Introduction to LED drive power supply topology principle

In LED lighting applications using AC-DC power supplies, the building blocks of power conversion include discrete components such as diodes, switches (FETs), inductors, capacitors, and resistors to perform their respective functions, while pulse width modulation (PWM) regulators are used to control power conversion. Isolated AC-DC power conversion, which usually incorporates a transformer in the circuit, includes flyback, forward, and half-bridge topologies, as shown in Figure 1. The flyback topology is the standard choice for low- to medium-power applications with power less than 30 W, while the half-bridge structure is most suitable for providing higher energy efficiency/power density. As for the transformer in the isolation structure, its size is related to the switching frequency, and most isolated LED drivers basically use "electronic" transformers.

　　Figure 1: Common isolation topologies.

　　In LED lighting applications using DC-DC power supplies, the LED drive methods that can be used include resistor type, linear regulator and switching regulator. The basic application diagram is shown in Figure 2. In the resistor type drive method, the forward current of the LED can be controlled by adjusting the current detection resistor in series with the LED. This drive method is easy to design, low cost, and has no electromagnetic compatibility (EMC) problems. The disadvantages are that it depends on voltage, requires binning of LEDs, and has low energy efficiency. Linear regulators are also easy to design and have no EMC problems. They also support current stabilization and overcurrent protection (fold back) and provide external current set points. The disadvantages are power dissipation problems, the input voltage must always be higher than the forward voltage, and the energy efficiency is not high. The switching regulator continuously controls the on and off of the switch (FET) through the PWM control module, thereby controlling the flow of current.

　　Figure 2: Common DC-DC LED drive methods.

　　Switching regulators have higher energy efficiency, are independent of voltage, and can control brightness. However, their disadvantages are relatively high cost, higher complexity, and electromagnetic interference (EMI) problems. Common topologies of LED DC-DC switching regulators include different types such as buck, boost, buck-boost, or single-ended primary inductor converter (SEPIC). Among them, the buck structure is used when the minimum input voltage under all working conditions is greater than the maximum voltage of the LED string, such as using 24 Vdc to drive 6 LEDs in series; on the contrary, the boost structure is used when the maximum input voltage under all working conditions is less than the minimum output voltage, such as using 12 Vdc to drive 6 LEDs in series; and when the input voltage and output voltage range overlap, the buck-boost or SEPIC structure can be used, such as using 12 Vdc or 12 Vac to drive 4 LEDs in series, but the cost and energy efficiency of this structure are the least ideal.

　　The method of using AC power to directly drive LEDs has also gained some development in recent years. See Figure 3 for its application diagram. In this structure, the LED strings are arranged in opposite directions, working in half cycles, and the LEDs are turned on only when the line voltage is greater than the forward voltage. This structure has its advantages, such as avoiding power losses caused by AC-DC conversion. However, in this structure, the LEDs are switched at a low frequency, so the human eye may perceive flickering. In addition, LED protection measures need to be added in this design to protect them from line surges or transients.

　　Figure 3: Schematic diagram of driving LEDs directly with AC.