Revealing the secrets of power drive

Detailed explanation of three commonly used LED driving power supplies (switching constant current source/linear IC power supply/resistance-capacitance step-down power supply)

What is an LED driving power supply?

LED driving power supply is a power converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Normally: the input of the LED driving power supply includes high-voltage power frequency AC (i.e., mains power), low-voltage DC , high-voltage DC, low-voltage high-frequency AC (such as the output of electronic transformers), etc.

The output of the LED driving power supply is mostly a constant current source that changes the voltage as the LED forward voltage drop value changes. Characteristics of LED driver power supply

1. High reliability is especially like the driving power supply of LED street lights. It is installed at high altitude and has a waterproof aluminum shell. If the quality is good, it will not break easily and reduce the number of maintenance times.

2. High-efficiency LEDs are energy-saving products, and the efficiency of the driving power supply must be high. This is especially important for structures where the power supply is installed inside the lamp. Because the luminous efficiency of LEDs decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The efficiency of the power supply is high, its power consumption is small, and the heat generated in the lamp is small, which also reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LED.

　3. High power factor Power factor is a requirement of the power grid for the load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factors of individual electrical appliances with low power will have little impact on the power grid, if everyone lights up lights at night and the same type of load is too concentrated, it will cause serious pollution to the power grid. For 30-watt to 40-watt LED driving power supplies, it is said that there may be certain index requirements for power factor in the near future.

　4. There are two current driving methods: one is a constant voltage source that supplies multiple constant current sources, and each constant current source supplies power to each LED individually. In this way, the combination is flexible. If one LED fails, it will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, with LEDs running in series or parallel. Its advantage is that the cost is lower, but the flexibility is poor. It also needs to solve the problem of a certain LED failure without affecting the operation of other LEDs. These two forms coexisted for some time. The multi-channel constant current output power supply method will be better in terms of cost and performance. Maybe it will be the mainstream direction in the future.

　5． Surge protection LED's ability to withstand surges is relatively poor, especially its ability to withstand reverse voltage. Strengthening protection in this area is also important. Some LED lights are installed outdoors, such as LED street lights. Due to the switching of the power grid load and the induction of lightning strikes, various surges will invade from the power grid system, and some surges will cause damage to the LED. Therefore, the LED driving power supply must have the ability to suppress surge intrusion and protect the LED from damage.

　6. In addition to the conventional protection functions of the protection function 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. In terms of protection, the lamp should be installed externally, the power supply structure should be waterproof and moisture-proof, and the outer casing should be light-resistant.

8. The life of the driving power supply must match the life of the LED.

9. Must comply with safety regulations and electromagnetic compatibility requirements.

Detailed explanation of three commonly used LED driving power supplies (switching constant current source/linear IC power supply/resistance-capacitance step-down power supply)

　LED driving power supply shortcomings

　LED driving power supply Reasons for the current shortcomings:

(1) The technical personnel who produce LED lighting and related do not know enough about switching power supplies. The power supplies they make can work normally, but there are some key evaluations and electromagnetic compatibility considerations. If it is not enough, there are still certain hidden dangers;

(2) Most LED power supply production is transformed from ordinary switching power supply to LED power supply, and the understanding of the characteristics and use of LED is not enough;

　(3) There are almost no standards for LEDs, and most of them refer to the standards of switching power supplies and electronic rectifiers;

　(4) Nowadays, most LED power supplies are not unified, so most of them are relatively small. If the purchase quantity is small, the price will be high, and component suppliers are not very cooperative;

(5) Stability of LED power supply: wide voltage input, high and low temperature operation, over-temperature, over-voltage protection, etc. The problems have not been solved one by one.

The development direction of LED drive power supply

1. Improve the life of the drive power supply

One of the most important advantages of LED is its long service life, about 30,000 to 100,000 Hour. Traditional LED driving power supplies contain electrolytic capacitors, and the life of electrolytic capacitors is relatively short, about 5,000 hours. This is the main reason that limits the life of LED driving power supplies. Therefore, electrolytic capacitors should be avoided as much as possible in LED driving power supplies.

　2. Improve the input power factor of the driving power supply and reduce the total harmonic distortion. The "ENERGYSTAR" solid-state lighting document issued by the U.S. Department of Energy stipulates that any power level requires mandatory power factor correction. This standard applies to a range of products, such as table lamps, cabinet lighting, etc. Among them, the power factor of the LED driving power supply for residential lighting must be greater than 0.7, and the power factor for commercial lighting must be greater than 0.9. The IEC61000-3-2 harmonic content standard stipulates that in lighting with a power level greater than 25W, the total harmonic distortion (THD) should be less than 35% and the power factor should not be less than 0.7. The introduction of these standards has put forward higher requirements for the design of LED lighting drive power supplies.

　3. Reduce the size of the driving power supply

The LED itself is relatively small, which is very beneficial for portable products. The LED driving power supply should also be as small as possible so that it can be successfully installed into the LED lamp holder. During the design process, the size of the driving power supply should be reduced as much as possible. ［5］

　4. Improve the reliability of the driving power supply

During the operation of LED, non-human factors such as high temperature and device aging will cause fatal damage to the LED body. Therefore, in the LED driving power supply

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