The topology structure of LED driver power supply
In LED lighting applications using AC-DC power supply, the construction module of power conversion includes discrete components such as diodes, switching transistors (FETs), inductors, capacitors, and resistors to perform their respective functions, while pulse width modulation (pWM) regulators are used to control power conversion. The isolated AC-DC power conversion with transformers usually added to the circuit includes topology structures such as flyback, forward, and half bridge, as shown in Figure 1. The flyback topology is the standard choice for medium to low power applications with power less than 30W, 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 isolation type LED drivers basically use "electronic" transformers.
In LED lighting applications using DC-DC power supply, the LED driving methods that can be used include resistance type, linear voltage regulator, and switch voltage regulator. The basic application diagram is shown in Figure 2. In the resistance type drive mode, the forward current of LED can be controlled by adjusting the current detection resistance in series with LED. This drive mode is easy to design, low in cost, and has no electromagnetic compatibility (EMC) problem. The disadvantage is that it depends on voltage, needs to screen LEDs, and has low energy efficiency. Linear voltage regulators are also easy to design and have no EMC issues. They also support current stabilization and overcurrent protection (foldback), and provide external current setting points. However, their shortcomings include power dissipation and the need for input voltage to always be higher than forward voltage, with low energy efficiency. The switch regulator continuously controls the opening and closing of the switch (FET) through the pWM control module, thereby controlling the flow of current.
Switching voltage regulators have higher energy efficiency, are voltage independent, and can control brightness. However, their shortcomings include relatively high cost, higher complexity, and electromagnetic interference (EMI) issues. The common topology structures of LEDDC-DC switching regulators include buck, boost, buck boost, or single ended primary inductor converters (SEpICs). When the minimum input voltage under all working conditions is greater than the maximum voltage of the LED string, a step-down structure is adopted, such as using 24Vdc to drive 6 series connected LEDs; On the contrary, when the maximum input voltage is less than the minimum output voltage under all working conditions, a boost structure is adopted, such as using 12Vdc to drive 6 series connected LEDs; When there is an overlap between the input voltage and output voltage range, a step-down boost or SEpIC structure can be used, such as using 12Vdc or 12Vac to drive four series connected LEDs. However, this structure has the least ideal cost and energy efficiency.
The use of AC power to directly drive LED has also made some progress in recent years. In this structure, LED strings are arranged in opposite directions, working in half a cycle, and the LED only conducts when the line voltage is greater than the forward voltage. This structure has its advantages, such as avoiding power loss caused by AC-DC conversion. However, in this structure, LED switches at low frequencies, so human eyes may notice flickering phenomena. In addition, LED protection measures need to be added in this design to protect it from the impact of line surges or transients.
