Silicon Control Rectifier(SCR) is a three-terminal, three-junction semiconductor device that performs various functions in an electric circuit. Silicon Control Rectifiers are used in high-power electronic devices to control the power supply. Silicon Control Rectifier belongs to the thyristor family. The Silicon Controlled Rectifier was developed in 1957 by electronic engineers. The developers of SCR linked SCR to thyristors. So we have discussed various types of SCR you must read:
In this article, I will highlight the definition of SCR, its construction, its workings, and the various uses of the silicon-controlled rectifier (SCR).
What Is SCR In Power Electronics?
A silicon-controlled rectifier is a four-layer device that is used to control the current. SCR in power electronics is used where high voltage and power are required. SCR has a four-layer semiconductor that forms two structures. The first is NPNP or PNPN. Besides this, SCR has three junctions that are J1, J2, and J3. Three terminals are the anode, cathode, and gate.
Working Principle Of SCR
SCR is a four-layer device that has a three-junction, p-n-p-n semiconductor device. There are three terminals in SCRs: anode, cathode, and gate.
The gate terminal in a silicon-controlled rectifier is used to control the flow of current between the anode and cathode. The silicon-controlled rectifier is in the off state, but applying high voltage to the gate helps the SCR turn on and pass current from the anode to the cathode. We don’t remove the high voltage from the anode until SCR is on. It is because the anode current is above a certain threshold, which is called the holding current.
Construction Of Silicon Controlled Rectifier (SCR)
A silicon-controlled rectifier has been constructed in four layers of P-type and N-type material. These layers form three junctions: J1, J2, and J3. The three terminals attached to it are anode, cathode, and gate
The anode is present at the primary terminal, and this entering terminal is of positive polarity. From this, the current flows or enters the device. The cathode is attached to the other terminal where the current leaves the devices, which means the leaving terminal is of opposing polarity.
The construction of power electronics is a major part of modern electrical engineering. It is important for things like renewable energy, electric cars, and factories. At Discrete Circuits, our team of experts is good at creating new and better ways to control and harness electrical power. We are always working to make the world cleaner and more efficient.
There are three main ways to build SCRs based on their power requirements and how they will be used:
1.Planar Type: This method is used for SCRs that conduct low power. It involves spreading the junction evenly through a process called diffusion.
2.Mesa Type: Used for high-power SCRs, this method also uses diffusion for the inner junction but combines it with another technique called alloying for the outer layers.
3.Press Pack Type: This type provides extra power and is used in very high power situations. A SCR is composed of tightly packed components that are often reinforced with materials like molybdenum or tungsten. This is then connected to a copper base which is connected to a heat sink to keep the SCR cool during operation.
These methods ensure that SCRs can handle varying levels of electrical power and operate reliably in a variety of electronic applications.
Modes Of Operation Of Silicon Controlled Rectifier (SCR)
There are three modes of operation for SCRs depending on how they are biased. Which are
- Forward blocking Mode
- Forward Conduction Mode and
- Reverse Blocking Mode
1. Forward Blocking Mode:
Here, the positive side of the SCR is connected to the anode and the negative side is connected to the cathode, leaving the gate open. This setup allows some current to flow, but the SCR acts like a closed gate, blocking most of it. It allows current to flow only when the voltage is high enough.
2. Forward Conduction Mode:
In this mode, the SCR is blocked and allows current to flow. This is done either by sending a pulse to the gate or by increasing the voltage across it. When this happens, an internal breakdown occurs, and the SCR turns on, just as a switch turns on.
It is important to note that the more current you send to the gate, the faster the SCR starts to conduct. The amount of current that triggers this transition is called the latching current.
Additionally, there is a level of current at which the SCR stops conducting and goes back to blocking mode. It is called holding current. As the current drops below this level, the SCR starts to act like a closed gate again.
3. Reverse Blocking Mode:
In this mode, the SCR is set in reverse, i.e. the positive side is connected to the cathode and the negative side is connected to the anode. In this configuration, the SCR blocks any current trying to flow in the opposite direction, useful for certain circuit applications.
In this condition, the cathode is given a positive charge compared to the anode. This causes junctions J1 and J3 to resist any current flow, while J2 allows current to flow. Picture it like flipping off a switch.
In this period, very little current is flowing, unless the voltage exceeds the reverse breakdown voltage (VBR) of the SCR. The current flowing through the SCR can suddenly increase if that happens; Too high a voltage can break the bond.
The excess current not only reduces the SCR’s power but also overheats it. Due to this, pushing the voltage above VBR can seriously damage the SCR.
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Applications Of SCR (Silicon Control Rectifier)
- Speed control of DC shunt motor
- Computer logic circuits
- AC power control circuits
- Battery Charging Regulators
- Power Switching Circuit
- Timing Circuits
- Inverters
- Temperature control systems
- SCR Crowbar
- Controlled Rectifier
