Author: LED lighting technical director room Haiming (Mark Fang)
Overview
Light-emitting diodes have the advantages of no pollution, low energy consumption, long life and fast response. As the EU is about to ban the widely used mercury-containing metal light sources in 2007, LEDs will become the main axis of the next generation of light sources. LEDs change with temperature, brightness increases, and LED heat dissipation technology has been improved. In 1992, the thermal impedance of an LED was 360 °C / W, and then dropped to 125 ° C / W, 75 ° C / W, 15 ° C / W, Now it is at the point of 6∼10°C/W. In short, in the past, for every watt of power consumed by LEDs, the temperature will increase by 360°C. Now it consumes 1 watt of electricity, but the temperature only rises by 6 ∼10. °C.
LEDs still need to overcome their luminous efficiency problems: the current stage of light energy efficiency can only reach 15%, and 85% are converted to heat, the application of LED light source, still need to be equipped with heat dissipation mechanism, if the heat dissipation function is not properly designed, for the light The diode itself will cause serious damage.
As the brightness of LEDs continues to increase, the heat dissipation technology of LEDs has also been improved. Therefore, the heat dissipation of LEDs will greatly increase the luminous efficiency and service life of LEDs. This article focuses on the impact of heat on LEDs, as detailed below.
The effect of heat on LEDs LED---cold light source (1) The principle of LED illumination is that electrons and holes are combined to emit light directly. No heat is needed in the process. LEDs can be called cold light sources. (2) LEDs need to be driven by current. Only about 15% of the input LED energy is converted into light, and most of it (about 85%) is converted into heat due to inefficiency. (3) The LED generates heat during the illumination process, and the LED is not a cold light source that does not generate heat.
The effect of heat on LED performance and structure
(1) The heat generated by the LED light is related to the working environment temperature (Ta), which will cause the LED chip junction temperature Tj to change. The LED is a temperature sensitive device. When the temperature changes, the LED performance and package structure are affected. Affect the reliability of LEDs (2) The heat is concentrated in the LED chips with small size. If there is no effective heat removal, the temperature of the chip rises, causing a non-uniform distribution of thermal stress, and the luminous efficiency of the chip and the lasing efficiency of the phosphor are reduced. When the temperature of the LED chip exceeds a certain value, the device failure rate increases exponentially. The statistical data shows that the reliability drops by 10% for every 2 °C rise in the temperature of the module.
(4) When multiple LEDs are densely arranged to form a white light illumination system, the heat dissipation problem is more serious. (5) Heat will affect the efficiency of the LED driver , damage the life of the magnetic components and output capacitors, and make the reliability of the LED driver . Reduce (the operating temperature of general semiconductor components should be controlled below 80 °C)
(6) The typical LED is encapsulated by optically transparent epoxy resin. When the temperature rises to the conversion temperature Tg of the epoxy resin glass, the epoxy resin is converted from a rigid material to an elastic material, and the coefficient of thermal expansion (CTE) will vary greatly. During the temperature change process of the encapsulating resin, the expansion and contraction are intensified, which will cause the displacement of the gold wire (or aluminum wire) bonding point to increase, and the gold wire (or aluminum wire) to be fatigued and damaged prematurely, causing the LED to open and suddenly fail. In order to avoid sudden LED failure, the LED junction temperature should always be below the Tg of the encapsulating resin.
The relationship between the luminous flux Fv and the junction temperature Tj Φv(Tj2)= Φv(Tj1)e(-kΔTj)
Where: Φv(Tj1)= luminous flux at junction temperature Tj1 Φv(Tj2)= luminous flux at junction temperature Tj2 ΔTj= Tj2 - Tj1 , k = temperature coefficient
Temperature coefficient of different material category LEDs
LED material category | Temperature coefficient k |
AlInGaP/GaAs Orange Red | 9.52 '10-3 |
AlInGaP/GaAs yellow | 1.11 '10-2 |
AlInGaP/GaP highlight red | 9.52 '10-3 |
AlInGaP/GaP yellow | 9.52 '10-2 |
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