Electrical Energy flow control and type change (AC ↔ DC) is an important scientific and engineering task – starting from the installation of electrical supply systems to wide usage of electricity in industries, transport and households – for more than 100 years. Due to semiconductor revolution, nowadays electromechanical and electromagnetic transformers are being replaced with advanced Power Electronic Converters with a very high efficiency. We can find Power Electronic Equipment with power amplitude starting from some watts to many megawatts in any urbanized and industrialized area of the World. What is Power Electronics? An adequate definition [1] is provided by Prof. R. W. De Doncker, (RWTH Aachen): “Power Electronics deals with the control and the low-loss conversion of electrical energy using electrical switches, nowadays – power semiconductors.”