DIODES
Power diodes provide uncontrolled rectification of power and are used in applications such
as electroplating, anodizing, battery charging, welding, power supplies (de and ac), and variablefrequency
drives. They are also used in feedback and the freewheeling functions of converters
and snubbers. A typical power diode has P-1-N structure, that is, it is a P-N junction with a nearintrinsic
semiconductor layer (!-layer) in the middle to sustain reverse voltage.
Figure 1.1 shows the diode symbol and its volt-ampere characteristics. In the forwardbiased
condition, the diode can be represented by a junction offset drop and a series-equivalent
resistance that gives a positive slope in the V-1 characteristics. The typical forward conduction
drop is 1.0 V. This drop will cause conduction loss, and the device must be cooled by the
appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small
Figure 1.2 Turn-off switching characteristics of a diode
leakage current tlows due to minority carriers, which gradually increase with voltage. If the
reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through
avalanche breakdown, which is when reverse current becomes large and the diode is destroyed
by heating due to large power dissipation in the junction.
The turn-otT voltage and current characteristics as functions of time, which arc indicated in
Figure 1.2, are particularly important for a diode. In the forward high-conduction region. the
conduction drop (V F) is small, as mentioned before. At this condition. the P and N regions near
the junction and the !-layer remain saturated with minority carriers.
If the device is open-circuited, the carriers die by a recombination process. which takes a
reasonably long time. Normally, a reverse de voltage (V Rl is applied to turn off the device. as
indicated in Figure 1.2. At timet= 0, the reverse voltage is applied when the current goes down
linearly because of series-circuit inductance. During time t2, the current is negative and the
minority carriers sweep out across the junction, but the excess carrier concentration keeps the
junction saturated, and therefore, the same negative current slope is maintained.
Power diodes provide uncontrolled rectification of power and are used in applications such
as electroplating, anodizing, battery charging, welding, power supplies (de and ac), and variablefrequency
drives. They are also used in feedback and the freewheeling functions of converters
and snubbers. A typical power diode has P-1-N structure, that is, it is a P-N junction with a nearintrinsic
semiconductor layer (!-layer) in the middle to sustain reverse voltage.
Figure 1.1 shows the diode symbol and its volt-ampere characteristics. In the forwardbiased
condition, the diode can be represented by a junction offset drop and a series-equivalent
resistance that gives a positive slope in the V-1 characteristics. The typical forward conduction
drop is 1.0 V. This drop will cause conduction loss, and the device must be cooled by the
appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small
leakage current tlows due to minority carriers, which gradually increase with voltage. If the
reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through
avalanche breakdown, which is when reverse current becomes large and the diode is destroyed
by heating due to large power dissipation in the junction.
The turn-otT voltage and current characteristics as functions of time, which arc indicated in
Figure 1.2, are particularly important for a diode. In the forward high-conduction region. the
conduction drop (V F) is small, as mentioned before. At this condition. the P and N regions near
the junction and the !-layer remain saturated with minority carriers.
If the device is open-circuited, the carriers die by a recombination process. which takes a
reasonably long time. Normally, a reverse de voltage (V Rl is applied to turn off the device. as
indicated in Figure 1.2. At timet= 0, the reverse voltage is applied when the current goes down
linearly because of series-circuit inductance. During time t2, the current is negative and the
minority carriers sweep out across the junction, but the excess carrier concentration keeps the
junction saturated, and therefore, the same negative current slope is maintained.
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