SCR from complementary BJTs; turning off SCR

Can two complementary BJTs make an SCR?

Yes, in principle. An SCR behaves like a PNP and an NPN transistor connected in a regenerative loop (two-transistor analogy): the collector of the PNP drives the base of the NPN and the collector of the NPN drives the base of the PNP.

   A
   |
  PNP (Q1) emitter
   |  \
   |   +--- base Q1 = collector Q2
  C1   |
   +---+--- G (base Q2)
       |
      NPN (Q2)
       |
       K

A small gate current turns Q2 ON, Q2 turns Q1 ON, and Q1 keeps Q2 ON, so the pair latches like an SCR (IA = (α2·IG + ICBO1 + ICBO2)/(1 − α1 − α2)). Such a discrete latch is used in low-power circuits, but it cannot replace a real SCR in power work because discrete BJTs have lower voltage blocking, poorer surge and dv/dt capability, and need careful biasing.

Turning OFF of SCR (commutation)

Once ON, an SCR cannot be turned off by the gate. To turn it OFF, the anode current must be reduced below the holding current IH, and then a reverse voltage must be maintained for at least the turn-off time tq (about 10–100 μs) so that the junctions recover their blocking ability.

Methods:

1. Natural (line) commutation: In AC circuits the current falls to zero at the end of each half cycle and the supply reverses, so the SCR turns off automatically. Used in AC controllers and controlled rectifiers.
2. Forced commutation: In DC circuits current never becomes zero naturally, so an extra circuit (L, C and an auxiliary SCR) forces it to zero:
   • Class A: self commutation by resonating load
   • Class B: self commutation by LC circuit
   • Class C: complementary commutation (C or L-C switched by another load-carrying SCR)
   • Class D: impulse commutation by an auxiliary SCR
   • Class E: external pulse source
3. Reducing anode current / opening the circuit: increasing load resistance or interrupting the anode circuit so that IA < IH.
4. Reverse biasing: applying a reverse voltage across anode and cathode.

 iA  |‾‾‾‾‾‾\
     |       \___  IH  ___
     |           \_____/      reverse recovery
 vAK |____________
     |            |___ reverse bias ≥ tq ___|‾‾ forward
                   <------- tq ------->

The circuit turn-off time must be greater than the device turn-off time tq; otherwise the SCR turns ON again when forward voltage reappears.