CHAPTER 1 Power Electronic Converters ANDRZEJ M. TRZYNADLOWSKI University of Nevada, Reno, Nevada This introductory chapter provides a background to the subject of the book. Fundamental principles of electric power conditioning are explained using a hypothetical genetic power converter. Ac to dc, ac to ac, dc to dc, and dc to ac power electronic converters are described, including select operating characteristics and equations of their most common representatives. 1.1 PRINCIPLES OF ELECTRIC POWER CONDITIONING Electric power is supplied in a "raw," fixed-frequency, fixed-voltage form. For small consumers, such as homes or small stores, usually only the single-phase ac voltage is available, whereas large energy users, typically industrial facilities, draw most of their electrical energy via three- phase lines. The demand for conditioned power is growing rapidly, mostly because of the progressing sophistication and automation of industrial processes. Power conditioning involves both power conversion, ac to dc or dc to ac, and control. Power electronic converters performing the conditioning are highly efficient and reliable. Power electronic converters can be thought of as networks of semiconductor power switches. Depending on the type, the switches can be uncontrolled, semicontrolled, or fully controlled. The state of uncontrolled switches, the power diodes, depends on the operating conditions only. A diode turns on (closes) when positively biased and it turns off (opens) when the conducted current changes its polarity to negative. Semicontrolled switches, the SCRs (silicon controlled rectifiers), can be turned on by a gate current signal, but they turn offjust like the diodes. Most of the existing power switches are fully controlled, that is, they can both be turned on and off by appropriate voltage or current signals. Principles of electric power conversion can easily be explained using a hypothetical "genetic power converter" shown in Fig. 1.1. It is a simple network of five switches, SO through $4, of which S1 opens and closes simultaneously with $2, and $3 opens and closes simultaneously with $4. These four switches can all be open (OFF), but they may not be all closed (ON) because they would short the supply source. Switch SO is only closed when all the other switches are open. It is assumed that the switches open and close instantly, so that currents flowing through them can be redirected without interruption.