1 Introduction
In recent years, multilevel converters have been recommended for medium and high-power applications . Basically, multilevel converter is a power electronic system that synthesises a desired voltage output from several levels of dc voltages as inputs . Generally, multilevel converters are classified into three kind’s structures, which are the diode-clamped converter , the flying capacitor converter and the cascade converter with separated dc sources . The diode-clamped converter uses more number of clamping diodes for higher levels and the balancing of voltage capacitors, which connected in series, is a big problem. For higher voltage levels, the flying capacitor converter requires a great number of capacitors and the capacitors voltage balancing is problem . Conventional H-bridge cascade converter is the best structure among classic multilevel converters, because this structure needs fewer numbers of power electronic components. The conventional cascade converter can be symmetric and asymmetric. In symmetric configuration, the values of the dc voltage sources are the same. However, the values of dc sources in asymmetric configuration are non-equal. The asymmetric configuration can produce more number of levels in output voltage in comparison with symmetric configuration for the same number of components. However, this structure needs many dc voltage sources and switches .