Power electronics digital notes b. Tech III year

Parallel Connection of an SCR 
Figure: 1. 28. Parallel connection of SCRs 
When the load current exceeds the SCR current rating, SCRs are connected in parallel to share the 
load current. But when SCRs are operated in parallel, the current sharing between them may not be 
proper. The device having lower dynamic resistance will tend to share more current. This will raise 
the temperature of that particular device in comparison to other, thereby reducing further its dynamic 
resistance and increasing current through it. This process is cumulative and continues till the device 
gets punctured. Some other factors which directly or indirectly add to this problem are difference in 
turn-on time, delay time, finger voltage and loop inductance.
Arrangement of SCRs in the cubicle also plays vital role. When the SCRs are connected in parallel, it 
must be ensured that the latching current level of the all the SCRs is such that when gate pulse is 
applied, all of them turn-on and remain on when the gate pulse is removed. Further the holding 

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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.