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Flying-capacitor multilevel inverter

Single-phase flying-capacitor multilevel half-bridge with output filter: pre-charge sequence, phase-shifted modulation and capacitor-voltage balancing studied in simulation.

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Flying-capacitor multilevel inverter
Flying-capacitor multilevel half-bridge with output filter and the simulated waveforms of the flying-capacitor voltages, output voltage and current.

What it is

A flying-capacitor multilevel inverter builds a multi-step output voltage from a single DC link by inserting or bypassing capacitors that float between the switch pairs. Each device only blocks a fraction of the DC-link voltage, the output has more levels than a two-level bridge and the effective switching frequency seen by the filter is multiplied by the number of cells.

This study looks at a single-phase half-bridge with an output LC filter: how the flying capacitors are charged before the inverter can start, how the phase-shifted carriers keep their voltages balanced, and what the output looks like when it is done properly.

What the note covers

  • Pre-charge sequence with series resistors and its timing.
  • Phase-shifted modulation of the cells and natural balancing of the capacitor voltages.
  • Device voltage stress and output-filter behaviour, from the simulation.

Figures

Phase-shifted carrier modulation: gate patterns of the switch pairs.
Pre-charge of the flying capacitors through the pre-charge resistors and the resulting device and capacitor voltages.