When positive potential applied to the anode with respect to the cathode, ideally no current will flow through the device and this condition is called forward – blocking state but when appropriate gate signal is applied, a large forward anode current starts flowing, with a small anode–cathode potential drop and the device becomes in forward-conduction state. Although after removing the gate signal, the device will remain in its forward-conduction mode until the polarity of the load reverses. Some thyristors are also controllable in switching from forward-conduction back to a forward-blocking state.
Application of Thyristor
As we already said that a thyristor is designed to handle large current and voltage, it is used mainly in electrical power circuit with system voltage more than 1 kV or currents more than 100 A. The main advantage of using thyristors as power control device is that as the power is controlled by periodic on – off switching operation hence (ideally) there is no internal power loss in the device for controlling power in output circuit. Thyristors are commonly used in some alternating power circuits to control alternating output power of the circuit to optimize internal power loss at the expense of switching speed.
In this case thyristors are turned from forward-blocking into forward-conducting state at some predetermined phase angle of the input sinusoidal anode–cathode voltage waveform.
Thyristors are also very popularly used in inverter for converting direct power to alternating power of specified frequency. These are also used in converter to convert an alternating power into alternating power of different amplitude and frequency.This is the most common application of thyristor.
Types of Thyristors
There are four major types of thyristors:
(i) Silicon Controlled Rectifier (SCR);
(ii) Gate Turn-off Thyristor (GTO) and Integrated Gate Commutated Thyristor (IGCT);
(iii) MOS-Controlled Thyristor (MCT)
(iv) Static Induction Thyristor (SITh).
Basic Construction of Thyristor
A high- resistive, n-base region, presents in every thyristor. As it is seen in the figure, this n-base region is associated with junction, J2. This must support the large applied forward voltages that occur when the switch is in its off- or forward-blocking state (non-conducting). This n-base region is typically doped with impurity phosphorous atoms at a concentration of 1013 to 1014 per cube centimeter. This region is typically made 10 to 100 micrometer thick to support large voltages. High-voltage thyristors are generally made by diffusing aluminum or gallium into both surfaces to create p-doped regions forming deep junctions with the n-base. The doping profile of the p-regions ranges from about 1015 to 1017 per cube centimeter. These p-regions can be up to tens of micrometer thick. The cathode region (typically only a few micrometer thick) is formed by using phosphorous atoms at a doping density of 1017 to 1018 cube centimeter. For higher forward-blocking voltage rating of thyristor, the n-base region is made thicker. But thicker n – based high-resistive region slows down on off operation of the device. This is because of more stored charge during conduction. A device rated for forward blocking voltage of 1 kV will operate much more slowly than the thyristor rated for 100 V. Thicker high-resistive region also causes larger forward voltage drop during conduction. Impurity atoms, such as platinum or gold, or electron irradiation are used to create charge-carrier recombination sites in the thyristor. The large number of recombination sites reduces the mean carrier lifetime (average time that an electron or hole moves through the Si before recombining with its opposite charge-carrier type). A reduced carrier lifetime shortens the switching times (in particular the turn-off or recovery time) at the expense of increasing the forward-conduction drop. There are other effects associated with the relative thickness and layout of the various regions that make up modern thyristors, but the major trade off between forward-blocking voltage rating and switching times and between forward-blocking voltage rating and forward-voltage drop during conduction should be kept in mind. (In signal-level electronics an analogous trade off appears as a lowering of amplification (gain) to achieve higher operating frequencies, and is often referred to as the gain-bandwidth product.)
Basic Operating Principle of Thyristor
Although there are different types of thyristors but basic operating principle of all thyristor more or less same. The figure below represents a conceptual view of a typical thyristor. There are three p–n junctions J1, J2 and J3. There are also three terminals anode (A), cathode (K) and gate (G) as levelled in the figure. When the anode (A) is in higher potential with respect to the cathode, the junctions J1 and J3 are forward biased and J2 is reverse biased and the thyristor is in the forward blocking mode. A thyristor can be considered as back to back connected two bipolar transistors. A p-n-p-n structure of thyristor can be represented by the p-n-p and n-p-n transistors, as shown in the figure. Here in this device, the collector current of one transistor is used as base current of other transistor. When the device is in forward blocking mode if a hole current is injected through the gate (G) terminal, the device is triggered on.
When potential is applied in reverse direction, the thyristor behaves as a reverse biased diode. That means it blocks current to flow in revere direction. Considering ICO to be the leakage current of each transistor in cut-off condition, the anode current can be expressed in terms of gate current.
Where α is the common base current gain of the transistor (α = IC/IE). The anode current becomes arbitrarily large as (α1 + α2) approaches unity. As the anode–cathode voltage increases, the depletion region expands and reduces the neutral base width of the n1 and p2 regions. This causes a corresponding increase in the α of the two transistors. If a positive gate current of sufficient magnitude is applied to the thyristor, a significant amount of electrons will be injected across the forward-biased junction, J3, into the base of the n1p2n2 transistor. The resulting collector current provides base current to the p1n1p2 transistor. The combination of the positive feedback connection of the npn and pnp BJTs and the current-dependent base transport factors eventually turn the thyristor on by regenerative action. Among the power semiconductor devices known, the thyristor shows the lowest forward voltage drop at large current densities. The large current flow between the anode and cathode maintains both transistors in saturation region, and gate control is lost once the thyristor latches on.
Transient Operation of Thyristor
A thyristor is not turned on as soon as the gate current is injected, there is one minimum time delay is required for regenerative action. After this time delay, the anode current starts rising rapidly to on-state value. The rate of rising of anode current can only be limited by external current elements. The gate signal can only turn on the thyristor but it cannot turn off the device. It is turned off naturally when the anode current tends to flow in reverse direction during the reverse cycle of the alternating current. A thyristor exhibits turn-off reverse recovery characteristics just like a diode. Excess charge is removed once the current crosses zero and attains a negative value at a rate determined by external circuit elements. The reverse recovery peak is reached when either junction J1 or J3 becomes reverse biased. The reverse recovery current starts decaying, and the anode–cathode voltage rapidly attains its off-state value. Because of the finite time required for spreading or collecting the charge plasma during turn-on or turn-off stage, the maximum dI/dt and dV/dt that may be imposed across the device are limited in magnitude. Further, device manufacturers specify a circuit-commutated recovery time, for the thyristor, which represents the minimum time for which the thyristor must remain in its reverse blocking mode before forward voltage is reapplied.
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