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1、Hengchun Mao, Member, IEEE, Fred C. Y. Lee, Fellow, IEEE, Dushan Boroyevich, Member, IEEE, and Silva Hiti, Member, IEEE1. IntroductionMotivated by the forthcoming stringent power-quality regulations, power-factor correction （PFC） has been an active research topic in power electronics. The single-pha

2、se PFC is already a common practice, and the industrial application of three-phase PFC techniques has also emerged. Up to this point, the research of three-phase converters has been heavily focused on inverter applications. Although most techniques developed in the inverter area can be used in PFC a

3、pplications, a PFC circuit has its unique characteristics and, therefore, deserves some special treatment. The primary differences between PFC and inverter applications include the following aspects.Special attention has to be paid to the quality of input current to reduce the pollution to the utili

4、ty, usually measured by the input current total harmonic distortion（THD）. Although there are no specific limits on the input current distortion of general high-power three-phase converters at present, it is a common practice to limit the input current THD of three-phase PFC converters at least below

5、 10%. This makes control design more critical than in inverters.The electromagnetic interference （EMI） emissions are a great concern in PFC applications. The high-speed switching action of a PFC converter generates both differential mode and common-mode noises at the input of the PFC converter at hi

6、gh frequencies. Passive filtering is widely used to reduce the EMI emissions into the utility.High switching frequencies are desirable to reduce the size and weight of reactive components （especially inductors） and to improve current-control performance. While a 20-kHz switching frequency is deemed

7、sufficient in most inverter applications, a PFC converter prefers a much higher switching frequency, e.g., 50100 kHz in tens of kilowatt power level. The effect of soft switching techniques is, therefore, very prominent in PFC applications.The input currents are generally in phase with the input vol

8、tages, and bidirectional power flow is usually not required in PFC circuits. These facts provide some flexibility to develop soft-switching techniques and control schemes specific to PFC converters.2. Simple Three-Phase PFC CircuitsTo reduce the converter cost and avoid the complexity of full-bridge

9、 three-phase converters, several simple topologies have been used for the low power end of three-phase PFC applications.A. Three-Phase Rectifiers Consisting Of Three Single-Phase ConvertersA simple way to implement a PFC converter in a three-phase application is to combine three single-phase boost r

10、ectifiers at the input side, one for each phase and each followed by a dcdc converter. This configuration is simplified in by directly coupling the outputs of the three single-phase PFC converters. This simplification could result in significant cost reduction, since only one dcdc converter is requi

11、red. The output capacitor is shared by the three converters, and the voltage across it does not have low-frequency ripple in balanced conditions. Therefore, fast voltage control can be used without distorting the input current references. The main problem of this configuration is that the current in

12、to a phase module is not the same as its return current, which causes control interference among the three phases.B. Three-Phase Single-Switch DCM RectifiersHigh power factor can be easily obtained when boost, buck-boost, flyback, Sepic, Cuk, and zeta converters are operated in discontinuous current

13、 mode （DCM） with constant duty cycles. There are several such three-phase PFC topologies developed which require only one active switch. The single-switch boost rectifier, is the most popular topology in this category, due to its simplicity and relatively good performance. Usually, the converter is

14、controlled by a slow voltage loop, which keeps the duty cycle of the main switch practically constant over a line cycle, so each input current has an envelope proportional to its corresponding phase voltage. The duty cycle determines the magnitude of the input currents and, thus, the input power, wh

15、ich provides a means to regulate the output voltage. Although the input current peak is proportional to the sinusoidal input voltage in each switching cycle, the average input current is distorted by the inductor current during the discharging stage, the duration of which is determined by the differ

16、ence between the output and input voltages. To reduce the distortion, the output voltage has to be sufficiently higher than the input voltage peak to limit the duration of the discharging stage.By changing the output stage of the boost topology, several other topologies have been proposed in. In the

17、 Sepic and Cuk topologies, the voltage gain can be reduced to facilitate the dcdc converter in the second stage. Single-stage PFC can be achieved by replacing the output inductor with a transformer and inputoutput isolation. However, the switch and the intermediate bulk capacitor are exposed to high

18、 voltage stress, and the conduction loss is increased due to the increased circulating current. In the so-called dither rectifier, the input inductors are changed to flyback transformers. Since the input current now is only the inductor current in the charging stage, nearly perfect sinusoidal curren

19、t can be obtained. The disadvantage of this flyback converter is the high voltage stress of the switch and the complex clamp circuit necessary to absorb the leakage energy of the flyback transformers.3. Three-Phase Buck RectifiersA buck rectifier has some attractive features compared to a boost rect

20、ifier, such as inherent short-circuit protection, easy inrush current control, and low output voltage. In addition, its input currents can be controlled in open loop, and much wider voltage loop bandwidth can be achieved. Generally, a buck rectifier has higher conduction loss than its boost counterp

21、art, because more semiconductor devices are in series, and the input currents are discontinuous. However, a buck rectifier usually has lower switching loss, especially at low line conditions, where the boost rectifiers switching loss （and also conduction loss） reaches its maximum. The worst case pow

22、er loss of a three-phase buck rectifier is not necessarily higher than that of a three-phase boost rectifier. At present, three-phase buck rectifiers are not used as widely as three phase boost rectifiers, probably because a single-phase buck rectifier is not a viable technique and three-phase curre

23、nt source inverters, which use the same topology as the buck rectifiers without the freewheeling diode, are not popular, except in very high-power SCR applications. However, it is possible that three-phase buck rectifiers could achieve certain performance/cost advantages over boost rectifiers for so

24、me applications, especially if the performance of bidirectional voltage devices can be significantly improved with the development of power semiconductor techniques in the future. 4. Control And System IssuesA. Control DesignControl of power converters usually can be divided into three functions: mo

25、dulation, current control, and regulation of an output variable （the output voltage in rectifiers）. In the three-phase inverter applications, the system dynamics is usually dominated by the slow electromechanical and/or large reactive components, so that the inverter dynamic performance is not very

26、critical. Additionally, accurate ac current control is not very important in many inverter applications （except for field-oriented drives）. On the contrary, high-quality current control, without the use of large reactive components, is the major objective in PFC applications. With high switching fre

27、quencies, which are made possible through the use of soft-switching techniques, high performance and very wide bandwidth control can now be designed. The control design is facilitated by recent improvements in the modeling of three phase converters. All standard modulation techniques developed for i

28、nverters can be used in rectifier applications. Sinusoidal PWM （SPWM） is well suited for analog implementation, but causes higher switching losses and current distortion. In boost rectifiers, SPWM can be used with third-harmonic injection to decrease the minimum output voltage by 15%. The same effec

29、t is automatically achieved with spacevector modulation （SVM）, which also significantly reduces switching loss and high-frequency current ripple. Many of the soft-switching techniques require the use of completely different modulation strategies or modifications of the standard PWM schemes, due to t

30、he requirement of synchronizing switch turn-on instants in the three phases. In the buck rectifiers, due to topological restrictions, three phases cannot operate independently. This prevents the direct use of hysteresis input current controllers. Instead, SVM or modified SPWM techniques are usually

31、used. Excellent input current quality can be achieved with open loop control.B. Filter Design and System InteractionSince a PFC converter has to meet EMI specifications, an input filter is usually required. The filter should provide enough high-frequency noise attenuation, while keeping a low input

32、displacement angle and should be as small and as light as possible to increase power density. A way to predict the conducted EMI emission of boost rectifiers is presented in, which can be easily extended to buck rectifiers. A proper damping network is necessary to ensure system stability.5. ConclusionsThis paper has given a comprehensive review of recent developments in three-phase PFC techniques, especially soft-switching techniques. It can be expected that soft-switching converters will be an increasingly viable alternative to the conventional hard-switc