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 nonsinusoidal 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.

In this dissertation, we address several issues concerning the application to single-phase PFC of various high-frequency switching converter topologies. The inherent PFC properties of second-order switching converters operating in Discontinuous Inductor Current Mode - DICM are well known, and Boost converters are widely used. However, their output voltage is always higher than the amplitude of the rectified-sinusoid input voltage. In addition, it is expected that the level of the differential-mode EMI is much higher in DICM, as compared to the Continuous Inductor Current Mode - CICM. Therefore, we first investigated the requirements for the EMI filter for a PFC stage based on a Boost converter operating in DICM.

The high-level of differential-mode EMI that is associated with DICM operation prompted our interest to investigate the application of two-switch fourth-order converters for PFC. The switching cell of these converters contains two inductors, which can operate in DICM or in CICM, and one capacitor, which can operate in Discontinuous Capacitor Voltage Mode - DCVM or in Continuous Capacitor Voltage Mode - CCVM. As a consequence, in these topologies several combinations of operating modes can be obtained, which have characteristics that otherwise cannot be obtained in second-order switching converters.

We analyze three fourth-order topologies operating in DCVM and CICM, which have both an input current with reduced high-frequency content and an inherent PFC property. One of the converters, i.e. the Buck converter with an LC input filter, is then selected for a more detailed analysis. In addition, a fourth-order topology with galvanic isolation and operating in DCVM and CICM is presented and analyzed, as well.

We also consider the operation in CCVM and CICM, which is analyzed for a fourth-order topology with step-down conversion ratio. The 'zero-ripple' technique is applied to obtain an input current having a very low high-frequency content, and average current mode control is used to shape the input current.

Methods for improving the efficiency of the PFC stage are addressed, too. We compare several Boost-type topologies that have lower conduction losses than the combined diode bridge and Boost converter, as well as one fourth-order topology that is able to operate with bipolar input voltage, in other words it can perform direct AC/DC conversion.

Finally, we propose a novel Zero Voltage Transition - ZVT topology, which reduces the switching losses by creating zero voltage switching conditions at the turn-on of the active switch. This topology can be used in a variety of converters, for DC/DC or PFC applications.