- Thyristor Definition: A thyristor, also known as a Silicon Controlled Rectifier (SCR), is a semiconductor device used to switch and control power in electrical circuits.
- Protection Importance: The protection of SCRs is crucial due to their sensitivity to over voltages, over currents, and high temperatures.
- Protective Measures: Various protective measures such as voltage clamps, circuit breakers, and snubber circuits are essential for maintaining SCR functionality.
- Heat Management: Effective thermal management through different mounting techniques is vital to prevent SCRs from overheating and failing.
- Protection of SCR: Key to SCR longevity and reliability is ensuring all forms of protection—electrical, thermal, and mechanical—are properly implemented.
Protection of a device is an important aspect for its reliable and efficient operation. Silicon Controlled Rectifier (SCR) are a very delicate semiconductor device. So we have to use it in its specified ratings to get desired output. SCR may face different types of threats during its operation due to over voltages, over currents etc. There are different types of thyristor protection schemes available for satisfactory operation of the device like
- Over voltage protection.
- Over current protection.
- High dv/dt protection.
- High di/dt protection.
- Thermal protection.
Over Voltage Protection
Over voltage protection is crucial since thyristors are highly sensitive to excessive voltages, which are the most common cause of thyristor failures.
A thyristor may be subjected to internal or external over-voltages.
Internal Over-Voltages : After commutation of a thyristor reverse recovery current decays abruptly with high di/dt which causes a high reverse voltage [as, V = L(di/dt) so if di/dt is high then V will be large] that can exceed the rated break-over voltage and the device may be damaged.
External Over-Voltages : These are caused due to various reasons in the supply line like lightning, surge conditions (abnormal voltage spike) etc. External over voltage may cause different types of problem in thyristor operation like increase in leakage current, permanent breakdown of junctions, unwanted turn-on of devices etc. So, we have to suppress the over-voltages.
Protective Measure: The effect of over-voltages can be minimized by using non-linear resistors called voltage clamping devices like metal oxide varistors. At the time of normal operation, it offers high impedance and acts as it is not present in the circuit. But when the voltage exceeds the rated voltage then it serves as a low impedance path to protect SCR.
Over Current Protection
Overcurrent mainly occurs due to different types of faults in the circuit. Due to overcurrent i2R loss will increase and high generation of heat may take place that can exceed the permissible limit and burn the device.
Protective Measure: To shield SCRs from overcurrent, Circuit Breaker (CBs) manage long-duration surges, while fast-acting current limiting fuses (FACLF) handle short, high surge currents effectively.
High dv/dt Protection
When a thyristor is in forward blocking state then only J2 junction is reverse biased which acts as a capacitor having constant capacitance value Cj (junction capacitance). As we know that current through capacitor follows the relation
Increased dva/dt i.e. or the rate of voltage change, can cause unwanted leakage current through the J2 junction, potentially turning on the SCR unintentionally. This issue, known as dv/dt triggering, is prevented using a Snubber circuit alongside the device.
Protective Measure :
Snubber Circuit: It consists of a capacitor connected in series with a resistor which is applied parallel with the thyristor, when S is closed then voltage Vs is applied across the device as well as Cs suddenly. At first Snubber circuit behaves like a short circuit. Therefore voltage across the device is zero. Gradually voltage across Cs builds up at a slow rate. So dv/dt across the thyristor will stay in allowable range.
Before turning on of thyristor Cs is fully charged and after turning on of thyristor it discharges through the SCR. This discharging current can be limited with the help of a resistance (Rs) connected in series with the capacitor (Cs) to keep the value of current and rate of change of current in a safe limit.
High di/dt Protection
When a thyristor is turned on by gate pulse then charge carriers spread through its junction rapidly. But if rate of rise of anode current, i.e. di/dt is greater than the spreading of charge carriers then localized heat generation will take place which is known as local hot spots. This may damage the thyristor.
Protective Measure: To avoid local hot spots we use an inductor in series with the device as it prevents high rate of change of current through it.
High Temperature Protection
Elevated junction temperatures can lead to insulation failure, making it essential to implement measures that limit temperature increases.
Protective Measure: We can achieve this by mounting the thyristor on heat sink which is mainly made by high thermal conductivity metals like aluminum (Al), Copper (Cu) etc. Mainly aluminum (Al) is used due to its low cost. There are several types of mounting techniques for SCR such as – Lead-mounting, stud-mounting, Bolt-down mounting, press-fit mounting, press-pack mounting etc.
Gate Protection of Thyristor
The gate circuit, similar to the thyristor, requires protection against overvoltages and overcurrents to prevent false triggering and reduce the risk of overheating.
Protective Measure: Overvoltages thyristor protection is achieved by using a zener diode and a resistor can be used to protect the gate circuit from overcurrent. Noise in gate circuit can also cause false triggering which can be avoided by using a resistor and a capacitor in parallel. A diode (D) may be connected in series or in parallel with the gate to protect it from high reverse voltage.
Overall Protection of a Thyristor
Lead mounting: In such mounting technique housing of SCR itself is used as heat radiator. Hence no need of additional heat zink arrangement. Hence, this technique of thyristor Protection is generally used for low current application, normally less than one ampere.
Stud mounting: The anode of the thyristor is in the form of threaded stud which is screwed to a metalling heat sink block.
Bolt-down mounting: Here the device is connected to the heat sink with the help of nut-bolt mechanism. It is mainly used in small and medium rating circuit.
Press fit mounting: This kind of mounting is obtained by inserting the whole SCR into the metallic block. It is used in high rating circuit.
Press-Pack mounting: This kind of mounting for thyristor protection is obtained by sandwiching the thyristor between to heat sink with the help of clamps. It is used for very high rating circuit.
