Due to the limitation of LED power level, it is usually necessary to drive multiple LEDs at the same time to meet the brightness requirements. Therefore, a special driving circuit is needed to light the LED. At present, several mainstream LED driving modes are as follows:
Resistance capacitance Depressurization
The impedance of the capacitor under AC is used to limit the input current, so as to obtain the DC level to supply power to the LED. This driving mode has the advantages of simple structure and low cost, but the input non isolation scheme has potential safety hazards. And the conversion efficiency is very low, so it is impossible to achieve constant current control.
Isolated flyback circuit
Using the flyback circuit, the DC level is generated at the secondary side through the transformer, and then the ripple of this level is fed back to the primary side through the optocoupler, so as to stabilize the self excitation. This kind of circuit meets the requirements of safety regulations, and has good output constant current accuracy and high conversion efficiency. However, due to the need of optocoupler and secondary constant current control circuit, the system is complex, large volume and high cost. At present, it has been gradually replaced by the original side scheme.
Primary side scheme
The primary side scheme is to control the output power supply and current completely on the AC primary side, which can achieve the most accurate constant current accuracy of 5%, and 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, relies on the current limiting resistance to control the primary side current, and multiplies the turn ratio to control the accuracy of the output current. The primary side scheme inherits the advantages of isolated flyback circuit. At the same time, it has simple architecture, small volume and low cost. At present, it has become the mainstream driver.
Constant current accuracy of primary side: because the production accuracy of voltage transformation is difficult to control, the output current drift of primary side scheme is large when using low-quality transformer. Therefore, the primary side scheme adds the secondary side constant current control circuit through improvement. Although it is more complex than the ordinary primary side scheme, compared with the flyback scheme, the optocoupler can still be omitted, and the system has the highest cost performance.
According to the power consumption rules of power grid and the characteristic requirements of LED driving power supply, the following points shall be considered when selecting and designing LED driving power supply:
1. High reliability
In particular, the driving power supply of LED street lamps is installed at high altitude, which is inconvenient to maintain and expensive to maintain.
2. High efficiency
LED is an energy-saving product, and the efficiency of driving power supply should be high. It is particularly important for the structure where the power supply is installed in the lamp. Because the luminous efficiency of LED decreases with the increase of LED temperature, the heat dissipation of LED is very important. The efficiency of the power supply is high, its power consumption is small, the calorific value in the lamp is small, and the temperature rise of the lamp is reduced. It is beneficial to delay the light attenuation of LED.
3. High power factor
Power factor is the requirement of power grid for load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single electrical appliance with small power is lower, it has little impact on the power grid, but when we light the lights at night, the similar loads are too concentrated, which will cause serious pollution to the power grid. For 30-40 watt LED driving power supply, it is said that in the near future, there may be certain index requirements for power factors.
4. Driving mode
Now there are two kinds of: 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, one LED failure will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, LED series or parallel operation. Its advantage is that the cost is a little low, but the flexibility is poor. It also needs to solve the problem that a certain led fault does not affect the operation of other LEDs. These two forms coexist for a period of time. The multi-channel constant current output power supply mode will be better in cost and performance. Maybe it's the mainstream direction in the future.
5. Surge protection
The anti surge ability of LED is relatively poor, especially the anti reverse voltage ability. It is also important to strengthen protection in this regard. Some LED lights are installed outdoors, such as LED street lights. Due to the start and fall of power grid load and the induction of lightning stroke, various surges will invade from the power grid system, and some surges will lead to the damage of LED. Therefore, the LED driving power supply should have the ability to suppress the invasion of 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 LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.
The lamps shall be of external installation type, the power structure shall be waterproof and moisture-proof, and the shell shall be sun resistant.
8. The service life of the driving power supply shall match the service life of the LED.
9. It shall meet the requirements of safety regulations and electromagnetic compatibility.
With the increasingly wide application of LED, the performance of LED driving power supply will be more and more suitable for the requirements of LED.