In recent years, the continuous progress of LED manufacturing technology and the development and application of new materials (nitride crystals and phosphors), breakthroughs in ultra-high brightness LEDs of various colors, the luminous efficiency has increased nearly 1000 times, chroma All the colors in the visible light range have been realized, the most important of which is the emergence of ultra-high brightness white LEDs, making it possible for LED applications to leap into the high-efficiency lighting source market.
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 LED street light lightning protection is discussed. The method of designing a high-quality LED driving circuit and the factors for selecting and designing the LED driving power supply are explained.
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 â‰ˆ 3.4V), the measured radiant flux Î¦ = 1.2lm, then this LED The lumen efficiency is Î· = 1.2 lm Ã— 1000 / 3.4 V Ã— 20 mA = 1,200 / 68 â‰ˆ 17.6 lm / W. Obviously, for a single LED, such as the applied electrical power Pe = VF Ã— 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 VFÃ—IF is on the LEDPN 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 rectification). The power consumed by the 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 the LED? What effect does the increase in junction temperature have on the 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 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.
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 constant current source is not used to drive the LED, the VF drop will cause the IF to increase exponentially. This process will accelerate the temperature rise of the LEDPN junction. The final temperature rise exceeds the maximum junction temperature, causing the LEDPN junction to fail. This is a positive feedback malignancy. 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.
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