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currents of the devices should not be so much different that at reduced load current one of the device
gets turned-off because of fall of current through it blow its holding current value. This is particularly
important because on increase in load current, the device which has stopped conducting cannot start
in the absence of gate pulse.
Another point to be considered is the on-state voltage across the device. For equal sharing of currents
by the devices voltage drop across the parallel paths must be equal. For operation of all the SCRs
connected in parallel
at the same temperature, it becomes necessary to use a common heat sink for
their mounting, as illustrated in figure. Resistance compensation used
for dc circuits is shown in
figure. In this circuit the resistors Rx and R2 are chosen so as to cause equal voltage drop in both
arms. Inductive compensation used for ac circuits is shown in figure The difference in characteristics
due to different turn-on time,
delay time, finger voltage, latching current,
holding current can be
minimized by using inductive compensation. Firing circuits giving high rate of rise can be used to
reduce mismatch of gate characteristics and delay time. Current sharing circuits must be designed so
as to distribute current equally at maximum temperature and maximum anode current. This is done to
ensure that the devices share current equally under worst operating conditions. Mechanical
arrangement of SCRs also plays an important role in reducing mismatching. Cylindrical construction
is perhaps the best from this point of view.
Derating:
Even
with all the measures taken, it is preferable to derate the device for series/parallel operation.
Another reason for derating is poor cooling and heat dissipation as number of devices operates in the
same branch of the circuit. Normal derating factors are 10 to 15% for parallel connection of SCRs
depending upon the number of devices connected in parallel. Higher voltage safety factor is taken
when SCRs are connected in series.
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