A thyristor is a four-layered semiconductor rectifier that consists of alternating p-type and n-type materials (see "Semiconductor Doping") forming three P-N Junctions. For thyristors, the flow of current between two electrodes, the anode and cathode, is triggered by a signal at a third electrode, the gate. Since the conducting state of thyristors is easily controlled by a signaling voltage, thyristors are commonly used as switching devices.
The thyristor is triggered into a conducting state by a short positive gate current upon which the thyristor becomes conductive in the forward direction and remains conductive as long as the forward current is greater than a particular value. This current value is called the holding current for the thyristor. If the current drops below the holding current, the thyristor switches to its nonconducting state.
One advantage that thyristors have over other switching devices, a transistor for instance, is that the signaling current does not need to be maintained for the thyristor to remain in its conducting state.
Thyristors exhibit three basic modes of operation:
- Reverse Blocking Mode is the state where the votage across the anode and cathode is reverse biased and the thyristor therefore does not conduct.
- Forward Blocking Mode is the state where the voltage across the anode and cathode is forward biased but the thyristor has not been triggered into its conducting state.
- Forward Conducting Mode is the state where the thyristor is forward biased and the thyristor has been trigger into its conducting state.
As described above, a thyristor in forward conducting mode remains in that state until the current through the thyristor drops below the holding current.
The thyristor's basic structure and the general properties of thyristors are illustrated in figure 1.
Figure 1. Thyristor structure and characteristics
Two transistor thyristor model
One way of modeling a thyristor is by the connection of two transistors - one pnp-transistor with one npn-transistor. This simple thyristor model is illustrated in figure 2.
Figure 2. Modeling of a thyristor by two connected transistors
The current gains β1 of the pnp-transistor and β2 of the npn-transitors are current dependent. Starting at a certain gate current, the combined current gain of the transistors is β1 • β2 ≥ 1, resutling in a feedback mechanism that causes the current increase to the point of saturation. Consequently, the thyristor goes into a conductive state. Due to this feedback mechanism, the thyristor remains in a conductive state even if the gate signal is removed.
If, however, the current through the thyristor drops below the holding current, the feedback mechanism of the connected transistors is diminished, and the thyristor stops conducting.
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