Page 177 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

POWER ELECTRONICS, RECTIFIERS, AND PULSE-WIDTH MODULATION INVERTERS

9.4 CHAPTER NINE

FIGURE 9.6 Voltage-current characteristics of an SCR.

below I . Therefore, once an SCR is triggered, its gate current can be removed without
H
changing the state of the device.
The SCR is commonly used for switching or rectification applications. It is available in
ratings ranging from a few amperes to a minimum of 3000 A. In summary, an SCR
1. Turns on when the voltage υ applied to it exceeds V
D BO
2. Has a breakover voltage V whose level is controlled by the amount of gate current i
BO G
present in the SCR
3. Turns off when the current i flowing through it drops below I
D H
4. Blocks all current flow in the reverse direction until the maximum reverse voltage is
exceeded

The Gate Turnoff Thyristor

A gate turnoff (GTO) thyristor is an SCR that can be turned off by a large enough nega-
tive pulse at its gate lead even if i exceeds i . These devices are becoming more popu-
H
D
lar because they eliminate the need for external components to turn off SCRs in dc
circuits (Fig. 9.7a).
Figure 9.7b illustrates a typical gate current waveform for a high-power GTO thyris-
tor. The gate current required to turn on a GTO thyristor is typically larger than that of an
ordinary SCR. The gate current for a large high-power device is around 10 A. A large neg-
ative current pulse of 20- to 30- s duration is required to turn off the device. The magni-
tude of this negative pulse must be one-fourth to one-sixth that of the current flowing
through the device.

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