The Power converter group at CERN (TE/EPC) is responsible for providing powering solutions for electro-magnets, RF tubes, particles sources and special equipment in experimental areas. This seminar summarises the ranges of power converters used at CERN and presents their design and operational challenges. After a brief introduction on the power converters roles, the basic principles of power electronics conversion is presented. The main challenges such as precision and reliability are illustrated and real design examples (LHC main quadrupoles, L4 ion source, PS main power converter, etc.) are presented.