Power Electronics has been gaining relevance in countless fields throughout industry, defence, transport, public buildings, communications, computing, home automation and electro-medical equipment. Although usually unnoticed, due to its subsidiary role of supplying energy to electrical equipment and systems, its applications and implantation are continuously growing. The power range is very wide, covering from over five thousand megawatts in High Voltage DC Transmission to a few tenths of a watt in micro-power converters for mobile telephones and personal electro-medical devices. Its multidisciplinary character and universality makes of power electronics a technological synergic vortex where semiconductor physics, electrical engineering, thermal management, control theory, industry communications, transport and most industrial disciplines are brought together. This paper, first of two, covers general aspects of Power Electronics and its applications in the high power range.