Thyristor turn-on and forced commutation

A thyristor is turned ON by making its anode current exceed the latching current while it is forward biased. The usual and best method is gate triggering; once ON, it can be turned OFF in a d.c. circuit only by forced commutation.

Methods of turn-on

1. Forward voltage triggering: raising $V_{AK}$ above $V_{BO}$ (not used; may damage the device).
2. Gate triggering: a positive gate pulse between gate and cathode (normal method).
3. dv/dt triggering: a fast rise of anode voltage drives a capacitive current through $J_2$ (unwanted).
4. Thermal triggering: high temperature raises leakage (unwanted).
5. Light triggering: light falling on the junction creates carriers (LASCR, used in HVDC).

Two-transistor analogy (gate turn-on mechanism): the PNPN structure is split into a PNP transistor $Q_1$ and an NPN transistor $Q_2$ connected so that the collector of each drives the base of the other.

          A
          |
        [Q1 pnp] E
          |  \
       B1 |   C1 ----+
          |          |
    +---- C2      B2 +----- G
    |  [Q2 npn]
    |     E
    |     |
    +-----K

With common-base gains $\alpha_1$, $\alpha_2$ and leakage currents $I_{CBO1}$, $I_{CBO2}$,

$$I_A = \frac{\alpha_2 I_g + I_{CBO1} + I_{CBO2}}{1 - (\alpha_1 + \alpha_2)}$$

When a gate current $I_g$ is injected, the emitter current of $Q_2$ rises, $\alpha_2$ increases, its collector current feeds the base of $Q_1$, whose collector current in turn feeds $Q_2$. This regenerative (positive feedback) action drives $\alpha_1 + \alpha_2 \to 1$, the anode current rises sharply and both transistors saturate: the SCR is latched ON. After that the gate can be removed.

Forced commutation techniques

In d.c. circuits the anode current never falls to zero by itself, so an external circuit (L, C and often an auxiliary thyristor) forces it to zero and reverse biases the SCR for longer than its turn-off time $t_q$.

 Class D (impulse) commutation
        +---- C ------[TA]---+
        |  (-   +)           |
 +Vdc --+--------[T1]--------+--- load ---+
                                          |
 GND -------------------------------------+

Firing $T_A$ places the charged capacitor across $T_1$ with reverse polarity; $T_1$'s current is diverted, it turns OFF, and the load current then charges $C$ until $T_A$ also turns OFF.