Parallel operation of SCRs

When current required by the load is more than the rated current of a single thyristor, SCRs are connected in parallel in a string.
For equal sharing of currents, V — I characteristics of SCRs during forward conduction must be identical as far as possible.
In Fig. (a) are shown two SCRs in parallel and their characteristics during forward conduction are shown in Fig. (b).
For parallel-connected SCRs, voltage drop VT across them must be equal. Fig. (b) shows that for the same voltage drop VT SCR₁ shares a rated current I₁ whereas SCR2 carries current I₂ much less than the rated current I₁ .
The total current carried by the unit is I₁ + I₂ and not the rated current 2I₁ as required.

Now consider n parallel connected SCRs. For satisfactory operation of these SCRs, they should get turned on at the same moment.
Consider that SCRI has large turn-on time whereas the remaining (n -1) SCRs have low turn-on time.
Under this assumption, (n -1) SCRs will turn on first but one SCRI with longer turn on time is likely to remain off.
The voltage drops across (n-1) SCRs falls to a low value and SCRI is therefore subjected to this low voltage.
For a given gate drive power, anode to cathode must have some minimum forward voltage, called finger voltage, for a thyristor to turn-on.

If voltage across SCRI drops to a value less than its finger voltage, then this thyristor will not turn on.
Therefore, the remaining (n-1) SCRs, which are already on, will have to share the entire load current.
As such, these SCRs may be overloaded and damaged because of heating caused by overcurrent.
If one SCRI in a parallel unit carries more current than other SCRs, then this SCRI 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.

Because of junction temperature rise, its dynamic resistance decreases and current shared by SCRI increases.
This process of anode current rise becomes cumulative and subsequently the junction temperature of SCRI exceeds its rated value, as a result SCRI is damaged.
This sequence of events may engulf another SCR and in this manner all SCRs in the string may be destroyed permanently.
Therefore, when SCRs are to be operated in parallel, it should be ensured that they operate at the same temperature.
This can be achieved by mounting the parallel unit on one common heat sink.

Unequal current distribution in a parallel unit is also caused by the inductive effect of current carrying conductors.
When SCRs are arranged unsymmetrically as (a), the middle conductor will have more inductance because ofmore flux linkages from two nearby conductors.
Consequently, less current flows through the middle SCR as compared to outer two SCRs.
This unequal current distribution can be avoided by mounting the SCRs symmetrically on the heat sink as shown in Fig.(b).

Since external resistance R relates to each SCR, the forward voltage drops across each arm should be equal.
Therefore, VT₁+I₁(R₁+RT₁) = VT₂+I₂(R₂+RT₂) Where R₁=R₂=R, RT₁, RT₂ are the junction resistance of SCR₁ and 2 respectively.

In case of ac circuits, the current equalization can be possible with the help of inductors. Fig. shows the parallel connection of SCRs with inductive coupling.
When the current flows through T1 and T2 are same (IT1=IT2), the voltage drop across inductance is zero due to mutual cancellation of inductance emfs in inductor L1 and L2.

If the current flows through T1 is greater than T2 (IT1>IT2), the induced emfs in the inductive coil is directly proportional to the unbalance currents.
Then current flow through the inductance L1 decreases but the current through the coil L2 increases. Hence, there is a tendency to share the current equally.