A comparative study of different multilevel inverters

The ordinary Voltage source inverter (VSI) has two output levels +V
DC
/2 and -V
DC
/2. For medium and high power applications, multi-stage VSI (series and/or parallel combinations) with suitable filter (for obtaining near sinusoidal waveform at the output) may be a solution but there are problems in matching of different stages, selection of bulky transformer and proper filter. The quality of both the output voltage and current waveforms with minimum ripple is obtained using high frequency switching along with various PWM techniques. These conventional two level inverters have some limitations in operating at high frequencies mainly due to switching losses and constraints in device rating. The problem is resolved by means of multilevel inverters (MLIs) which utilize lower switching frequencies and give high voltages with improved total harmonic distortion (THD) without use of filter. This paper gives a brief review on different MLI techniques with advantages and disadvantages of each technique. The main disadvantages of MLI are requirement for isolated power supplies, design complexity and switching control circuits for proper elimination of lower order harmonics.