Power Diode:
A power diode is more complicated in structure and operational characteristics than the small-signaldiode. It is a two-terminal semiconductor device with a relatively large single pn junction, which consists of a two-layer silicon wafer attached to a substantial copper base. The base acts as a heat sink, a support for the enclosure and one of electrical leads of the device. The extra complexity arises from the modifications made to the small-signal device to be adapted for power applications. These features are common for all types of power semiconductor devices.
Characteristics:
In a diode, large currents cause a significant voltage drop. Instead of the conventional exponential output relationship for small-signal diodes, the forward bias characteristic of the power diode is approximately linear. This means the voltage drop is proportional both to the current and to ohmic resistance. The maximum current in the forward bias is a function of the area of the pn junction.Today, the rated currents of power diodes are thousands of amperes and the area of the pn junction may be tens of square centimeters. The structure and the method of biasing of a power diode are displayed in Fig. 1.9. The anode is connected to the layer and the cathode to the p substrate layer n. In the case of power diode, an additional n layer exists between these two layers. This layer termed as a drift region can be quite wide for the diode. The wide lightly doped region adds significant ohmic resistance to the forward-biased diode and causes larger power dissipation in the diode when it is conducting current.
Forward biasing:
Most power is dissipated in a diode in the forward-biased on-state operation.For small-signal diodes, power dissipation is approximately proportional to the forward current of the diode. For power diodes, this formula is true only with small currents. For large currents, the effect of ohmic resistance must be added. In a high frequency switching operation,significant switching losses will appear when the diode goes from the off-state to the on-state, or vice versa. Real operation currents and voltages of power diodes are essentially restricted due to power losses and the thermal effect of power dissipation. Therefore, in power devices cooling is very important.Reverse biasing:
In the case of reverse-biased voltage, only the small leakage current flows through the diode. This current is independent of the reverse voltage until the breakdown voltage is reached. After that, the diode voltage remains essentially constant while the current increases dramatically. Only the resistance of the external circuit limits the maximum value of current. Large current at the breakdown voltage operation leads to excessive power dissipation that should quickly destroy the diode. Therefore, the breakdown operation of the diode must be avoided.To obtain a higher value of breakdown voltage, the three measures could be taken. First, to grow the breakdown voltage, lightly doped junctions are required because the breakdown voltage is inversely proportional to the doping density. Second, the drift layer of high voltage diodes must be sufficiently wide. It is possible to have a shorter drift region (at the same breakdown voltage) if the depletion layer is elongated. In this case, the diode is called a punch-through diode.The third way to obtain higher breakdown voltage is the boundary control of the depletion layer. All of these technological measures will result in the more complex design of power diodes.
Switching:
For power devices, switching process is the most common operation mode. A power diode requires a finite time interval to switch over from the off state to the on state and backwards. During there transitions, current and voltage in a circuit vary in a wide range. This process is accompanied with energy conversion in the circuit components. A power circuit contains many components that canstore energy (reactors, capacitors, electric motors, etc.). Their energy level cannot vary instantaneously because the power used is restricted. Therefore, switching properties of power devices are analyzed at a given rate of current change, as shown transients in Fig. 1.10.
The most essential data of power switching are the forward voltage overshoot U(F max) when a diode turns on and the reverse current peak value I(R max) when a diode turns off.
Summary:
Power diode is adapted for switching power applications. In addition to bulk resistance, it has high ohmic resistance. To withstand the essential losses that appear when the diode goes from the off state to the on state and backward, cooling is very important. To obtain a higher value of breakdown voltage, some measures are usually taken, such as lightly doped junctions, sufficiently wide drift layer, and the boundary control of the depletion layer. These measures result in a more complex design of power diodes but shorten the reverse recovery time and increase their lifetime.
Special-Purpose Diodes:
Rectifier diodes are used in the circuits of 50 Hz to 50 kHz frequencies. They are never intentionally
operated in the breakdown region because this may damage them. They cannot operate properly under abnormal conditions and high frequency. Devices of other types have been developed for such kind of operations.
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