A Capacitor Voltage Buildup Method for Flying Capacitor Multilevel Inverters

A voltage buildup method for flying capacitor multilevel inverter(FCMLI) was presented.Before FCMLI worked in steady state,all flying capacitors should be charged to corresponding DC voltage levels.Both voltage stress and current stress should be held within certain limits during voltage buildup process.The method has used the existing high voltage input and internal Buck converter of FCMLI produced high voltage stress,so it could’t meet the requirements.In this paper,some reverse Boost DC/DC converters of FCMLI were discovered.As flying capacitors were considered to be the output of those Boost converters,a new low voltage input was added to the old output side.Meanwhile the circuit worked in high frequency PWM mode and charged up these flying capacitors.And it was also pointed out that voltage buildup processes of different flying capacitors must follow strict order of voltage level,or else the voltage stress could not be kept in certain limit.By the use of voltage-current two-loop control method,the current stress could be adjusted at the same time.So the new method can fill both requirement of function and stress;in addition,the topology of FCMLI need not to be changed,and a small quantity of hardware devices need to be added;and the new method is also flexible.Simulation and experiment of a flying capacitor five-level inverter verified the correctness of these analyses.