Power-Supply Fundamentals

Nov. 15, 2002
Electronic equipment requiring dc power must adjust the ac input to a usable level with transformers or switching transistors and use a rectifier stage to convert the ac to dc.

Electronic equipment requiring dc power must adjust the ac input to a usable level with transformers or switching transistors and use a rectifier stage to convert the ac to dc. Primary (nonrechargeable) or secondary (rechargeable) batteries generally power portable equipment and use a wide variety of chemistries.

Modern power supplies typically use solid-state rectifiers for converting ac line voltage to some desired value of dc. The dc output voltage may be unregulated or regulated to any degree of precision and stability. Power supplies have different filter arrangements for eliminating ac ripple components from the dc output. They also vary in size, weight, efficiency, voltage range, and many other important characteristics.

Four forms of power supplies are widely used today: unregulated, ferroresonant, series-regulated, and switching-regulator types. Differences include maintaining constant voltage output, cost, efficiency, size, weight, and amount of ripple.

Unregulated power supplies find very limited use today, since the output dc voltage is proportional to the input ac voltage. Output voltage fluctuates in step with input variations as well as changes in load impedance. Also, ripple voltage can run as high as 10% of the output dc voltage. Even with filter networks on the output, the remaining ripple and load regulation may be insufficient for most modern electronic circuits. But some components -- relays, solenoids, and indicating lamps, for example -- do not usually require a precision and filtered voltage. Thus, an unregulated power supply may be desirable for these circuits because unregulated supplies are simple, reliable, and inexpensive. Additionally, unregulated power supplies are typically more efficient and dissipate less heat than regulated power supplies with the same output voltage and current.

Regulated power supplies are the type most widely used. Two fundamental designs are the linear series-regulated and the switching power supply. Although switching power supplies are considered by many to be the new wave of technology, they have been around for many years. But, because of recent improvements in semiconductor components, capacitors, and transformers, they can be made more reliable, simpler, and less costly than just a few years ago. They are well suited for applications requiring small size, high efficiency, and moderately good regulation. On the down side, however, switching power supplies generate voltage spikes and noise in their outputs and can radiate EMI/RFI because of switching transients. These unwanted signals can seriously interfere with legitimate signals and completely mask others. Thus, carefully designed power supplies should include effective shielding, bypassing, and filtering to prevent propagating noise into sensitive circuits.

Linear-regulated power supplies are typically larger and less efficient than switching power supplies, but cost less and are more precisely regulated. The linear power supply is used by engineers needing high gain, wide dynamic range, low noise, and fast response for laboratory and systems power requirements.

High gain brings better regulation of load and source effects and results in higher precision and more accurate voltage settings. Wide dynamic range makes it possible to control or program the power supply over a wide voltage range, either by internal or external command signals. Low noise or ripple produces better resolution of control. Response speed indicates how rapidly the power supply can respond to a control input or to a load disturbance to take action.

There is a large and growing number of uses that demand "smart" linear power supplies. Such supplies usually connect to controllers (often via the IEEE-488 bus) to integrate, sum, step, range, and otherwise interact with the command loop of modern automatic test equipment (ATE) systems.

Ferroresonant power supplies: Another power supply that should be considered is known as a ferroresonant type. Regulation takes place within the step-up or step-down transformer by core saturation means. This type costs less than series-regulated and switching power supplies. Ferroresonant power supplies are not as precisely regulated as the other two, but produce sufficiently precise voltages for many applications.

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