Power electronics digital notes b. Tech III year

Series Connection of an SCR 
Figure: 1. 27. Series connection of SCRs 
(i)
Unequal distribution of voltage across SCRs 
(ii)
Difference in recovery characteristics.
Care must be taken to share the voltage equally. For steady-state conditions, voltage sharing is 
achieved by using a resistance or a Zener diode in parallel with each SCR. For transient voltage 
sharing a low non-inductive resistor and capacitor in series are placed across each SCR, as shown in 
figure. Diodes D1 connected in parallel with resistor Rl, helps in dynamic stabilization. This circuit 
reduces differences between blocking voltages of the two devices within permissible limits. 
Additionally the R-C c
ircuit can also serve the function of ‘snubber circuit‘. Values of R1 and C1 can 
primarily be calculated for snubber circuit and a check can be made for equalization. If ΔQ is the 
difference in recovery charge of two devices arising out of different recovery current for different 
time and ΔV is the permissible difference in blocking voltage then 
C1 = ΔQ/ ΔV
The value of resistance Rx should be sufficient to over damp the circuit. Since the capacitor C1 can 
discharge through the SCR during turn-on, there can be excessive power dissipation, but the 
switching current from C1 is limited by the resistor R1 This resistance also serves the purpose of 

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damping out ‘ringing’ which is oscillation of C1 with the circuit inductance during commutation. All 
the SCRs connected in series should be turned-on at the same time when signals are applied to their 
gates simultaneously. 
String efficiency = 
This phenomenon increases the reliability of the string, but reduces the utilization of each SCR. Thus 
string efficiency decreases. Reliability of string is measured by derating factor (DRF) which is given 
by the expression 
DRF = 1- string efficiency