The Efficiency Objective and the Switch
As simple a circuit element as the light switch like the one illustrated below gives a reminder that the extreme requirements in power electronics are not especially novel. Ideally, when a switch is on, it provides vswitch = 0, and will carry any current imposed on it. When a switch is off, it blocks the flow of current (iswitch = 0), regardless of the voltage across it. The device power, vswitch/iswitch, is identically zero at all times. The switch controls energy flow with no loss. Reliability is high, too. Household light switches perform over decades of use and perhaps 100,000 operations. Of course, a mechanical light switch does not meed all the practical needs. A switch in a power supply often operates 100,000 times each second. Even the best mechanical switch will not last beyond a few million cycles.
A circuit built from ideal switches will be lossless. The advantages of switching circuits are so significant that many people equate power electronics with the study of switching power converters. Other lossless elements such as capacitors, inductors, and conventional transformers, might also be useful for conversion. Such a system consists of an energy source, an electrical load, a power electronic circuit, and control functions. The power electronic circuit contains switches, lossless energy storage elements, and magnetic transformers. The controls take information from the source, load, and designer, then determine how the switches operate to achieve the desired conversion. Usually, the controls are built up with conventional low-power analog and digital electronics.