A simple AC powered linear power supply uses a transformer to convert the alternating current from the wall to the desired voltage. An array of diodes then rectifies the alternating current to direct current. Finally, a low-pass filter smooths out the high-frequency ripple that is left after the rectification.
In a switching power supply the incoming power is passed through a transistor array that inverts (changes the positive voltage to negative and vice-versa) hundreds of thousands of times per second. This means that a smaller, less expensive, lighter transformer can be used, because the alternating current is being made to alternate faster, and thus there is less time for the magnetic core of the transformer to saturate.
Some power supplies are regulated. These power supplies incorporate circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is maintained despite any variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input. Both linear and switching power supplies can be regulated.
Switching power supplies can be used as DC to DC converters. In this application, the power supply is designed to accept a limted range DC input and 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.
The most exotic power supplies are used in aviation to enable reliable restarting of a stalled engine.
In jet transports, the engine is restarted from the power produced by the other engine(s), 400 Hz three-phase AC. Most of the starting torque generated by the engine's motor generator is provided by the current at the peak of the alternating current.
If the aircraft electronics used simple linear power supplies, they would steal current from this peak, because the diodes would conduct only during the peaks of the 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. They do this by using a switching power supply, but leaving the transistors on longer near the bases of the wave, and shorter on the peaks. This trick is called "power factor correction," and is the reason why some modern power supplies are computer-controlled.