Electrical Power Systems Technology

Control Devices 457

be exceeded for this to take place. Note that the IH level is just above the

knee of the IAK curve after it returns to the center.

Quadrant III of the I-V characteristic curve shows the reverse break-

down condition of operation. This characteristic of an SCR is similar to that

of a silicon diode. Conduction occurs when the peak reverse voltage (PRV)

value is reached. Normally, an SCR would be permanently damaged if the

PRV is exceeded. Today, SCRs have PRV ratings of 25 to 2000 V.

For an SCR to be used as a power control device, the forward VBO

must be altered. Changes in gate current will cause a decrease in the VBO.

This occurs when the gate is for-ward biased. An increase in IG will cause

a large reduction in the forward VBO. An enlargement of quadrant I of the

I-V characteristics is shown in Figure 8-9, which also shows how different

values of IG change the VBO. With 0 IG it takes a VBO o of 400 V to produce

conduction. An increase in IG reduces this quite significantly. With 7 mA

of IG the SCR conducts as a forward-biased silicon diode. Lesser values of

Figure 17-4. I-V characteristics of an SCR