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Characteristics of Thyristors

If the anode voltage VAK 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 VBO. On the application of a positive gate pulse, the thyristor starts to conduct at a lower forward voltage VAK.

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 VBO: If the forward voltage VAK is increased beyond VBO, 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. Characteristics of Thyristors

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.

Cross Sections of Thyristors 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 ICBO as-

{I_C} = \alpha {I_E} + {I_{CBO}}

{I_{C1}} = {\alpha _1}{I_A} + {I_{CBO1}}
{I_{C2}} = {\alpha _2}{I_K} + {I_{CBO2}}
{I_A} = {I_{C1}} + {I_{C2}} = {\alpha _1}{I_A} + {I_{CBO1}} + {\alpha _2}{I_K} + {I_{CBO2}}
{I_A} = \frac{{{\alpha _2}{I_G} + {I_{CBO1}} + {I_{CBO2}}}}{{1 - \left( {{\alpha _1} + {\alpha _2}} \right)}}
[note that IK=IA+IG]Two Transistor Model of Thyristors
The current gain α1 varies with emitter current IE1 which is equal to IA; and α2 varies with emitter current IE2 which is equal to Ik.

Current Gain with Emitter Current

A typical variation of current gain a with emitter current IE is shown in figure below. If the gate current IG is increased from zero to some positive value, this will increase the anode current IA as shown by last equation. An increase of IA which is an increase of IE1 would increase α1 (as shown in figure below) and also α2.

Current Gain with Emitter Current
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.
Two Transistor Model
  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 Cj2 can be expressed as {i_{j2}} = \frac{{d\left( {{q_{j2}}} \right)}}{{dt}} = \frac{d}{{dt}}\left( {{C_{j2}},{V_{j2}}} \right) = {V_{j2}}\frac{{d{C_{j2}}}}{{dt}} + {C_{j2}}\frac{{d{V_{j2}}}}{{dt}}
  3. If the rate of rise of voltage dv/dt is large, then ij2 would be large, which would result in increased leakage currents ICBO1 and ICBO2. High enough values of ICBO1 and ICBO2 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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