Eight classic questions and answers on LED design

　I have collected some classic basic problems in LED application design and shared them with you. The content involved includes the analysis of the similarities and differences between the lumen efficiency of a single LED and the lumen efficiency of a lamp using LED as a light source, the principle of LED junction temperature and the impact of increased junction temperature on LED, the principle of electrostatic damage and the list of some types of LEDs that are easily damaged by electrostatic damage and fail, the discussion of whether a varistor can be used for lightning protection of LED street lights , the interpretation of the method of designing high-quality LED drive circuits and the factors to consider when selecting and designing LED drive power supplies .

　　Q: 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?

　　A: For a specific LED, add the specified forward bias, such as adding IF = 20mA forward current (corresponding to VF ≈ 3.4V), the measured radiant 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 to lumens per watt is the lumen efficiency of a 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 drive circuit makes its lumen efficiency lower than the lumen efficiency 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 drive circuit.

　　Q: What is the junction temperature of an LED? What effect will the increase in junction temperature have on the LED?

　　A: The basic structure of LED is a PN junction of a semiconductor. When current flows through the 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.

　　When the temperature of the PN junction (such as the ambient temperature) rises, the ionization of impurities inside the PN junction accelerates, and the intrinsic excitation accelerates. 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 semiconductor resistivity due to the decrease in 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 electrons/holes 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, and the energy of the vibration also increases. When it exceeds a certain value, the electrons/holes will exchange energy with the lattice atoms (or ions) when they transition 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 increase at 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 energy level splitting, resulting in changes in the spectrum produced during electron transitions. 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.), and in severe cases will cause the LED to fail. Therefore, 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 easily damaged by electrostatic discharge and fail?

　　A: Static electricity is actually composed of accumulated charges. In daily life, especially in dry weather, people will 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 accumulated static electricity 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 surface 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, and the LED fails.

　　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.

　　Q: Can a varistor be used for lightning protection of LED street lights?

　　Answer: Lightning protection design for street lamps is not a simple question. First of all, you need to understand the overall architecture of your street lamp system. For example, if you use AC-->switching power supply-->constant current source--> LED light source , then you should first consider lightning protection for the switching power supply. Lightning strikes are often introduced by AC wires, and the first subject to be attacked should be the switching power supply. The DC voltage output by the switching power supply is theoretically a clean power supply, so the impact of lightning strikes on the constant current source is already very small. LED street lamps are generally purchased with ready-made switching power supplies, so when you choose, you should specifically purchase a switching power supply that can protect against 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 application?

　　A: Isolation means that there is no direct electrical connection between input and output. From the perspective of safety, the voltage between input and output is generally required to be above 3KV. Currently, most isolation solutions are AC/DC flyback circuit solutions that use transformers as isolation components. Therefore, the circuit is relatively complex and the cost is relatively high. Non-isolated solutions are basically DC/DC boost or buck circuits that use power inductors. The circuit is relatively simple, so the cost is relatively low.

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

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

　　Q: What is the principle and process of LED breakdown by static electricity? What are the effects?

　　Answer: LED is a semiconductor component, and its PN junction is directly exposed to the outside, which is very easy to be exposed to static electricity. When the charges of different polarities on the two electrodes of the LED accumulate to a certain extent (generated charges or transferred charges), and cannot be released in time, once the charge energy exceeds the maximum tolerance of the LED chip, the charge will be discharged between the two electrode layers of the LED in a very short moment (nanosecond level), generating power joules of heat, and a high temperature of more than 1400℃ will be formed locally between the conductive layers (often the area with the smallest resistance value and around the electrodes). The high temperature will melt the conductive layers into some small holes, resulting in leakage, dim light, dead light, electrical drift and other phenomena.

　　The electrostatic energy that breaks down the LED is not a high voltage, but an energy that depends on two core factors: the amount of charge and the duration. The shorter the discharge time, the greater the power, and the more charge, the greater the power. Static electricity breaking down the LED is a very complex process, so the simulation design when testing the LED anti-static is also a very complex and rigorous test.

　　If the LED is seriously damaged by static electricity, it will often be dead light or leakage. If it is slightly damaged by static electricity, the LED will generally not be abnormal, but at this time, the LED has certain hidden dangers. When it is damaged by static electricity for the second time, it will appear dim, dead light, and leakage will increase.

　　Q: How to design a high-quality LED driver circuit?

　　A: LED light source is a long-life light source, with a theoretical life of up to 50,000 hours, but unreasonable application circuit design, improper selection of circuit components, and poor heat dissipation of LED light sources will affect its service life. Especially in the application circuit, the electrolytic capacitor used as the output filter of the AC/DC rectifier bridge has a service life of less than 5,000 hours, which has become a stumbling block in the manufacture of long-life LED lamp technology. Designing and producing a new generation of LED driver ICs that can save electrolytic capacitors in the application circuit is a feasible solution.

　　In addition, the design of the new generation of LED driver ICs must break the traditional DC/DC topology design concept, such as using constant power, not using hysteresis control buck type but using fixed frequency and constant current control, solving the problem of light source flickering and multiple lamps not lighting up in parallel caused by using halogen lamp electronic transformers, etc.; it is also necessary to solve the problem of LED driver ICs passing EMC, safety regulations, CE, UL and other certifications in various application circuits; the application circuit should be simple and use fewer components; isolation and non-isolation applications have always been the focus of the business in the battle between safety and efficiency; improve the duty cycle of the PWM controller, etc.

　　Q: What factors should be considered when selecting and designing an LED driver?

　　A: LED driver power supply is a voltage converter that converts power supply into a specific voltage and current to drive LED to emit light. Usually, the input of LED driver power supply includes high-voltage power frequency AC (i.e. mains), low-voltage DC, high-voltage DC, low-voltage high-frequency AC (such as the output of electronic transformer), etc. The output of LED driver power supply is mostly a constant current source that can change the voltage as the forward voltage drop value of the LED changes. According to the power consumption rules of the power grid and the characteristics of LED driver power supply, the following points should be considered when selecting and designing LED driver power supply:

　　1. High reliability: Especially for the driving power supply of LED street lamps, which is installed at high altitude, it is inconvenient to maintain and the maintenance cost is also high.

　　2. High efficiency: LED is an energy-saving product, and the efficiency of the driving power supply must be high. This is especially important for the structure where the power supply is installed in the lamp. Because the luminous efficiency of LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The high efficiency of the power supply means that its power consumption is small, and the heat generated in the lamp is small, which also reduces the temperature rise of the lamp. This is beneficial to delaying the light decay of the LED.

　　3. High power factor: Power factor is the load requirement of the power grid. Generally, there is no mandatory index for electrical appliances below 70 watts. Although a low power factor of a single low-power electrical appliance has little impact on the power grid, when everyone turns on lights at night, the same load is too concentrated, which will cause serious pollution to the power grid. For 30-40 watt LED driver power supplies, it is said that in the near future, there may be certain index requirements for power factor.

　　4. Driving mode: There are two common methods now: one is that a constant voltage source supplies multiple constant current sources, and each constant current source supplies power to each LED separately. This method is very flexible in combination, and the failure of one LED does not affect the operation of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, and the LEDs are operated in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and the problem of a certain LED failure not affecting the operation of other LEDs must be solved. These two forms coexist for a period of time. The multi-channel constant current output power supply method will be better in terms of cost and performance. Perhaps it will be the mainstream direction in the future.

　　5. Surge protection: LEDs are relatively poor at resisting surges, especially reverse voltage. It is also important to strengthen this protection. 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 will invade from the grid system, and some surges will cause damage to the LEDs. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from being damaged.

　　6. Protection function: In addition to the conventional protection function of the power supply, it is best to add LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

　　7. For external installation of protective lamps, the power supply structure should be waterproof and moisture-proof, and the outer shell should be sun-resistant.

　　8. The life of the driver should match the life of the LED.

　　9. It must comply with safety regulations and electromagnetic compatibility requirements.