Analysis of several mainstream LED driving methods
LEDs are sensitive semiconductor devices with negative temperature characteristics, so they need to be stable and protected in the application process, which leads to the concept of driving. The requirements of LED devices for driving power are almost harsh. Unlike ordinary incandescent light bulbs, LEDs can be directly connected to 220V AC mains. LED is a low-voltage drive of about 3 volts. Complex conversion circuits must be designed. Different uses of LED lights must be equipped with different power adapters. In the international market, foreign customers have very high requirements for the efficiency conversion, effective power, constant current accuracy, power supply life, and electromagnetic compatibility of LED driving power supplies. When designing a good power supply, these factors must be comprehensively considered because the power supply is in the entire lamp. Is as important as the human heart.
Due to the limitation of LED power level, it is usually necessary to drive multiple LEDs at the same time to meet brightness requirements. Therefore, a special driving circuit is required to light the LEDs.
The impedance of the capacitor under AC is used to limit the input current to obtain a DC level to power the LED. This drive method has a simple structure and low cost, but there is a potential safety hazard in entering a non-isolated solution. And the conversion efficiency is very low, and constant current control cannot be achieved.
2.Isolated flyback circuit
Utilizing a flyback circuit, a DC level is generated on the secondary side through a transformer, and this level ripple is fed back to the primary side through an optocoupler, thereby self-exciting and stable. This type of circuit meets the safety certification requirements, and the output constant current accuracy is better, and the conversion efficiency is higher. However, due to the need for an optocoupler and a secondary-side constant current control circuit, the system is complicated, bulky, and expensive. It has gradually been replaced by the original plan.
The primary side solution is to control the output power and current completely on the AC primary side. The most accurate can achieve 5% constant current accuracy. The secondary side only needs a simple output circuit. The primary side mainly relies on the feedback of the auxiliary side to control the output voltage, the control of the primary side current by the current limiting resistor, and at the same time multiplying the turns ratio to control the accuracy of the output current. The primary solution inherits the advantages of the isolated flyback circuit, and has a simple structure, which can achieve small size and low cost. It has become the mainstream driver.
The problem of constant current accuracy on the primary side: Because the production precision of the transformer is difficult to control, when the low-quality transformer is used in the primary side solution, the output current drifts greatly. Therefore, the primary side solution has been improved by adding a secondary side constant current control circuit. Although it is more complicated than the ordinary primary side solution, compared with the flyback scheme, it can still eliminate the optocoupler and so on.
According to the power consumption rules of the power grid and the characteristics of LED drive power, the following points must be considered when selecting and designing LED drive power:
Especially like the driving power of LED street lights, installed at high altitude, the maintenance is not convenient, and the maintenance cost is also large.
LED is an energy-saving product, and the efficiency of the driving power should be high. It is especially important for the structure in which 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 consumption is small, the heat generation in the lamp is small, and the temperature rise of the lamp is reduced. It is beneficial to delay the light decay of LED.
3.High power factor
Power factor is the load requirement of the grid. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the lower power factor of a single power consumer has a smaller impact on the power grid, everyone lights up at night, and similar loads are too concentrated, which will cause serious pollution to the power grid. For 30W to 40W LED drive power, it is said that in the near future, there may be certain index requirements for the power factor.
There are two kinds of traffic now: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is flexible, and LED failures along the way will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, LEDs run in series or parallel. Its advantage is lower cost, but poor flexibility. It also needs to solve the problem of one LED failure, which does not affect the operation of other LEDs. These two forms coexist for a period of time. Multi-channel constant current output power supply method will be better in terms of cost and performance. Maybe the mainstream direction in the future.
LED anti-surge capability is relatively poor, especially anti-reverse voltage capability. It is also important to strengthen this protection. Some LED lights are installed outdoors, such as LED street lights. Due to the induction of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause damage to the LEDs. Therefore, the LED drive power must have the ability to suppress the intrusion of the surge and protect the LED from damage.
6, protection function
In addition to the conventional protection function of the power supply, it is best to add negative feedback of the LED temperature to the constant current output to prevent the LED temperature from becoming too high.
For external installation of lamps, the power supply structure must be waterproof and moisture-proof, and the shell must be resistant to sunlight.
8. The life of the driving power supply must match the life of the LED.
9, to meet the requirements of safety regulations and electromagnetic compatibility.
With the increasing application of LED, the performance of LED driving power will be more and more suitable for LED requirements.