Thyristor V-I, application and opto-isolation

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 $V_{BO}$.

Application in a power circuit

Example: single-phase half-wave controlled rectifier for a d.c. load (heater, d.c. motor field).

  vs ~ ---+---[SCR]---+
          |           |
          |           R  vo
          |           |
          +-----------+
 vo    __          __
      | \         | \
  ----+--\--------+--\----
     a    pi     2pi+a

The SCR is fired at angle $\alpha$ in each positive half cycle and turns OFF naturally at $\pi$. The average load voltage is

$$V_o = \frac{V_m}{2\pi}(1 + \cos\alpha)$$

so the power to the load is controlled by $\alpha$. SCRs are similarly used in full converters, AC voltage controllers (light dimmers, fan regulators), inverters, choppers, and HVDC valves.

Isolation using an opto-coupler

Opto-coupler isolation: the gate signal generator (logic level, grounded to the control circuit) and the power circuit (hundreds of volts, cathode floating) must be electrically separated. An opto-coupler does this with light.

   control side     |      power side
                    |          + Vaux
    R1              |          |
 o--/\/\--+         |          R2
 pulse    |         |          |
         LED ~~~~>  |   photo-transistor
          |  light  |   (C top, E below)
 o--------+         |          |
 GND                |          +------> G
                    |          |
                    |          R3
                    |          |
                    |  Vaux(-) +------> K

1. The control pulse drives a current through the LED via the current-limiting resistor $R_1$.
2. The LED emits infrared light, which falls on the phototransistor (or photo-SCR) inside the same package.
3. The phototransistor turns ON and connects the auxiliary supply $V_{aux}$ (referenced to the cathode) through $R_2$ to the gate, giving gate current. $R_3$ prevents false triggering by noise.
4. When the pulse ends, the LED is dark, the phototransistor turns OFF and gate current stops.

There is no electrical path between the two sides, so the isolation can withstand a few kV, and noise from the power circuit does not reach the logic circuit.