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Key points in LED driver power supply

Click: Time:2022-11-17 11:20:53

In order to save energy and electricity, LED has been greatly promoted, but LED requires a power supply driver, which directly affects the lifespan of the LED. Therefore, how to do a good LED driver power supply is the top priority of LED power supply designers. This article introduces some issues with LED driver power supplies, hoping to provide some help to engineers.

1. The driving circuit directly affects the lifespan of LED

The LED driver we refer to includes two types: digital driver and analog driver. Digital driver refers to digital circuit driver, including digital dimming control, RGB full color transformation, etc. Analogical drive refers to analog circuit drive, including AC constant current switching power supply and DC constant current control circuit. The driving circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life, and any failure of a component will lead to the failure of the entire circuit or partial functional failure. The service life of LED is 50000 to 100000 hours, calculated as 50000 hours, continuously lit, with a lifespan of nearly 6 years. The lifespan of switch power supplies is difficult to reach 6 years. The warranty period of switch power supplies sold on the market is generally 2-3 years, and the price is 4-6 times that of ordinary power supplies. It is difficult for general lighting manufacturers to accept. So the faults of LED lighting fixtures are mostly driver circuit faults.

2. Heat dissipation issues

LED is a cold light source, and the working junction temperature should not exceed the limit, and a certain margin should be left during design. The design of the entire lighting fixture should consider many aspects such as beautiful appearance, convenient installation, light distribution, heat dissipation, etc. It is necessary to seek a balance point among many factors, so that the overall lighting fixture is the best. The development time of LED lighting fixtures is not long, and there are not many experiences to learn from, and many designs are constantly improving. Some LED lamp manufacturers use outsourced or purchased power supplies, and lamp designers have limited knowledge of power supplies. They provide more heat dissipation space for LEDs and less heat dissipation space for power supplies. Usually, after designing the lighting fixtures, a suitable power supply is found to match them, which brings certain difficulties to the matching of the power supply. Often encountered, due to the small internal space or high internal temperature of the lamp, and the low cost control, it is unable to be matched with a suitable power supply. Some LED lamp manufacturers have the ability to research and develop power supplies. They can solve the above problems by conducting evaluations in the early stages of lamp design and synchronizing power supply design. In the design, it is necessary to comprehensively consider the heat dissipation of LED and power supply, and overall control the temperature rise of the lighting fixtures, in order to design better lighting fixtures.

3. Problems in power supply design

a. Power design. Although LED has high light efficiency, 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, there will be a temperature rise of 45-55 degrees inside the lamp. If the power supply works in a high-temperature environment for a long time, it is necessary to increase the power margin to ensure its lifespan, usually leaving a margin of 1.5-2 times.

b. Component selection. When the internal temperature of the lamp is between 45 and 55 degrees, and the internal temperature rise of the power supply is about 20 degrees, the temperature near the component should reach 65 to 75 degrees. The parameters of some components will drift at high temperatures, and the life of some components will be shortened. Therefore, the components should be selected that can be used at high temperatures for a long time, and special attention should be paid to electrolytic capacitor and wires.

C. Electrical performance design. The switch power supply is designed for LED parameters, 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 is uneven. Moreover, changes in temperature can also cause the output current of the power supply to shift. Generally, the batch error is controlled within+/-5% to ensure consistent brightness of the lights. There is a deviation in the forward voltage drop of the LED, and the constant current voltage range of the power supply design should include the voltage range of the LED. When multiple LEDs are used in series, the minimum voltage drop multiplied by the number of serial connections is the lower limit voltage, and the maximum voltage drop multiplied by the number of serial connections is the upper limit voltage. The constant current voltage range of the power supply is slightly wider than this range, and generally there is a margin of 1-2V between the upper and lower limits.

d. PCB layout design. LED lighting fixtures leave a smaller size for the power supply (unless it is external to the power supply), so there are high requirements in PCB design and many factors to consider. Sufficient safety distance should be left, requiring isolated input and output power supply. The primary and secondary circuits should withstand 1500-2000VAC, and a minimum distance of 3MM should be left on the PCB. If it is a metal shell lamp, the entire power supply board should also consider the safety distance between the high-voltage part and the shell. If there is no space to ensure a safe distance, other measures should be taken to ensure insulation, such as drilling holes on the PCB, adding insulation paper, and sealing insulation adhesive. In addition, the cloth board should also consider heat balance, and the heating elements should be evenly distributed and cannot be placed in a centralized manner to avoid local temperature rise. Electrolytic capacitor is far away from heat source, slowing down aging and extending service life.

e. Certification issues. At present, there is no standard for LED lighting fixtures in China, and relevant national departments are studying and formulating it. The certification of lighting fixtures sold in China is based on the standards of lighting fixtures, while those sold abroad are certified for CE or UL, and some are also based on foreign LED lighting fixture standards. Therefore, in response to this situation, it is difficult for the design of switching power supplies to simultaneously meet the above standards. We can only meet different certifications for different requirements.

4. Use parameters.

The selection of outsourced power sources mainly depends on the voltage range of constant current and constant current. The constant current value is selected as the lower standard current of the LED. The selection of voltage range should be moderate, and try not to choose a larger range to avoid power waste.

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