What is SCR?
SCR is a silicon control rectifier that belongs to the thyristor family. The SCR is a unidirectional semiconductor device that allows current to flow in one direction only. The SCR is four layers three-terminal device. The three terminals of the SCR are anode, cathode, and gate. The SCR can be turned on by applying the trigger pulse to the gate terminal. We can well understand the SCR working by studying the vi characteristics of SCR.
Voltage-ampere(V-I) Characteristics of SCR
There are three modes of operation of SCR.
- Forward Blocking Mode
- Forward Conduction Mode
- Reverse Blocking Mode
Let us discuss the VI characteristics of SCR in different modes of operation.
Forward Blocking Mode of SCR
In the forward blocking mode, SCR remains in forwarding bias, but SCR does not conduct. When the anode voltage is positive with respect to the cathode and gate current is zero, the device remains in the off state.
In the forward blocking mode,
- J1 and J3 junction is in forward bias
- J2 junction is in reverse bias
- Very low leakage current flows through the device.
- SCR offers very high resistance.
- SCR remains in off State
However, we can turn on SCR by applying a high voltage between anode and cathode even if the gate current is zero. The voltage at which the SCR turns on when the gate current is zero is the forward break-over voltage.
Forward Conduction Mode of SCR
In forward conduction mode, we can make the SCR turn on at the lesser anode to cathode voltage on the application of small gate voltage momentarily. The voltage cause gate current to flow. Thus, the gate current pulse is sufficient to switch on the SCR at the lesser anode to cathode voltage. The SCR remains on after the removal of the gate current pulse. The circuit diagram for getting the forward conduction characteristics of the SCR is as given below.
The SCR is in reverse bias when the positive voltage between gate and cathode. Now on the application of gate current through switch S1, the J2 junction of SCR starts conducting and SCR is in forwarding bias. Thus all three junctions(J1, J2, and J3) of SCR are now in forwarding bias. Therefore, SCR starts conducting. The SCR offers very low resistance in forwarding conduction mode.
Once the SCR starts conducting, the SCR remains in its on state even after removing of gate current pulse.
Now, if the gate current is further increased, the device can be turned on even at the much lesser forward anode to cathode voltage. The forward conduction, the forward blocking, and the reverse blocking region are as depicted below.
The device will remain in the on- state even after removing the gate pulse. The device will remain in its on-state if the anode current is more than the latching current. When the anode current reduces below the holding current the device turns off. The holding current is associated with the turn-off mechanism of SCR. The latching current is always more than the holding current.
Reverse Blocking Mode of SCR
When we connect the SCR in reverse bias- when the anode is negative with respect to the cathode, the device remains in an off state because junction J1 and J3 are in reverse bias while junction J2 is forward bias. The SCR acts as two diodes connected in the series with a reverse bias applied across it. The device offers very high resistance.
The SCR can be turned on if the voltage across anode to cathode is above the reverse breakdown voltage of the device.
The maximum reverse voltage at which the SCR starts conducting heavily is known as the reverse break-over voltage.
Important Terms in the V-I Characteristics of SCR
We use the following terms frequently while studying the V-I characteristics of the SCR.
- Forward Brekover Voltage
- Latching current
- Holding current
- Forward anode current rating
- Peak reverse voltage
- Circuit fusing rating
1. Forward Breakover Voltage
The minimum forward voltage at which SCR starts conducting in the absence of the gate current. This minimum voltage is the forward break-over voltage of SCR.
For example, the SCR of forwarding break over voltage rating 500 volts remains in its off state when the voltage is below 500 volts, and it starts conducting when the voltage is equal to or above 500 volts.
2. Latching Current
The latching current is the minimum anode current at SCR that remains in on state after removing of gate current. if the value of anode current is less than the latching current value. the SCR will not continue to conduct.
3. Holding Current
It is the maximum anode current at which SCR turns off from its on the state. If the holding current is 10 mA, the SCR will turn off if the anode current is less than 10 mA.
3. Peak Reverse Voltage
The peak reverse voltage is the maximum reverse voltage across SCR ( Cathode -positive and anode – negative) that can be safely applied without conducting the SCR.
This is a very important parameter. when SCR is used for controlled rectifies, the reverse voltage applies during the negative half cycle and SCR must remain in its off state. The peak reverse voltage of SCR should be always more than the peak negative voltage of AC voltage.
4. Forward Current Rating
The maximum anode current can flow through the SCR and it does not cause damage to SCR. The SCR is available in different anode current ratings. A 50 ampere forward current rating SCR can carry the current safely.
If the current exceeds above SCR anode current rating, the device may fail. The semiconductor devices are vulnerable to temperature. Therefore, the devices must be selected according to the required current rating and surrounding ambient temperature.
5. Circuit Fusing Rating
The circuit fusing rating indicates the maximum forward surge current capability of SCR. The circuit fusing rating depends on the current and time. The fusing rating of SCR is I2t. The heating of SCR must be below the rated fusing rating for reliable SCR operation.
Related Posts on Semiconductor Devices