A Brief Analysis of Constant Current Drive for LED Lighting

LED is a characteristic-sensitive semiconductor device with negative temperature characteristics. Therefore, it needs to be stabilized and protected during application, which gives rise to the concept of driver . LED devices have almost harsh requirements for driver power supply . Unlike ordinary incandescent bulbs, LED can be directly connected to 220V AC mains. LED is driven by a low voltage of about 3 volts, so complex conversion circuits must be designed. Different LED lamps for different purposes 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 driver power supply. Designing a good power supply must comprehensively consider these factors, because the role of power supply 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 the brightness requirements. Therefore, a special driving circuit is needed to light up the LED. The following is a brief introduction to the current mainstream LED driving methods:

1. Resistor-capacitor voltage reduction

The impedance of the capacitor under AC is used to limit the input current, thereby obtaining a DC level to power the LED. This driving method has a simple structure and low cost, but the input is not isolated, which poses a safety hazard. In addition, the conversion efficiency is very low and constant current control cannot be achieved.

2. Isolation flyback circuit

By using a flyback circuit, a DC level is generated on the secondary side through a transformer , and the ripple of this level is fed back to the primary side through an optocoupler, thereby achieving self-excitation stability. This type of circuit meets the requirements of safety regulations, and has good output constant current accuracy and high conversion efficiency. However, due to the need for an optocoupler and a secondary side constant current control circuit, the system is complex, large in size, and high in cost. It has gradually been replaced by the primary side solution.

3. Original side solution

The primary side solution is to completely control the output power and current on the AC primary side, and the most accurate constant current accuracy can be 5%. 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, and relies on the current limiting resistor to control the primary current, and multiplying the turns ratio to control the accuracy of the output current. The primary side solution inherits the advantages of the isolated flyback circuit, and has a simple architecture, small size and low cost. It has become the mainstream driver.

The problem of constant current accuracy on the primary side: Since the production accuracy of the transformer is difficult to control, the output current drift is large when the primary side solution uses a low-quality transformer. Therefore, the primary side solution is improved by adding a secondary side constant current control circuit. Although this is more complicated than the ordinary primary side solution, compared with the flyback solution, it can still save the optocoupler, etc., and the system has the highest cost performance.

According to the power usage rules of the power grid and the characteristics of the LED driver power supply, the following points should be considered when selecting and designing the LED driver power supply:

1. High reliability

Especially the driving power supply of LED street lamps , which is installed at high altitude, is inconvenient to maintain and the maintenance cost is also high.

2. High efficiency

LED is an energy-saving product , and the efficiency of the driving power supply must be high. This is especially important for the structure where the power supply is installed in the lamp. Because the luminous efficiency of LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The more efficient the power supply is, the less power it consumes, and the less heat it generates in the lamp, which reduces the temperature rise of the lamp. This is beneficial to delaying the light decay of the LED.

3. High power factor

The power factor is the load requirement of the power grid. Generally, there is no mandatory index for electrical appliances below 70 watts. Although a low power factor of a single low-power electrical appliance has little impact on the power grid, when everyone turns on lights at night, the same load is too concentrated, which will cause serious pollution to the power grid. For 30-40 watt LED driver power supplies, it is said that in the near future, there may be certain index requirements for power factor.

4. Drive mode

There are two common methods now: one is that a constant voltage source supplies multiple constant current sources, and each constant current source supplies power to each LED separately. This method has flexible combination, and the failure of one LED does not affect the operation of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, and the LEDs run in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and the problem of a certain LED failure not affecting the operation of other LEDs must be solved. 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. Perhaps it will be the mainstream direction in the future.

5. Surge protection

LEDs have poor surge resistance, especially reverse voltage resistance. It is also important to strengthen protection in this area. 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 surges will cause damage to the LEDs. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.

6. Protection function

In addition to the conventional protection functions 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.

7. Protection

For external installation type of lamps, the power supply structure should be waterproof and moisture-proof, and the outer shell should be sun-resistant.

8. The life of the driving power supply should be compatible with the life of the LED.

9. It must comply with safety regulations and electromagnetic compatibility requirements.

As LEDs are increasingly used, the performance of LED driver power supplies will become more and more suitable for LED requirements.