In order to save energy and electricity, LEDs have been greatly promoted, but LEDs need a power driver, the quality of which will directly affect the life of the LED. Therefore, how to make an LED driver power supply is the top priority for LED power supply designers. . This article introduces some LED driving power issues, hoping to provide some help to engineers.
1. The drive circuit directly affects the life of the LED. What we call LED drivers include digital drivers and analog drivers. Digital drivers refer to digital circuit drivers, including digital dimming control, RGB full-color changes, etc. Analog drive refers to analog circuit drive, including AC constant current switching power supply and DC constant current control circuit. The drive circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life. Failure of any component will lead to the failure of the entire circuit or partial functional failure. The service life of LED is 50,000-100,000 hours. Calculated at 50,000 hours, if it is continuously lit, it has a lifespan of nearly 6 years. It is difficult for a switching power supply to have a service life of 6 years. The warranty period of switching power supplies sold on the market is generally 2-3 years. Power supplies with a 6-year warranty are military grade and the price is 4-6 times that of ordinary power supplies. Generally, It is difficult for the lighting factory to accept this. Therefore, the faults of LED lamps are mostly caused by drive circuit faults. < /p>
2. Heat dissipation problem LED is a cold light source, and the operating junction temperature cannot exceed the limit, and a certain margin must be left during design. The design of the entire lamp must consider many aspects such as beautiful appearance, easy installation, light distribution, heat dissipation, etc. It is necessary to find a balance point among many factors, so that the overall lamp is the best. LED lamps have not been developed for a long time, and we can learn from them. I don’t have much experience, and many designs are constantly being improved. Some LED lamp manufacturers use outsourced or outsourced power supplies. Lamp designers don’t know much about power supplies, so there is a large space for heat dissipation for LEDs and a small space for heat dissipation for power supplies. Generally, after the lamps are designed, a suitable power supply is found to match them, which brings a certain degree of difficulty to the matching of the power supply. It is often encountered that the lamp cannot be equipped with a suitable power supply due to the small internal space or high internal temperature, and low cost control. Some LED lighting manufacturers have power supply R&D capabilities. They can solve the above problems by conducting evaluations at the early stages of designing lamps and simultaneously designing power supplies. In the design, the heat dissipation of the LED and the heat dissipation of the power supply must be comprehensively considered, and the temperature rise of the lamp should be controlled as a whole, so that a better lamp can be designed.
3.Problems in power supply design a. Power design. Although LED light efficiency is high, there is still 80-85% heat loss, resulting in a temperature rise of 20-30 degrees inside the lamp. If the room temperature is 25 degrees, the temperature inside the lamp will be 45-55 degrees. The power supply is in a high temperature environment for a long time. When working under low conditions, to ensure the service life, the power margin must be increased, generally leaving a margin of 1.5-2 times. b. Component selection. When the internal temperature of the lamp is 45-55 degrees, the internal temperature rise of the power supply is still about 20 degrees, so the temperature near the components should reach 65-75 degrees. The parameters of some components will drift at high temperatures, and the life of some components will be shortened. Therefore, components should be selected that can be used at higher temperatures for a long time, paying special attention to electrolytic capacitors and wires. c. Electrical performance design. The parameter design of the switching power supply for LEDs is mainly constant current parameters. The size of the current determines the brightness of the LED. If the batch current error is large, the brightness of the entire batch of lights will be uneven. Moreover, changes in temperature can also cause the power supply output current to deviate. Generally, the batch error must be controlled within +/-5% to ensure consistent brightness of the lamp. The forward voltage drop of LEDs has deviations, and the constant current voltage range of the power supply design must include the voltage range of the LED. When multiple LEDs are connected in series, the minimum voltage drop multiplied by the number of series connections is the lower limit voltage, and the maximum voltage drop multiplied by the number of series connections is the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range. Generally, the upper and lower limits are different. Leave 1-2V margin. d. PCB layout design. LED lamps have a smaller size left for the power supply (unless the power supply is external), so the requirements for PCB design are higher and there are many factors to consider. Enough safety distance must be left, the input and output power supply must be isolated, the primary side circuit and the secondary side circuit must withstand a voltage of 1500-2000VAC, and at least a sufficient distance of 3MM must be left on the PCB. If it is a lamp with a metal shell, the layout of the entire power supply should also consider the safe distance between the high-voltage part and the shell. If there is no space to ensure a safe distance, other measures must be used to ensure insulation, such as drilling holes in the PCB, adding insulating paper, potting insulating glue, etc. In addition, the heat balance must be considered when laying out the board. The heating elements must be evenly distributed and cannot be placed centrally to avoid local temperature increases. Keep electrolytic capacitors away from heat sources to slow down aging and extend service life. e. Certification issues. At present, there are no standards for LED lamps in China. Relevant national departments are studying and formulating them. Certification of lamps sold domestically is based on the standards of lighting lamps, while those sold abroad are certified by CE or UL, and some are based on foreign LED lamp standards. Therefore, in this case, it is difficult for the design of switching power supply to meet the above standards at the same time. We can only meet different certifications for different requirements.
4. Problems in power supply applications
5. Use parameters. The selection of outsourced power supplies mainly depends on the constant current and constant current voltage range. The constant current value is selected to be lower than the standard current of the LED. The choice of voltage range should be moderate, and try not to choose a larger range to avoid wasting power.