LED application knowledge sharing

Publisher:影子猎人Latest update time:2011-11-28 Source: OFweek半导体照明网Keywords:LED Reading articles on mobile phones Scan QR code
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1. What are the similarities and differences between the lumen efficiency of a single LED and the lumen efficiency of a lamp that uses LED as a light source?

For a specific LED, add the specified forward bias, for example, add IF = 20mA forward current (corresponding to VF ≈ 3.4V), the measured radiation luminous flux Φ = 1.2lm, then the lumen efficiency of this LED is:

η=1.2lm×1000/3.4V×20mA=1200/68≈17.6lm/W

Obviously, for a single LED, if the applied power Pe = VF × IF, then the radiant flux measured at this power converted into lumens per watt is the lumen efficiency of the single LED.

However, as a lamp, no matter how much power VF×IF is actually added to the LED PN junction , the electrical power of the lamp is always the electrical power sent to the lamp input port, which includes the power consumed by the power supply part (such as the voltage stabilizer, the current stabilizer, the AC rectifier to the DC power supply part, etc.). In the lamp, the existence of the driving circuit makes its lumen efficiency lower than the lumen efficiency of a single LED in the test . The greater the circuit loss, the lower the lumen efficiency. Therefore, it is extremely important to find a high-efficiency LED driving circuit .

2. Why is the radiant luminous flux of a blue light LED several times or even dozens of times higher than that of a blue light LED after being coated with a special phosphor to form a white light LED ?

We already know how white light LEDs are made. One of the methods is to apply a layer of YAG phosphor on a blue light LED chip . Some blue light photons excite the YAG phosphor to form a photoelectric conversion. The phosphor is excited to produce yellow light photons. The blue light and yellow light are mixed to become white light, which becomes a white light LED. This mixture of light of different wavelengths after photoelectric conversion will make its spectrum wider. White light LEDs generally have a much wider spectrum than LED blue light spectrum. For white light LEDs made of blue light chips and YAG phosphors, compared with monochromatic LEDs, the visual function of the human eye should be the integrated average of the visual functions of various wavelength components. This value can be calculated to be about 296lm, that is, when this white light LED emits white light with a light power of 1W, its radiant luminous flux is about 296lm, which is 7.2 times greater than the radiant luminous flux of a blue LED with a light power of 1W.

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

The basic structure of LED is a PN junction of a semiconductor . Experiments show that when current flows through an LED device, the temperature of the PN junction will rise. Strictly speaking, the temperature of the PN junction area is defined as the junction temperature of the LED. Usually, since the device chip has a very small size, we can also regard the temperature of the LED chip as the junction temperature.

The window layer substrate or the material of the junction area and the conductive silver paste all have certain resistance values, which are added together to form the series resistance of the LED. When the current flows through the PN junction, it will also flow through these resistors, which will also generate 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 medium, most of the light generated inside the chip cannot smoothly overflow the interface, but is totally reflected 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 becomes heat in the form of lattice vibration, causing the junction temperature to rise.

4. Why does the increase in temperature on the LED PN junction cause its photoelectric parameters to degrade?

As an impurity semiconductor, the PN junction also has problems such as impurity ionization, intrinsic excitation, impurity scattering and lattice scattering during its operation, which changes the number and efficiency of composite carriers converted into photons. When the temperature of the PN junction (such as the ambient temperature) rises, the ionization of impurities inside the PN junction is accelerated, and the intrinsic excitation is accelerated. When the concentration of composite carriers generated by intrinsic excitation far exceeds the impurity concentration, the impact of the increase in the number of intrinsic carriers is more serious than the impact of the change in the resistivity of the semiconductor with reduced mobility, resulting in a decrease in internal quantum efficiency. The increase in temperature leads to a decrease in resistivity, which reduces VF under the same IF. If the LED is not driven by a constant current source, the VF drop will cause the IF to increase exponentially. This process will accelerate the temperature rise on the LED PN junction, and eventually the temperature rise will exceed the maximum junction temperature, causing the LED PN junction to fail. This is a vicious process of positive feedback.

The temperature rises on the PN junction, causing the process of emitting photons when the excited electron-hole in the semiconductor PN junction recombines from the high energy level to the low energy level. This is because when the temperature on the PN junction rises, the amplitude of the semiconductor lattice increases, causing the vibration energy to increase. When it exceeds a certain value, the electron-hole will exchange energy with the lattice atoms (or ions) when it transitions from the excited state to the ground state, thus becoming a transition without photon radiation, and the optical performance of the LED degrades.

In addition, the temperature rise on the PN junction will also cause the lattice field formed by the ionized impurity ions in the impurity semiconductor to cause the ion energy level fission. The energy level splitting is affected by the PN junction temperature, which means that the temperature affects the lattice vibration, causing the symmetry of the lattice field to change, thereby causing the energy level splitting, resulting in changes in the spectrum produced during electron transition . This is why the LED emission wavelength changes with the PN junction temperature rise.

In summary, the temperature rise on the LEN PN junction will cause changes in its electrical, optical and thermal properties. Excessive temperature rise will also cause changes in the physical properties of LED packaging materials (such as epoxy, phosphor, etc.), which may lead to LED failure in severe cases. Therefore, reducing the temperature rise of the PN junction is an important key to the application of LEDs.

5. Why is it said that improving light efficiency can reduce junction temperature?

The light energy generated by a unit of input electrical power is usually called photoelectric conversion efficiency, or light efficiency for short. According to the law of conservation of energy, the input power of the LED will eventually be released in the form of light and heat. The higher the light efficiency, the less heat is released, and the smaller the temperature rise of the LED chip. This is the basic principle that improving light efficiency can reduce junction temperature.

6. How to achieve LED dimming and color adjustment?

Since the luminous intensity IV (or light radiation flux) of LED is linearly related to its working current IF within a certain current range, that is, as the current IF increases, IV also increases accordingly, therefore, changing the IF of LED can change its luminous intensity and achieve dimming.

According to the principle of colorimetry, if the three primary colors of red, green and blue are mixed, under the appropriate combination of the three primary colors brightness ratio, countless colors can be obtained in theory. This can be achieved by using three LEDs with three wavelengths of light, for example, 470nm (blue), 525nm (green) and 620nm (red). By lighting and IF control, color regulation, that is, color adjustment, can be achieved.

7. What is electrostatic damage? Which types of LEDs are easily damaged by electrostatic discharge and fail?

Static electricity is actually composed of accumulated charges. In daily life, especially in dry weather, people feel "electric shock" when they touch doors and windows with their hands. This is the "discharge" of static electricity from doors and windows to the human body when the static electricity accumulates to a certain level. For wool fabrics and nylon chemical fiber items, the voltage of static electricity accumulation can be as high as more than 10,000 volts. The voltage is very high, but the power of static electricity is not large and will not threaten life. However, it can be fatal to some electronic devices and cause device failure.

The GN-based devices in LEDs have a high resistivity due to the wide bandgap semiconductor material. For the InGaN/AlGaN/GaN double heterojunction blue light LED, the thickness of the InGaN active layer is generally only tens of nanometers. Since the two positive and negative electrodes of this LED are on the same side of the chip and 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, increasing its leakage. In severe cases, the PN junction breaks down and short-circuits, causing the LED to fail.

Because of the threat of static electricity, anti-static measures must be taken for the processing plant, machines, tools, instruments, and employee clothing during the processing of LED chips and devices of the above structure to ensure that the LED is not damaged. In addition, anti-static materials should also be used in the packaging of chips and devices.

Keywords:LED Reference address:LED application knowledge sharing

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