Page 175 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

POWER ELECTRONICS, RECTIFIERS, AND PULSE-WIDTH MODULATION INVERTERS

9.2 CHAPTER NINE

Figure 9.2 illustrates the voltage-current characteristic
of a diode. A large current flows when a voltage is
applied to the diode in the forward direction.
The current is limited to a very small value
(microamperes or less) when a voltage is applied to
the diode in the reverse direction. The diode will
break and allow current in the reverse direction if the
reverse voltage is large enough.
FIGURE 9.1 The symbol of a diode. Diodes are rated by the maximum reverse voltage
they can withstand before breaking down and by the
amount of power they can safely dissipate. The power dissipated by a diode during for-
ward direction is equal to υ i . This power is limited to prevent overheating the diode. The
D D
peak inverse voltage (PIV) is the maximum reverse voltage of a diode. This value must be
high enough to ensure that the diode will not conduct in the reverse direction.
Diodes are also rated by their switching time. It is the time required to go from the off
state to the on state, and vice versa. Power diodes are very large, high-power devices that
store large charge in their junctions. They switch states much more slowly than diodes
found in electronic circuits. Essentially, all power diodes can be used in 50- to 60-Hz
rectifiers because they can switch states fast enough for this application. However, pulse-
width modulators (PWMs) require power diodes to switch states at rates higher than 10,000
Hz. Special diodes called fast-recovery, high speed diodes are used for these very fast
switching applications.

The Two-Wire Thyristor or PNPN Diode
Thyristor is a name of a family of semiconductor devices which have up to four semi-
conductor layers. The two-wire thyristor is a member of this family. It is also known as
the PNPN diode or reverse-blocking diode-type thyristor in the IEEE standard for
graphic symbols (Fig. 9.3).

FIGURE 9.2 Voltage-current characteristic of a diode.

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