LED application articles
1, A single led lumen efficiency and LED light source for the composition of the luminous efficiency of the lamp What is the difference?
For a particular LED, plus the provisions of the forward bias, for example, plus IF = 20mA forward current (corresponding to VF ≈ 3.4V), the measured radiation flux Φ = 1.2lm, then the LED lumens The efficiency is:
Η = 1.2 lm × 1000 / 3.4 V × 20 mA = 1200/68 ≈ 17.6 lm / W
Obviously, for a single LED, such as the applied electric power Pe = VF × IF, then the measured luminous flux at this power is converted to the lumen value per watt is the lumen efficiency of a single LED.
However, as a fixture, regardless of the actual power on the LED PN junction VF × IF is how much, the lamp power is always the input power of the lamp input port, which includes the power part (such as regulators, AC rectifier into a DC power supply, etc.). In the fixture, the presence of the drive circuit makes its lumen efficiency lower than that of a single LED. The greater the circuit loss, the lower the luminous efficiency, therefore, looking for a high efficiency LED driver circuit is extremely important.
2, Why a blue LED coated with a special phosphor composed of white LED, the radiation flux than blue light several times or even ten times?
From the front we already know what method is produced by white LED, one of which is in the blue LED chip coated with a layer of YAG phosphor, part of the blue photon excited YAG phosphor, the formation of light conversion, fluorescence Powder is excited to produce yellow photons, blue light and yellow light mixed into white light, become white LED. This combination of light at different wavelengths after light conversion will make its spectrum broaden, and white LEDs generally have a much wider spectrum than the LED blue-light spectrum. For a white LED made with a blue chip plus a YAG phosphor, the visual function of the human eye should be an integral mean of the visual function of the various wavelength components compared to a monochrome LED, which can be calculated by about 296 lm , That is, this white LED, when emitting light power of 1W white light, the radiation flux is about 296lm, this value than the emission of light power 1W blue LED radiated flux 41 increased by 7.2 times.
3, What is the junction temperature of the LED? How did it produce
The basic structure of the LED is a semiconductor PN junction. Experiments show that when the current flows through the LED device, PN junction temperature will rise, strictly speaking, put the PN junction temperature is defined as the LED junction temperature. Usually because the device chip has a very small size, so we can also LED chip temperature as the junction temperature.
Window layer of the substrate or the junction of the material and conductive silver plastic and so there is a certain resistance value, the resistance value of each other base, constitute the LED series resistance. When the current flows through the PN junction, but also through these resistors, which will also produce Jouer heat, causing the chip temperature or junction temperature rise; LED chip materials around the circumference, with a much greater refractive index , So that most of the light generated inside the chip can not smoothly overflow the interface, and the chip and the media interface to produce total reflection, back to the chip and through a number of internal reflections eventually absorbed by the chip material or substrate, and lattice vibration Become hot, to promote junction temperature rise.
4, Why LED PN junction temperature rise will cause its photoelectric parameters degradation?
PN junction as impurity semiconductor in its work process, the same existence of impurity ionization, intrinsic excitation, impurity scattering and lattice scattering and other issues, so that the compound flow into the number and efficiency of photons change. When the temperature of the PN junction (such as ambient temperature) increases, the impurity ionization in the PN junction is accelerated and the intrinsic excitation is accelerated. When the concentration of the composite carrier generated by the intrinsic excitation far exceeds the impurity concentration, the effect of the increase in the number of intrinsic carriers is more serious than that of the semiconductor resistivity with reduced mobility, Efficiency decreases, the temperature rise and lead to a decrease in resistivity, so that the same IF, VF reduced. If you do not use constant current source to drive the LED, the VF drop will cause the IF index to increase, this process will make the LED PN junction temperature rise faster, the final temperature rise exceeds the maximum junction temperature, resulting in LED PN junction failure, which is a positive feedback The vicious process.
PN junction temperature rise, so that the semiconductor PN junction in the excited state of the electron-hole recombination from high-energy level to low-level transition when the process of photon emission degradation. This is due to the temperature rise of the PN junction, the amplitude of the semiconductor lattice increases, so that the vibration of the energy also increased, when it exceeds a certain value, the electron hole from the excited state transition to the ground state back to the lattice (Or ion) exchange of energy, so as a non-photon radiation transition, LED optical degradation.
In addition, the temperature rise of the PN junction also causes the lattice field formed by the ionization impurity ions in the impurity semiconductor to cause the ion energy level to fission. The energy level division is affected by the PN junction temperature, which means that the lattice vibration The symmetry of its lattice field changes, resulting in energy level splitting, resulting in electronic transition generated when the spectrum changes, which is the LED light emission wavelength with the PN junction temperature rise of the reasons.
In combination with the above, the LEN PN junction temperature rise can cause its electrical, optical and thermal performance changes, too high temperature rise will cause LED packaging materials (such as epoxy, phosphor, etc.) changes in physical properties, LED failure, so reduce the PN junction temperature rise, is the key to the application of LED.
5, Why increase the light effect can reduce the junction temperature?
Usually the unit input power generated by the optical energy called photoelectric conversion efficiency referred to as light efficiency. According to the law of conservation of energy, LED input power will eventually be released through the two forms of light and heat, the higher the luminous efficiency of the less heat, the smaller the LED chip temperature rise, which is to improve the light efficiency can reduce the junction temperature Fundamental.
6, How to achieve LED dimming, color?
As the LED luminous intensity IV (or optical radiation flux) and its working current IF in a certain current range was a county relationship, that is, with the current IF increases, IV also increases, therefore, change the LED IF , You can change its luminous intensity, to achieve dimming.
By the principle of chromaticity can know that if the red, green and blue primary colors for mixing, in the appropriate combination of the three primary colors brightness ratio, theoretically can be obtained numerous colors, which can use three kinds of light-emitting wavelength LED, as long as the For example: 470nm (blue), 525 nm (green) and 620 nm (red) of the three wavelengths of the LED through the lighting and IF control, you can achieve color control, that is, color.
7, What is static damage? What types of LEDs are susceptible to electrostatic damage leading to failure?
Static electricity is actually composed of charge accumulation. People in their daily lives, especially in dry weather, when the hand to touch the doors and windows items will feel "electric shock", which is the windows and doors items accumulated to a certain degree of static electricity on the human body "discharge." For wool fabric, nylon chemical fiber products, the accumulation of static electricity can be as high as 10,000 volts, the voltage is very high, but the static power is not big, not life-threatening, but for some electronic devices can be fatal, resulting in device failure.
LED in the GN with the composition of the device, because it is a wide bandgap semiconductor material, its high resistivity, ForGaN / AlGaN / GaN double heterojunction blue light LED, the InGaN active layer thickness is generally only a few Ten nanometer, and then because of the two positive and negative electrodes on the same side of the chip, the distance between the two is very small, if the static charge at both ends accumulated to a certain value, the static voltage will PN breakdown Leakage increases, serious PN junction breakdown short circuit, LED failure.
Because of the existence of electrostatic threats, for the structure of the LED chips and devices in the processing of the plant, machinery, tools, equipment, including staff clothing to take anti-static measures to ensure that no damage to the LED. In addition, in the chip and device packaging also use anti-static material.