The turn-off circuit consists npn transistor

At t � t1, a pulse of VG volts will appear at the SCR gate. The circuit conditions established with this input are shown in Fig. 21.4a. The potential VG was chosen suf-ficiently large to turn Q2 on (VBE2 � VG). The collector current of Q2 will then rise to a value sufficiently large to turn Q1 on (IB1 � IC2). As Q1 turns on, IC1 will increase, resulting in a corresponding increase in IB2. The increase in base current for Q2 will result in a further increase in IC2. The net result is a regenerative increase in the col-lector current of each transistor. The resulting anode-to-cathode resistance (RSCR �V/IA) is then small because IA is large, resulting in the short-circuit representation for the SCR as indicated in Fig. 21.4b. The regenerative action described above results in SCRs having typical turn-on times of 0.1 to 1 �s. However, high-power devices in the range 100 to 400 A may have 10- to 25-�s turn-on times.

The next question of concern is: How long is the turn-off time and how is turn-off accomplished? An SCR cannot be turned off by simply removing the gate signal, and only a special few can be turned off by applying a negative pulse to the gate ter-minal as shown in Fig. 21.3a at t � t3.

The two possibilities for current interruption are shown in Fig. 21.5. In Fig. 21.5a, IA is zero when the switch is opened (series interruption), while in Fig. 21.5b, the same condition is established when the switch is closed (shunt interruption).

21.4 SCR CHARACTERISTICS

AND RATINGS