Electronic power supply
From Free net encyclopedia
An electronic power supply, often referred to somewhat incorrectly as an AC adaptor, is an electronic device that produces a particular DC voltage or current from a source of electricity such as a battery or wall outlet. The direct current (DC) is usually used to power the internal circuits of an electronic device, such as a computer. An external AC adaptor (often rectangular in shape, black in color) is also commonly referred to as power brick.
Types
A simple AC powered linear power supply uses a transformer to convert the voltage from the wall outlet to a lower voltage. An array of diodes called a diode bridge then rectifies the AC voltage to DC voltage. A low-pass filter smooths out the voltage ripple that is left after the rectification. Finally a linear regulator converts the voltage to the desired output voltage, along with other possible features such as current limiting.
In a switched-mode power supply the incoming power is passed through a transistor and transformer network that switches on and off thousands to millions of times per second. This means that a smaller, less expensive, lighter transformer can be used, because the voltage is being made to alternate faster, and thus a smaller magnetic core can be used.
The voltage regulation of power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input.
Switching power supplies can be used as DC to DC converters. In this application, the power supply is designed to accept a limited range DC input and then output a different DC voltage. This is particularly useful in portable devices, as well as power distribution in large electronic equipment. A transformerless switching power supply that outputs a voltage higher than its input voltage is typically called a boost converter. A transformerless switching power supply that outputs a voltage lower than its input voltage is typically called a buck converter. These transformerless switching power supplies use an inductor as the primary circuit element in converting the voltage. Circuitry is used to pass current through the inductor to store a certain amount of electrical energy as a magnetic field. The current flow is then stopped, and the magnetic field collapses causing the stored energy to be released as current again. This is done rapidly (up to millions of times per second). By carefully metering the amount of energy stored in the inductor, the current released by the inductor can be regulated thus allowing the output voltage to be tightly regulated. A switching power supply incorporating a transformer can provide many output voltages simultaneously, and is typically called a flyback converter. Switching power supplies are typically very efficient if well designed, and therefore waste very little power as heat. Because of these efficiencies, they are typically much smaller and lighter than an equivalently rated linear supply.
Uses in aviation
The most exotic power supplies are used in aviation to enable reliable restarting of stalled engines.
In jet transports, an engine is restarted from the power produced by the 400 Hz, three-phase AC generators attached to the shafts of the other engine(s). Most of the starting torque generated by the engine's motor/generator is provided by the current at the peaks of the AC waveform.
If the aircraft electronics used simple rectifying power supplies, they would use current only from these peaks, since the diodes conduct only during the voltage peaks where the input voltage is higher than the output voltage. This could prevent the pilot from restarting an engine in an emergency.
Therefore, aircraft power supplies take energy evenly from all parts of the AC waveform. this is done by using a switching power supply technique called "power factor correction" which creates a balanced current draw over the entire AC waveform.