Quasi-Two-Level Operation of a Five-Level Flying-Capacitor Converter

This paper deals with the quasi-two-level operation of a five-level Flying-Capacitor converter. The combination of the quasi-two-level operation and multilevel converters enables several opportunities. The series connection of low-voltage devices allows higher switching frequencies even at high DC-link voltages. The use of intermediate levels decreases the dv/dt of the output voltage during the switching transients and improves the voltage spectrum. Furthermore, regarding the flying capacitors, the installed capacitance required can be significantly reduced. The redundant switching states are usually used to balance the capacitor voltages. However, this can be associated with enormous complexity, especially with a high number of levels. This paper considers two methods of voltage balancing for the quasi-two-level operation, which do without the redundant switching states and are based solely on the adaptation of the dwell times of the individual intermediate levels. The first considers the flying capacitors separated so that the change of the dwell time happens independently. However, this leads to a variable dv/dt of the output voltage. The second method considers all flying capacitors of a phase together and assigns the dwell time according to the capacitor voltages. This results in an approximately constant dv/dt of the output voltage. The functionality of the quasi-two-level operation and the two balancing methods are explained and confirmed by simulation.