LED application articles

- Mar 20, 2019 -

1. What are the similarities and differences between the lumen efficiency of a single led and the lumen efficiency of a fixture using LEDs 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 on 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. Why does a blue LED have a radiant flux that is several times or even ten times higher than that of a blue light after it is coated with a white LED made up of a special phosphor?

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 a white LED made with a blue chip plus YAG phosphor, the human eye's visual function should be an integral average of the visual functions of various wavelength components compared to a monochromatic LED. This value can be calculated to be approximately 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 emitting light of 1 W.

 

3. What is the junction temperature of the LED? How is it produced?

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 regard 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 returns 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, which causes the junction temperature to rise.

 

4. Why does the temperature rise of the LED PN junction cause its photoelectric parameters to degrade?

In the working process of the PN junction as an impurity semiconductor, there are also problems of ion ionization, intrinsic excitation, impurity scattering and lattice scattering, so that the number and efficiency of conversion of composite carriers into photons are changed. When the temperature of the PN junction (for example, the ambient temperature) rises, the ionization of the impurities inside the PN junction is accelerated, and the intrinsic excitation is accelerated. When the concentration of composite carriers generated by the intrinsic excitation far exceeds the impurity concentration, the influence of the increase in the number of intrinsic carriers is more serious than the change in the semiconductor resistivity with reduced mobility, resulting in internal quantum. The efficiency decreases, and the temperature rise causes the resistivity to drop, so that the VF is lowered under the same IF. If the constant current source is not used to drive the LED, the VF drop will cause the IF to increase exponentially. This process will increase the temperature rise of the LED PN junction, and eventually the temperature rise exceeds the maximum junction temperature, causing the LED PN junction to fail. This is a positive feedback. The vicious process.

The temperature rise on the PN junction degrades the process of emitting photons when transitioning from a high energy level to a low energy level when the electrons in the semiconductor PN junction are in an excited state. This is because when the temperature on the PN junction rises, the amplitude of the semiconductor lattice increases, and the energy of the vibration also increases. When it exceeds a certain value, the electrons and holes return from the excited state to the ground state and return to the lattice atom. (or ions) exchange energy, thus becoming a transition of photon-free radiation, and the optical performance of the LED is degraded.

In addition, the temperature rise on the PN junction also causes the lattice field formed by the ionized impurity ions in the impurity semiconductor to fission the ion level, and the energy level split is affected by the PN junction temperature, which means that the temperature affects the lattice vibration, so that The symmetry of the lattice field changes, causing the energy level to split, resulting in a change in the spectrum generated during the electronic transition. This is why the LED emission wavelength changes with the PN junction temperature rise.

In summary, the temperature rise of the LEN PN junction causes changes in its electrical, optical, and thermal properties. Excessive temperature rise can also cause changes in the physical properties of LED packaging materials (such as epoxy, phosphor, etc.), which can lead to serious LED failure, so reducing the temperature rise of the PN junction is an important key to the application of LED.

 

5. Why do you think that improving the light efficiency can lower the junction temperature?

Generally, the light energy generated by the unit input electric power is referred to as photoelectric conversion efficiency. According to the law of conservation of energy, the input power of the LED will eventually be released through both light and heat. The higher the light efficiency, the less heat is emitted, and the smaller the temperature rise of the LED chip, which is to improve the junction temperature. Fundamental.

 

6, how to achieve LED dimming, color?

Since the luminous intensity IV (or optical radiant flux) of the LED is in a county relationship with its operating current IF within a certain current range, that is, as the current IF increases, the IV also increases, thus changing the IF of the LED. , you can change its luminous intensity and achieve dimming.

It can be known from the principle of colorimetry that if the three primary colors of red, green and blue are mixed, under the combination of the appropriate brightness ratios of the three primary colors, theoretically, a myriad of colors can be obtained, and it is possible to use three kinds of LEDs of the illumination wavelength, as long as For example, LEDs of three wavelengths of 470 nm (blue), 525 nm (green), and 620 nm (red) can achieve color control, that is, color grading, by lighting and IF control.

 

7. What is electrostatic damage? What types of LEDs are susceptible to static damage and cause failure?

Static electricity is actually composed of charge accumulation. In daily life, especially in dry weather, people feel “electric shock” when they touch the door and window items by hand. This is the “discharge” of the human body when the static electricity of the door and window items accumulates to a certain extent. For wool fabrics and nylon chemical fiber articles, the voltage accumulated by static electricity can be as high as 10,000 volts, and the voltage is very high, but the electrostatic power is not large and will not be life-threatening. However, for some electronic devices, it can be fatal and cause device failure.

A device composed of a GN-based LED has a high resistivity because of its wide bandgap semiconductor material. For a double-heterojunction blue LED of InGaN/AlGaN/GaN, the thickness of the active layer of InGaN is generally only a few. Ten nanometers, because the two positive and negative electrodes of the LED are on the same side of the chip, the distance between them is very small. If the electrostatic charge at both ends accumulates to a certain value, the electrostatic voltage will break down the PN, making it The leakage increases, and in severe cases, the PN junction breaks through and the LED fails.

Because of the static electricity threat, LED chips and devices with the above structure should adopt anti-static measures for processing factories, machines, tools, instruments, including employee clothing during the processing to ensure that LEDs are not damaged. In addition, antistatic materials should also be used on the packaging of chips and devices.