Three-Level Unipolar/Bipolar Buck-Boost AC-AC Converters with Less Active Switches and No Commutation Issues

This paper proposes three-level buck-boost direct ac-ac converters based on switching-cell configuration with coupled-magnetics. The proposed converters use only six active switches and can produce non-inverting buck, non-inverting boost, versatile non-inverting buck-boost, and inverting buck-boost output voltages. Unlike the conventional ac-ac converters, PWM dead- and overlap-times are allowed in the proposed converters without switch voltage and current overshoots, respectively, making it free of commutation issues without requiring extra circuitry and control. Moreover, high-speed power MOSFETs can be utilized without the conduction of their body diodes and related poor reverse recovery issues. The proposed converters generates three-level input and output voltages, increasing the voltage levels of input/output inductors and reducing their voltage stresses. In addition, a phase-shift PWM control is utilized which enhances the actual frequency of input and output voltages by twice the operating frequency of the switching devices. Consequently, input and output inductors can be designed much smaller while reducing input/output current ripples. The proposed circuits also feature shared-ground point between ac source and load, and provide continuous input and output currents. Two circuit topologies are proposed; Type-I is a non-inverting (unipolar) buck-boost converter with less magnetic components and diodes, and Type-II is a bipolar buck-boost converter. The experimental test results on a 400 W laboratory circuit are given to verify the operation of the proposed converter.