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The construction of the transistor and the thyristor is very similar. The word “silicon” is a byproduct of the fact that it is a multi-layer semiconductor device. The “controlled” part of the name means that it needs a gate signal to turn “ON,” and the “rectifier” part of the name means that once “ON,” it behaves like a rectifying diode. The thyristor actually resembles a controlled rectifying diode, according to the circuit symbol for this device.
The Thyristor is a four layer (P-N-P-N) semiconductor device that contains three PN junctions in series, in contrast to the junction diode, which is a two layer (P-N) semiconductor device, and the widely used bi-polar transistor, which is a three layer (P-N-P, or N-P-N) switching device.
Like a diode, a thyristor is a unidirectional device that can only conduct current in one direction,. But unlike a diode, depending on how the thyristor’s gate is activated,. It can be made to function as either an open-circuit switch or as a rectifying diode. To put it another way, thyristors can only be used for switching and cannot be amplified.
The silicon controlled rectifier (SCR), along with Triacs (Triode AC’s), Diacs (Diode AC’s), and UJTs (Unijunction Transistor), is one of many power semiconductor devices that are all capable of acting as very quick solid state AC switches for controlling large AC voltages and currents. This makes solid state devices for controlling AC motors, lamps, and phase control very useful for electronics students.
The thyristor is a three-terminal device with the labels “Anode,” “Cathode,” and “Gate,”. And it consists of three PN junctions that can be switched “ON” and “OFF” at a very fast rate. Or “ON” for varying periods of time during half cycles to provide a chosen amount of power to a load. The best way to understand how the thyristor works is to imagine it as being constructed of two transistors connected back-to-back to form a pair of complementary regenerative switches, as is depicted.
A Thyristors Two Transistor Analogy
The two transistor equivalent circuit demonstrates. That while the collector current of TR1 feeds into the base of TR2, the collector current of TR2 feeds into the base of the NPN transistor TR2. Since each transistor receives its base-emitter current from the other’s collector-emitter current,. These two interconnected transistors are dependent on one another for conduction. Therefore, even when an anode-to-cathode voltage is present, nothing will happen until one of the transistors receives some base current.
The two outer P-N junctions are reverse biased, but the centre N-P junction is forward biased. When the thyristor’s anode terminal is negative in relation to the cathode,. And the device functions very similarly to an ordinary diode. The flow of reverse current is thus prevented by a thyristor until a high voltage level is reached at which the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the use of a Gate signal.
This is a significant drawback of the thyristor because it means that it may unintentionally conduct when exposed to reverse overvoltages, high temperatures, or spikes in the dv/dt voltage.
The two outer P-N junctions are now forward biassed but the centre N-P junction is reverse biassed if the anode terminal is made positive in relation to the cathode. As a result, forward current is also stopped. The collector current that results from injecting a positive current into the base of NPN transistor TR2 flows into the base of transistor TR1. As a result, the PNP transistor TR1 experiences collector current flow,. Which raises the base current of TR2 and so on.
Due to their connection in a non-stop regenerative feedback loop,. The two transistors quickly force each other to conduct to saturation. The forward resistance of the device when conducting can be very low at less than 1,. So the voltage drop across it and power loss are also minimal. Once the device is triggered into conduction, the current flowing through it between the Anode and the Cathode is only constrained by the resistance of the external circuit.
When a positive current is applied to the base of the transistor TR2,. Which is referred to as the “Gate” terminal for a silicon controlled rectifier, a thyristor can be turned “ON” . And made to behave like a typical rectifying diode. This enables us to see that a thyristor blocks current in both directions of an AC supply in its “OFF” state.
The operating voltage-current I-V characteristics curves for the operation of a Silicon Controlled Rectifier are given as:
VI Characteristics Curves
The gate signal loses all control once the thyristor is turned “ON”. And is flowing current in the forward direction (anode positive) because of the regenerative latching action of the two internal transistors. After regeneration is started, applying any gate signals. Or pulses will have absolutely no impact because the thyristor is already conducting and fully-ON.
The SCR cannot be biased, unlike the transistor, to remain in an active region along a load line between its blocking and saturation states. Since conduction is managed internally, the device’s performance is not greatly impacted by the size and duration of the gate “turn-on” pulse. The device will conduct and stay permanently “ON”. Even if the gate signal is completely removed by applying a brief gate pulse to it.
Since the thyristor has two stable states—”OFF” or “ON,” it can also be viewed as a bistable latch. This is because a silicon controlled rectifier blocks current in both directions of an AC waveform. When there is no gate signal applied, and once it triggers conduction, the regenerative latching action prevents it from being turned “OFF” again simply by using its Gate.
So how do we turn “OFF” the thyristor?
Once the thyristor has self-latched into its “ON” state and is producing current, it can only be turned “OFF” again by either completely cutting off the supply voltage,. Which also eliminates the anode current (IA),. Or by externally reducing the anode to cathode current to a level below. What is known as the “minimum holding current,” IH.
Therefore, before applying a forward voltage to the device once more without causing it to automatically self-conduct,. The anode current must be reduced below this minimal holding level for an amount of time sufficient for the thyristors’ internally latched pn-junctions to return to their blocking state. The Anode current, which is also known as the Load current,. Must therefore be greater than the Holding current value for a Thyristor to conduct in the first place. That is when IL > IH.
The SCR will automatically turn itself “OFF” at some value close to the cross over point of each half cycle. When used on a sinusoidal AC supply because it has the capability to do so whenever the Anode current is reduced below this minimum holding value. As we now know, the SCR will remain “OFF” until the application of the next Gate trigger pulse.
Since an AC sinusoidal voltage continually reverses in polarity from positive to negative on every half-cycle, this allows the thyristor to turn “OFF” at the 180o zero point of the positive waveform. This effect is known as “natural commutation” and is a very important characteristic of the silicon controlled rectifier.
Thyristors used in circuits fed by DC supplies cannot experience this natural commutation condition because the DC supply voltage is constant. As a result, a method of turning the thyristor “OFF” at the proper moment is required because once triggered, it will continue to conduct.
However, natural commutation happens every half cycle in AC sinusoidal circuits. The thyristor is then forward biassed (anode positive) during the positive half cycle of an AC sinusoidal waveform and can be turned “ON” by using a Gate signal or pulse. The Anode turns negative while the Cathode remains positive during the negative half cycle. This voltage reverse biases the thyristor, preventing it from conducting even in the presence of a Gate signal.
The thyristor can therefore be made to conduct until the end of the positive half cycle by applying a Gate signal at the proper moment during the positive half of an AC waveform. As a result, phase control, as it is also known, can be used to trigger the thyristor at any point along the positive half of the AC waveform. This makes power control of AC systems, as shown, one of the many applications for a silicon controlled rectifier.
At the start of each positive half-cycle the SCR is “OFF”. On the application of the gate pulse triggers the SCR into conduction. And remains fully latched “ON” for the duration of the positive cycle. If the thyristor is triggered at the beginning of the half-cycle ( θ = 0o ),. The load (a lamp) will be “ON” for the full positive cycle of the AC waveform (half-wave rectified AC) at a high average voltage of 0.318 x Vp.
As the application of the gate trigger pulse increases along the half cycle ( θ = 0o to 90o ),. The lamp is illuminated for less time and the average voltage delivered to the lamp will also be proportionally less reducing its brightness.
Then we can use a silicon controlled rectifier as an AC light dimmer. As well as in a variety of other AC power applications. Such as: AC motor-speed control, temperature control systems and power regulator circuits, etc.
As we have seen so far, a thyristor is fundamentally a half-wave device that, regardless of the Gate signal, conducts only in the positive half of the cycle when the anode is positive. And blocks current flow like a diode when the anode is negative.
The term “Thyristor” refers to a variety of semiconductor devices,. Some of which can conduct in both directions,. Some of which are full-wave devices, and some of which can be turned “OFF” by the Gate signal.
Such devices include “Gate Turn-OFF Thyristors” (GTO), “Static Induction Thyristors” (SITH), “MOS Controlled Thyristors” (MCT), “Silicon Controlled Switch” (SCS), “Triode Thyristors” (TRIAC) and “Light Activated Thyristors” (LASCR) to name a few, with all these devices available in a variety of voltage and current ratings making them attractive for use in applications at very high power levels.
Thyristors, also known as silicon controlled rectifiers,. Are three-junction PNPN semiconductor devices that can be compared to two interconnected transistors for switching large electrical loads. They can be turned “ON” by applying a single pulse of positive current to their gate terminal,. And they will stay that way until the current flowing from the anode to the cathode falls below the minimum latching level.
Static Characteristics of a Thyristor
- The only mode of operation for semiconductor thyristors is switching mode.
- Thyristors work with current; a small Gate current regulates a larger Anode current.
- only conducts current when the gate is being triggered and forward biased.
- When the thyristor is turned “ON,” it behaves like a rectifying diode.
- To keep conduction going, anode current needs to be higher than holding current.
- Regardless of whether Gate current is applied, reverse biased devices block current flow.
- As long as the anode current is greater than the latching current,. Once triggered “ON,” the device will remain latched “ON” conducting.
Thyristors are high-speed switches that can be used in many circuits in place of electromechanical relays. Because they don’t have any moving parts, aren’t susceptible to contact arcing, corrosion, or dirt buildup. Thyristors can be used to control the mean value of an AC load current. In addition to simply turning large currents “ON” and “OFF” without expending a lot of energy. Electric lighting, heaters, and motor speed control are a few applications of thyristor power control that are well-illustrated.