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Thyristors Triacs and Diacs


Figure 1. Thyristor Packages

Figure 1, Typical Thyristor Packages

Examples of common thyristor packages are shown at Figure 1. The construction of a typical medium power thyristor (also called a Silicon Controlled Rectifier or SCR) is shown in Figure 2. It consists of four layers of silicon in a p−n−p−n structure. Its circuit symbol in Figure 3 shows that it is basically a diode, but with an additional terminal, called the GATE.

Figure 2. Typical Thyristor Construction

Figure 2, Typical Thyristor Construction

Figure 3. Thyristor Circuit Symbol

Figure 3, Thyristor Circuit Symbol

The purpose of the gate is to enable the device to be switched from a non-conducting (forward blocking) mode into a low resistance, forward conducting state. Thus a small current applied to the gate is able to switch a much larger current (at a much higher voltage) applied between anode and cathode. Once the thyristor is conducting however, the gate current may be removed and the device will remain in a conducting state.

To turn the thyristor off, the current flowing between anode and cathode must be reduced below a certain critical "holding current" value, (near to zero); alternatively the anode and cathode may be reverse biased.

The thyristor is normally made to conduct by applying a gating pulse, while the main anode and cathode terminals are forward biased. When the device is reverse biased a gating pulse has no effect.

The main application for thyristors is in the switching of high power loads. They are the switching element in many domestic light dimmers and are also used as control elements in variable or regulated power supplies.

Figure 4 shows a typical characteristic curve for a thyristor. It can be seen that in the reverse biased region it behaves in a similar way to a diode. All current, apart from a small leakage current is blocked (reverse blocking region) until the reverse breakdown region is reached, at which point the insulation due to the depletion layers at the junctions breaks down.

In the forward biased mode, unlike a normal diode, no current apart from a small leakage current flows. This is called the forward blocking mode. If a gating pulse is applied however, the thyristor "fires" and the forward resistance of the device falls to a very low value, allowing very large (several amperes) currents to flow in the forward conducting mode. Thyristors can also be made to fire by applying a very large forward voltage between anode and cathode, but this is not desirable as the device is not then being used to CONTROL conduction.

Figure 4. Typical Thyristor Characteristics

Figure 4, Typical Thyristor Characteristics

The actual operation of the thyristor can be described by referring to Figure 5, which shows simplified diagrams of the thyristor structure with the p n layers and junctions labelled. (Figures 5a and b)

To understand the operation of a thyristor, think of it as a two-transistor (pnp and npn) model as shown in Figure 5a b and c. If no gate signal is applied, but a voltage is applied (less than forward breakdown voltage) between the top emitter terminal (marked A) and the bottom emitter terminal (marked K) so that A is positive with respect to K, both transistors will be turned off. No current is flowing so the voltage on the gate and cathode will be the same.

Figure 5. The Thyristor "Two Transistor Model"

Figure 5, The Two Transistor Model

When the gate is made positive with respect to K by the application of a gating pulse, Tr2 will turn on and its collector voltage will fall rapidly. This will cause the pnp transistor Tr1 base emitter junction to become forward biased, turning on Tr1. A large current will now be flowing between A and K. The action described happens very quickly as the switching on of Tr2 by Tr1 is a form of positive feedback with each transistor collector supplying large current changes to the base of the other.

As Tr1 collector is connected to Tr2 base, the action of switching on Tr1 connects Tr2 base virtually to the high positive voltage at A. This ensures that Tr2 ( and therefore Tr1) remains in conduction, even when the gating pulse is removed.

To turn the transistors off, the voltage across A and K must be reversed or the current flowing through the transistors must be reduced to a very low level, so the base emitter junctions no longer have sufficient forward voltage to maintain conduction.

Because of the thyristor´s ability to switch very large currents at very high (hundreds of volts) voltages, the thyristor is a useful device in power control circuits. It is quite capable of handling AC mains and is used in such circuits as lighting dimmers, motor speed controls etc. They are also widely used as fast acting protection devices in DC power supplies. The switching speed of thyristors is very fast and they are able to switch from fully off to fully on, typically in 1µs.

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