The Power Diode
In the earlier topics we saw that a semiconductor signal diode can only conduct current in one direction from its anode to its cathode, but cannot in the reverse direction acting just like an electrical one way valve. A extensively used application of this feature is in conversion of the alternating voltage (AC) into the continuous voltage (DC). Or we can say that Rectification. Small signal diodes can be taken in use as rectifiers in low current (less than 1-amp) rectifiers, low-power, or applications, but were larger forward bias currents or higher reverse bias blocking voltages are involved the PN junction of a small signal diode would ultimately overheat and melt so instead larger more robust Power Diodes are used.
The power semiconductor diode, called as the Power Diode, has a larger PN Junction region compared to its smaller signal diode cousin, resulting in high forward current capability of nearly several hundred amps (KA) and a reverse blocking voltage of around thousand volts (KV). As the power diode has large PN junction, it is not suitable for the high frequency applications above 1MHz, but expensive and special high frequency, high current diodes are available. For the high frequency rectifier applications Schottky Diodes are usually used because of the short reverse recovery time and low voltage drop in the forward bias condition of them.
Power diodes offer uncontrolled rectification of power and are used in the applications like battery charging and DC power supplies also the AC rectifiers and inverters. Because of their high current and voltage characteristics they can be used as freewheeling diodes and snubber networks also. Power diodes are designed to have the forward "ON" resistance of fractions of an Ohm while their reverse blocking resistance is in mega-Ohms range. Some larger value power diodes are designed to be "stud mounted" onto heat sinks decreasing their thermal resistance to 0.1 to 1oC/Watt.
If an alternating voltage is applied across the power diode, during the positive half cycle the diode will conduct passing current and during the negative half cycle the diode will not conduct blocking the flow of current. Then conduction through the power diode only occurs during positive half cycle and is therefore unidirectional that is DC as shown.
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