IACS CO., LIMITED (Yiwu Fuwei e-commerce firm)

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 composed of LED as a light source. The junction temperature principle of LED and the effect of rising junction temperature on LEDs, the principle of electrostatic damage 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 to be considered when designing 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 ≈ 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 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 junction temperature increase 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 regard 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 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 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 opticalperformance 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 changes, causing the energy level to split, resulting in a change in the spectrum produced by 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 packaging 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? What 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, 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 doors and windows is accumulated 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.

Q: Can LED street light lightning protection use a varistor?

A: The lightning protection design of street lamps is not a simple problem. First of all, you need to understand the overall architecture of your street lighting system, such as AC-->Switching Power Supply-->Constant Current Source-->LED Light Source Solution. Then, you should first consider the lightning protection of switching power supply. The intrusion of lightning strike is often The main body that is invaded by the AC wire should be the switching power supply. The DC voltage output by the switching power supply is theoretically a clean power supply. So far, the impact of the lightning strike on the constant current source is very small. LED street lights are generally purchased with ready-made switching power supplies. Therefore, when you choose, you can purchase a switch power supply that can prevent lightning strikes. That is, a lightning protection circuit has been set at the input end of the switching power supply.

Q: What are the advantages and disadvantages of isolated and non-isolated drive solutions? How to choose in the application?

Answer: The so-called isolation means that there is no direct electrical connection between the input and the output. From the safety requirements, it is generally required that the input/output withstand voltage is above 3KV. The current isolation schemes are mostly AC/DC flyback circuit schemes in which the transformer is used as an isolation component, so the relative circuit is complicated and costly. The non-isolated type is basically a DC/DC boost (bost) or buck (buck) circuit with power inductors. The relative circuit is simpler and the cost is relatively low.

Due to the large differences in circuits, most chips do not have the possibility to implement both solutions at the same time. From the perspective of constant current accuracy, the isolation type can be achieved within ±5%, while the non-isolated type is difficult to achieve.

At present, in the LED lamps that use the mains as the input power source (especially the lamps that are integrated with the light source), the principle of safety first is basically no need to adopt a non-isolated scheme. However, there are exceptions. LED fluorescent tubes still use non-isolated solutions due to structural and space constraints. In low-voltage-powered LED luminaires, non-isolated solutions are the best choice based on efficiency and cost priority.

Q: What is the principle and process of LED breakdown by static electricity? What will happen?

A: LED is a semiconductor component, its PN junction is directly exposed to the outside, it is easy to contact static electricity. When the charge of different polarity on the two electrodes of the LED accumulates (the charge generated or the transferred charge) to a certain extent, and is not released in time, once the charge energy exceeds the maximum value of the LED chip, the charge will be extremely short. Instantaneous (nanosecond level) discharge between the two electrode layers of the LED, generating heat of Joules of power. Locally (often the smallest resistance, the periphery of the electrode) will form a high temperature above 1400 °C, and the high temperature will be high. Will melt the conductive layer into small holes, resulting in leakage, dark, dead lights, electrical drift and so on.

This electrostatic energy that breaks down the LED is not a high voltage. Academically speaking, it is an energy, which depends on the amount of charge and the length of time. The shorter the time of the discharge, the greater the power, and the more the charge, the greater the power. Electrostatic breakdown of LEDs is a very complicated process. Therefore, the analog design for testing LED antistatic is also a very complicated and rigorous test.

If the LED is damaged by static electricity, it is often a dead light or a leakage. Slight electrostatic damage, LED generally has no abnormalities, but at this time, the LED has a certain hidden danger, when it is subject to secondary electrostatic damage, it will appear dark, dead lights, leakage increases.

Q: How do I design a high quality LED driver circuit?

Answer: LED light source is a long-life light source with a theoretical life of up to 50,000 hours. However, the application circuit design is unreasonable, the circuit components are not properly selected, and the LED light source is not well cooled, which will affect its service life. Especially in the application circuit, the electrolytic capacitor, which is the output filter of the AC/DC rectifier bridge, has a service life of less than 5,000 hours. This has become a roadblock for manufacturing long-life LED luminaire technology, which can be designed and produced in the application circuit. A new generation of LED driver ICs that eliminate electrolytic capacitors is a viable solution.

In addition, the design of a new generation of LED driver ICs must break the traditional DC/DC topology design concept, such as constant-power, step-down type without hysteresis control, fixed-frequency constant current control, and use of halogen-type electronic transformers. The generated light source flashes and the multiple lamps are not lit in parallel, etc.; the LED driver IC must also be able to pass EMC, safety, CE, UL and other certifications in various application circuits; the application circuit should be simple and use components. Less; isolated and non-isolated applications have always been the focus of business security and efficiency; increase the duty cycle of the PWM controller and so on.

Q: What factors should I consider when selecting and designing an LED driver?

A: The LED driver power supply is a voltage converter that converts the power supply to a specific voltage and current to drive the LED to emit light. Normally, the input of the LED driving power source includes high-voltage power frequency AC (ie, mains), low-voltage DC, high-voltage DC, Low-voltage high-frequency AC (such as the output of an electronic transformer). The output of the LED driver is mostly a constant current source that changes the voltage as the LED's forward voltage drop changes. According to the power consumption rules of the power grid and the characteristics of the LED drive power supply, the following points should be considered when selecting and designing the LED drive power supply:

1. High reliability: Especially like the driving power of LED street lamps, it is installed at high altitude, it is inconvenient to repair, and the maintenance cost is also large.

2. High efficiency: LED is an energy-saving product, and the efficiency of driving the power supply is high. It is especially important for the structure in which the power supply is installed in the luminaire. Since the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The power source has high efficiency, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LEDs.

3. High power factor: The power factor is the grid's load requirements. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single power consumer with a small power has little effect on the power grid, at night, everyone lights up, and the similar load is too concentrated, which will cause serious pollution to the power grid. For 30 watts to 40 watts of LED driver power, it is said that in the near future, there may be certain indicators for power factors.

4. Drive mode: There are two types of traffic: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is very flexible, one LED failure, does not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current supply, with LEDs running in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it is necessary to solve a certain LED failure without affecting the operation of other LEDs. These two forms coexist for a while. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Perhaps it is the mainstream direction in the future.

5. Surge protection: LED's ability to withstand surge is relatively poor, especially against reverse voltage capability. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the start-up of the grid load and the induction of lightning strikes, various surges are invaded from the grid system, and some surges can cause LED damage. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.

6. Protection function: In addition to the conventional protection function, it is better to increase the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

7. The protective lamp is installed outside, the power structure should be waterproof and moisture-proof, and the outer casing should be light-resistant.

8. The life of the drive power supply is adapted to the life of the LED.

9. To comply with safety and electromagnetic compatibility requirements.

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