In ancient times, thunder, lightning and electric fish [1] were the natural phenomena related to electricity. They were treated as myths, but not as energy until the discovery of electrostatic phenomena by Thales of Miletus in 640–۵۴۰ BCE [2,3]. Much later in 1752, B. Franklin discovered electricity [4], and in 1820 H.C. Örsted revealed electromagnetism [5]. Since then, a series of great discoveries on the principles of electricity and magnetism has been achieved by Volta, Coulomb, Gauß, Henry, Faraday and others, leading to many inventions such as batteries (1800), generators (1831), electric motors (1831), telegraphs (1837) and telephones (1876), to name just a few. In the early 19th century electricity has been established as a scientific discipline, and in the late 19th century the greatest progress has been witnessed in electrical engineering [6]. In 1882, the first power grid, which was a direct-current (DC) distribution system invented by T. Edison, was set up in New York to provide 110 V DC power supplying over 1000 bulbs in a short distance. At that time, the problem was how to transfer energy at a low loss from power plants to customers over a long distance through transmission lines [7]. It is now well known that electricity must be transmitted at high voltages and in the form of alternating current (AC), because DC voltage could not be increased or decreased by DC systems at that time [8]. In 1885, L. Gaulard and J.D. Gibbs developed a device named transformer, which can increase or decrease the electrical voltage of AC systems. Thereafter, G. Westinghouse applied transformers in AC distribution systems to transmit electricity efficiently over long distances, which has promoted the development of electrical engineering [5]. Transformers played a vital rôle in electricity transmission, especially in energy conversion between different voltages. Transformers can, however, only increase or decrease AC voltage (AC-AC) at the same frequency. Moreover, energy loss in transformers, magnetic radiations, huge volume and high cost of copper limited their wider use [9]. In practical applications, electric energy was expected to be converted from one form to another, for instance, between AC and DC, or just to different voltages or frequencies, or some combinations of those — demands that cannot be fully met by transformers. Therefore, novel techniques were required to solve those problems. With the development of semiconductor switches, power electronics has come into being [10]. Power electronics refers to electric power, electronics and control systems. Electric power deals with static and rotating power equipment for generation, transmission and distribution of electric power; while electronics is concerned with solid-state semiconductor power devices and circuits together with control systems for power conversion specified to meet the desired control objectives. Power electronics is one of the main technologies to realise energy conversion with high efficiency. It is known that about 70% of electric energy is converted by power electronics devices before it reaches the consumer. Nowadays, power electronics has become a fundamental technology critical for the development of energy conservation, especially for renewable energy [11]. The history of power electronics is linked to the breakthrough and the evolution of power-semiconductor devices.