Engineer's ten-year experience summary: LED design questions and answers
Through more than ten years of research and study on LED design. I have collected some classic basic questions in LED application design and share it with everyone. It involves the lumen efficiency of a single LED and the lumen efficiency of a lamp made up of LEDs. The junction temperature principle of LED and the effect of rising junction temperature on LEDs, the principle of electrostatic breakdown and enumeration of some types The LED is easily damaged by static electricity, and the problem of using a varistor for lightning protection of LED street lamps is discussed. The method of designing high-quality LED driver circuit and the factors for selecting and designing LED driver power are discussed.
Q: What are the similarities and differences between the lumen efficiency of a single LED and the lumen efficiency of a fixture made up of LEDs?
A: For a specific LED, plus the specified forward bias, such as adding IF = 20mA forward current (corresponding VF ≈ 3.4V), the measured radiant flux Φ = 1.2lm, then this LED The lumen efficiency is η = 1.2 lm × 1000 / 3.4 V × 20 mA = 1,200 / 68 ≈ 17.6 lm / 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, AC). The power consumed by rectification into a DC power supply section, 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.
Q: What is the junction temperature of LED? What effect does the junction temperature increase have on LED?
A: The basic structure of the LED is a semiconductor PN junction. When current flows through the LED device , the temperature of the PN junction will rise. In a 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.
When the temperature of the PN junction (e.g., ambient temperature) rises, the ionization of impurities inside the PN junction is accelerated, and the intrinsic excitation is accelerated. When the concentration of the 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 a decrease in 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 LED is not driven by a constant current source, 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 at the PN junction degrades the process of emitting photons when transitioning from a high energy level to a low energy level in the excited electron/hole recombination in the semiconductor PN junction. 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 electron/hole transitions from the excited state to the ground state and 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. The symmetry of the lattice field is changed, 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 temperature rise of the PN junction.
In summary, the temperature rise on 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 package materials (eg, epoxy, phosphor, etc.). In severe cases, the LED will fail, so reducing the temperature rise of the PN junction is an important key to the application of LEDs.
Q: What is electrostatic damage? Which types of LEDs are susceptible to static damage and cause failure?
A: Static electricity is actually composed of charge accumulation. In daily life, especially in dry weather, when you touch the door and window items by hand, you will feel “electric shockâ€. 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 there is the threat of static electricity, led chip and device for the above-mentioned structures have to take anti-static measures for processing plants, machines, tools, instruments, clothing including employees in the process, to ensure that does not damage the LED. In addition, antistatic materials should also be used on the packaging of chips and devices.
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