PWM Strategy for improved natural balancing of a four-level H-bridge Flying Capacitor Converter

Abstract

Flying capacitor based multilevel PWM converter may be an attractive choice due to the natural balancing property of the capacitor voltages. Previous research on natural balancing of a 4-level H-bridge Flying Capacitor Converter (FCC) with Phase Shifted (PS) PWM showed slow natural balancing rate for small modulation indices. Theoretical natural balancing time constant becomes infinitely large for zero output voltage (modulation index). This was probably the reason that many researchers became interested in active balancing methods for this and other capacitive multilevel voltage source inverters. The paper addresses natural voltage balancing improvement possibilities by employing modified PWM strategies different from classical PS PWM. Dramatic improvement in natural voltage balancing rate for small output voltages is achieved due to making a better use of switching states redundancy. Suggested is a family of improved PWM strategies and the best strategy is identified based on natural balancing damping rate considerations. Natural balancing mechanisms are analyzed using time domain averaging methodology that results in simple easy to interpret expressions regarding parameter influence.