A Review of Multilevel Inverter Topology and Control Techniques <s> C. Flying Capacitor Multilevel Inverter (FCMI) <s> Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of DC capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separate DC sources. The operating principle, features, constraints, and potential applications of these converters are discussed. <s> BIB001 </s> A Review of Multilevel Inverter Topology and Control Techniques <s> C. Flying Capacitor Multilevel Inverter (FCMI) <s> Traditional two-level high-frequency pulse width modulation (PWM) inverters for motor drives have several problems associated with their high frequency switching which produces common-mode voltage and high voltage change (dV/dt) rates to the motor windings. Multilevel inverters solve these problems because their devices can switch at a much lower frequency. Two different multilevel topologies are identified for use as a power converter for electric drives: a cascade inverter with separate DC sources; and a back-to-back diode clamped converter. The cascade inverter is a natural fit for large automotive all-electric drives because of the high VA ratings possible and because it uses several levels of DC voltage sources which would be available from batteries or fuel cells. The back-to-back diode clamped converter is ideal where a source of AC voltage is available such as a hybrid electric vehicle. Simulation and experimental results show the superiority of these two power converters over PWM-based drives. <s> BIB002
The structure of this inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, it uses capacitors in their place, hence the name implies. The circuit topology of the flying capacitor multilevel inverter is shown in Fig. 4  . This topology has a ladder structure of DC side capacitors, where the voltage on each capacitor differs from that of the next capacitor. The voltage increment between two adjacent capacitor legs gives the size of the voltage steps in the output waveform BIB001 . Unlike the NPCMI, the flying capacitor inverter does not require all of the switches that are ON (conducting) be in a consecutive series. Moreover, the flying-capacitor inverter has phase redundancies BIB002 . These redundancies allow a choice of charging and discharging specific capacitors, and can be incorporated in the control system for balancing the voltages across the various levels. One advantage of the flying-capacitor-based inverter is that it has redundancies for inner voltage levels. In another words, two or more valid switching combinations can synthesize an output voltage. Another merit of FCMI is the availability of phase redundancies for balancing the voltage levels of the capacitors. Real and reactive power flow can equally be controlled. The large numbers of capacitors enable the inverter to ride through short duration outages and deep voltage sags. It requires a single isolated dc supply voltage source  , BIB002 . Like other topologies it has disadvantages of complicated control in tracking the voltage levels for all of the capacitors. Also, pre-charging all of the capacitors to the same voltage level and startup are complex. Switching utilization and efficiency are poor for real power transmission. The large numbers of capacitors are both more expensive and bulky than clamping diodes in multilevel diode-clamped converters. Packaging is more difficult in inverters with higher number of levels  .