LEDs have the advantages of low energy consumption and long life, so many special applications use LEDs as their source of illumination. However, since the power required for the LED is DC and low voltage, constant current output, energy conversion efficiency, and power factor should be considered in the design of the LED driver for illumination. 1, low frequency transformer and half wave or full wave rectifier circuit Low-frequency transformers and half-wave or full-wave rectifier circuits are shown in Figure 1. The advantage of this circuit is that the structure is simple and the cost is low; the disadvantage is that the volume is large, and the voltage mode is adopted, the brightness of the LED changes with the change of the voltage, the constant current output cannot be provided, and the surge current is large. 
The low-frequency transformer and half-wave or full-wave rectification circuit shown in Figure 1 provides a simple way to provide LED power, requiring only a low-frequency transformer, rectifier, filter capacitor, and a variable resistor to adjust the brightness. The number of LEDs in series is mainly determined by the turns ratio of the transformer. Once the turns ratio is selected, it is difficult to change the number of LEDs in series. Only the number of LEDs can be increased in parallel, but this circuit is difficult to achieve parallel connection. There is the same current between the LEDs, resulting in inconsistent brightness of each LED. 2, switching converter circuit The switching converter circuit is shown in Figure 2. The advantage of this circuit is that the technology is very mature, and there are many types of integrated switching converters, so it is used by most of the current LED lighting power sources; the disadvantage is that the parallel current LEDs can not achieve better current matching, resulting in uneven brightness of each LED. . 
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