LED street lamp is a driver with low voltage and high current. Its luminous intensity is determined by the current flowing through the LED. Too strong current will cause led attenuation, and too weak current will affect the luminous intensity of the LED. Therefore, constant current power supply needs to be provided for LED driving to ensure the safety of high-power LED and achieve ideal luminous intensity at the same time. Driving high-power LED with mains power needs to solve the problems of voltage reduction, isolation, PFC (power factor correction) and constant current. It also needs relatively high conversion efficiency, small volume, long-time operation, easy heat dissipation, low cost, anti electromagnetic interference, over temperature, over-current, short circuit, open circuit protection, etc. The scheme adopts outdoor LED street lamp power supply designed by active PFC functional circuit, with built-in complete EMC circuit and high-efficiency lightning protection circuit, which meets the requirements of safety regulations and electromagnetic compatibility. Finally, the test results also show that the PFC switching power supply designed in this scheme has good performance, reliability, economy and high efficiency, and has achieved satisfactory results in the use of LED street lamps.
1. Overall system block diagram
The switching power supply controlled by isolation transformer and PFC outputs constant voltage and current to drive LED street lamps. The overall block diagram of the circuit is shown in Figure 1.
The anti surge ability of LED is relatively poor, especially the anti reverse voltage ability. It is also important to strengthen protection in this regard. LED street lamps installed outdoors should strengthen surge protection. Due to the start and fall of power grid load and the induction of lightning stroke, various surges will invade from the power grid system, and some surges will lead to the damage of LED. Therefore, the LED driving power supply shall have the ability to suppress surge intrusion and protect the LED from damage. EMI filter circuit mainly prevents harmonic interference on the power grid from entering the module in series, which affects the normal operation of the control circuit.
The three-phase AC becomes pulsating DC after full bridge rectification, and the DC voltage is output under the action of filter capacitor and inductance. The main switch DC / AC circuit converts DC into high-frequency pulse voltage at the secondary output of the transformer. The high-frequency pulse output by the transformer passes through high-frequency rectification, LC filtering and EMI filtering to output the DC power supply required by LED street lamps.
The PWM control circuit adopts voltage and current double loop control to adjust the output voltage and limit the output current. The feedback network adopts constant current and constant voltage device tsm101 and comparator, and the feedback signal is sent to PFC device L6561 through optocoupler. Due to the use of PFC device, the power factor of the module reaches 0.95
2. Design of DC / DC converter
There are many types of DC / DC converters. In order to ensure power safety, the design scheme is isolated. Isolated DC / DC conversion can be further subdivided into forward, flyback, half bridge, full bridge and push-pull. Among them, half bridge type, full bridge type and push-pull type are usually used in high-power output occasions. Their excitation circuit is complex and difficult to realize; The forward and flyback circuits are simple and easy, but because the flyback is more suitable for the change of input voltage than the forward, and the PFC output voltage in the power supply system will change greatly, the flyback mode is adopted for DC / DC conversion, which is conducive to ensuring the stability of output voltage.
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 added with an isolation transformer, as shown in Figure 2. In the flyback topology, the transformer is used as the energy storage element. When the switch is on, the transformer stores energy, and the load current is provided by the output filter capacitor; When the switch is turned off, the transformer transmits the stored energy to the load and output filter capacitor to compensate the energy consumed when the capacitor provides load current alone.
In the figure, T1 is a high-frequency isolation transformer, VQ1 is a CMOS power triode 17n80c3, vd7 and vd8 are transient suppression diodes, vd6 are fast recovery diodes, vd5 are double diodes, 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 L and 2 of the transformer form a winding to provide working power for PFC devices, and the leads 11 and 12 form a winding to provide working power for constant current and constant voltage devices and comparators.
3 feedback network circuit design
3.1 constant current and constant voltage circuit
This design uses the constant current and constant voltage controller tsm101 to adjust the output voltage and current to make it stable. The circuit is shown in Figure 3. Through the control function of tsm101, the constant current (CC) and constant voltage (CV) of the power supply are ensured. In Figure 3, uout + and uout – are the voltages filtered by the isolation transformer through the double diode and electrolytic capacitor, and then the outputs filtered by the inductor L4 and capacitor are uout + and uout -, which are the output voltages of the power supply module and are directly added to the LED street lamps. The adjustable resistors rv1 and RV2 adjust the output voltage and current respectively. R10 and R11 are 22m Ω resistors, which respectively sample the voltage and current output by the power supply. The output tout of tms101 is sent to pin 5 of L6561 through optocoupler, thyristor and triode, and constant current control is realized through feedback circuit. Pin 8 of the device is connected to the auxiliary power supply, and pin 4 is connected to the secondary side ground of transformer T1.
3.2 comparator circuit
The comparator lm258 is adopted, and the circuit is shown in Figure 4.
The voltage signals VR + and VR – at both ends of the sampling resistance at the output end are sent to the comparator lm258. By comparing with the preset voltage, the voltage feedback signal dout.vf is generated as the auxiliary power supply generated by the secondary winding of transformer T1.
4 PFC circuit design
This design uses the most common active power factor correction controller L6561. PFC circuit is shown in Figure 5.
Pin 8 of L6561 is the power input, which is provided by the secondary winding of transformer T1; Pin 7 is the drive signal output pin, which directly drives MOS transistor VQ1; Pin 6 is the reference ground, which is connected with the ground of the main circuit; Pin 5 is the zero crossing detection pin, which is used to determine when to turn on the MOS tube. The winding composed of pin 1 and pin 2 of transformer T1 transmits the zero crossing signal of inductive current to pin 5 of the device through resistance, and the signal dout generated by comparator lm258 is transmitted to pin 5 of the device through optocoupler, triode, thyristor, etc. to detect the output current. Pin 4 is the MOS transistor current. The device compares the signal detected by this pin with the inductive current signal generated inside the device to determine when to turn off the MOS transistor. In Fig. 2, the resistor R4 is used as the current detection resistor to sample the MOS tube current. One end of the resistor is connected to the system ground, and the other end is at the source of the MOS tube and the pin 4 of the device. Pin 3 is an input end of the internal multiplier of the device, which is connected with the output voltage of the rectifier bridge circuit to determine the waveform and phase of the input voltage, which is used to generate the inductance current reference signal inside the device. In Figure 5, ubout is divided by three resistors and transmitted to pin 3. Pin 2 is the other input of the internal multiplier and the output of the voltage error amplifier, and pin 1 is the input of the system feedback voltage. The output tout of the constant current and constant voltage device transmits the voltage feedback to the pin 1 of the device through the optocoupler to form a negative feedback loop of the output voltage. Resistor R28 and capacitor C18 are connected between pin 1 and pin 2 of the device to form a compensation network of voltage ring.
5 test results
After the power module is electrically installed, add the load and test the key points with an oscilloscope. Figure 6 (a) shows the output voltage ubrout + of the rectifier bridge, Figure 6 (b) shows the waveform of the voltage ubout + after ubrout + flows through the inductor, and Figure 6 (c) shows the waveform of the output voltage ucout + of the double diode.
The test results show that the power is 90W and the power factor is up to 0.95. According to the needs of users, the negative feedback of LED temperature can be added in the constant current output to prevent the LED temperature from being too high.
The scheme adopts outdoor LED street lamp power supply designed by active PFC functional circuit, with built-in complete EMC circuit and high-efficiency lightning protection circuit, which meets the requirements of safety regulations and electromagnetic compatibility. It adopts voltage loop feedback, voltage limiting and constant current, high efficiency, accurate constant current and wide range, realizes wide input, stable voltage and constant current output, avoids the current change caused by the change of led forward voltage, and the constant current makes the brightness of LED stable. The test results also show that the PFC switching power supply designed in this scheme has good performance, reliability, economy and high efficiency, and has achieved satisfactory results in the use of LED street lamps.