Characteristics of Thyristors

If the anode voltage V_AK is increased to a sufficiently large value, the reverse biased junction J2 would breakdown. This is known as avalanche breakdown and the corresponding voltage is called the forward breakover or forward breakdown voltage V_BO. On the application of a positive gate pulse, the thyristor starts to conduct at a lower forward voltage V_AK.

The important points on this characteristic are :

1. Latching Current IL: This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed.
2. Holding Current IH: This is the minimum anode current required to maintain the thyristor in the on-state.
3. Forward Breakover Voltage V_BO: If the forward voltage V_AK is increased beyond V_BO, the thyristor can be turned on. But such a turn-on could be destructive.
4. Once the thyristor is turned on by a gate signal and its anode current is greater than the holding current, the device continues to conduct due to positive feedback even if the gate signal is removed. This is because the thyristor is a latching device and it has been latched to the on-state.

Cross Sections of Thyristors

This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor. A thyristor can be considered as two complementary transistors. One being pnp and the other npn. The two-transistor model is shown in figure below.

Two Transistor Model of Thyristors

The collector current IC of a transistor is related to the emitter current IE and the leakage current of the collector base junction I_CBO as-

[note that I_K=I_A+I_G]

The current gain α₁ varies with emitter current I_E1 which is equal to I_A; and α₂ varies with emitter current I_E2 which is equal to I_k.

Current Gain with Emitter Current

A typical variation of current gain a with emitter current I_E is shown in figure below. If the gate current I_G is increased from zero to some positive value, this will increase the anode current I_A as shown by last equation. An increase of I_A which is an increase of I_E1 would increase α₁ (as shown in figure below) and also α₂.

If α₁ and α₂ approach unity, the denominator of last equation approaches zero and a large value of anode current is produced causing the thyristor to turn on as a result of the application of a small gate current.

Two Transistor Model

The capacitance of the pn junctions are shown in figure.

1. Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor.
2. If a thyristor is in the blocking state and a rapidly rising voltage is applied to the device, high currents would flow through the junction capacitors. The current through capacitor C_j2 can be expressed as
3. If the rate of rise of voltage dv/dt is large, then i_j2 would be large, which would result in increased leakage currents I_CBO1 and I_CBO2. High enough values of I_CBO1 and I_CBO2 may cause α₁ and α₂ to approach unity, resulting in undesirable turn on of the thyristor. A large current through the junction capacitors may cause damage to the device.

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