For some industrial applications, the demand for current ratings is so high that a single SCR cannot fulfill such requirements. Hence SCRs are connected in parallel as given in Fig. 1a, in order to meet the  high current (h.c.) demand.
String efficiency = Actual current rating of the whole string/(Individual current rating of one SCR x Number of SCR in the string)
Derating factor (DRF) = 1-String efficiency
String efficiency is the degree of utilization of SCR and Derating factor is the measure of the reliability of string.

#### I-V Characteristics

For equal sharing of currents, I-V characteristics of SCR during forward conduction must be identical as far as possible.
• For parallel connected SCRs, voltage drop VT across them must be equal.
• I-V characteristics shows that for the same voltage drop VT, SCR1 shares a rated current I1, whereas SCR2 carries current I2.
• String efficiency = (I1+I2)/(I1*2) = (1/2)*(1+I2/I1)
The unequal current distribution and other operating problems concerning the parallel operation of SCR are discussed below:

#### Simultaneous Turn-ON

Consider n parallel connected SCRs. For satisfactory operation of these SCRs, they should get turned on at same moment.
• Example: Consider that SCR1 has large turn-ON time whereas the remaining (n-1) SCRs have low turn-ON time.
• Under this assumption, (n-1) SCR will turn on first but one SCR1 with longer turn ON time is likely to remain OFF.
• The voltage drop across (n-1) SCRs falls to a low value and SCR1 is therefore subjected to this low voltage.
• As a consequence, the remaining (n-1) SCR will have to share the entire load current.

#### Same Temperature Rise

• If one SCR1 in a parallel unit carries more current than other SCRs then this SCR1 will have greater junction temperature rise.
• As a result, its dynamic resistance (dVT/dIa) during forward conduction decreases and this further increases the current shared by this SCR.
• RD at I'=oa/ab and RD at I"=oa/ac
• Current changes from I' to I" because of junction temperature rise.
• Therefore, when SCRs are to be operated in parallel they should operate at the same temperature.
• This  can be achieved by mounting the parallel unit on one common heat sink.

#### Uniform Current Sharing

Current sharing in parallel connected SCRs can be made uniform by connecting a suitable external resistance in series with each SCR.
RT1+R1=RT2+R2=RTn+Rn  (Where: RTn is external resitance and Rn is dynamic resistance)

#### Symmetrical Arrangement

Unequal current distribution in a parallel unit is also caused by the inductive effect of current carrying conductors.
• When SCRs are arranged asymmetrically, the middle conductor will have more inductance because of more flux linkages from two nearby conductors.
• As a consequence, less current flows through the middle SCR as compared to outer two SCRs.
• To avoid this unequal current distribution, symmetrical arrangement is used.

#### Mid Point Reactor

In ac circuits, current distribution can be made more uniform by the magnetic coupling of the parallel paths.
• The tapped point A is the mid point of the reactor.
• When I1>I2, flux linkage induce emfs in L1 and L2.
• emf across reactor L1 opposes the flow of I1, whereas that across L2 aids the flow of I2.
• Thus there is a tendency to buck I1 and boost I2 so as to minimize the unbalance of currents in the parallel unit.

#### Reference

Dr. P. S. Bimbhra, "Power Electronics", Khanna Publishers, Fifth Edition, 2013

#### Video Tutorial

Parallel connection of thyristor using MATLAB Simulink.

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