Thyristor Protection or SCR Protection and Gate Protection

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. Thyristor / SCR overvoltage crowbar circuit lt uses just four components: a silicon controlled rectifier or SCR, a zener diode, a resistor and a capacitor. Thyristor overvoltage protection circuit. The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground.There are different types of thyristor protection schemes available for satisfactory operation of the device like:

Over voltage protection

• It is the most important protection scheme w. r. t. others as thyristors are very sensitive to over voltages. Maximum time thyristor failures happen due to over-voltage transients.
• A thyristor may be subjected to internal or external over-voltages.
• Internal over-voltage:It is due to the operating condition of SCR.During the commutation of SCR ,when the anode current decays to zero anode current reverses due to stored changes. First the reverse current rises to peak value, then reverse current reduces abruptly with large di/dt.During series inductance of SCR large transient large voltage i.e Ldidt/ is generated.
• 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.
• Suppression of overvoltage:The effect of overvoltage is minimized using RC circuits and non linear resistor called voltage clamping device.
  
  
  
  Voltage clamping device is a non linear resistor.It is connected between cathode and anode of SCR. The resistance of voltage clamping device decreases with increasing voltages. During normal working condition Voltage clamping (V.C) device has high resistance, drawing only leakage current. When voltage surge appears voltage clamping device offers a low resistance and it create a virtual short circuit across the SCR. Hence voltage across SCR is clamped to a safe value.When surge condition over voltage clamping device returns to high resistance state. e.g. of voltage clamping device (a)Seleniumthyrector diodes (b)Metal Oxide varistors (c)Avalanche diode supressors
• 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: SCR can be protected from overcurrent by using Circuit Breaker (CB) and fast acting current limiting fuses (FACLF). CBs are used for protection of thyristor against continuous overloads or against surge currents of long duration as a CB has long tripping time. But fast-acting fuses is used for protecting SCR against high surge current of very short duration.
• Electronic crowbar protection: As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for overcurrent protection of power converters using SCRS. An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs.
  
  
  
  A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds. The input terminals are then short-circuited by crowbar SCR and it shunts away the converter overcurrent. The crowbar thyristor current depends upon the source voltage and its impedance. After some time, main fuse interrupts the fault current. The fuse may be replaced by a circuit breaker if SCR has adequate surge current rating.

High dv/dt Protection 

• With forward voltage across the anode and cathode of a thyristor, the two outer junctions are forward biased but the inner junction is reverse biasedThis reverse biased junction j₂, has the characteristics of a capacitor due to charges existing across the junction .As we know that current through capacitor follows the relation : i = dQ/dt = d/dt (C_jV_a) = C_j(dV_a)/(dt) + V_a(dC_j)/(dt) As C_j the capacitance of junction J₂ is almost constant, the current is given by i = C_jdV_a/dt  where Q = Charge and i = charging current 
• If the rate of rise of forward voltage d/dt (v_a) is high, the charging current i will be more. This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit.False turn-on of a thyristor by large du/dt can be prevented by using a snubber circuit in parallel with the device.
• Snubber Circuits:A snubber circuit consists of a series combination of resistance R_s and capacitance C_j in parallel with the thyristor as shown in Fig.
  
  
  
  Strictly speaking, a capacitor C_s in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor  C_s behaves like a short circuit therefore voltage across SCR is zero.With the passage of time, voltage across C_s builds up at a slow rate such that dv/dt across C_s and therefore across SCR is less than the specified maximum dv/dt rating of the device.
• Before turning on of thyristor C_s 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 R_s connected in series with the capacitor C_s 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 

• Thyristors are also sensitive to high temperatures, which can cause thermal runaway and device failure. Over Temperature protection circuits are designed to monitor the temperature of the thyristor and trigger a shutdown if the temperature exceeds a safe limit.
• With the increase in the temperature of the junction, insulation may get failed. So we have to take proper measures to limit the temperature rise.
• Protective Measure: We can achieve this by mounting the thyristor on heat sink which is mainly made by high thermal conductivity metals like aluminum (AI), 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 Thyristors

• Gate circuit should also be protected against overvoltages and overcurrents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. 
• Protective Measure:over-voltages Protection against is achieved by connecting a zener diode ZD across the gate circuit. A resistor Rs connected in series with the gate circuit provides protection against overcurrents.A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to false. operation of the main SCR. Gate protection against such spurious firing is obtained by using shielded cables or twisted gate leads. A varying flux caused by nearby transients cannot pass through twisted gate leads or shielded cables. As such no e.m.f. is induced in these cables and spurious firing of thyristors is thus minimised. A capacitor and a resistor are also connected across gate to cathode to bypass the noise signals.

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.
• 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.

FAQ

Q1:What is thyristor protection?

Thyristor or SCR is an intricated semiconductor device and so its protection during operation is quite an important aspect that is needed to be dealt with. For thyristors, it is generally said that the device operates in an efficient manner when the operation takes place under specified ratings.

Q2:What are the different types of SCR protection?

Some of the protection techniques employed for an SCR include over voltage protection, over current protection, dv/dt protection and di/dt protection. Also, to operate the SCR in permissible temperature limits, heat produced at the junctions must be dissipated. This can be accomplished by using heat sinks.

Q3:What is need for protection of thyristors?

A thyristor must be provided following protection, in order to obtain reliable operation.There are several reasons why thyristors need protection:

1. Overvoltage protection: Thyristors are designed to operate within a certain voltage range, and exceeding this range can damage the device. Overvoltage protection circuits are used to limit the voltage across the thyristor and prevent it from exceeding its maximum voltage rating.
2. Overcurrent protection: Thyristors are also designed to handle a certain current range, and exceeding this range can cause overheating and damage to the device. Overcurrent protection circuits are used to limit the current through the thyristor and prevent it from exceeding its maximum current rating.
3. Over-temperature protection: Thyristors can also overheat due to high current or ambient temperature. Over-temperature protection circuits are used to monitor the temperature of the thyristor and prevent it from overheating.
4. Reverse voltage protection: Reverse voltage can cause damage to thyristors, so reverse voltage protection circuits are used to prevent reverse voltage from being applied to the device

Q4:What are local hot spots in a thyristor?

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.