temperature of 85°C.
The following LED parameters also depend on temperature:
The amount of luminous flux. Figure 2 shows the dependence of the relative luminous flux on temperature for LEDs from the CREE series. As can be seen from the graph, as the LED junction temperature increases, the luminous flux decreases, and vice versa - with good cooling, the flux increases.
Forward voltage drop. As the temperature changes, the forward voltage drop across the LED (Vf) also changes. As the temperature increases, the voltage decreases. The amount of voltage change depends on the specific model. Table 1 shows the coefficients of voltage versus temperature for the MKR and MKR2 series of LEDs. It is important to take into account the value of this parameter and select a driver for the lighting system so that it can provide the required voltage over the entire operating temperature range of the LED.
Table 1. Values of voltage versus temperature coefficients for the MKR and MKR2 LED series
As can be seen from the graphs (Figures 1, 2), at temperatures below 100°C the luminous flux decreases slightly, and at a temperature of 85°C it is 100%. Recently, LEDs have been tested at a transition temperature of 85°C, so at temperatures below 85°C the graphs show an increase in luminous flux. This temperature will be considered the operating temperature for CREE LEDs.
Rice. 1. Lifetime of XPG LEDs, depending on temperature
Rice. 2. Dependence of luminous flux on transition temperature using the example of an MKR series LED
Now let’s describe the method for calculating and selecting a heat sink for high-power LEDs. An LED, like any other electronic device, is not 100% efficient, which means that part of the power consumed is converted into heat. Modern LEDs have an efficiency of about 30...40%, that is, on average 60...70% of power consumption is converted into heat. For example, when using a 20-watt LED matrix, 12 watts of heat must be dissipated, which is quite a lot. CREE in its “XLampThermalManagement” document recommends using the assumption that 75% of the power consumed is converted into heat, this assumption allows you to play it safe when designing a heat sink. The power that needs to be dissipated can be calculated using the formula:
Pt—thermal power (W);
Vf - forward voltage drop across the LED (V);
If is the current through the LED (A).
Before describing the methodology for calculating the cooling system, let's say a few words about the theory of heat transfer.
The main contributions to the cooling of LED lamps are thermal conduction and convection.
Thermal conduction is the process of transferring heat from a more heated body to a less heated one. In luminaires, due to direct contact, heat is transferred from the LED to the printed circuit board, and then to the radiator, or, if the LED is installed directly on the radiator, then directly to the radiator. To calculate the amount of heat transferred due to thermal conductivity, you can use the formula:
(2)
Qcond is the amount of heat transferred through the material (W);
k is the thermal conductivity coefficient of the material (W/(m*K));
A is the area of intersection of materials through which heat passes (m2);
DT—temperature gradient (K);
Dx is the distance that heat travels (m).
Convection is transmission due to the movement of flows of liquids or gases. Usually in LED lamps this is the transfer of heat from the radiator to the environment (usually air). There are two options for convection: natural and forced. With natural convection, heat is transferred due to pre-existing air currents caused by temperature differences. In forced convection, the movement of liquid or gas flows is created by additional devices such as a fan, pump, etc.
The amount of heat dissipated by convection can be calculated using the formula:
Qconv is the amount of heat dissipated by convection (W);
h - heat transfer coefficient (W/(m 2 *K));
A is the surface area of the radiating element (m2);
DT is the difference between the temperature of the radiating element and the ambient temperature (K).
The main problem in calculating the amount of heat dissipated by convection is determining the coefficient h. The value of the coefficient h can vary significantly, depending on the geometry of the radiator, boundary conditions and other parameters. For example, with natural convection, the coefficient h is in the range of 5...20 W/(m 2 *K). And for systems with forced convection, the heat transfer coefficient can reach values of 100 W/(m 2 *K) with air cooling, and up to 1000 W/(m 2 *K) with liquid cooling. LED lighting usually uses natural air cooling; for calculations of such systems, the heat transfer coefficient can be taken equal to 10 W/(m 2 *K).
The LED cooling system can be represented as an equivalent circuit of thermal resistances connected in series and parallel. As an example to create an equivalent circuit, let's take a matrix of n LEDs mounted on a printed circuit board attached to a heatsink (Figure 3).
Rice. 3.
In this case, the equivalent circuit will consist of n thermal resistances “LED junction - contact” (indicated in the diagram as Qj-sp), connected in parallel. Then - from n thermal resistances “contact - printed circuit board” (Qsp-pcb). It is also necessary to take into account the thermal resistances between the printed circuit board and the thermally conductive material (Qpcb-tim), between the thermally conductive material and the heatsink (Qtim-hs) and, finally, between the heatsink and the environment (Qhs-a).
At the nodes of this equivalent circuit, the temperature can be measured, for example, at the Theatsink point, the temperature of the radiator can be measured.
If the lighting device uses only one LED, the equivalent circuit will be a chain of thermal resistances connected in series. In turn, the thermal resistance of the entire cooling system is the sum of all thermal resistances. For a lamp consisting of one LED installed on a printed circuit board and on a radiator, the thermal resistance of all cooling systems is calculated using the following formula:
The lower the total thermal resistance value, the better the heat is dissipated from the LED. The thermal resistance between elements a and b is calculated by the formula:
Qa-b is the thermal resistance between elements a and b (°C/W);
Ta—temperature of element a (°C);
Tb—temperature of element b (°C);
Pt is power calculated using formula 1.
In the documentation for its LEDs, CREE offers a graph of maximum current versus temperature. An example of such a graph is shown in Figure 4. Knowing the maximum current and the estimated ambient temperature, you can calculate the value of the power that needs to be dissipated, and, accordingly, you can obtain the value of the maximum thermal resistance of the cooling system, which will allow you to select a radiator and heat-conducting materials.
Rice. 4. Dependence of maximum current on temperature for MKR LEDs
Let's take a closer look at how elements such as a printed circuit board, heat-conducting materials and a heatsink contribute to the overall thermal resistance.
Printed circuit board. Most CREE LEDs must be mounted on a board (for LED power and mechanical wiring). The thermal resistance largely depends on the choice of PCB material and topology. For example, for standard FR4 boards the thermal resistance can be 20...80 °C/W, while for boards on a metal substrate the thermal resistance will be a few °C/W. CREE offers an "optimizing pcb thermal performance" guide for LED PCB design that provides recommendations for PCB layout to reduce thermal resistance. You can also use LEDs mounted directly on the radiator. In this case, the PCB will not contribute to the total thermal resistance.
Thermal conductive materials necessary to create good thermal contact between the PCB and the heatsink or between the LED and the heatsink. In addition to providing reliable thermal contact, some thermally conductive materials, depending on the design of the cooling system, can perform other functions, such as insulating electrical components of the circuit or providing mechanical support. Table 2 below presents the characteristics of the main heat-conducting materials.
Table 2. Characteristics of heat-conducting materials
| Thermal conductive material | Advantages | Flaws |
|---|---|---|
| Thermal pastes | High volumetric thermal conductivity, small size of the adhesive layer, low viscosity, do not harden | Considered quite messy to produce |
| Phase Change Materials | High viscosity gives higher reliability compared to thermal pastes, much more convenient to use, no delamination | Lower thermal conductivity compared to thermal pastes, surface resistance may be higher than thermal pastes, pressure must be applied to improve efficiency |
| Gels | Lower thermal conductivity compared to thermal pastes, less adhesion than hot melt adhesives | |
| Hot melt adhesives | Fills surface unevenness well | Cleaning process required |
When choosing a heat-conducting material, it is necessary to take into account many parameters, not only the value of thermal conductivity. The thickness of the adhesive layer of the material is often overlooked, and as follows from formula (5) below, the thermal resistance directly depends on this parameter. Manufacturers of thermally conductive materials provide information about the main parameters in the documentation, and in order to correctly select a thermally conductive material, it is very important to understand the influence of each of these parameters on the operation of the cooling system. Sometimes a thinner adhesive layer with poor thermal conductivity value will have lower thermal resistance compared to a thicker layer with better thermal conductivity value. Both of these conditions must be taken into account when choosing materials. The thermal resistance of a heat-conducting material is described by the formula:
Qtim is the thermal resistance of the heat-conducting material (°C/W);
L—layer thickness (m);
K—thermal conductivity (W/m*K);
A is the contact area (m2).
Radiator— this is perhaps the most important element in the LED cooling system; it removes heat from the PCB or directly from the LED, and dissipates the heat into the air. The following requirements are imposed on the radiator: the radiator material must have a high thermal conductivity value, the surface area of the radiator must be maximum. In addition to cooling, a radiator can perform other functions; most often it can act as a housing or holder. Table 3 shows the thermal conductivities of some materials. Moreover, radiators made of the same material, but made using different surface treatment methods, may have different thermal conductivity coefficients. For example, an anodized aluminum radiator, due to radiation, has a higher thermal conductivity coefficient than a conventional aluminum radiator.
Table 3. Thermal conductivity of some materials
| Material | Thermal conductivity, W/(m*K) |
|---|---|
| Air | 0,024 |
| Aluminum | 120…240 |
| Ceramics | 15…40; 100…200 |
| Conductive polymers | 3…20 |
| Copper | 401 |
| Stainless steel | 16 |
| Thermal paste/epoxies | 0,1…10 |
| Water | 0,58 |
Often, LED lamps are subject to quite serious dimensional requirements, which may result in the need to design a radiator for specific requirements. When designing a radiator, it is necessary to take into account the weight of the final product, cost, thermal parameters, and the possibility of further production.
Typically cast or forged aluminum radiators are used. The anodized aluminum radiator has a high emissivity.
Designing a radiator can be a rather complex task, in which it is necessary to take into account dimensional limitations, cost, weight, and the possibility of mass production. Below are some guidelines for radiator design:
- The surface area of the radiator should be as large as possible;
- As a rough estimate, we can take the following assumption: for 1 W of heat dissipated, a radiator with an area of 32...65 cm 2 is required;
- for the correct location of the radiator, to ensure good air flow between its fins, it is necessary to have a good idea of how the LED lamp will ultimately be mounted;
- a material with good thermal conductivity is required;
- use radiators with good emissivity. Anodizing dramatically increases the heat emissivity of an aluminum radiator;
- use programs to simulate cooling systems;
- select the radiator production method. Some methods of producing radiators may impose restrictions on the thickness and length of radiator fins and the materials used. The most common production methods: stamping, casting, forging. Each production method has its pros and cons.
Forced cooling
Let's say a few words about active cooling, the options of which are discussed in Table 4.
Table 4. Types of forced cooling
| Type | Dissipated thermal power, W | Description |
|---|---|---|
| Cooler | <170 | Mounts directly on the radiator. Additional nutrition is required. |
| Heat pipes | <140 | Heat pipes don't dissipate heat, they transfer it to another location, so a heatsink is still needed. |
| Liquid cooling | <200 | Designed to dissipate large amounts of heat, it is a fairly expensive solution, about 10 times more expensive than heat pipes. |
| Peltier modules | <80 | Ineffective, limited cooling, expensive. Additional nutrition is required. |
| Jet cooling | <80 | Comparable to a cooler, but runs quieter and is highly reliable. A special heatsink design is required. |
| SynJet Cooling Systems | <240 | Smaller dimensions compared to conventional radiators. Quieter than coolers. Long service life. The disadvantages include the need for a separate power source. |
If natural cooling is not enough to remove heat, then forced cooling must be used. There are many active cooling options, from coolers to water cooling. If the use of active cooling is unavoidable, it should be taken into account that LEDs can operate for tens...hundreds of thousands of hours, therefore it is necessary to provide for a system for protecting LEDs from overheating in the event of failure of active cooling devices, otherwise failure of forced cooling devices will almost immediately result in leads to failure of the LED due to overheating. In addition to service life, important parameters are efficiency, reliability, low noise level, price, ease of maintenance, power consumption. Often, forced cooling devices require additional power, which leads to a decrease in the efficiency of the system as a whole.
Several examples of heat sink calculations
Example of heat sink calculation for . These LEDs are mounted directly on the radiator (Figure 5).
Rice. 5. Mounting the CXA1304 LED on a radiator
The equivalent circuit for calculating the thermal regime for this case consists of the thermal resistance “junction - LED pad”, the thermal resistance “contact pad - heat-conducting material”, the resistance “thermal-conducting material - heatsink” and, finally, the thermal resistance “heatsink - air” ( Figure 6).
Rice. 6. Equivalent circuit for CXA1304 LED
Let's make the calculation for ambient temperatures of 25°C and 55°C. Let's assume that the LED is operating at maximum current and that the junction temperature is 85°C. Using the PCT application on the CREE website ( http://pct.cree.com/), we obtain the Vf value for the junction temperature at maximum current, the data is listed in Table 5. We will use the most common thermal paste KPT-8 as a heat-conducting material, and take the thermal conductivity equal to 0.7 W/(m*°C).
Table 5. Calculation data for LED CXA1304
| Transition temperature, °C | 85 |
| If, A | 0,25 |
| Vf, V | 43,7 |
| P = Af x Vf, W | 10,92 |
| Pdiss = 0.75 x P, W | 8,19 |
| LED contact area, mm 2 | 178,22 |
For CXA series LEDs, the documentation does not indicate the value of the thermal resistance “junction - contact pad”. To determine the system parameters, it is recommended to use a graph from which you can obtain the value of the maximum thermal resistance between the LED contact and the air (Figure 7).
Rice. 7.
From this graph we find that for an air temperature of 25°C the maximum resistance will be 6°C/W, and for 55°C - 2°C/W. Let's calculate the thermal resistance of the heat-conducting material using formula (5). Let's take the thickness of the thermal paste layer to be 0.1 mm. Then the value of thermal resistance will be as follows: Qtim = 0.8°C/W.
Therefore, for the case of 25°C, the value of the thermal resistance of the radiator should be less than 5.2°C/W, for 55°C - less than or equal to 1.2°C/W. For example, we will use radiators from MechaTronix ( http://www.led-heatsink.com/). Suitable for 55°C as a radiator LPF11180-ZHE(Figure 8). The thermal resistance of this radiator is 1.07°C/W. For the case with a temperature of 25°C, the choice of radiators is wider.
Rice. 8. Radiator LPF11180-ZHE manufactured by MechaTronix
Now let's look at the calculation of the cooling system for LEDs MK-R2 and (also for two temperature options). Data for calculations are included in table 6.
Table 6. Calculation of the cooling system for MK-R2 and CXA3070 LEDs for two temperature options
| Parameter | Name | ||
|---|---|---|---|
| MK-R2 | |||
| Maximum current, A | 0,42 | 0,7 | |
| Forward voltage drop, V | 37,86 | 34,28 | |
| Transition temperature, °C | 85 | 85 | |
| Contact surface area, mm 2 | 29,5 | 748 | |
| Power dissipation, W | 11,93 | 18 | |
| Thermal resistance Tj-sp, °С/W | 1,7 | — | |
| Total thermal resistance, °C/W | for 25°С | 5 | 4,5 |
| for 55°С | 4,6 | 3 | |
| Thermal resistance of heat-conducting material, °C/W | 0,8 | 0,2 | |
| Thermal resistance of the printed circuit board, °C/W | 3 | — | |
| Thermal resistance of the radiator, °C/W | for 25°С | 1,2 | 4,3 |
| for 55°С | 0,8 | 2,8 | |
For MK-R2 LED, if the ambient temperature is 55°C, the junction temperature will be higher than 85°C. Table 6 shows the data when the LED junction temperature will be 110°C. Also, due to the fact that the MK-R2 LED is first mounted on the printed circuit board, and then on the heatsink, another thermal resistance appears in the equivalent circuit. Table 6 shows the thermal resistance for a board with a metal base. The last line indicates what thermal resistance the radiator should have. A heatsink is suitable for cooling these LEDs. SpotLight Led HeatSink 34W companies Nuventix(nuventix.com).
For these CXA3070 LEDs, we will offer several cooling options and compare their characteristics. To cool these LEDs, we’ll take a regular radiator, a radiator with a cooler, and a cooling system SynJet manufactured by Nuventix.
The option with passive cooling is the simplest, since it does not require additional power sources, but to remove significant heat, a fairly large radiator may be required, and this leads to an increase in cost and makes the lighting device quite massive and large. Therefore, passive cooling is best used for low-power lighting systems. A radiator is suitable for cooling the CXA3070 LED LSB99. This radiator has the following dimensions: diameter 100 mm, height 50 mm, radiator weight 470 g, which is significantly heavier compared to active cooling.
For active cooling with a cooler we use a radiator and cooler assembly . To power the cooler, you will need an additional 12 V power supply with a power of 0.3 W, the dimensions of the cooling system will be 86 mm in diameter and 52 mm in height, weight within 300 g. The presence of the cooler creates additional noise, the stated operating time of the cooler is at a temperature of 60° C - about 70 thousand hours.
For cooling by the SynJet system you will need a module SynJet Par20 Cooler 24. The maximum possible dissipation power is 24 W. The dimensions of the entire lamp assembly will be within 45 mm in height and 65.5 mm in diameter with a weight of 140 g. But for forced cooling, an additional 12 or 5 V power supply with a power of 1 W will be required, because of this in general The energy efficiency of the system is slightly reduced. The declared operating time of such a forced cooling system is about 100,000 hours.
The reliability and durability of LED devices directly depends on the quality of the cooling system design, which is why it is so important to pay special attention to the design of a reliable heat sink. To cool low-power LED systems, a conventional radiator will be sufficient; in some cases, active cooling may be required to remove heat from high-power luminaires. Also, when developing new lighting devices, it is strongly recommended to carry out calculations and modeling of the cooling system. The CREE website provides many heat dissipation calculation methods and useful applications for properly selecting cooling elements.
5. XLamp Thermal Management
6. Optimizing PCB Thermal Performance.
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