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What is Thyristor and How it Works?

Thyristor, commonly known as SCR, its standard term is reverse blocking three terminal thyristor. Thyristors are high-power semiconductor devices with switching and rectifying functions, and are used in various circuits, such as controllable rectifying and converting, inverter and contactless switch. As long as it provides D and weak trigger signals, it can control the strong current output. Therefore, it is a bridge for semiconductor devices to enter the strong field from the weak field.

What is thyristor

Like thyristors, transistors are also used in switching devices. Transistors are tiny electronic components that change the world. We can find them in various devices such as TV, mobile phone, laptop, calculator and headset. Because of their adaptability or versatility, it doesn’t mean that they can do it. We can use them as amplification and switching devices, but they can’t withstand higher current. Transistors also need continuous switching current. Therefore, for all these problems and to overcome them, we use thyristors.

Generally, SCR and thyristor are interchangeable, but SCR is a kind of thyristor. Thyristors include many types of switches, some of which are SCR (silicon controlled rectifier), GTO (gate off), IGBT (insulated gate controlled bipolar transistor), etc. But SCR is the most widely used device, so the word thyristor becomes synonymous with SCR. In short, SCR is a kind of thyristor.

SCR or thyristor is a four layer three junction semiconductor switch device. It has three terminals: anode, cathode and grid. Thyristors are also unidirectional devices like diodes, which means they flow current in only one direction. It consists of four in series three PN junctions. The gate terminal triggers the SCR by providing a small voltage to the terminal, which we also call the gate trigger method to turn on the SCR.

Types of Thyristor

There are two commonly used types:

  • ordinary thyristors (also called unidirectional thyristors)
  • TRIAC(triode for alternating current

How does Thyristor work

The two types of thyristors, unidirectional thyristors and three-terminal TRIAC, are briefly introduced below.

Working Principle of Unidirectional Thyristor

The internal structure of the unidirectional thyristor is shown in figure 1 (a). It can be seen from figure 1 (a) that the unidirectional thyristor is composed of four layers semiconductors P1N1P2N2. There are three PN junctions in the middle: the junction J1, J2, and J3. The anode A is drawn from P1, the cathode K is drawn from N2, and the control electrode (or gate) G is drawn from the middle P2. The circuit symbol of the unidirectional thyristor is shown in figure 1 (b).

Schematic Diagram and Circuit Symbol of Unidirectional Thyristor
Schematic Diagram and Circuit Symbol of Unidirectional Thyristor OLYMPUS DIGITAL CAMERA

In order to understand the working principle of the unidirectional thyristor, the unidirectional thyristor can be equivalently regarded as a combination of a PNP transistor T1 and an NPN transistor T2. The middle layer Pand layer N1 are shared by two transistors. The anode A is equivalent to the emitter of T1, and the cathode K is equivalent to the emitter of T2, as shown in figure 2.

Working Principle of Unidirectional Thyristor
Working Principle of Unidirectional Thyristor

Working Principle of TRIAC

A TRIAC is a three-terminal element with a five-layer structure of N1P1N2P2N3. It has three electrodes: a main electrode A1, a main electrode A2, and a control electrode (or gate) G. It is also a gate control switch. Regardless of its structure or characteristics, it can be regarded as a pair of anti-parallel ordinary thyristors. Its structure, equivalent circuit and symbols are shown in figure 3.

Symbol, Structure and Equivalent Circuit of the TRIAC
Symbol, Structure and Equivalent Circuit of the TRIAC

The main electrodes A2 and A1 of the triac are connected in series with the control object (load) RL, which is equivalent to a non-contact switch. The “on” or “off” of this switch is controlled by a signal uG (called a trigger signal) on the control electrode G. When there is a voltage (u ≠ 0) between the main electrodes A2 and A1, the moment the trigger signal uG appears, it will be conductive between A2 and A1 of the TRIAC, which is equivalent to the closed state of the switch. And once it is turned on, even if uG disappears, it can be kept on until u = 0 or the current in the series circuit of the main electrode and the load is reduced to a certain value, then it is turned off. After the cutoff, it is equivalent to the off state of the switch. In this way, the small current signal on the control electrode can be used to control the large current in the main electrode circuit.

 Volt-ampere Characteristic Curve of TRIAC
 Volt-ampere Characteristic Curve of TRIAC

Generally speaking, regardless of the voltage polarity between the two main electrodes A2 and A1 of TRIAC, as long as a certain amplitude of positive and negative pulses is applied to the control electrode, it can be turned on. So i represents the current in the main electrode and u represents the voltage between A2 and A1. The functional relationship between the two (called the volt-ampere characteristic curve) is shown in figure 4. It can be seen from the curve that the TRIAC has basically the same symmetrical performance in the first quadrant and the third quadrant.

According to the voltage u on the main electrode and the polarity of the trigger pulse voltage uG on the control electrode, combined with the volt-ampere characteristic curve, the TRIAC can be divided into four trigger modes, which are defined as follows:

  • (1) I+trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a positive trigger relative to A1.
  • (2) I-trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a negative trigger relative to A1.
  • (3) Ⅲ+trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a positive trigger relative to A1.
  • (4) Ⅲ-trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a negative trigger relative to A1.

Among these four trigger modes, I+ and III- have higher sensitivity, and are two commonly used trigger modes.

In the control circuit of the new type electric heating electric appliance, the trigger signal applied to the control electrode of TRIAC is output by a single chip microcomputer or an integrated circuit. Some output a continuous positive (or negative) voltage signal, and some output a series of zero-crossing trigger pulses synchronized with a 50Hz sinusoidal AC power supply. The former is called a potential trigger, while the latter is called a pulse trigger.

Characteristics of Thyristor or SCR

The basic circuit to obtain the VI characteristics of the thyristor is given below. The anode and cathode of the thyristor are connected to the main power supply through the load. The gate and cathode of the thyristor are fed from the source es to provide the gate current from the gate to the cathode.

Thyristor-basic-circuit-for-obtaining V-I characteristic

As per the characteristic diagram, there are three basic modes of SCR: reverse blocking mode, forward blocking mode, and forward conduction mode.

  • Forward Blocking Mode–In this mode, with switch s on, the cathode is positive relative to the anode. Nodes j 1 and j 3 are biased in the reverse direction, while J 2 is biased in the forward direction. When a reverse voltage (which should be less than VBR) is applied to the thyristor, the device will provide a high impedance in the reverse direction. Therefore, the thyristor is considered as a disconnect switch in reverse blocking mode. VBR is the reverse breakdown voltage of avalanche. If the voltage exceeds VBR, the thyristor may be damaged.
  • Forward Conduction Mode–When the anode is positive relative to the cathode, turn on the grid switch. As shown in the figure, thyristors are called forward biases, junctions j 1 and j 3 are forward biased, and j 2 are reverse biased. In this mode, the small current flowing is called forward leakage current, because the forward leakage current is small and not enough to trigger SCR. Therefore, even in the forward blocking mode, the SCR is considered an off switch.
  • Triggering Methods of SCR or Thyristor

There are many methods to triggering the SCR like:

  • Forward Voltage Triggering
  • Gate Triggering
  • dv/dt triggering
  • Temperature Triggering
  • Light Triggering

Forward Voltage Triggering–By applying a positive voltage between the anode and the cathode while keeping the grid circuit open, junction J2 is reversed biased. As a result, a depletion layer was formed on J2. With the increase of the positive voltage, the phase of depletion layer disappearing is coming. It is said that there is avalanche breakdown in J2. Therefore, the thyristor enters the on state. The voltage at which the avalanche occurs is called the forward breakdown voltage VBO.

Gate Triggering–This is one of the most common, reliable and effective ways to open a thyristor or SCR. In the gate triggering, in order to make the SCR conductive, a positive voltage is applied between the gate and the cathode, which will generate the gate current, and the charge will be injected into the internal p layer, and a positive breakdown will occur. The higher the gate current is, the lower the forward breakdown voltage is. As shown in the figure, there are three junctions in SCR. To turn on SCR now, junction J2 should be off. By using the gate triggering method, with the applied gate pulse, junction J2 is disconnected, junction j1and J2 become forward biased, or SCR enters the on state. Therefore, it allows current to flow through the anode to the cathode. According to the two transistor models, when the anode is positive relative to the cathode. The current does not flow through the anode to the cathode until the gate pin is triggered. When current flows into the gate pin, it turns on the lower transistor. When the lower transistor turns on, it turns on the upper transistor. This is an internal positive feedback, so by providing a pulse at the gate once, the thyristor remains on. When both transistors are on, the current begins to flow through the anode to the cathode. This state is called forward conduction, which is the way the transistor “locks” or maintains permanent conduction. To turn off the SCR, you cannot turn it off just by removing the grid current, in which case the thyristor is independent of the grid current. Therefore, to turn off the power, you must turn off the power circuit.

dv/dt Triggering–In the reverse biased node J2, due to the charge at both ends of the node, it has the same characteristics as a capacitor, which means that the behavior of node J2 is like a capacitor. If the forward voltage is suddenly applied, the charging current flowing through the junction capacitance CJ will turn on the SCR.

The charging current iC is given by;

iC = dQ/dt = d(Cj*Va) / dt                  (where, Va is forward voltage appears across junction J2)
iC = (Cj * dVa /dt) + (Va* dCj / dt )
as the junction capacitance is nearly constant, dCj / dt is zero, then
iC = Cj dVa / dt

Therefore, if the rate of rise of forward voltage dVa /dt is high, the charging current iwould be more. Here, the charging current plays the role of gate current to turn On the SCR even the gate signal is zero.

Temperature Triggering— When the Thyristor is in forward blocking mode, most of the applied voltage collects over the junction J2, this voltage associated with some leakage current. Which increases the temperature of the junction J2. So, with the increase in temperature the depletion layer decrease and at some high temperature (within the safe limit), depletion layer breaks and the SCR turns to ON state.

Light Triggering— For triggering a SCR with light, a recess (or hollow) is made inner p-layer as shown in figure below. The beam of light of particular wavelength is directed by optical fibres for irradiation. As, the intensity of the light exceeds to a certain value, SCR get turn ON. These type of SCR called as Light Activated SCR (LASCR). Sometimes, these SCR triggered using both light source and gate signal in combination. High gate current and lower light intensity required to turn ON the SCR.

LASCR or Light triggered SCR are used in HVDC (High Voltage Direct Current) transmission system.

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