Characteristics of Thyristor or Characteristics of SCR

A thyristor is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n junctions, functioning as an electrical switch for high power operations. It has three basic terminals, namely the anode, cathode and the gate mounted on the semiconductor layers of the device. The symbolic diagram and the basic circuit diagram for determining the characteristics of thyristor is shown in the figure below,

V-I Characteristics of a Thyristor

From the circuit diagram, it’s clear that the anode and cathode are connected to the supply voltage through the load. A secondary supply, E_s, is also applied between the gate and cathode terminal, providing the positive gate current when the switch, S, is closed.
Upon supplying power, we observe the required V-I characteristics of a thyristor, as shown below for anode to cathode voltage (V_a) and anode current (I_a). A detailed study of these characteristics reveals three basic modes of operation: reverse blocking mode, forward blocking (off-state) mode, and forward conduction (on-state) mode. These are discussed in detail below to understand the overall behavior of a thyristor.

Reverse Blocking Mode of Thyristor

Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage E_s is detached initially by keeping switch S open. For understanding this mode we should look into the fourth quadrant where the thyristor is reverse biased.

In this setup, Junctions J₁ and J₃ are reverse biased, while Junction J₂ is forward biased. This arrangement mimics two diodes connected in series with a reverse voltage applied across them, allowing only a minimal leakage current of a few microamps to flow.
This is the reverse blocking mode, or the off-state, of the thyristor. Increasing the reverse voltage to the critical breakdown voltage (VBR) causes an avalanche at J₁ and J₃, and the reverse current spikes. This surge, associated with V_BR, leads to increased losses and heating in the SCR, potentially damaging the thyristor if the junction temperature exceeds safe limits. It is crucial to ensure that the reverse voltage never exceeds V_BR. When reverse voltage applied across a thyristor is less than V_BR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit.

Forward Blocking Mode

Now considering the anode is positive with respect to the cathode, with gate kept in open condition. The thyristor is now said to be forward biased as shown the figure below.

As we can see the junctions J₁ and J₃arenow forward biased but junction J₂ goes into reverse biased condition. In this particular mode, a small current, called forward leakage current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage.

In this particular mode, the thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J₂ will have an avalanche breakdown at a voltage called forward break over voltage V_B0 of the thyristor. But, if we keep the forward voltage less than V_BO, we can see from the characteristics of thyristor, that the device offers a high impedance. Thus even here the thyristor operates as an open switch during the forward blocking mode.

Forward Conduction Mode

When the forward voltage from anode to cathode increases, with the gate circuit open, the reverse-biased junction J2 undergoes an avalanche breakdown at the forward break over voltage (V_BO), turning on the thyristor. Once activated, as illustrated in the characteristics diagram, the operational point shifts immediately from M towards N, then settles anywhere between N and K, representing the forward conduction mode. In this mode, the thyristor conducts maximum current with minimal voltage drop, effectively functioning as a closed switch.