Sinrace Power Supply Technology (Shenzhen) Co., Ltd.
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| The design and implementation of a high power factor power supply |
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At this stage power factor correction PFC (Power Factor Correction) is divided into two kinds of active and passive. Passive PFC has simple structure, mainly using principle of inductive coil internal current being not mutated to adjust the phase difference of current and voltage in circuit, thereby change the power factor, but its structure is heavy, easy to produce low-frequency noise and maximum power factor is only 70%. Active PFC usually is active power factor adjustments, can be summarized as step-up switching power supply circuit, have advantages of small size, wide input voltage and the higher power factor, power factor is close to 100%.
Design Proposal of high power factor power Power factor monitor The design uses a phase measurements, which were detected on the transformer secondary voltage and current signals by the comparator before shaping, and then calculate the phase difference between voltage and current, then the cosine operation, you can get the system power factor. Load output voltage and current through the sample to get the system apparent power. According to P = S × cosQ = S2-P2 (Q, said reactive power) computing power of the active power, reactive power and other parameters. This method is easy to operate, but also through other method to measure relative accuracy can be achieved very high accuracy, which can well meet the system requirements. Power factor correction The system uses active power factor correction, can improve the power supply input power factor, reducing input current harmonics. The main way to achieve two kinds: (1) two PFC technology that rectifier and DC / DC power stage active PFC circuit is inserted between the pre-class, used to improve power factor and achieve the DC / DC stage input pre-steady, the technology is generally used where high power output; (2) single-stage PFC technique, about the PFC stage and DC / DC level of the shared components, to achieve a unified control and is usually shared device is MOSFET. The design and optimization approach is particularly important for low-power applications. Here UCC28019 PFC controller in CCM mode selection to achieve the ultimate power factor correction. The device is soft-start mechanism, a good dynamic response, combined with the external circuit the input voltage protection, open-loop protection, output over-voltage protection, soft over-current control (SOC) and peak current limiting. System output voltage from the device VSENSE pin resistor divider connected with the internal 5 V reference decision. By the formula Rfb2=(Vref*Rfb1)/(Vout-Vref) available, by adjusting the voltage divider ratio of the output voltage of the digital variable. Systems solutions The system uses the MSP430F449 core for control and computing, such as precision measurement phase by measurement of system power factor. UCC28019 PFC places as the core, the use of hardware circuits form a closed loop feedback circuit, real-time monitoring of output voltage and current. Microcontroller to provide overcurrent protection relay and the samples and to control the display voltage and current. Use the keyboard to select various functions. LCD display of real-time operating data, friendly interface. Figure 1 is a block diagram of the system as a whole. System hardware circuit design Power-factor measurement circuit At the sides of the transformer current transformer and voltage transformer by sampling AC signal, and then by the dual comparator LM393 reconstructive surgery and other precision measurement using the phase difference, get the system power factor. Sampling circuit of output current and voltage Load in the circuit plus a precision high-power output resistance, the use of I / V converter output current can be obtained. Measuring the output voltage through the voltage divider to convert the MSP430 internal A / D converter voltage can be identified and then sampled. Power factor correcting circuit UCC28019 power factor correction circuit to the core values of external components through a series of power factor correction to 98%. Figure 3 uses the UCC28019 power factor correction circuit design. Current foldback circuit Single-chip real-time by sampling the output current. When the current is too large chip control relay module to disconnect the system power; When troubleshooting, the measured current value is less than the predetermined value, the microcontroller again issued instructions to relay closed circuit re-work. System software design System software controls the power factor measurement part of the test phase circuit work, and real-time sampling the output voltage and current. When the detected output current is greater than 2.5 A, the control relay off and reset the circuit, LCD display real-time power supply current parameters set by the key step to achieve the output voltage adjustable. By measuring the final power factor, the system carries UCC28019 to the core of PFC ,which can let the power factor be as high as 98%, with over-current protection and self-recovery, and the hardware circuit is simple, has a certain practicality. |
