PFC switch design based on LED street light

The LED street lamp is a low-voltage, high-current driving device. The intensity of its illumination is determined by the current flowing through the LED. If the current is too strong, the LED will be attenuated. If the current is too weak, the LED will be affected. Therefore, the LED driver needs to provide constant illumination. Stream power supply to ensure the safety of high-power LEDs while achieving the desired luminous intensity. Driving high-power LEDs with utility power needs to solve the problems of buck, isolation, PFC (power factor correction) and constant current, and also requires relatively high conversion efficiency, small volume, long-term operation, easy heat dissipation, and low cost. , anti-electromagnetic interference, and over temperature, over current, short circuit, open circuit protection. This paper introduces the design of PFC switching power supply based on LED street light. The solution adopts an outdoor LED street lamp power supply designed with active PFC function circuit, built-in complete EMC circuit and high efficiency lightning protection circuit, which meets the requirements of safety regulations and electromagnetic compatibility. The final test results also show that the PFC switching power supply designed by this scheme has good performance, reliability, economy and high efficiency, and has achieved satisfactory results in the use of LED street lamps.

1 system overall block diagram

The switching power supply realized by the isolation transformer and PFC control outputs a constant voltage and constant current voltage to drive the LED street light. The overall block diagram of the circuit is shown in Figure 1.

The ability of LEDs to withstand surges is relatively poor, especially against reverse voltage capability. It is also important to strengthen protection in this area. LED street lights should be installed outdoors to enhance surge protection. Due to the start of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause LED damage. Therefore, the LED driving power supply should have the ability to suppress surge intrusion and protect the LED from damage. The EMI filter circuit mainly prevents harmonic interference on the power grid from entering the module and affects the normal operation of the control circuit.

After the three-phase AC is rectified by the full bridge, it becomes a pulsating DC. Under the action of the filter capacitor and the inductor, the DC voltage is output. The main switch DC/AC circuit converts DC power into a high frequency pulse voltage at the secondary output of the transformer. The high frequency pulse outputted by the transformer is subjected to high frequency rectification, LC filtering and EMI filtering to output the DC power required for the LED street lamp.

The PWM control circuit uses voltage-current double-loop control to achieve regulation of the output voltage and output current. The feedback network uses a constant current constant voltage device TSM101 and a comparator. The feedback signal is optically coupled to the PFC L6561. The power factor of the module is 0.95 due to the use of the PFC device.

2DC/DC converter design

There are many types of DC/DC converters. In order to ensure the safety of electricity, this design is chosen to be isolated. The isolated DC/DC converter form can be further subdivided into forward, flyback, half bridge, full bridge and push-pull. Among them, half-bridge, full-bridge and push-pull are usually used in high-power output applications, and the excitation circuit is complicated and difficult to implement; while the forward and flyback circuits are simple and easy, but due to the flyback Compared with the forward mode, the input voltage is changed, and the PFC output voltage of the power supply system will change greatly. Therefore, the DC/DC conversion adopts the flyback mode, which is beneficial to ensure that the output voltage is stable.

The flyback switching power supply is mainly used when the output power is 5~150W. This power supply structure is derived from the Buck-Boost structure and is added to the isolation transformer, as shown in Figure 2. In a flyback topology, a transformer acts as an energy storage component. When the switch is turned on, the transformer stores energy, and the load current is provided by the output filter capacitor. When the switch is turned off, the transformer transfers the stored energy to the load and the output filter capacitor to compensate for the energy consumed when the capacitor separately supplies the load current.

In the figure, T1 is a high-frequency isolation transformer, VQ1 is a CMOS power transistor 17N80C3, VD7 and VD8 are transient suppression diodes, VD6 is a fast recovery diode, VD5 is a double diode, and C3, C4, C5 and C6 are electrolytic capacitors. Ubout is the pulsating DC signal from the rectifier bridge, and GD is the control signal from the power factor correction circuit. The leads 1 and 2 of the transformer form a winding to provide operating power to the PFC device, and the leads 11 and 12 form a winding to provide operating power for the constant current constant voltage device and the comparator.

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