Topological issues in single phase power factor correction

Abstract

The equipment connected to an electricity distribution network usually needs some kind of power conditioning, typically rectification, which produces a non-sinusoidal line current due to the nonlinear input characteristic. With the steadily increasing use of such equipment, line current harmonics have become a significant problem. Their adverse effects on the power system are well recognized. They include increased magnitudes of neutral currents in three- phase systems, overheating in transformers and induction motors, as well as the degradation of system voltage waveforms. Several international standards now exist, which limit the harmonic content due to line currents of equipment connected to electricity distribution networks. As a result, there is the need for a reduction in line current harmonics, or Power Factor Correction - PFC. There are two types of PFC’s. 1) Passive PFC, 2) Active PFC. The active PFC is further classified into low-frequency and high-frequency active PFC depending on the switching frequency. Different techniques in passive PFC and active PFC are presented here. Among these PFC’s we will get better power factor by using high-frequency active PFC circuit. Any DC-DC converters can be used for this purpose, if a suitable control method is used to shape its input current or if it has inherent PFC properties. The DC-DC converters can operate in Continuous Inductor Current Mode – CICM, where the inductor current never reaches zero during one switching cycle or Discontinuous Inductor Current Mode - DICM, where the inductor current is zero during intervals of the switching cycle. In DICM, the input inductor is no longer a state variable since its state in a given switching cycle is independent on the value in the previous switching cycle. The peak of the inductor current is sampling the line voltage automatically. This property of DICM input circuit can be called “self power factor correction” because no control loop is required from its input side. In CICM, different control techniques are used to control the inductor current. Some of them are (1) peak current control (2) average current control (3) Hysteresis control (4) borderline control. These control techniques specifically developed for PFC boost converters are analyzed. For each control strategy advantages and drawbacks are highlighted and information on available commercial IC's is given. This high frequency switching PFC stage also has drawbacks, such as: it introduces additional losses, thus reducing the overall efficiency; it increases the EMI, due to the highfrequency content of the input current. Some of the EMI requirements are discussed. But the level of high-frequency EMI is much higher with a considerable amount of conduction and switching losses. This highfrequency EMI will be eliminated by introducing an EMI filter in between AC supply and the diode bridge rectifier. The efficiency will be improved by reducing the losses using soft switching techniques such as ‘Zero Voltage Switching’- (ZVS), ‘Zero Voltage Transition’ (ZVT), and ‘Zero Current Switching’- (ZCS). We study circuit techniques to improve the efficiency of the PFC stage by lowering the conduction losses and/or the switching losses. Operation of a ZVT converter will be discussed, in which the switching losses of the auxiliary switch are minimized by using an additional circuit applied to the auxiliary switch. Besides the main switch ZVS turned- on and turned-off, and the auxiliary switch ZCS turned-on and turned-off near ZVS. Since the active switch is turned- on and turned-off softly, the switching losses are reduced and the higher efficiency of the system is achieved.