Thyristor V-I and firing circuit

A thyristor (SCR) is a four-layer P-N-P-N, three-junction ($J_1, J_2, J_3$) semiconductor switch with three terminals: anode (A), cathode (K) and gate (G). It can be turned ON by a small gate pulse when forward biased, but once ON the gate loses control; it turns OFF only when its anode current falls below the holding current.

V-I characteristic

              IA
               ^
               |  |<- ON state (VT = 1-2 V)
               |  |
         IL ---|--|
         IH ---|--+.
               |    `.   Ig2 > Ig1 > Ig=0
               |      `-.___      ___
     -VBR      |            `----'   |
  -----+-------+---------------------+--> VAK
       |       |  forward blocking  VBO
       |       |  (leakage current)
       | reverse blocking
       | (leakage current)
       v reverse avalanche

1. Reverse blocking mode: cathode is positive with respect to anode. Junctions $J_1$ and $J_3$ are reverse biased and $J_2$ is forward biased, so only a small reverse leakage current flows. If the reverse voltage reaches the reverse breakdown voltage $V_{BR}$, avalanche breakdown occurs at $J_1$ and $J_3$; the current rises sharply and the device is usually destroyed.
2. Forward blocking mode: anode is positive and gate is open. $J_1$ and $J_3$ are forward biased but $J_2$ is reverse biased, so only a small forward leakage current flows. The SCR is OFF and blocks the forward voltage.
3. Forward conduction (ON) mode: if the forward voltage is raised to the forward breakover voltage $V_{BO}$ (with $I_g = 0$), junction $J_2$ breaks down by avalanche and the SCR switches suddenly to the ON state (the negative-resistance jump). The voltage across it drops to about 1 to 2 V and the current is limited only by the load. With gate current, $J_2$ breaks down at a lower voltage: the larger $I_g$, the smaller the breakover voltage ($I_{g2} > I_{g1}$). In practice the SCR is always turned ON by a gate pulse at a voltage well below $V_{BO}$.

Two important currents on the curve are the latching current $I_L$ (minimum current just after turn-on to stay ON) and the holding current $I_H$ (minimum current to remain ON); $I_L > I_H$.

Thyristor firing circuit

A firing (triggering) circuit supplies a gate pulse of correct magnitude, width and timing (firing angle $\alpha$) and is synchronised with the supply.

Resistance-capacitance (RC) firing circuit (half wave):

      +------- load -------+
      |                    |
   ~ Vs                  A |
      |                  SCR
      |        R      D2   |K
      +---/\/\/\-----|>|-- G
      |     (variable)     |
      |   D1         C     |
      +---|<|---+---||-----+

1. In the negative half cycle the capacitor $C$ charges through $D_1$ to $-V_m$ (upper plate negative).
2. In the positive half cycle $C$ charges positively through the variable resistor $R$.
3. When the capacitor voltage reaches the gate trigger voltage $V_{gt}$ plus the drop of $D_2$, gate current flows and the SCR fires.
4. Increasing $R$ slows the charging, so firing is delayed. The firing angle can be varied from about $0^\circ$ to $180^\circ$ (an R-only circuit gives only $0^\circ$ to $90^\circ$). $D_2$ blocks reverse gate voltage.

A UJT relaxation oscillator or a digital (microcontroller) circuit with a pulse transformer or opto-coupler is used when sharp pulses, wider range and isolation are needed.