RCA P95400EB SCR (transcalent); and Westinghouse T7S7-55, T7S7-50, T7SH-40, T7SH-50, T7SH-45, and T7 2H-35 SCR's. Due to the high frequency and high currents required by mass drivers, the di/dt will be a limiting factor. If exceeded, high current densities occur within the SCR wafer causing hot spots and destruction of the SCR. This can be overcome by paralleling SCR's and reducing operating frequency. If dV/dt should be exceeded, then the SCR will turn on without being triggered on by the gate. This is not acceptable in the mass driver and can be reduced by the addition of snubber cir- cults. The it rating is usually not a problem in mass driver designs since the on time for the SCR is relatively short (< lOOps). Peak surge forward current is the real limiting factor facing mass drivers. Most designs would like 3-10 times what is available per SCR. This can be overcome by paralleling SCR's but results in a much more complicated drive and trigger circuit. If this spec is exceeded, thermal shock to the SCR will result in destruction of the SCR. Turn off time is the period in which the SCR recovers after commutating off and can withstand forward voltage without turning on a- gain. This results in dead spots in the drive current and reduces the duty cycle of that phase. Finally, the voltage rating of the SCR must never be exceeded. If an overvoltage transient should occur (depending on the severity), immediate destruction of the SCR will result. The specific mass (Kg/watt) for disk pack phase control SCR's made by Westinghouse (nos. T520, T620, T625, T720, T920, T9G0, and TA20) are shown in Fig. 7.3. The mass used was for the total unit (silicon wafer and case). It does not include any mounting hardware (bolts, etc.) to assemble it to the feeder strips. Earlier MDRE designs have used compared obtained from Fig. 7.3. One should note the T920, T9G0, and TA20 SCR's have voltage ratings of 2200
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