[Inventory] LED drive mode comparison and LED drive power supply selection

LEDs are characteristic-sensitive semiconductor devices with negative temperature characteristics, so they need to be stably operated and protected during application, thus creating the concept of driving. LED devices are almost demanding on the power supply. Unlike ordinary incandescent bulbs, LEDs can be directly connected to 220V AC mains. The LED is driven by a low voltage of about 3 volts. It is necessary to design a complicated conversion circuit. LED lamps for different purposes must be equipped with different power adapters. In the international market, foreign customers have very high requirements for efficiency conversion, effective power, constant current accuracy, power supply life and electromagnetic compatibility of LED drive power . To design a good power supply, these factors must be considered comprehensively because the power supply is in the entire luminaire. The role is just 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 requirement. Therefore, a special driving circuit is needed to illuminate the LED. At present, several mainstream LED driving methods are as follows:

Resistor pressure reduction

The input current is limited by the impedance of the capacitor under AC to obtain a DC level to power the LED. The driving method is simple in structure and low in cost, but the input non-isolation scheme has security risks. Moreover, the conversion efficiency is very low and constant current control cannot be achieved.

Isolated flyback circuit

The flyback circuit is used to generate a DC level on the secondary side through the transformer, and then the ripple of this level is fed back to the primary side through the optocoupler to be self-oscillated. Such circuits comply with safety certification requirements, and the output constant current accuracy is better, and the conversion efficiency is higher. However, due to the need for optocoupler and secondary constant current control circuits, the system is complicated, bulky, and costly. It has gradually been replaced by the original side plan.

Primary scheme

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

The problem of constant current accuracy of the primary side: Since the production accuracy of the transformer is difficult to control, the output current drifts greatly when the primary side scheme uses a low-quality transformer. Therefore, the primary side scheme has improved the secondary side constant current control circuit through improvement, which is more complicated than the ordinary primary side scheme. However, compared with the flyback scheme, the optocoupler can be omitted, and the system has the highest cost performance.

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