1. The lumen efficiency of a single led and the lumen efficiency of a lamp constructed using LED as a light source
For a specific LED, plus the specified forward bias, for example, after adding IF=20mA forward current (corresponding VF≈3.4V), the measured radiation flux Φ=1.2lm, then the lumen of this LED The efficiency is:
η=1.2lm×1000/3.4V×20mA=1200/68≈17.6lm/W
Obviously, for a single LED, such as the applied electrical power Pe = VF × IF, then the measured luminous flux at this power is converted to lumens per watt, which is the lumen efficiency of a single LED.
However, as a luminaire, no matter what the actual power VF×IF is added to the LED PN junction, the electric power of the luminaire is always the electric power input from the input port of the luminaire. It includes the power supply part (such as voltage regulator, steady current source, The power consumed by AC rectification into a DC power supply, etc.). In a luminaire, the presence of a driver circuit makes its lumen efficiency lower than that of testing a single LED. The greater the circuit loss, the lower the lumen efficiency. Therefore, it is extremely important to find a high-efficiency LED driver circuit.
2, the blue LED is coated with a special phosphor to form a white LED, the radiation flux will be more than ten times higher than
From the front we already know how to make white LEDs. One method is to apply a layer of YAG phosphor on the blue-emitting LED chip , and some blue photons excite the YAG phosphor to form light-to-light conversion and fluorescence. The powder is excited to produce a yellow photon, and the blue light and the yellow light are mixed into white light to become a white LED. This mixing of light of different wavelengths after light-to-light conversion will broaden its spectrum, and white LEDs generally have a much broader spectrum than the blue spectrum of LEDs. For white LEDs made with blue chip and YAG phosphor, the human eye's visual function should be the integral average of the visual functions of various wavelength components compared to the monochromatic LED. This value can be calculated to be about 296 lm. That is, such a white LED, when emitting white light with an optical power of 1 W, has a radiation flux of about 296 lm, which is 7.2 times larger than the radiation flux 41 of the blue LED that emits 1 W.
3. How does the junction temperature of the LED produce?
The basic structure of an LED is a semiconductor PN junction. Experiments have shown that when current flows through the LED device, the temperature of the PN junction will rise. In the strict sense, the temperature of the PN junction region is defined as the junction temperature of the LED. Usually because the device chip has a small size, we can also consider the temperature of the LED chip as the junction temperature.
The material of the window layer substrate or the junction region and the conductive silver paste have a certain resistance value, and these resistance values ​​are mutually added to form a series resistance of the LED. When current flows through the PN junction, these resistors also flow through, which also produces Joule heat, causing the chip temperature or junction temperature to rise; because the LED chip material has a much larger refractive index than the surrounding dielectric. As a result, most of the light generated inside the chip cannot overflow the interface smoothly, and the total reflection at the interface between the chip and the medium is returned to the inside of the chip and is finally absorbed by the chip material or substrate through multiple internal reflections, and is in the form of lattice vibration. It turns into heat, causing the junction temperature to rise.
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