Introduction:
Silicon controlled rectifier (SCR) also known as Thyristor is a three-terminal and four-layer unidirectional current-controlling semiconductor device. It is made up of silicon materials and is mainly used for controlling high power and conversion of high ac current into dc current. Hence the named silicon-controlled rectifier.
As like the p-n junction diode, SCR allows electric current in one direction and it blocks electric current in another direction. A normal p-n junction diode consists of two semiconductor layers namely p-type material and n-type material. Whereas an SCR consists of four semiconductor layers alternating p-type and n-type materials. In addition to Anode and Cathode terminal, there is also a Gate terminal on SCR.
The principle of four-layer (P-N-P-N) switching was developed by Tanenbaum, Goldey, Moll, and Holonyak of Bell Laboratories in 1956 and the silicon-controlled rectifier (SCR) was developed by a power engineers team led by Gordon Hall and commercialized by Frank W. Frank W. “Bill” Gutzwiller in 1957.
The name “silicon controlled rectifier” is General Electric’s trade name for a type of Thyristor. SCR is mostly used member of the Thyristor family and is more popular than other Thyristors like TRIAC, SCS, DIAC, etc. Hence the device is often referred by Thyristor.
SCRs are widely used in different applications like rectification, regulation of power and inversion, AC Dimmer etc. It is an essential component in the construction of almost all modern switching power supplies by controlling the amount of power going into the output terminal.
Silicon Controlled Rectifier (SCR) Symbol:
A Silicon Controlled Rectifier (SCR) consists of three terminals namely Anode (A), Cathode (K), and Gate (G) terminal. The device can be turned ON or OFF by controlling the gate input or biasing condition of SCR. The schematic symbol of SCR is shown in the figure below. The arrow of the diode represents the direction of conventional current flow.
Construction of Silicon Controlled Rectifier (SCR):
Basically, Silicon controlled rectifier (SCR) is a four-layer and three-terminal semiconductor device. It is made up of four semiconductor layers of alternating p-type and n- materials which form a PNPN or NPNP structure. Hence it has three p-n junctions J1, J2, and J3. These junctions may alloyed or diffused based on the type of construction.
The three terminals Anode (A), Cathode (K), and Gate (G) are arranged in such a way that Gate (G) terminal is attached to the p-type layer nearer to the Cathode (K) terminal in the PNPN structure. A typical structure of SCR with P-N-P-N layers (PNPN form) is shown in the figure below.
Here, Anode (A) is a positively charged electrode and the conventional current enters into the device through this terminal. Cathode (K) is a negatively charged electrode and conventional current leaves the device through this terminal. Gate (G) is a control terminal that controls the flow of current between Anode (A) and Cathode (K).
In the structure of SCR with PNPN form, the Anode (A) terminal is connected to the first P-type layer, and Cathode (K) terminal is connected to the last N-type layer. The gate (G) terminal is connected to the second p-type layer nearer to Cathode (K) as shown in the above figure. The outer layers (first P-type and last N-type layer) of SCR are heavily doped whereas middle P and N- type layers are lightly doped. In SCR, silicon is used as an intrinsic semiconductor to form P-type and N-type layers because silicon has a very small leakage current in comparison to germanium.
From the structure of SCR, it is seen that a single SCR is a combination of one PNP transistor (Q1) and one NPN transistor (Q2). The emitter of Q1 acts as the Anode terminal and the emitter of Q2 acts as the cathode terminal of SCR. Further, the base of Q1 is connected to the collector of Q2 and the collector of Q1 is connected to the base of Q2. The Gate terminal of SCR is connected to the base of Q2. This analogy of SCR as a combination of two transistors is called two transistor model. The structure of SCR as two transistor model is shown in the figure below.
Modes of operation in Silicon Controlled Rectifier (SCR):
Depending on the biasing given to SCR, there are three modes of operation. They are
- Forward Blocking Mode (OFF State)
- Forward Conducting Mode (ON State)
- Reverse Blocking Mode (OFF State)
1. Forward Blocking Mode (OFF State):
In this mode of operation, a positive voltage (+) is given to the Anode (A) terminal of SCR, and a negative voltage (-) is given to Cathode (K). The Gate (G) terminal is open-circuited as shown in the figure below.
Under this condition, junctions J1 and J3 are forward biased whereas junction J2 is in reverse biased condition. The depletion region at junction J2 blocks the flow of current from junction J1 to junction J3 as it acts obstacle or wall between them. However, a small amount of leakage current flows between these junctions J2 and J3.
When the applied voltage across the SCR reaches a breakdown voltage, the avalanche breakdown occurs due to high energy minority carriers. The current starts flowing through the SCR at this breakdown voltage and there is no current flow below the breakdown voltage because SCR offers very high resistance to the current below the breakdown voltage and acts as an open switch by blocking the forward current. Hence it will be in an OFF state.
From above, it is observed that the SCR is in forward biasing condition but there is no current flow through it. Hence this mode of operation is named forward locking mode.
2. Forward Conducting Mode (ON State):
In this mode of operation, the SCR comes into the conduction mode from blocking mode. It can be done in two ways, i.e. either by increasing the forward bias voltage (voltage across Anode and Cathode) beyond the breakdown voltage or by applying positive pulse or voltage at the Gate terminal. The biasing of SCR in this mode is shown in the figure below.
In the first case, the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown voltage, the depletion region breakdown occurs at J2, and the current starts flowing through SCR. In this condition, the SCR will be in an ON state. After the occurrence of junction breakdown, the current flow in SCR increases rapidly as shown in V-I Characteristics below.
In the second case, a small positive pulse or voltage VG is applied to the Gate terminal of SCR as shown in the figure above. When the gate voltage is applied to the gate terminal, the reverse biased junction J2 in forward blocking mode will become forward biased, and the depletion region width becomes very narrow. In this condition, a small forward bias voltage between Anode and Cathode can easily penetrate this narrow depletion region. Therefore on applying a small forward bias voltage, an electric current starts flowing through the SCR and it will be in an ON state.
Once the SCR starts conducting, no more gate voltage is needed to maintain it in the ON state. The minimum current required to maintain the SCR in the ON state on the removal of gate voltage VG is called latching current.
Any one of these methods results in avalanche breakdown at junction J2 and hence the SCR turns into conduction mode and acts as a closed switch thereby current starts flowing through it. Here, the SCR is forward biased and current flows through it. Hence this mode of operation is named as forward conducting mode.
3. Reverse Blocking Mode (OFF State):
In this mode of operation, a positive voltage (+) is given to Cathode (K) terminal, and a negative voltage (-) is given to Anode (A), Gate (G) terminal is an open circuit as shown in the figure below.
Under this condition, junctions J1 and J3 are reverse biased whereas junction J2 is in forward biased condition. As junctions J1 and J3 are reverse-biased, there is no current flow through the SCR. But due to the drift of the charge carrier in a forward-biased junction J2, there is small leakage current flow in SCR which is not sufficient to turn ON the device. Hence the SCR will be in an OFF state and acts as an open switch.
The SCR offers high impedance in this mode of operation until the applied voltage is less than the reverse breakdown voltage VBR. If the reverse applied is greater than the reverse breakdown voltage, the avalanche breakdowns occur at junction J2 and hence increase reverse current flow in the SCR device. This reverse current causes more losses in SCR and produces heat on more increasing it. When the reverse voltage applied to SCR is more than VBR, There will be considerable damage to the device.
V-I Characteristics of Silicon Controlled Rectifier (SCR):
The V-I characteristics of SCR are shown in the figure below. In this V-I characteristic, the horizontal line represents the amount of voltage applied VA across the SCR and the vertical line represents the amount of current flow IA in the SCR.
Here, the V-I characteristics of SCR are divided into three regions. They are:
1. Forward Blocking Region
The region OA in V-I characteristics is called the forward blocking region. This region represents the forward-blocking mode of SCR operation. In this region, the forward bias voltage is given to SCR where positive voltage is given to Anode, the negative is given to SCR and Gate is open-circuited. In this condition, the junctions J1 and J3 become forward biased whereas junction J2 becomes reverse biased. A small leakage current flows from the Anode terminal to the Cathode terminal of SCR which is known as a forward leakage current. The SCR does not conduct electric current and the device is in an OFF state in this region.
2. Forward Conduction Region
The region BC in V-I characteristics is called the forward conduction region. This region represents the forward conduction mode of SCR operation. In this region, the current flowing from Anode to Cathode increases rapidly. When the forward bias voltage applied between Anode and Cathode is increased beyond the breakdown voltage, the depletion region breakdown occurs at junction J2 and the current starts flowing through the SCR and it will be in the ON state. The current flow in this region increases rapidly after junction J2 breakdown occurs. The voltage at which the junction breakdown occurs when the Gate is open is known as forward breakdown voltage (VBF)
The region AB in V-I characteristics indicates that as soon as the SCR becomes ON, the voltage across the SCR drops to some volts.
3. Reverse Blocking Region
The region OE in the V-I characteristics is called the reverse blocking region. This region represents the reverse blocking mode of SCR operation. In this region, the reverse bias voltage is applied to SCR where a positive voltage is given to Cathode, a negative voltage is given to Anode, and the Gate terminal is open-circuited. In this condition, junctions J1 and J3 are reverse biased whereas the junction is forward biased. As junction J1 and J3 are in reverse biased condition, there is no current flow through SCR. But due to the drift of the charge carrier in forward-biased junction J2, there is small leakage current flow in SCR which is not enough to turn ON the device. Hence the SCR will be in an OFF state in this region.
When the reverse bias voltage between Anode and Cathode is increased beyond the reverse breakdown voltage VBR, an avalanche breakdown occurs, and the current increases rapidly. The region EF in V-I characteristics is known as the reverse avalanche region.
Applications of SCR:
The main application of SCR is switching and power control. The followings are some applications that use switching and power control properties of SCR.
- It is used as a switch
- It is used b in AC voltage stabilizers
- It is used in choppers (DC to Dc converters)
- It is used for inverters (DC to AC converters)
- It is used in battery charger
- It is used for power control circuits
- It is used in DC circuit breaker
- It is used for AC power control with a solid relay
- It is used to control motors speed
- It is used to adjust the light dimmer
- It is used in fan speed regulators
Advantages of SCR:
The followings are some advantages of SCR.
- It can handle large voltage, current, and power.
- The voltage drop across conducting SCR is small which will reduce the power dissipation.
- Triggering circuits are simple.
- Easy to turn ON.
- It has a higher switching speed.
- It can be protected with the help of a fuse.
- It is simple to control.
Disadvantages of SCR:
The followings are the disadvantages of SCR.
- It can conduct only in one direction. So power control can be done only during half cycle of ac.
- The gate current cannot be negative
- It cannot be used at high frequency as it can be operated at a maximum frequency of 400 HZ.
- It needs to be turned on each cycle in ac circuits.
