Development and implementation of a new modular active snubber cell for inverters

Usage of semiconductor power converters at AC energy systems, motor control, uninterruptable and switching mode power supplies etc. have been increasing consistently. Low Total Harmonic Distortion (THD) and high Power Factor (PF) for AC side, low Ripple Factor (RF) for DC side and also low size and cost and high power density and efficiency and low Electro Magnetic Interference (EMI) for whole system are desired at applications. To provide these wishes, limiting rate of rising of current and voltage, suppressing switching losses and increasing switching frequency make a great contribution with using Soft Switching (SS) techniques instead of Hard Switching Techniques (HS). SS is used commonly at DC-DC applications those have few switches. On the other hand, SS is rarely used at converters have many switches as inverters because of application difficulty. There are conduction and switching losses at operation of power converters. The conduction losses depend on duty cycle whereas they don't depend on switching frequency. The switching losses occur with crossing voltage and current of semiconductors and also increase proportional to switching frequency. Therefore, switching losses are important limiting factor for increasing switching frequency and also EMI is result of switching rate. On the other hand THD, PF and RF values get better with increasing frequency. SS is limiting rise of voltage and current and also minimizing or eliminating crossing of current and voltage of semiconductors so switching losses at switching time on principle. So, SS provides low EMI and switching losses and also high switching frequency operation. On the other hand PF and THD values can be improved and dimensions and cost of the converters can be reduced due to SS. SS techniques are Zero Current Switching (ZCS), Zero Voltage Switching (ZVS), Zero Voltage Transition (ZVT), Zero Current Transition (ZCT). At this thesis, a new active and modular SS cell, easily applicable and attractive for every converter, is improved. The proposed SS cell has been applied to single phase of grid connected T type three phase three level inverter (T-3LI) that gains popularity in recent years. At this new SS converter, the main switches turn on with ZVT and turn off with ZVS. Also, auxiliary switches turn on ZCS and turn off ZVS. There are no additional voltage stresses on the semiconductors. The steady state analysis has been already made and this analysis is verified with simulation and 1 kW – 100 kHz application.