| The design of single active power factor correction (APFC) circuit with high power factor and low electromagnetic interference (EMI) has been performed in this paper.The traditional APFC circuit is analyzed in detail. Based on the comparison between single stage and dual stage structures for APFC circuit, a single stage one is used in present work. Also, a single-phase Boost chop-mode converter is chosen as the power stage circuit, and its composing and working processes are described. Further, the comparisons and analyses are made for both the current modes of APFC and the corresponding control methods, and thus the continuous current mode and the average current mode control are adopted in this design. In addition, UC3854 is used as the control integrated circuit (IC) in APFC, and its working principle and pin function are introduced. For the control input, multiplier, voltage loop and current loop included in control unit for UC3854, the detailed analyses are given and the traditional design methods are presented.For APFC with the determinations of the structure, power stage circuit, current modes, control methods and control IC, it can be shown that this traditional APFC has the following disadvantages, i.e. the greater ripple of output voltage, the longer response time of voltage loop and a great deal of switch ripple lying on the input current. Therefore, the optimizations and improvements have been systemically made for the traditional APFC by means of the simulation calculations on APFC.The apparoch of pulse compensation is applied to resolve the problem of the greater ripple of output voltage. On the basis of the detailed calculations and analyses for the band-pass filter used as a part getting the second harmonics at the output end of APFC circuit, an improved pulse compensation circuit is suggested and its parameters have been optimized. The improved pulse compensation circuit can decrease the ripple of the output voltage of APFC circuit as greatly as possible while retaining the APFC's stability. Consequently, a capacitor with low capacity at output end of APFC circuit may be used.According to the improved pulse compensation circuit, the relation between pulse compensation and crossover frequency of the voltage loop is analyzed, and the parameters of the voltage loop are optimized. By theoretical caculations and simulation, it is evident that the improved pulse compensation circuit increases similarly the crossover frequency of the voltage loop, resulting in the band broadening and the rapid response of the voltage loop.To eliminate the switch ripple lying on the input current, an input filter is added at input end of the APFC. The structures of various input filters are compared and the parameters of the selected input filter are analysed and calculated in detail. The relation between output impedance of the input filter and input impedance of APFC circuit and its affection on APFC's stability are discussed at length. Also, the parameters of the input filter are optimized.Based on the systemic optimizations made above for APFC circuit and at the input voltage of 220 V/50 Hz, the parameters of several constitutes, i.e. the optimized input filter, power stage part and the pulse compensation part of APFC circuit are caluclated, and the simulation computations are done using Saber soft. It is, therefore, shown that the simulated results are in good agreement with the theoretical designs, and the designs proposed in this work improve obviously the performances of the system when compared with the traditional APFC circuit, which demonstrates the correctness of the optimizations presented in this paper. |
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