Part I: How LED works and optical introduction http://
3. LED thermals
3. LED thermals
3.1 Introduction
LED is an electronic device that can be illuminated after being powered. It was mainly used as an indicator light on various devices. Now its application has been extended to other lighting fields, such as direct use as an indoor and outdoor lighting source. LEDs have higher luminous efficiency than traditional light sources such as incandescent lamps, and their efficiency is not limited by shape and size, which is superior to fluorescent sources.
However, like all electronic devices, LEDs are sensitive to temperature and performance is greatly affected by ambient temperature. LED lamps that lack a reasonable heat dissipation design will cause the LED package to overheat for a long time, which will cause the life to be reduced until it is completely scrapped. Therefore, in order to obtain long-life, high-performance LED lamps, it is necessary to design an excellent cooling and cooling system (see Figure 3.1.1). Thermal management can be said to be the most important part of LED lamp design.
Figure 3.1.1: a) Thermal simulation results; b) Photographs taken by the thermal imaging lens for the same fixture.
3.2 Introduction to heat transfer
Heat transfer is a branch of thermodynamics that studies the exchange of energy and heat between physical systems. Heat transfer is divided into three main mechanisms (Figure 3.2.1):
Figure 3.2.1: Three main heat transfer mechanisms: conduction, convection, and thermal radiation.
Conduct
When there is a temperature difference in an entity, heat is transferred from the high temperature area to the low temperature area. Thermal conduction is the most common heat transfer mechanism between solids and several solids in contact.
convection
Thermal convection is a way of transferring heat through the movement of fluids, most commonly in liquids and gases. Thermal convection is divided into natural convection and spontaneous convection. Natural convection is spontaneous inside the fluid, not heat transfer from external equipment such as pumps, fans, and drainers. Artificial convection is caused by external means such as agitators and pumps. The heat transfer generated.
Heat radiation
Any entity with temperature emits electromagnetic radiation externally, and the resulting heat transfer is called thermal radiation. This form does not require the presence of any medium. Thermal radiation is determined by various characteristics of the surface of the object. If an object and its surface are in thermodynamic equilibrium and its surface is completely absorbed by radiation of any wavelength, we call this object a black body. Light-colored objects such as white or metallic objects absorb less visible light and are therefore less likely to be heated. Smooth and shiny metal surfaces have low absorption rates for both visible and far-infrared radiation, thus reducing the occurrence of thermal radiation.
3.3 Energy conversion
The principle of all light sources is to convert electrical energy into visible light and thermal energy, the difference being the specific ratio. The output of incandescent bulbs is mainly concentrated in infrared radiation, which accounts for only 8% of visible light. The fluorescent light source emits more efficiently, reaching 21%, but still emits a lot of infrared, ultraviolet and heat. LEDs produce very little infrared light, which converts 40% of the electrical energy into visible light (see Figure 3.3.1). The rest of the energy is converted into heat, which needs to be conducted out of the LED working area, in turn through the base printed circuit board. The heat dissipation system and the outer casing are finally released into the air.
Figure 3.3.1: Energy conversion rate of a "white light" source (Visible light, Heat removed by co nduction and convection, heat removed by heat radiation by Heat removed by thermal radiation)
For LED light sources, heat radiation is almost non-existent, so most of the heat can only be dissipated through conduction and convection. LED light sources are highly efficient, and thermal management can be said to be the most challenging part of the design process. Proper cooling and cooling system design is critical for LED luminaires.
Socket efficiency (electrical light conversion efficiency WPE)
To improve the structural design, it is necessary to calculate the energy consumed by the heat more accurately (that is, how much energy in the light source is converted into heat), for which we introduce the concept of socket efficiency (electrical light conversion efficiency WPE). WPE is defined as the ratio of the energy of visible light produced by an LED source to the input electrical energy (that is, how much electrical energy is converted into visible light). Usually LED socket efficiency is 40%, which means that about 60% of the electrical energy becomes heat. At OMS, the WPE of a conventional light source is typically estimated to be 20%.
Assuming that all of the following sources have an input power of 20 W, the heat dissipated by conduction and convection is calculated as follows:
Case 1: Incandescent light source
20W x (1-WPE) x 19% = 20W x 80% x 19% = 3.04 W
Case 2: Fluorescent light source
20W x (1-WPE) x 42% = 20W x 80% x 42% = 6.72 W
Case 3: LED light source
20W x (1-WPE) x 60% = 20W x 60% x 60% = 7.2 W
About 60% of the energy in the LED chip is converted into heat, which needs to be dissipated by conduction and convection. Without efficient thermal management and cooling and cooling systems, the LED chips can overheat, causing changes in LED characteristics. This change will directly affect the short-term and long-term performance of the LED. The short-term effect is that color drift and light output drop, and the long-term consequences will lead to accelerated light decay, thus shortening the service life.
3.4 LED performance
The application of LEDs has grown by leaps and bounds over the past few years. Initially, the cooling of the LEDs was not a problem, because the main use of small power LEDs at the time. However, with the use of high-power LEDs , the heat generation is greatly increased. In order to ensure high efficiency, reliability and long life of LEDs, heat dissipation must be solved.
The basic parameters for measuring LED performance are (see Figure 3.4.1):
• Node temperature - Tj
• Thermal resistance - Rj-a
Figure 3.4.1: Basic parameters for measuring LED performance
(LED DIE: LED chip; CIRCUIT BOARD: circuit board; HEAT SINK: heat sink; HEAT IS REMOVED FROMLED CHIP TO AMBIENT: heat is dissipated from the LED chip to the environment)
Thermal resistance
Thermal resistance is defined as the ratio of temperature difference to power, reflecting the efficiency of heat transfer between materials/components. Its definition formula is as follows:
R – thermal resistance between two points;
ΔT – the temperature difference between two points;
P – heat transfer power between two points.
Junction temperature
When the LED light source is energized and illuminating, it will generate heat, and the junction temperature must be kept at a low level as much as possible. The term "node" refers to the pn junction inside the LED semiconductor chip. The recommended values ​​for the maximum allowable temperature can be found in the parameter table for each LED product.
Since the temperature at the junction is highest throughout the LED, it is a key indicator of LED lifetime. From a thermodynamic point of view, the junction temperature is affected by many factors, such as cooling and cooling systems, environment, interface materials, and so on.
The formula for calculating the junction temperature is expressed as follows:
Where Rjc represents the thermal resistance of the junction to the outer casing and is provided by the manufacturer; P represents the heating power, which can be calculated by using electric power and WPE; Tc represents the temperature of the outer casing. If the calculated Tj is higher than the highest allowable temperature of the node provided by the manufacturer, then the fixture must be redesigned.
LED performance degradation
Excessive junction temperatures can cause LED performance degradation, especially for lifetime, color quality, and lumen output. If the rated maximum junction temperature is exceeded, the LED's lifetime will drop by 30% to 50% for every 10 degrees increase in operating temperature.
When the junction temperature rises, it will also cause a significant color drift to the short-wave direction of the spectrum, which has a great influence on the "white light" LED light source. Most of the so-called "white light" LEDs actually emit blue light, which turns into white light after being converted by phosphors. When the temperature rises, the blue light drifts toward the red spectrum, and the effect of the phosphor changes, and as a result, the color tone of the final light changes.
The last major parameter affected by the LED thermal management system is the lumen output. Increasing the current increases the lumen output of the LED, but a large current also causes an increase in heat generation. Therefore, an optimal balance must be chosen between system performance and service life when determining the current value.
3.5 LED lamp thermal design
From a thermal design point of view, a typical LED luminaire includes an LED source, a printed circuit board (PCB), and a cooling system. The LED light source includes a semiconductor chip (light emitting portion), an optical component, a package, and a heat dissipating block for guiding the heat of the chip, and the heat dissipating block is soldered on the PCB board (usually a metal PCB board, referred to as MCPCB for short).
To shorten the development cycle of LED lamps, it is necessary to estimate the heating of the LED light source inside the lamp. With the increase in high-power, high-lumen packaging products, the heat dissipation problem of LED lamps has become a challenge. Thermal design has become an indispensable part of the development of LED lamps. In the early design phase, the thermodynamic simulation program based on the finite element method is currently the most widely used tool software. Figure 3.5.1 shows a simplified model of an LED luminaire based on several basic input/output parameter simulations.
Figure 3.5.1: Simplified model of LED luminaires based on simulation of several basic input/output parameters
Thermodynamic modeling
There are three major factors that affect the LED junction temperature: drive current, heat transfer path, and ambient temperature. Generally speaking, the larger the drive current, the higher the heat generated by the chip. In order to maintain the desired light output, lifetime and color temperature, heat must be removed from the chip in time. The amount of heat transfer depends on the ambient temperature and the heat transfer path from the chip to the surrounding environment.
Model description
Figure 3.5.2: 3D model of LED luminaires
Figure 3.5.2 shows a 3D model of a set of LED luminaires. In this model, the shell is made of a variety of different materials to find the one that is most conducive to heat dissipation. The lamp does not have a heat sink, and the lamp housing itself plays a role of heat conduction and heat dissipation. Under these conditions, the shape and material of the outer casing are the key factors.
Boundary conditions
In order to do computational fluid dynamics (CFD) analysis, we assume the following conditions:
The electric power of the lamp is 32W
At steady state
Ambient temperature 35 ° C
Heat is dissipated through natural conduction and convection
The radiation effect of all parts is set to 0.8
The luminaire is in the horizontal direction (the most unfavorable situation)
The calculation range is 800*800*800mm3
Material properties
Table 3.5.3 shows the properties of various materials in thermodynamic simulations.
result
The housing is available in two different materials: ADC12 and AI6082, ADC12 is suitable for die casting and AI6082 is suitable for machining. Through numerical analysis, the model using AI6082 showed a 6% increase in the model using ADC12. The maximum allowable case temperature of the LED chip is 78 ° C, both of which meet this condition, but the case 1 is closer to the critical value. So the best choice for this luminaire is the AI6082. Figure 3.5.4 shows the simulation results.
Figure 3.5.4: Simulation results for two different material shells
3.6 cooling system
Excessive heat directly affects the short-term and long-term performance of the LED source.
• Short-term: color drift, reduced light output
• Long-term: accelerated light decay and reduced life
Natural (passive) and manual (active) cooling systems are often used to dissipate heat. (See Figure 3.6.1)
Passive cooling
“Passive†means that the system does not contain energy-consuming mechanical equipment such as heat pumps, fans or fans. The most common passive heat sink in LED luminaires is the heat sink. Generally speaking, the heat sink is composed of a plurality of sets of metal segments, which can quickly conduct the heat accumulated in the LED light source. Since the heat sink itself does not consume energy, it is the most energy efficient cooling system. However, as the power of the LED light source increases, the heat dissipation area is required to be larger and larger, which requires the design of a heat sink having a complicated shape, which adversely affects the design of the light fixture.
Active cooling
“Active†means that the cooling system contains energy-consuming mechanical equipment such as pumps, fans or fans. Active cooling systems are necessary for small luminaires that use high-power, high-light-packaged LED light sources because they make the luminaire structure and size smaller.
Figure 3.6.1 Passive heat dissipation (a. LED fixtures using heat sinks) and active heat dissipation (b, LED fixtures using heat sinks and fans)
3.7 Passive cooling system design
The most common type of LED luminaire is the passive cooling system. Several factors must be considered when designing such a system, such as the layout of the LED light source, the properties of the luminaire material, the shape and surface treatment of the heat sink, and other factors described below.
LED layout spacing
Most of the power consumed by the LED is converted into heat, and the tighter the LED particle layout, the less the heat dissipation space, and the higher the junction temperature. Therefore, the LED particles should be as large as possible under the conditions of packaging and optical characteristics. (See Figure 3.7.1)
Figure 3.7.1 Example of LED Layout Spacing
Material properties
Thermal conductivity is a physical quantity used to measure the efficiency of heat transfer. The thermal conductivity of a material reflects the thermal conductivity of the material. Some materials are good conductors of heat compared to other materials (see Table 3.7.2). For example, pure copper has a thermal conductivity of 400 W/mK, while air has a thermal conductivity of only 0.025.
Aluminum is a common material for making heat sinks, not only because of its high cost performance, but also because aluminum is easy to process, die cast, and squeeze. Another feature of the heat sink is the geometric shape, while the aluminum profile is easy to shape. In addition, aluminum has advantages such as light weight, corrosion resistance and good stability. In general, aluminum is an excellent material for making heat sinks.
Table 3.7.2 Thermal conductivity of materials
shape
Convection is a fluid process that removes heat from the surface of a object by the flow of a gas or liquid. The larger the surface area, the more convection occurs. An example is the heat sink, which is designed to be the current shape to maximize the surface area of ​​the convection. This multi-bladed structure is capable of greatly increasing the surface area with a constant volume. (Fig. 3.7.3)
Figure 3.7.3 Fin-shaped lamp body
Surface treatment
The emissivity is a physical quantity that reflects the relative ability of the surface of the object to release energy, usually written as ε or e. It is defined as the ratio of the radiant energy of a material's surface to the radiant energy of a standard blackbody at the same temperature. The ideal black body has ε=1, while the real material ε<1.
High emissivity coatings increase the rate of heat exchange. Generally, the darker and darker the surface, the closer the emissivity is to 1. The higher the reflectivity of the material, the closer the emissivity is to zero.
Printed circuit board (PCB )
The LEDs are mounted on a multi-layer FR4 or metal printed circuit board (MCPCB). For best performance, the thermal resistance of the PCB should be as low as possible.
FR4 board (FR4 PCB)
FR4 is the standard material for making PCBs. The number of LED particles mounted on each PCB is determined by the LED input power and boundary conditions. The heat on the PCB is transferred to the cooling system through the vents. These vents are plated through holes (PTH) that can be opened, blocked, or closed. The thermal resistance of the final board is determined by the number and density of louvers on the board, the thickness of the copper layer, and the thickness of the plating through-hole.
Metal circuit board (MCPCB)
Figure 3.7.4 shows the structure of the MCPCB. A MCPCB board consists of a copper substrate, an insulating layer and a heat sink, aluminum or copper. Increasing the thickness of the copper substrate or thinning the thickness of the insulating layer can greatly reduce the thermal resistance. Figure 3.7.5 shows what a real FR4 board and metal board look like.
Figure 3.7.4 Cross section of a metal circuit board (MCPCB) (Note: not year-on-year magnification)
Figure 3.7.5 FR4 PCB(a) and MCPCB(b)
Roughness
When the heat sink is connected to the package LED, the two parts of the surface are required to be as close as possible. Unfortunately, no matter how well handled, the solid surface cannot be completely smooth. Due to the unevenness of the microstructure, all surfaces have a certain roughness. The presence of these small protrusions, small dimples or twisted shapes is superimposed to form a rough, uneven surface visible to the naked eye. When two such surfaces are in contact, only the small protrusions on the two faces are actually in contact with each other, and the small cavities are still separated to form an air gap.
Thermal interface material
Thermal Interface Materials (TIMs), also known as thermal conductive materials, are used to increase the thermal conductivity between bonded solid surfaces, such as PCB boards and heat sinks, to improve heat dissipation efficiency. Because if not filled, the air-filled gap between the surfaces of the two mechanically joined materials can be a poor conductor of heat (see Figure 3.7.6).
Figure 3.7.6 Space between two surfaces: a) unfilled voids; b) TIMs material fill
The most common thermal interface materials are white or thermal paste. The most common is thermal grease , which is doped with alumina, zinc oxide or boron nitride. Some brands of thermal interface materials use finely ground silver powder. Another large class of thermal interface materials are phase change materials. These materials are solid at room temperature and liquefy at the working temperature of the chip.
Production Process
The most common technique for utilizing natural convection is to make several holes in the top and bottom of the package, allowing the airflow to pass up and down to dissipate heat from the LED. Compared to the two processes of die casting and extrusion, the aluminum profile treated by extrusion has a higher density (making fewer bubbles inside the heat sink) (see Figure 3.7.7). Since the difference in thermal conductivity between air and aluminum is so large, a little bit of air residue can cause a significant change in the thermal conductivity of the material. The thermal conductivity of die-cast aluminum heat sinks is on average 20-30% lower than extruded aluminum heat sinks of the same volume and shape.
Figure 3.7.7 Heat sink processed by extrusion
Shell design and installation method
When designing the LED housing, consider keeping the thermal conduction path from the PCB backplane to the housing. It is common practice to mount the back side of the PCB directly onto the LED housing to allow maximum contact between the two (see Figure 3.7.8).
Figure 3.7.8 Installation method
The improvement of this installation method is to add a heat conduction plate between the PCB board and the outer casing, and the heat conduction plate can be better fitted with the PCB board to increase the contact area of ​​heat conduction.
Similarly, the most common technique for utilizing natural convection is to make several holes in the top and bottom of the package, allowing the airflow to pass up and down to dissipate heat from the LED.
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