electronic power supply
electronic power supply[i‚lek′trän·ik ′pau̇·ər sə‚plī]
Electronic power supply
A source of electric power (voltage and current) to operate electronic circuits. Active electronic circuits contain such devices as transistors or vacuum tubes and require external power to amplify, filter, modify, or create electrical signals. The most common source of energy for electronic circuits is obtained by converting the electrical energy available in the conventional alternating-current (ac) electric power mains to an appropriate voltage or current. These converters, or electronic power supplies, can be implemented with a wide variety of circuits. Other power sources include batteries, mechanically driven generators, photovoltaic (solar) cells, and fuel cells. See Alternating current, Converter
Most electronic circuits require a direct-current (dc) or constant voltage. If ac power is required, an oscillator or a simple transformer is used. Although some dc-to-dc converters are used, most dc power supplies convert the alternating power from the ac main to dc power. These ac-to-dc power supplies are classified according to the type of circuits used to realize the conversion: rectification, filtering, and regulation. Simple battery chargers are examples of power supplies that do not require filtering or regulation.
An essential step in the conversion of ac to dc is a process called rectification. Rectification converts ac voltage to a waveform with average or dc value by passing only one polarity (half-wave) or by generating the magnitude or absolute value (full-wave). Three types of rectifier (diode) circuits are commonly used. Only one diode is required to obtain half-wave rectification. Full-wave rectification can be obtained with four diodes connected in a bridge configuration or with two diodes and a center-tapped transformer. Transformers are normally used at the input of the rectifiers to increase or decrease the voltage and isolate the dc output from the ac input for safety purposes. See Diode, Rectifier, Transformer
For most applications the ac or alternating portion of the rectified output is unwanted and may cause undesirable results, such as an annoying hum in audio systems. A capacitor can be used to reduce or filter the ac portion of the rectified waveform. The capacitor is charged through the diodes to the peak ac voltage minus the diode forward voltage. Some of the charge stored on the capacitor is delivered to the load each cycle, but the next voltage peak recharges the capacitor. See Capacitor
Regulators are often used to make the power supply output insensitive to input voltage amplitude variations and further reduce the ripple voltage. The regulator may also be used to adjust or change the dc output voltage and limit the amount of current delivered by the power supply. Regulators are a form of dc-to-dc converter.
The oldest and simplest type of regulator is the linear regulator. A simple linear regulator is the shunt type. It consists of a Zener diode and a current-limiting resistor. The Zener diode establishes a fixed, or reference, voltage if it is properly reverse biased. Another common type of linear regulator is the series-pass regulator. See Transistor, Zener diode
More modern power supplies have switching regulators, and are informally called switchers. There are more than a dozen different topologies (basic block diagrams) for switching regulators, but they all use one or more transistors acting as switches; either ON or OFF. In addition to the solid-state (transistor) switches, a typical switching regulator uses capacitors and inductors to store energy and diodes to direct the current.
Other common types of switching regulators are called buck-boost (or flyback) and push-pull or (buck-derived). Switching regulators can be used to generate multiple outputs at different voltage levels. The improved efficiency of switching regulators is due to the fact that energy is stored very efficiently in inductors and capacitors. The remaining losses in the control circuitry, switches, and the diodes are small compared to linear regulators. See Inductor
Ferroresonant transformer-based power supplies
These have some advantages for high-current applications such as battery chargers. Ferroresonant power supplies use nonlinear magnetic properties and a resonant circuit to regulate the output voltage, and they have efficiencies similar to switching power supplies. Power supplies driven by three-, six-, or twelve-phase ac are easier to filter, and they generate much lower harmonic distortion of the current in the ac power system.