Thyristors:
Thyristor name is derived from the Greek “thyra” and means “door”, that is allowing something to pass through. The main group of thyristors is composed by SCR, and others are the special-purpose devices.Structure:
A silicon-controlled rectifier (SCR) consists of a four-layer silicon wafer with three pn junctions.It has four doped regions, the anode (A), the cathode (C), and the gate (G). The gate is the control lead. The SCR is triggered into conduction by applying a gate-cathode voltage, which causes a specific level of gate current. The device is returned to its non-conducting state by either anode current interruption or forced commutation.When the SCR is turned off, it stays in a non-conducting state until it receives another trigger. Therefore, the SCR can be termed as one operation thyristor-or
rectifier thyristor.The structure, biasing circuit, and possible symbols of thyristors are shown in Fig. 1.48. First of them displays the anode-side SCR with an n-gate lead, the second is the cathode-side thyristor with a p-gate lead, and the last is the most common device. High-voltage high-power thyristors sometimes also have a fourth terminal, called an auxiliary cathode,used for connection to the triggering circuit. This prevents the main circuit from interfering with the gate circuit.
Thyristors are commonly used in adjustable ac rectifier circuits, especially in power units up to 100 MVA. Their frequency capabilities are not high, in fact lower than 10 kHz.
Output characteristics:
Fig. 1.49 illustrates the output curves and idealized output characteristics of a thyristor. The device has two operating regions: non-conducting and conducting. The current–voltage output characteristics for different gate currents show the forward bias. The output characteristic of athyristor in the reverse bias is very similar to the same curve of the diode with a small leakage current.Using the same arguments as for diodes, the thyristor can be represented by the idealized characteristic in analyzing the circuit-desired topologies.
When it is non-conducting, the thyristor operates on the lower line in the forward blocking state (off
state) with a small leakage current. The thyristor is in off state until no current flows in the gate. The
short firing pulse below the breakover voltage from the gate driver triggers the thyristor. This current pulse may be of triangle, rectangle, saw-tooth, or trapezoidal shape.When a thyristor is supplied by ac, the moment of a thyristor firing should be adjusted by shifting the control pulse relative to the starting point of the positive alternation of anode voltage. This delay is called a
control angle or firing angle.In dc circuits, the use of thyristors is complicated due to their a turning on/off.After the pulse of the gate driver is given, the thyristor breaks over and switches along the dashed line to the conducting region. The dashed line in this graph indicates an unstable or temporary condition.The device can have current and voltage values on this line only briefly as it switches between the two stable operating regions. Once turned to the on state and the current higher than the
holding current,the thyristor remains in this state after the end of the gate pulse.When the thyristor is conducting, it is operating on the upper line. The current (up to thousands of amperes) flows from the anode to the cathode and a small voltage drop (1 to 2 V) exists between them. If the current tries to decrease to less than the holding border, the device switches back to the non-conducting region.
Turning off by gate pulse is impossible. Thyristor turns off when the anode current drops under the
value of the holding current.
Input characteristics:
Fig. 1.50 illustrates the input characteristics of the thyristor. The curves show the relation between the gate current and the gate voltage. This relation has a broad coherence area with a width that depends on the temperature and design properties of the device.
The gate current has an effect upon the form of the characteristic. The value of the breakover voltage is the function of the gate current. The more is the gate current the lower is the voltage level required to switch on the thyristor. Maximum breakover voltage of a thyristor reaches up to thousands of volts. If the applied voltage exceeds the breakover level, SCR triggers without the gate pulse. This prohibited mode should be avoided.
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