October is power supply month! For the rest of the month, we’ll be posting about ways to utilize or create power supplies for your projects, with the most obvious way being an AC-DC power adapter. The following is an excerpt from Charles Platt’s definitive Encyclopedia of Electronic Components, Vol. 1:
What It Does
An AC-DC power supply converts alternating current (AC) into the direct current (DC) that most electronic devices require, usually at a lower voltage. Thus, despite its name, a power supply actually requires an external supply of power to operate.
Larger products, such as computers or stereo equipment, generally have a power supply contained within the device, enabling it to plug directly into a wall outlet. Smaller battery-powered devices, such as cellular phones or media players, generally use an external power supply in the form of a small plastic pod or box that plugs into a wall outlet and delivers DC via a wire terminating in a miniature connector. The external type of power supply is often, but not always, referred to as an AC adapter.
Although an AC-DC power supply is not a single component, it is often sold as a pre-assembled modular unit from component suppliers.
The two primary variants are a linear regulated power supply and switching power supply.
Linear Regulated Power Supply
A linear regulated power supply converts AC to DC in three stages:
* A power transformer reduces the AC input to lower-voltage AC.
* A rectifier converts the AC to unsmoothed DC. Rectifiers are discussed in the entry on diodes in this encyclopedia.
* A voltage regulator, in conjunction with one or more capacitors, controls the DC voltage, smooths it, and removes transients. The regulator is properly known as a linear voltage regulator because it contains one or more transistors, which are functioning in linear mode—that is, responding linearly to fluctuations in base current, at less than their saturation level. The linear voltage regulator gives the linear regulated power supply its name.
This type of power supply may be described as transformer-based, since its first stage consists of a transformer to drop the AC input voltage before it is rectified.
Because the rectifier in a power supply generally passes each pulse of AC through a pair of silicon diodes, it will impose a voltage drop of about 1.2V at peak current. A smoothing capacitor will drop the voltage by about 3V as it removes ripple from the current, whereas a voltage regulator typically requires a difference of at least 2V between its input and its output. Bearing in mind also that the AC input voltage may fluctuate below its rated level, the output from the power transformer should be at least 8VAC higher than the ultimate desired DC output. This excess power will be dissipated as heat.
The basic principle of the linear regulated power supply originated in the early days of electronic devices such as radio receivers. A transistorized version of this type of power supply remained in widespread use through the 1990s. Switching power supplies then became an increasingly attractive option as the cost of semiconductors and their assembly decreased, and high-voltage transistors became available, allowing the circuit to run directly from rectified line voltage with no step-down power transformer required.
Some external AC adapters are still transformer-based, but are becoming a minority, easily identified by their relatively greater bulk and weight. An example is shown to the right: A relatively old, cheap AC adapter contains only the most rudimentary set of components, and does not supply the kind of properly regulated DC power required by electronic equipment.
Switching Power Supply
Also known as a switched-mode power supply, an SMPS, or switcher, it converts AC to DC in two stages.
* A rectifier changes the AC input to unsmoothed DC, without a power transformer.
* A DC-DC converter switches the DC on and off at a very high frequency using pulse-width modulation to reduce its average effective voltage. Often the converter will be the flyback type, containing a transformer, but the high-frequency switching allows the transformer to be much smaller than the power transformer required in a linear regulated power supply. See the DC-DC converter entry in this encyclopedia for an explanation of the working principles.
A simplified schematic of a switching power supply is shown in picture to the right. It’s a greatly simplified schematic showing the principal components of a switching power supply. Note the absence of a 115VAC power transformer. The transformer that is inserted subsequently in the circuit functions in conjunction with the high switching frequency, which allows it to be very much smaller, cheaper, and lighter.