How to heat the hottest LED lamps

How to heat LED lamps

1. Too high node temperature will lead to the decline of LED performance, especially the service life, light color quality and lumen output. If the rated maximum junction temperature is exceeded, the service life of the LED will be reduced by 30% to 50% for every 10 degrees of increase in the working temperature

2. After the node temperature rises, it will also lead to obvious color drift towards the high end of the spectrum (longer wavelength), which has a great impact on the "white" LED light source. Most of the so-called "white" LEDs actually emit blue light, which turns into white light after being converted by phosphors. When the temperature rises, the blue light will drift towards the red spectrum, and the interaction with the phosphor will change, resulting in the change of the tone of the final light

3. The last major parameter affected by LED thermal management system is lumen output. Increasing the current can improve the lumen output of LED, but high current will also lead to increased calorific value. Therefore, when determining the current value, we must choose an optimal balance between system performance and service life

in short, too much heat will directly affect the short-term and long-term performance of LED light sources:

short term: color drift, light output reduction

long term: light decay accelerates, and the service life is shortened

therefore, in order to obtain long-life and high-performance LED lamps and lanterns, we must design a set of excellent cooling and heat dissipation system. Thermal management can be said to be the quantity of LED indicating this kind of resistance, which is called the most important link in the design of viscosity lamps and lanterns. People usually use natural (passive) and artificial (active) cooling systems to dissipate heat

① passive heat dissipation:

"passive" means that this system does not contain energy consuming mechanical equipment, such as heat pumps, fans or fans. The most common passive heat dissipation device in LED lamps is the heat sink. Generally speaking, the heat sink is composed of many groups of metal fans, which can quickly transmit the heat accumulated in the LED light source. Because the heat sink itself does not need to expand the equipment to consume poor energy, it is the most energy-saving heat dissipation system. However, with the improvement of LED light source power, the heat dissipation area is required to be larger and larger, which requires the design of heat sinks with complex shapes, which will adversely affect the design of lamps and lanterns

② active heat dissipation:

"active" means that this heat dissipation system contains energy consuming mechanical equipment such as pumps, fans or fans. Active heat dissipation system is necessary for small lamps using high-power and high luminous flux LED light source, because it can make the lamp structure and size smaller

how to design the passive cooling system

the most common LED lamp is the passive heat dissipation system. When designing this system, several main factors must be considered, such as the layout of LED light source, the properties of lamp material, the shape and surface treatment of heat sink, and other factors described below

1 led layout spacing

most of the electric energy consumed by the LED is converted into heat and emitted. The closer the LED particle layout is, the less the heat dissipation space is, and the higher the node temperature is. Therefore, the larger the spacing of LED particles, the better under the conditions of packaging and optical characteristics

▲ led particle layout

2 material properties

thermal conductivity is a physical quantity used to measure heat conduction efficiency, and the thermal conductivity of materials reflects the heat conduction capacity of materials. Some materials are good conductors of heat compared to other materials. For example, the thermal conductivity of pure copper is 400w/m.k, while that of air is only 0.025w/m.k

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 extrude. Another more important feature of the heat sink is the geometry, and the aluminum profile is easy to process and shape. In addition, aluminum has advantages such as light weight, corrosion resistance and good structural stability. In general, aluminum is an excellent material for making heat sinks

3 shape

convection is a fluid process that takes heat away from the surface of an object through the flow of gas or liquid. The larger the surface area, the more convection occurs. An example is the heat sink, which is designed to maximize the surface area of convection. This multi blade structure can greatly increase the surface area without changing the volume

4 surface treatment

radiation coefficient is a physical quantity that reflects the relative ability of an object's surface to radiate and release energy. It is usually written as ε Or E. It is defined as the ratio of the radiant energy of a material surface to the radiant energy of a standard blackbody at the same temperature. Ideal blackbody ε= 1. Real materials ε 1。 High emissivity coatings can improve the rate of heat exchange. Generally speaking, the darker and darker the surface color, the closer the emissivity is to 1. The higher the reflectivity of the material, the closer the radiation coefficient is to 0. Printed circuit board (PCB) LEDs are installed on multilayer FR4 or metal printed circuit board (MCPCB). In order to obtain the best performance, the thermal resistance of PCB should be as low as possible

5 FR4 circuit board (FR4 PCB)

fr4 is the standard material for making PCB. The number of LED particles installed on each PCB depends on the LED input power and boundary conditions. The heat on the PCB board is transferred to the heat dissipation system through the heat dissipation holes, which are electroplated through holes (PTH), which can be opened, blocked or closed. Finally, the thermal resistance of the whole circuit board is determined by the number and density of heat dissipation holes on the board, the thickness of copper foil layer and the coating thickness of electroplated through holes

6 metal circuit board (MCPCB)

the following figure shows the structure of MCPCB. A MCPCB includes copper layer, insulating layer, heat dissipation plate, aluminum or copper sheet. Increasing the thickness of copper layer or thinning the thickness of insulation layer can greatly reduce the thermal resistance

7 surface roughness

when connecting the heat sink with the packaged semiconductor, the solid surfaces of the two parts should be in full contact as much as possible. Unfortunately, no matter how well treated, the solid surface cannot be completely smooth. Due to the uneven microstructure, all surfaces have a certain roughness. The existence of these small protrusions, small pits or distorted shapes is superimposed to form a rough and uneven surface visible to the naked eye. When two such surfaces contact, in fact, only the small protrusions on the two surfaces are in contact with each other, and the small cavities are still separated, forming an air gap

thermal interface materials (TIMS), also known as heat conduction 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 it is not filled, the air filled gap between the surfaces of two mechanically bonded materials will be a poor conductor of heat

8 thermal interface materials

the most common thermal interface materials Sinochem Group: Sinochem Group is one of China's four major national oil companies. The material is white glue or thermal conductive glue, and the most common is thermal conductive silicone grease. First of all, it should take the resource-saving development path, mixed with alumina, zinc oxide or boron nitride. Some brands of thermal interface materials will use finely ground silver powder. Another large class of thermal interface materials is phase change materials, which are solid at room temperature and will liquefy as grease at the working temperature of the chip

9 production process

the most common technology using natural convection is to make several holes at the top and bottom of the packaging shell, so that the air flow can be connected up and down to heat the LED. Compared with die-casting and extrusion processes, the aluminum profile processed by extrusion method will have a higher density (less bubbles in the radiator). Because the thermal conductivity difference between air and aluminum is so large, a little air residue will cause the thermal conductivity of the material to change greatly. On average, the thermal conductivity of die cast aluminum fins is% lower than that of extruded aluminum fins with the same volume and shape

10 shell design and installation methods

when designing the LED shell, it is also necessary to consider the heat conduction path from the PCB backplane to the shell. Usually, the back of the PCB is directly installed on the LED shell, so that the two can contact each other in the largest area

the improvement scheme of this installation method is to add a heat conduction plate between the PCB board and the shell, which can better fit the PCB board and increase the contact area of heat conduction

similarly, the most common technology using natural convection is to make several holes at the top and bottom of the package shell, so that the air flow can pass up and down, and heat the LED