Recent Advances in Single-Stage Power Factor Correction Khalid Rustom and Issa Batarseh School of Electrical Engineering and Computer Science University of Central Florida Orlando, FL 32816 E-mail:
[email protected] factor correction approaches in term of their topological structures, followed by common control techniques. At the end, we emphasized the single-stage approach by highlighting its common drawbacks and recent solutions.
Abstract - This paper presents an overview of various interesting power factor correction techniques for single-phase applications. The discussion includes commonly-used control strategies and various types of converter topologies. Included is a comparative study of these strategies, with the major advantages and disadvantages is highlighted. We will emphasize the single-stage topologies, its drawhacks and some promising solutions.
2 CLASSIFICATION O F POWER FACTOR CORRECTION APPROACHES The general approaches to improve power factor can be widely classifieds as passive and active approaches [6]. The passive approaches use capacitive inductive filters to achieve PCF, while the active approaches use a switchedmode power supply to shape the input current. These approaches are discussed briefly next.
1 INTRODUCTION It is well known that power supplies connected to AC mains introduce harmonic currents in the utility. Such harmonic currents cause several problems including voltage distortion, heating, radiated and conducted noises and can reduce the capability of the line to provide energy [I]. As a result, national and international standards or recommendations have been adopted that make the use of power factor correction circuits in power supplies a necessity.
2.1 Passive Approaches
Unity power factor is not necessary to meet the regulations. For example, both IEEE 519 and iEC 1000-32 [2-31, allow the presence of harmonics in the line current. This fact has lead to the publication of a great number of papers in recent years, with solutions that range from a simple LC filter to the two-stage approach. The two-stage approach to achieve power factor correction requires the presence of a PFC stage prior to the D C D C regulation stage. An alternative solution to realize the goal was to integrate the active PFC stage with the isolated high quality output D C D C stage into one stage, which is known as a single-stage converter with least components and simplest controller. Theoretically, changing the two-stage scheme to single-stage scheme can substantially alleviate the cost and complexity of Single-Stage (S’) PFC ACDC. The question arises, why hasn’t such a concept been extensively adopted in today’s power supply industry? Perhaps the answer is because there are still some existing technical challenges, with respect to the development of viable Sz PFC A C D C converters. These challenges include high voltage and current stresses, and low efficiency, etc. Also, the single-stage PFC is attractive to low-power applications. Several existing review papers have been focused on the general PFC topics with some comparison [4]. Other papers specialized in a single-stage configuration [ 5 ] . in this paper, we present a general classification of the power
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In the passive approaches, a full bridge rectifier with an LC filter is used to reduce the line current harmonic limits. Generally the LC filter can be placed in either the AC-side or the DC-side of the rectifier as shown in Figure l(a). Placing the LC filter in the AC-side will result in more pure sinusoidal input current. Passive PFC can meet the regulation with high efficiency, superior reliability, low cost, and low EM1 [781, On the other hand, the filter capacitor voltage vanes with the line voltage, which bas a detrimental effect on the performance and efficiency of the DCDC converter. When considering a hold-up time for the power supply, the bulk capacitance has to be increased and becomes very bulky compared to what it would be without this varying voltage. As a result, the passive approaches seem to be more attractive in low-power applications, up to 300Watts, and are more suitable for narrow line voltage range. Other drawbacks are the size and weight of the filter choke inductor. However, the majority of power supplies manufachlred in low-power and cost-sensitive applications have adopted the passive PFC approaches.
2.2 Active Approaches In active PFC approaches, a switched mode converter is employed to overcome the limitations of the passive approaches. Assuming unity power factor, the line current should be sinusoidal and in phase with the line voltage. That will result in pulsating output power than contains in addition to the real (average power) - an alternating component with double-line frequency. Since the power demanded by most loads is constant, an energy storage element is needed. Since the inductor-stored energy cannot
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match this excessive energy, another storage component is needed. This storage capacitor is normally located between the two stages and should handle the double-line frequency ripple component, which make it bulky. This second, harmonic problem that presents itself on
the output of the PCF stage cannot be internally solved. Usually, a compromise between PFC and output voltage ripple should he made, but most of the time this output voltage is not good enough to supply the load. As a result, another D C D C converter is needed, or what is so called post regulator, to solve this problem and achieve tight output regulation. The result in the most powerful PFC configuration and is the active two-stage PFC shown in Figure l(b).
(a)
General Struchlres of the Passive PFC Approaches
& & -
2.2.1 Two-Stage PFC converter
This configuration implies the use of two converters to achieve both power factor correction and output regulation in addition to the rectification circuit and the input EM1 filter. These converters are independent, which means each one bas its own switches and control circuit. The PFC converter performs the input current shaping using one of the popular converter topologies (buck, boost, buck-boost, flypack, SEPIC, Cuk, ZETA) in addition to one of the PFC controlling techniques. The boost converter is widely used due to its advantages, which include good power factor, grounded switch, input inductor and simplicity. However, this PFC converter normally has a low bandwidth control, which implies a loosely regulated output voltage across the storage capacitor. In universal line voltage applications, the DC bus voltage may vary between 380-400V. Because of the relative high voltage on the storage capacitor, the value of the capacitance can be optimized to provide the necessary hold up time. The D C D C converter is connected to the storage capacitor to provide the necessary tight output voltage regulation with the appropriate gain and, most of the time, provides isolation.
@) System Configuration ofTwo-Stage PFC Power Supply
(c) System Configuration of Single-Stage PFC Power Supply
Figure 1 General StnrchlTes ofthe PFC Conveners
2.3 Approach Comparisons Generally, the passive approach should be considered in low power applications, especially when designing to meet the minimum regulation requirements with a narrow line voltage range.
2.2.2 Single-Stage PFC converter The single-stage PFC configuration came about to reduce the cost and complexity of the two-stage structure, and it can be viewed more as a modification on the twostage PFC rather than a class by itself. As can be seen from Figure l(c), the PFC and the DCDC cell share the control circuit and can also share the switches in this configuration. The energy storage capacitor between the two stages serves as a buffer for frequency isolation and to provide the converter with the necessary hold up time. However, in single stage configuration, the voltage across the storage capacitor is not regulated, -because the controller is used to regulate the output voltage. As a result, this voltage can vaty greatly, usually between 130500V in universal line application. This will have a negative impact on the design and cost of the PFC converter. More about single-stage converters is presented in Section IV.
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