The SCR Silicon Controlled Rectifier is an important and commonly used member of the thyristor family. SCR can be used for various applications such as optimization, power regulation, and reversal. Like a diode, an SCR is a non-directive device that transmits current in one direction and resists it in the other.
SCR silicon-controlled rectifier is a three-pole and four-layer semiconductor current control device. It is mainly used in high-power control devices. Silicon-controlled rectifiers are sometimes called SCR diodes, 4-layer diodes, 4-layer devices, or thyristors. It consists of a silicon material that controls high strength and converts strong alternating current to direct current (correction). Therefore, it is called a silicon-controlled rectifier.
SCR silicon-controlled rectifier is a non-directional semiconductor device made of silicon. This device is equivalent to the solid state of the thyratron. Hence it is also called thyristor or thyroid transistor. SCR Silicon Controlled Rectifier is a trading name given to a thyristor by General Electric. Basically, an SCR is a three-pin, four-layer semiconductor device made up of alternate P-type and N-type materials.
Construction of SCR Silicon Controlled Rectifier,
The SCR is a four-level and three-pin device. The four layers, consisting of P and N layers, are arranged alternately to form the three junctions J1, J2, and J3. This transition depends on the type of construction, alloy, or stretch.
The silicon-controlled rectifier consists of three PN junctions. Its designated J1, J2, J3, and four layers of semiconductor material, connected in the form of NPNP and PNPN. In PNPN type structures, the anode lead is connected to the P-type material layer, and the cathode lead is connected to the N-type material layer.
Similarly, the gate terminal of a silicon-controlled rectifier is connected to a P-type material layer close to the cathode type material layer. The diffusion of doped semiconductor material forms layers of semiconductor material into a highly doped semiconductor material.
In fact, it is a conventional rectifier (PN) and a transistor (NPN), which together form a PNPN device in a block. Three terminals have been seized. One is from an outer P-type material called anode A. The other is from an outer layer of N-type material called cathode K, and the third is from the base of the transistor section and is called gate G.
A thyristor or silicon-controlled rectifier is a multimode semiconductor device and is similar to a transistor. The silicon-controlled rectifier consists of three leads, anode, cathode, and gate, instead of the bipolar diodes, anode, and rectifier’s cathode. Diodes are called uncontrolled rectifiers because they operate in the forward-biased state without any control when the diode plate voltage exceeds the cathode voltage.
Characteristics of SCR
The characteristic of SCR or silicon controlled rectifier is characteristic of current voltage. The current through the thyristor changes when the terminals between the anode and the cathode change and the voltage at the terminals between the gate and the cathode changes.
A graphical representation of the current by the SCR and the voltage between the anode and the cathode terminal is known as the SCR properties.
The SCR is a 4-layer 3-junction P-N-P-N semiconductor device consisting of at least three P-N junctions, which acts as an electrical switch for high-power operations. It has three main leads, namely anode, cathode, and gate, mounted on the device’s semiconductor layers.
The SCR has three modes of operation, 1Reverse blocking mode, 2 Forward blocking mode, and 3 Forward conduction mode. Let’s discuss each of the three methods one by one.
Reverse Blocking Mode of SCR
The Reverse SCR blocking mode is the operating mode in which it provides high resistance to current and therefore does not act. SCR behaves like an open switch in reverse lock mode. Therefore, this style is also known as SCR of State.
In general, the rated reverse blocking voltage and rated forward blocking voltage are the same. Applications specific to SCR reverse blocking are power source inverters. In this case, the J1 junction and J3 junction are moved in opposite directions, while the J2 junction becomes biased further.
Since Junction J1 and Junction J3 are reverse biased, there is no current flowing through the SCR circuit. But there is a small leakage in the forward bias junction J2 due to the charge carrier’s flow. This small leakage current is not enough to activate the SCR. Thus, SCR will be off.
Forward Blocking Mode SCR
Forward blocking mode is the SCR operating mode in which it does not work even if biased forward. The term forward-biased SCR implies that its anode terminal is positive concerning the cathode terminal when the gate switch S is open.
In this mode of operation, a positive voltage (+) is applied to the anode A (+), a negative voltage (-) is applied to the cathode K (-), and the gate G is open, as shown in the figure below. In this case, the J1 junction and the J3 junction are forward biased, while the J2 junction becomes reverse biased.
Due to the reverse bias voltage, the width of the depletion region increases at junction J2. This depletion region at J2 acts as a wall or obstruction between J1 and J3. It blocks the current flowing between J1 and J3. Therefore, most of the current does not flow between junction J1 and junction J3. However, a small leakage current flows between junction J1 and junction J3.
Forward Conduction Mode of SCR
Forward conduction mode is the only mode in which the SCR will continue to run. We can adjust the thyristor in two ways. First, we can increase the forward bias voltage above the fault voltage or apply a positive voltage to the gate terminal.
When the forward voltage between the anode and cathode increases as soon as the gate is opened, the overvoltage VBO at the reverse junction J2 will cause a forward breakdown avalanche, causing the thyristor to turn on. As soon as the thyristor is turned on, we can see from the thyristor’s image characteristic that M immediately moves towards N, and then somewhere between N and K.
If you want to use SCR for low voltage applications, you can apply a positive voltage to the SCR gate. Applying a positive voltage will help conduct SCR. In this mode of operation, the thyristor will move forward, and the current will flow through it.
Otherwise, there is no need to apply a large voltage between the anode and the cathode. A small voltage between the anode and the cathode and a positive voltage at the gate terminal is sufficient to move the SCR from blocking mode to conduction mode. In this process, the thyristor moves forward, and the current flows through it. Therefore, it is called Forward Conducting Mode.
Uses or Application of SCR
SCR Silicon Control Rectifiers are mainly used in devices where high power control is required, possibly in combination with high voltage. SCRs are capable of controlling the power delivered to the load.
Their work makes them suitable for use in medium and high-voltage AC power management systems. It is important to vary the power supplied to the load depending on the load requirements, such as motor speed control and dimmer.
SCR and similar devices are used to improve high power AC power when transmitting high voltage DC power. In AC circuits, phase control is the most common form of SCR power control. With phase control, power control is achieved by changing the alpha angle at the gate terminal.
SCR is widely used to convert power signals due to its long service life, fast operating speed. And lack of other defects associated with mechanical and electromechanical switches. It is also possible to change the SCR to a specific operating frequency so that the load’s current flow varies.
An example of such a circuit is the PWM-based SCR circuit, which provides a variable output signal to the load. It is used as a switch in various devices—the traditional DC speed control method. The shirt is designed to change the field of motor stimulation. Early solid-state pinball machines used them to digitally control lights, solenoids, and other functions instead of mechanics, hence the name solid.