The effect of heat on the LEDs
LEDs are sensitive to temperature and performance is greatly affected by ambient temperature. Mainly in the following three aspects:
1. Excessive junction temperature can cause LED performance degradation, especially life, light color 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.
2. When the junction temperature rises, it will also cause obvious color drift to the high end of the spectrum (wavelength becomes longer), 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.
3. 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.
In summary, excessive heat directly affects the short- and long-term performance of LED sources:
• Short-term: color drift, reduced light output
• Long-term: accelerated light decay and reduced life
Therefore, in order to obtain long-life, high-performance LED lamps, it is necessary to design an excellent cooling and cooling system. Thermal management can be said to be the most important part in the design of LED lamps. Natural (passive) and manual (active) cooling systems are often used to dissipate heat.
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. In general, the heat sink consists of a number of sets of metal segments that quickly conduct heat from the LED 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 lamp.
2 active cooling:
“Active” means that the cooling system contains energy-consuming mechanical equipment such as pumps, fans or fans. For small luminaires that use high-power, high-light-packaged LED light sources, an active cooling system is necessary because it allows for a smaller luminaire structure and size.
How to design a passive cooling system?
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 source, the properties of the luminaire material, the shape and surface treatment of the heat sink, and other factors described below.
1.347v-480v LED high bay layout spacing
Most of the energy consumed by the LED is converted into heat, and the tighter the LED particle layout, the less heat dissipation, and the higher the junction temperature. Therefore, the LED particles should be as large as possible under the conditions allowed by the package and optical characteristics.
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. For example, the thermal conductivity of pure copper is 400W/m.K, and the thermal conductivity 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 of aluminum, easy processing, die casting, and extrusion. 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 structural stability. In summary, aluminum is an excellent material for making heat sinks.
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 can greatly increase the surface area without changing the volume.
The emissivity is a physical quantity that reflects the relative ability of the surface to radiate energy from the surface of the object, usually written as ε or e. It is defined as the ratio of the radiant energy of a material’s surface to the standard and the energy radiated by the black body 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 one. The higher the reflectivity of the material, the closer the emissivity is to zero. Printed circuit board (PCB) LEDs are mounted on multi-layer FR4 or metal printed circuit boards (MCPCB). For best performance, the thermal resistance of the PCB should be as low as possible.
5.FR4 board (FR4 PCB)
FR4 is the standard material for making PCBs. The number of LED particles installed on each PCB depends on the LED input power and boundary conditions. The heat on the PCB is transferred to the heat dissipation system through the heat dissipation holes. These heat dissipation holes are plated through holes (PTH), which can be opened, blocked or closed. Finally, the thermal resistance of the entire board is determined by the number and density of the heat sink holes on the board, the thickness of the copper foil layer, and the plating thickness of the plated through holes.
6.Metal circuit board (MCPCB)
The following figure shows the structure of the MCPCB. An MCPCB board consists of a copper layer, an insulating layer and a heat sink, aluminum or copper. Increasing the thickness of the copper layer or thinning the thickness of the insulating layer can greatly reduce the thermal resistance.
7. Surface roughness
When the heat sink is connected to the packaged semiconductor, the two parts of the solid 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 superimposes 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 Materials (TIMs), also known as thermal conductive materials, are used to increase the heat transfer coefficient 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.
8.Thermal interface material
a) unfilled voids; b) TIMs material filling
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.
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 the extrusion process will be denser (making fewer bubbles inside the heat sink). 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.
10.Shell design and installation method
When designing the LED housing, consider also leaving a thermal path from the PCB backplane to the enclosure. It is common practice to mount the back side of the PCB directly onto the LED housing to maximize contact between the two.
The improvement of this installation method is to add a heat conducting plate between the PCB board and the outer casing, and the heat conducting board 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 LEDs.