Power Supplies and DC-DC Conversion
Power supply circuits for radios, portable communications, television sets, and son on, are a commonplace although often neglected element of electrical equipment. This application dominates the power electronics industry in many ways, because of the huge quantities involved. Some typical supplies intended as components of larger systems are illustrated below. The earliest power supplies for vacuum tube electronics were rectifiers, followed by filtering circuits to create a smooth DC output. Until quite recently, most power supplies took this same form, usually with the addition of a transformer at the AC input to provide the correct output level.
This conventional power supply style matured after about 1967, when integrated series regulator circuits were developed and routinely used at the supply output. A series regulator circuit is a corm of amplifier, which provides a very tightly fixed output even from a somewhat noisy rectified signal. The combination of transformer, rectifier, and regulator is referred to as a linear power supply since the output filtering circuit is based on a linear amplifier, though the circuit as a whole is still nonlinear. Classical rectifier power supplies in this form are among the most widely used electronic circuits, and form the power supply for hundreds of millions of small appliances and electronic accessories.
As costs of electronics decline, the power supply becomes a larger fraction of system cost and design effort. It is not uncommon for power supply costs to make up 50% of the total cost of a typical electronic product. This situation makes new technology developments in power supplies critically important. Late in the 1960s, use of DC sources in aerospace applications led to the application of DC-DC conversion circuits for power supplies. The basic circuit arrangements are much older, and grew out of rectifier applications. Although the basic ideas are old, power semiconductors can be used to make these circuits inexpensive and reliable. In a typical arrangement, an AC source from a wall outlet is rectified without any transformation; the resulting high DC voltage is converted through a DC-DC circuit to the 5 V, 12 V, or other level required by the application. These switched-mode power supplies have rapidly supplanted linear supplies across the full spectrum of circuit applications.
A personal computer often requires three different 5 V supplies, two 12 V supplies, a -12 V supply, a 24 V supply, and perhaps a few more. Only a switched-mode supply can support such complex requirements without high costs. The bulk and weight of linear supplies make them infeasible for cell phones, laptops, and similar equipment.
Switched-mode supplies often take advantage of FET semiconductor technology. Trends towards high reliability, low cost, and miniaturization have reached the point at which a 5 V power supply sold today might last 500,000 hours (more than 50 years!), and provide a few hundred watts of output in a package with volume of less than 20 cubic centimeters, for a price of less than $0.50 per watt. This type of supply brings an interesting, if mundane, dilemma: The AC line cord to plug it in actually takes up more space than the power supply itself. Innovative concepts such as integrating a power supply within a connection cable are often seen to address this.
Device technology for supplies is being driven by expanding needs in the automotive industry, the telecommunications industry, and markets for portable electronics and equipment. The automotive industry uses well-defined voltage levels, but the amount and complexity of electronic hardware in a typical car continues to increase. Power conversion for this industry must be cost-effective, yet rugged enough to survive the high vibration and wide temperatures range to which a passenger car is exposed. Global communications is possible only when sophisticated equipment can be used almost anywhere in the world. This brings a special challenge, since electrical supplies are neither reliable nor consistent in much of the world. In North America, voltage swings in the domestic AC supply are often less than +/-5% around a nominal value. In many developing nations, the swing can easily be +/-25% (when power is available). Communications equipment must tolerate these swings, and must also match a wide range of possible backup sources. Given the enormous size of possible markets for telephones and consumer electronics in developing countries, the need for flexible-source equipment is clear. Portable equipment challenges designers to obtain the best possible performance from small batteries. Equipment must use as little energy as possible. The low voltages used for portable battery packs, which range from less than 2 V up to only about 10 V, make any conversion circuit difficult.