The paper proposes a new three-phase four-wire multilevel inverter.Besides the conventional six-switch three-phase full-bridge inverter output

the inverter cascades to a half bridge configuration and has a bootstrap circuit with the load neutral line connecting to the anode of input power.Under the ripple-tracing segmental SPWM(Sinusoidal Pulse Width Modulation)

the cascaded half-bridge electricity is obtained when the bootstrap circuit works.The circuit topology structure and its control method reduce the independent direct-current power sources of the multilevel inverter and raise the inverter output voltage.Using the piecewise calculation method

the paper analyzes the ripples and current of the bootstrap capacitor with regular sampling SPWM modulation

and the harmonic waves generated.The analyses and experiments in this paper show that a bootstrap capacitor's capacitance

charging loop resistance

load and modulation degree can affect the capacitor's ripples and current.