Introduction
Although the large semiconductor diode was a predecessor to thyristors, the modern power electronics area truly began with advent of thyristors. Thyristors (also known as the Silicon Controlled Rectifiers or SCRs) have come a long way from this modest beginning and now high power light triggered thyristors with blocking voltage in excess of 6kv and continuous current rating in excess of 4kA are available. They have reigned supreme for two entire decades in the history of power electronics. Along the way a large number of other devices with broad similarity with the basic thyristor (invented originally as a phase control type device) have been developed. They include inverter grade fast thyristor, IGBT, Silicon Controlled Switch (SCS), light activated SCR (LASCR), Asymmetrical Thyristor (ASCR) Reverse Conducting Thyristor (RCT), Diac, Triac and the Gate Turn-Off Thyristor (GTO).
From the construction and operational point of view a thyristor is a four layer, three terminal, minority carrier semi-controlled device. It can be turned on by a current signal but can not be turned off without interrupting the main current. It can block voltage in both directions but can conduct current only in one direction. During conduction it offers very low forward voltage drop due to an internal latch-up mechanism. Thyristors have longer switching times (measured in tens of μs) compared to a BJT. This, coupled with the fact that a thyristor can not be turned off using a control input, have all but eliminated thyristors in high frequency switching applications involving a DC input (i.e, choppers, inverters). However in power frequency ac applications where the current naturally goes through zero, thyristor remain popular due to its low conduction loss its reverse voltage blocking capability and very low control power requirement. In fact, in very high power (in excess of 50 MW) AC – DC (phase controlled converters) or AC – AC (cyclo-converters) converters, thyristors still remain the device of choice.
Constructional Features of a Thyristor
Fig 1 shows the circuit symbol, schematic construction and the photograph of a typical thyristor.
Figure 1
(a) Circuit Symbol, (b) Schematic Construction, (c) Photograph
As shown in Fig 1 (b) the primary crystal is of lightly doped n- type on either side of which two p type layers with doping levels higher by two orders of magnitude are grown. As in the case of power diodes and transistors depletion layer spreads mainly into the lightly doped n- region. The thickness of this layer is therefore determined by the required blocking voltage of the device. However, due to conductivity modulation by carriers from the heavily doped p regions on both side during ON condition the “ON state” voltage drop is less. The outer n+ layers are formed with doping levels higher then both the p type layers. The top p layer acts as the “Anode” terminal while the bottom n+ layers acts as the “Cathode”. The “Gate” terminal connections are made to the bottom p layer.
Basic operating principle of a thyristor
The actual operation of the thyristor can be described by referring to Figure 2, which shows simplified diagrams of the thyristor structure with the p n layers and junctions labelled. Figures 2 (a) and (b).
To understand the operation of a thyristor, think of it as a two-transistor (pnp and npn) model as shown in Figure 2 (a), (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.
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.
Thyristor's Characteristics
In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to remain in the on state), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).
A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.
After the current in a thyristor has extinguished, a finite time delay must elapse before the anode can again be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors can be made by diffusing heavy metal ions such as gold or platinum which act as charge combination centers into the silicon. Today, fast thyristors are more usually made by electron or proton irradiation of the silicon, or by ion implantation. Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon.
Basic operating principle of a thyristor
The actual operation of the thyristor can be described by referring to Figure 2, which shows simplified diagrams of the thyristor structure with the p n layers and junctions labelled. Figures 2 (a) and (b).
To understand the operation of a thyristor, think of it as a two-transistor (pnp and npn) model as shown in Figure 2 (a), (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 2
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.
Thyristor's Characteristics
In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to remain in the on state), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).
Figure 3. Switching Characteristics
A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.
After the current in a thyristor has extinguished, a finite time delay must elapse before the anode can again be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors can be made by diffusing heavy metal ions such as gold or platinum which act as charge combination centers into the silicon. Today, fast thyristors are more usually made by electron or proton irradiation of the silicon, or by ion implantation. Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon.
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