TRENDS IN POWER SUPPLY BASIC AND TUTORIALS

As costs of electronics decline, the power supply becomes a larger fraction of system cost and design effort. One major manufacturer estimates that power supply cost will soon reach 50% of the total cost of a typical electronic product such as a cordless telephone or personal computer.

Thus, new technology developments in power supplies are critically important. In the past, bulky linear power supplies were designed with transformers and rectifiers from the ac line frequency to provide low-level dc voltages for electronic circuits.

Late in the 1960s, use of dc sources in aerospace applications led to the development of power electronic dc-dc conversion circuits for power supplies. In a typical power electronics arrangement today, 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 5V, 12 V, or other level required.

These switched-mode power supplies are rapidly supplanting linear supplies across the full spectrum of circuit applications. A personal computer commonly requires three different 5V supplies, two ‡12 V supplies, a ÿ12 V supply, a 24 V supply, and perhaps a few more. This does not include supplies for video display or peripheral devices.

Only a switched-mode supply can support such complex requirements without high costs. The bulk and weight of linear supplies make them infeasible for hand-held communication devices, calculators, notebook computers, and similar equipment. Switched-mode supplies often take advantage of MOSFET semiconductor technology.

Trends toward high reliability, low cost, and miniaturization have reached the point at which a 5 V power supply sold today might last 1,000,000 hr (more than 100 yr), provide 100Wof output in a package with volume <15 cm3, and sell for a price of <$0:30 watt. This type of supply brings an interesting 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 will be used in the future. Device technology for power supplies is being driven by expanding needs in the automotive and telecommunications industries as well as in markets for portable equipment.

The automotive industry is making a transition to 42 V systems to handle increasing electric power needs. Power conversion for this industry must be cost effective, yet rugged enough to survive the high vibration and wide temperature range to which a passenger car is exposed.

Global communication is possible only when sophisticated equipment can be used almost anywhere. This brings a special challenge, because electrical supplies are neither reliable nor consistent throughout much of the world.

While in North America voltage swings in the domestic ac supply are often < #5% around a nominal value, in many developing nations the swing can be +/-25%—when power is available. Power converters for communications equipment must tolerate these swings, and must also be able to make use of a wide range of possible backup sources.

Given the enormous size of worldwide markets for telephones and consumer electronics, there is a clear need for flexible-source equipment. Designers are challenged to obtain maximum performance from small batteries, and to create equipment with minimal energy requirements.