Design of LED drive power PFC circuit

　　The LED driving power supply is a power converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Normally: the input of the LED driving power supply includes high-voltage power frequency AC (ie, mains power), low-voltage DC, high-voltage DC, and low-voltage high voltage. Frequency AC (such as the output of an electronic transformer), etc.

　　The output of the LED driving power supply is mostly a constant current source that changes the voltage as the LED forward voltage drop value changes.

　　1 System working principle and technical indicators

　　1.1 Working principle

　　LED drivers are mainly used for high-power lighting. The design must meet normal lighting requirements; meet high efficiency, low cost and small size; the output of the power driver must adopt constant current control; safety work must be fully considered.

　　The AC input first enters the lightning surge protection and EMI circuit. This part of the circuit prevents external interference from intruding into the power supply, and at the same time prevents internal interference from entering the AC transmission system. Normal input AC passes through the rectifier filter circuit and enters the power factor correction circuit. The voltage and current waveforms are modulated to achieve power factor and current harmonic requirements; constant current output is achieved through the constant current control circuit to achieve normal operation of the LED. The working principle diagram is shown in Figure 1.

　　1.2 Technical indicators

　　System design technical index requirements are shown in Table 1.

　　2pFC drive circuit design

　　The system uses chips that are cheap, easy to design, and have no turn-on loss.

　　When the boost tube is turned off, it is naturally turned off with zero current, and the peak it withstands is small. The critical conduction control mode (CRM) has the advantages of correspondingly smaller losses. The working principle block diagram of the CRM mode is shown in Figure 2 [6-8]. There are many chips used for CRM mode control. The L6562D chip of ST Company has the characteristics of excellent performance, high reliability and low price, so this chip was selected. This system uses the control chip L6562D to design the pFC circuit. It has a simple external circuit, pulse-by-pulse current limiting, and programmable output overvoltage protection and open-loop protection functions. The circuit principle is shown in Figure 3

　　2.1 High frequency filter capacitor

　　The minimum design requirements, the lowest input voltage, and the maximum output power are V (nln) = 17V, V (唧) = 264V, po) = 90W, pFC circuit operating frequency f~ = 90kHz, output voltage Vo = 400V, pFC circuit efficiency 7/=96%, according to formula (1), Im=O. 58A.

　　2.4 Number of turns of transformer

　　Therefore, the inductor here not only acts as an inductor, but also acts as a transformer to generate the power required by the control IC. It can be calculated that the operating frequency is when the input voltage is 170VAC. As a secondary side power supply of the transformer, it is necessary to ensure that the IC can work normally. The operating voltage is set to l8V here. Choose TDK's pQ26/2o magnetic core: Ae=l19mm. Take the maximum magnetic flux density as: AB=O. 25T. According to formula (5), Nr. =49.5, choose 50 turns. According to equation (6), D=0.4 can be calculated. According to equation (7), Ns=5 turns.

　　2. The effective current value of the primary side current of the MOSFET of 2.5pFC is the maximum when it is at the lowest voltage, which is calculated by the approximate calculation of equation (8). 0.58A. In fact, when the MOSFET is turned off, no current flows through it. You can choose a field effect transistor Spp20-N60C3.

　　3 Auxiliary circuit design

　　3.1 Protection function

　　The system design requires output protection functions such as over-voltage, short circuit and over-temperature. VIN0=2.2uF. L_FC3=0.1uF

　　Output overvoltage protection is by detecting the output voltage. When the output voltage exceeds the protection point. The trigger signal is transmitted to the primary through photovoltaic isolation, and the COMp terminal of the control chip L6565D is pulled down, causing the power supply to have no output.

　　Output short circuit protection is to sample the output current through a resistor. When the output current exceeds the set protection point (2A), the comparator flips over, driving the photovoltaic to turn on. The secondary of the photovoltaic will control the COMp terminal of the chip L6565D, causing the power supply to have no output. When the short circuit condition is removed, the output can be automatically restored.

　　The output over-temperature protection is realized by connecting a 105°C normally closed temperature relay ST-22 in series with the power supply terminal VCC of the control chip L6565D. The temperature relay is mounted on the radiator of the main power tube. When the temperature exceeds 105°C, the temperature relay opens and the power output turns off. When the temperature returns to about 95°C, the temperature relay closes again. Power output is restored.

　　3.2BUCK constant current source circuit

　　The BUCK circuit topology is simple, with few peripheral components and high efficiency. The circuit principle is shown in the figure. The control chip is MP4688 from Mps Company. The circuit is shown in Figure 4.

　　3.3 Anti-surge, lightning protection, and EMI filter circuit

　　This power supply is used to power LED street lights. Surge protection and lightning protection are basic requirements

　　First, the large input power (about 90W) makes EMI filtering an important indicator requirement. For these three technical indicators, the designed circuit is shown in Figure 5.

　　Varistor RV. To prevent surges, RV:, RV3, VG1 (gas discharge tube) together form a lightning protection circuit, common mode inductor, L. , cIC constitutes an EMI filter circuit, fuse F1 provides input short-circuit protection, ~3 is C. , provide discharge path.

　　4 System test The test block diagram is shown in Figure 6. A4.1 Insulation resistance test Use an insulation withstand voltage tester to test the input to output of the power supply under test. The insulation resistance Ra of the input and output to the shell should be greater than or equal to 50MI}. Test voltage: DC500V. Short-circuit the input terminals and output terminals respectively.

　　4.2 Insulation strength test: Use an insulation withstand voltage tester to test the power supply under test. Apply the following conditions and maintain for lmin. There should be no breakdown or flashover. Short-circuit the input terminals and output terminals respectively.

　　4.3 Short-circuit protection function test: Connect the power supply, voltage regulator, intelligent power meter, power supply under test, electronic load and various measuring instruments as shown in the test block diagram. Adjust the voltage regulator so that the input voltage is AC230V. The power supply under test is working normally. Use the short-circuit function of the electronic load to short-circuit the output of the power supply under test, and then remove the short-circuit state. The power supply under test should be able to return to normal working condition. The short-circuit protection function of the power supply under test is normal.

　　4.4 High and low temperature working test

　　Place the power supply under test into the temperature test chamber, and connect the instruments and electronic loads according to the test block diagram. Start the temperature test chamber and cool it down to -45℃±2℃. After 2 hours of constant temperature, power on and test. After completing the test, start heating up. During the heating process, the rated working status of the power supply under test is maintained, the temperature rises to +45oC±2oC, and the test is conducted after powering on at a constant temperature for 2 hours.

　　Through testing, the system solutions are effective and improve the efficiency of the converter, making the overall efficiency of the power supply meet the technical requirements. From the experimental data results and waveforms, it is concluded that the LED driving power supply based on pFC design and implementation has a reasonable design, reliable and stable performance during debugging and testing, and both performance and indicators meet the design requirements.

　　5Conclusion

　　LED is considered to be the most promising lighting source in the future, and LED driving power is the most important factor affecting and restricting the life of LEDs. This paper is based on this prospect and develops a pFC-based LED driving power supply. The driving power supply has low loss, high power factor and efficiency. The drive power circuit has a simple form, few peripheral components, small size, high power density, high reliability, good waterproof and dustproof effects, and all performances meet the standard requirements.

　　Technical Zone: How to accurately measure ripple, the core indicator of power supply. Seven principles to determine the quality of LED drive power supply. How to solve the problem of vulnerability of LED drive power supply. The importance of new energy and drive. How to identify the quality of LED drive power supply? Design of LED drive power pFC circuit

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