In this note, we are going to learn about Characteristics of SCR, about Forward Characteristics and Reverse Characteristics of SCR. Welcome to Poly Notes Hub, a leading destination for Engineering Notes.
Author Name: Arun Paul.
What is SCR?
SCR stands for “Silicon Controlled Rectifier.” It is a thyristor-based semiconductor device. SCRs are frequently employed in electrical circuits and power control systems. It functions as a controlled switch, allowing current to flow only when a predetermined triggering signal is applied to its gate terminal. Once activated, an SCR will continue to conduct current until the forward voltage across it falls below a given threshold or the current through it falls below a certain limit. This makes SCR ideal for applications that require precise control over the flow of electrical power, such as motor speed control, voltage regulation, and power switching circuits.
Some Important points about Silicon Controlled Rectifier:
- Control Rectification: SCR is a semiconductor device that controls rectification and regulates electrical power.
- Three-Terminal Device: It contains three terminals: anode, cathode, and gate. The gate terminal serves to bring the SCR into conduction.
- Unidirectional Conduction: Like diodes, SCRs only allow current to flow in one direction: from anode to cathode.
- Triggering Mechanics: To transition from the non-conducting state to the conducting state, the SCR requires a certain trigger voltage applied to its gate terminal.
- Latch Up Behavior: An SCR that has been triggered into conduction will continue to do so even after the gate signal is removed, until the forward voltage or the current flowing through it both drop below a certain threshold.
- High Power Handling Capability: They can handle high current and voltage levels, making them ideal for power control applications.
- Thermal Considerations: SCRs may require heat sinks or cooling devices due to the heat created during operation, particularly at high current levels.
VI Characteristics of SCR
The forward and reverse characteristics of an SCR are shown above and we describe it below:
Forward characteristics of SCR
When the SCR is forward -biased and the gate current is equal to Zero, a small forward leakage current flows. If the forward voltage is increased, a point reaches when SCR starts conduction and voltage at this point is called Forward Breakover voltage.
If the gate current is increased to IG1 the value of forward voltage required for conduction (VF1) is much less.
If the gate current, is further increased to IG2. the SCR will start conducting at a much less voltage VF2.
Reverse characteristics of SCR
When the SCR is reverse -biased, a small leakage current flows and is known as reverse blocking current. As the reverse Voltage is increased, and reaches the value known as reverse breakdown voltage, the reverse current increase very rapidly.
If this current is not limited, the SCR will be destroyed.
The region before reverse breakdown is known as reverse blocking region.
Some Important Definitions
Below we listed some important definition on the V-I Characteristics of SCR –
- Forward Blocking Region: The region of the forward characteristics before switch-on occurs is known as forward, blocking region.
- Forward conduction Region: The region after switch on is called forward conduction Region.
- Holding Current: It is the minimum forward current that will keep SCR in conduction. Below this value of current, the SCR will be switched off.
- Forward Breakover Voltage: It is minimum forward voltage, gate being open, at which SCR starts conduction i.e. turned on.
- Latching Current: When a gate current is applied to an SCR, anode current starts increasing. Latching Current is the minimum anode current to keep the SCR in conducting state after the gate voltage is removed. Latching Current is generally two to three times the holding current.
- Reverse Breakdown voltage: The maximum reverse voltage at which SCR starts conducting heavily is called Reverse break down voltage.