A Systematic Method for Studying the Use of DC/DC Converters With Three Discontinuous Conduction Modes as Automatic PFCs

This article describes a general method for studying the behavior of converters with three discontinuous conduction modes as automatic power factor correctors (PFCs). Some converters with a single discontinuous conduction mode (such as Buck-Boost, Flyback, SEPIC, Ćuk, and Zeta converters) can behave as automatic PFCs when operating in that discontinuous conduction mode, with a nearly constant duty cycle during each cycle of the line voltage. There are also dc/dc converters that, due to multiple diodes and inductors, can operate in different discontinuous conduction modes. A systematic method for studying these three discontinuous conduction modes was recently presented. This article extends that method to using the aforementioned dc/dc converters as part of an ac/dc converter with low harmonic injection in the line, identifying when the converters behave as “ideal automatic resistor emulators” (i.e., configuring ideal automatic PFCs) and when they behave as “quasi-ideal automatic resistor emulators” (i.e., as quasi-ideal automatic PFCs). As an example, the article examines a SEPIC converter used as a resistor emulator. In this case, three discontinuous conduction modes are possible, if the inductance of the input inductor is low enough to allow the input bridge rectifier diodes to stop conducting when the transistor is
off
. The study allows us to determine of the line current waveform when the converter operates in any of the possible discontinuous conduction modes. The results show that quasi-ideal automatic PFC behavior can be achieved in discontinuous conduction modes with low inductance values. Moreover, the results for the SEPIC can be easily extended to the Ćuk converter operating in the same manner. Finally, the theoretical predictions for the line current from the proposed study were verified through simulation using PSIM (in the case of both the SEPIC and Ćuk topologies), and through experimentation (in the case of SEPIC).