Since LED are characteristic-sensitive semiconductor devices and have negative temperature characteristics, they need to be stabilized and protected during application, resulting in the concept of driving. LED devices have very strict requirements on driving power. Unlike ordinary incandescent bulbs, LED can be directly connected to 220V AC mains. LED are driven by a low voltage of 2 to 3 volts, and complex conversion circuits must be designed. LED lights for different purposes should be equipped with different power adapters. In the international market, foreign customers have very high requirements on the efficiency conversion, effective power, constant current accuracy, power supply life, and electromagnetic compatibility of LED drive power supply. To design a good power supply, these factors must be comprehensively considered, because the power supply is in the whole lamp. It is as important as the human heart.
The driving power supply converts the power supply into a specific voltage and current to drive the LED light-emitting voltage converter. Usually, the input of the LED driving power supply 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), etc. The output of the LED drive power supply is mostly a constant current source that can change the voltage with the change of the LED forward voltage drop value. The core components of the LED power supply include switch controllers, inductors, switching components (MOSfet), feedback resistors, input filter devices, output filter devices, and so on. According to the requirements of different occasions, there are also input overvoltage protection circuit, input undervoltage protection circuit, LED open circuit protection, overcurrent protection and other circuits.
First,the constant current drive of the LED
When using LED as a display or other lighting equipment or backlight, it needs to be driven by constant current, the main reasons are:
- Avoid driving current beyond the maximum rating and affect its reliability.
- Obtain the expected brightness requirements, and ensure the consistency of the brightness and chromaticity of each LED.
- It can effectively avoid lightning strike, power grid surge, over-current, over-voltage protection, so that the life of LED is improved.
However, the problem is to deal with the heat dissipation problem. If the heat dissipation problem is not dealt with properly, it will affect the life of the LED.
Second, the considerations for LED driver design
At present, the LED are all driven by DC, so a power adapter, that is, the LED driving power supply, needs to be added between the mains and the LED. Its function is to convert AC mains into DC power suitable for LED. According to the power consumption rules of the power grid and the driving characteristics of the LED, the following points should be considered when selecting and designing the LED driving power supply:
Especially like the driving power supply of LED street lights, it is installed at high altitude, which is inconvenient to maintain, and the maintenance cost is also large.
LED is an energy-saving product, and the efficiency of the driving power supply is high. It is especially important for the structure where the power supply is installed in the luminaire. Because the luminous efficiency of the LED decreases with the increase of the LED temperature, the heat dissipation of the LED is very important. The efficiency of the power supply is high, its power loss 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 LED.
3.High power factor
Power factor is the requirement of the grid for the load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single electrical appliance with low power is low, it has little impact on the power grid, but everyone turns on the lights at night, and the same load is too concentrated, which will cause serious pollution to the power grid. For the 30W~40W LED drive power supply, it is said that in the near future, there may be certain index requirements for power factor.
One is that a constant voltage source supplies multiple constant current sources, and each constant current source supplies power to each LED individually. In this way, the combination is flexible, and the failure of one LED will not affect the work of other LED, but the cost will be slightly higher.
The other is direct constant current power supply, with LED running in series or parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it also needs to solve the problem of a certain LED failure without affecting the operation of other LED. 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. Maybe the mainstream direction in the future.
The ability of LED to resist surge is relatively poor, especially the ability to resist reverse voltage. Strengthening this protection is also important. 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 of the surges will cause damage to the LED. Therefore, the LED drive power supply must have the ability to suppress the intrusion of surges and protect the LED from damage.
In addition to the conventional protection function of the power supply, it is better to increase the negative feedback of the LED temperature in the constant current output to prevent the LED temperature from being too high.
For external installation type of lamps, the power supply structure should be waterproof and moisture-proof, and the shell should be resistant to sunlight.
8.The life of the drive power should match the life of the LED
9.To comply with safety regulations and electromagnetic compatibility requirements
Third, the LED power supply is classified according to the driving method
The output current of the constant current drive circuit is constant, but the output DC voltage varies within a certain range with the load resistance value. The smaller the load resistance value, the lower the output voltage. the higher
The constant current circuit is not afraid of short circuit of the load, but it is strictly forbidden to open the load completely.
The constant current drive circuit is ideal for driving LED, but the price is relatively high.
Attention should be paid to the maximum withstand current and voltage values used, which limit the number of LED used.
When the parameters in the voltage regulator circuit are determined, the output voltage is fixed, but the output current changes with the increase or decrease of the load.
The voltage stabilizing circuit is not afraid of the load open circuit, but it is strictly forbidden to completely short the load.
The LED is driven by a regulated driving circuit, and each string needs to be added with a suitable resistor to make each string of LED display an average brightness;
Brightness is affected by voltage changes from rectification.
Advantages and disadvantages of overall constant current and channel-by-channel constant current working methods
Compared with the overall constant current, although the channel-by-channel constant current has many disadvantages, the cost is also relatively high. But it can really protect the LED and prolong the life of the LED, so the constant current is the future trend.
Fourth, the lack of LED power supply
1.Reasons for the current shortage of LED drive power supply:
The technical personnel of the company producing LED lighting and related products do not know enough about the switching power supply.
Most of the LED power supply manufacturers have transformed from ordinary switching power supplies to make LED power supplies, and they do not know enough about the characteristics and use of LED.
At present, there are almost no standards for LED, and most of them refer to the standards of switching power supplies and electronic rectifiers.
Now most of the LED power supplies are not unified, so most of the quantities are relatively small. The purchase volume is small, the price is high, and the component suppliers are not very cooperative.
Stability of LED power supply: wide voltage input, high temperature and low temperature operation, over temperature, over voltage protection and other issues have not been solved one by one.
2.Problems that need to be solved in LED power supply
The first is the overall life of the drive circuit, especially the life of key components such as capacitors at high temperatures, which directly affects the life of the power supply; the second is that LED drivers should challenge higher conversion efficiency, especially when driving high-power LED. Because all the power that is not used as light output is dissipated as heat, the power conversion efficiency is too low, which affects the energy-saving effect of LED; once again, in applications with low power (1-5W), the cost of constant current drive power The proportion is close to 1/3, which is close to the cost of the light source, which affects the market promotion to a certain extent.