Working of SCR

SCR working is explained in following modes.
   1) Forward Blocking mode:-

In this mode junction (J1 & J3) is in forward bias condition, junction (J2) is in reverse bias & Gate to Cathode voltage is open. In this condition junction (J2) will have avalanche breakdown at a voltage called Forward Breakdown Voltage, after this voltage conduction takes place & SCR get turn on. In this condition a small current flows called Forward Leakage Current.

   2) Reverse Blocking mode:-

In this mode junction (J2) is in forward bias condition, junction (J1 & J3) is in reverse bias & Gate to Cathode voltage is open. If reverse voltage is increase then at a critical breakdown level voltage called Reverse Breakdown Voltage (VBR). A large current flow gives rise to more losses in SCR. This may lead to damage the SCR.

   3) Forward Conduction mode:-

When anode to cathode forward voltage is increased with gate circuit open, reverse bias junction (J2) will have an avalanche breakdown at a voltage called Forward Breakover Voltage (VBO). After this breakdown, thyristor gets turned on.

V-I Characteristics of SCR:-

Fig.-1 V-I Characteristics of SCR

Latching Current (IL):-

The minimum value of anode current by which SCR turns on & maintain conduction when gate signal is removed called Latching Current (IL).

Holding Current (IH):-

The minimum value of anode current below which it must fall for turning off the SCR called Holding Current (IH).

Note:-

   1) Latching Current is two or three times higher than the Holding Current.

   2) Latching Current is associated with Turn ON process.

   3) Holding Current is associated with Turn OFF process.