A power diode is a crystalline semiconductor device used mainly to convert alternating current (AC) to direct current (DC), a process known as rectification. Found in the power supply circuits of virtually all modern-day electrical and electronic equipment, a power diode's function is akin to a mechanical one-way valve. It conducts electric current with minimal resistance in one direction, known as its forward direction, while preventing current from flowing in the opposite direction. Typically capable of passing as much as several hundred amps forward, power diodes have much larger P-N junctions and hence higher forward current carrying capacity than their smaller signal diode relatives used in consumer electronics to regulate and reduce current. This makes power diodes better suited for applications where larger currents and higher voltages are involved.
Manufacturers generally produce a range of power diodes suitable for particular uses. They are rated according to the maximum current they can carry in the forward direction and the maximum reverse voltage they can withstand. Due to resistance, a small drop in voltage occurs when passing an electric current through a power diode in the forward direction. Conversely, a power diode can only withstand a certain amount of voltage flowing in the reverse direction before it breaks down and ceases functioning.
Power diodes are made primarily of silicon, though small quantities of other materials, such as boron, gallium arsenide, germanium or phosphorous are also used. A single power diode can be used to convert AC to DC, but this produces what is known as half-wave varying DC. More commonly, two or three or more diodes are connected in circuit to produce full-wave varying DC. The most important of these is the bridge rectifier, in which four connected diodes convert both positive and negative sections of an AC wave into DC, thus producing full-wave rectification.
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Electric power utilities the world over typically use three-phase AC to distribute electricity. Though it delivers varying DC from incoming AC, a full-wave or bridge rectifier does not deliver DC at the constant voltage necessary to power most modern-day electrical and electronic equipment. Therefore, a reservoir capacitor is usually connected to the output end of the rectifier in order to smooth out the rippled voltage. For example, in a typical U.S. household, three-phase AC from the electrical main circuits passes through three pairs of power diodes. The resulting DC is then smoothed out and delivered at a voltage sufficiently constant for use by passing it through a smoothing capacitor.