With the advancement of LED epitaxial materials, chip technology and packaging technology, the luminous efficiency of LEDs continues to increase, which makes it possible to replace traditional light sources with LED light sources. In theory, LED has the advantages of long life and high efficiency, but in some practical applications, it has left the impression of large light decay and short life, which greatly affects the popularity and promotion of semiconductor lighting . The reason is mainly the driving power problem of LED.
Long life and high efficiency of LEDs are premised, that is, suitable working conditions. The main factor affecting the life and luminous efficiency is the working junction temperature of the LED. The test data provided by mainstream LED manufacturers shows that the luminous efficiency of LED is almost inversely proportional to the junction temperature, and the lifetime decreases almost exponentially with the increase of junction temperature. Therefore, controlling the junction temperature within a certain range is the key to ensuring LED lifetime and luminous efficiency. In addition to the heat dissipation measures, it is necessary to incorporate the junction temperature into the control parameters of the driving power supply.
1 LED junction temperature detection
The junction temperature of the LED refers to the temperature of the PN junction. It is difficult to actually measure the junction temperature of the LED, but it can be measured indirectly according to the temperature characteristics of the LED.
The volt-ampere characteristics of LEDs are similar to those of ordinary diodes. Typical volt-ampere characteristics of blue LEDs used for white illumination are shown in Figure 1.
Figure 1 volt-ampere characteristics of LED
The volt-ampere characteristics of LEDs have the same negative temperature coefficient as other diodes, that is, the I/V curve appears to shift left when the junction temperature rises, as shown in the following figure.
Figure 2 Temperature characteristics of volt-ampere characteristics
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