What is SCR and its working?

A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum, Goldey, and Holonyak of Bell Laboratories in 1956.The practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs, which are bidirectional (i.e. charge carriers can flow through them in either direction). SCRs can be triggered normally only by a positive current going into the gate as opposed to TRIACs, which can be triggered normally by either a positive or a negative current applied to its gate electrode.

Thyristor Construction.

A thyristor is a semiconductor device that is widely used as a switch or a controlled rectifier in electronic circuits.A thyristor has characteristics similar to a thyratron tube. Its construction includes four layers of alternating N-type and P-type semiconductor material, forming a P-N-P-N structure.

The four layers of a thyristor are known as the anode layer, the cathode layer, and two additional layers known as the P1 and N1 layers. The anode layer is typically made of P-type material, while the cathode layer is made of N-type material. The P1 and N1 layers are also made of P-type and N-type material, respectively.

The P1 and N1 layers form a junction known as the gate, which is used to control the flow of current through the thyristor. When a voltage is applied to the gate, it triggers a process called "regeneration," in which a small amount of current flows from the anode to the cathode. This current then triggers a much larger flow of current, known as "latching," which continues until the voltage across the thyristor is reduced to zero.

The construction of a thyristor is designed to allow for efficient switching of electrical currents, with a minimal amount of power loss.

COMPARISON OF THYRATRONS WITH POWER TRANSISTOR 

Thyratrons and power transistors are two different types of electronic components that can be used in power applications.

Some comparisons between Thyratrons and Power transistor:

• Operation: Thyratrons are gas-filled tubes that conduct current when a control voltage is applied to their control electrode. They can be used as switches or amplifiers. Power transistors, on the other hand, are semiconductor devices that can be used as switches or amplifiers. They operate by controlling the flow of current through a semiconductor material.
• Power Handling Capability: Thyratrons are typically used in high power applications, where they can handle very high currents and voltages. Power transistors are also capable of handling high power, but their power handling capability is generally lower than that of thyratrons.
• Switching Speed: Thyratrons have a slower switching speed than power transistors, which means that they may not be suitable for applications that require very fast switching. Power transistors, on the other hand, have a fast switching speed, which makes them suitable for high-frequency applications.
• Cost: Thyratrons are typically more expensive than power transistors, mainly because they are larger and more complex. Power transistors are smaller and less complex, which makes them more cost-effective.
• Efficiency: Power transistors are generally more efficient than thyratrons, as they waste less energy in the form of heat. Thyratrons tend to generate more heat and are less efficient.

Thyratrons and Power transistors have different strengths and weaknesses, and the choice between the two will depend on the specific requirements of the application. Thyratrons are typically used in high-power applications where speed is not critical, while power transistors are used in high-frequency applications where speed and efficiency are important.

Main differences between a thyratron and an SCR 

Thyratrons and SCRs (Silicon Controlled Rectifiers) are both types of electronic switching devices that are used to control the flow of current in electronic circuits.

 The main differences between them:

• Operation: Thyratrons are gas-filled tubes that rely on the ionization of gas to conduct current, while SCRs are solid-state devices that use semiconductors to control the flow of current.
• Switching speed: Thyratrons are slower to switch than SCRs, with switching times typically in the range of microseconds to milliseconds, while SCRs can switch on and off in nanoseconds.
• Current rating: Thyratrons can handle higher currents than SCRs, with some types capable of handling currents in excess of 10,000 amps, while SCRs are typically limited to a few hundred amps.
• Voltage rating: Thyratrons are capable of withstanding high voltages, up to several hundred kilovolts, while SCRs are typically rated for lower voltages, in the range of a few hundred volts.
• Applications: Thyratrons are typically used in high-power applications such as industrial welding, high-voltage power supplies, and radar systems, while SCRs are used in a wide range of applications including motor control, lighting control, and power supplies for electronic devices.
• Cost: Thyratrons are generally more expensive than SCRs, due to their larger size, higher current and voltage ratings, and more complex manufacturing process.

Static V-1 Characteristics of a Thyristor

Thyristor is a four-layered semiconductor device that acts as a switch in power electronic circuits. It has three terminals: an anode, a cathode, and a gate.Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. anode and cathode are connected to main source through the load. The gate and cathode are fed from a source E, which provides positive gate current from gate to cathode. 

Thyristor can operate in three modes: 

1. Reverse blocking mode(off-state),
2. Forward blocking (off-state) mode and
3. Forward conduction (on-state) mode

These three modes of operation are now discussed below:

Reverse Blocking Mode

When a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two diodes connected in series. A small leakage current flows. This is the reverse blocking mode. If the reverse voltage is increased, then at critical breakdown level, called the reverse breakdown voltage (VBR), an avalanche occurs at J1 and J3 and the reverse current increases rapidly. SCRs are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region. Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for a reverse blocking SCR is in current-source inverters.

An SCR incapable of blocking reverse voltage is known as an asymmetrical SCR, abbreviated ASCR. It typically has a reverse breakdown rating in the tens of volts. ASCRs are used where either a reverse conducting diode is applied in parallel (for example, in voltage-source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).

Asymmetrical SCRs can be fabricated with a reverse conducting diode in the same package. These are known as RCTs, for reverse conducting thyristors.

Forward blocking mode

In this mode of operation, the anode (+, p-doped side) is given a positive voltage while the cathode (−, n-doped side) is given a negative voltage, keeping the gate at zero (0) potential i.e. disconnected. In this case junction J1and J3 are forward-biased, while J2 is reverse-biased, allowing only a small leakage current from the anode to the cathode. When the applied voltage reaches the breakover value for J2, then J2 undergoes avalanche breakdown. At this breakover voltage J2 starts conducting, but below breakover voltage J2 offers very high resistance to the current and the SCR is said to be in the off state.

Forward conducting mode

An SCR can be brought from blocking mode to conduction mode in two ways: Either by increasing the voltage between anode and cathode beyond the breakover voltage, or by applying a positive pulse at the gate. Once the SCR starts conducting, no more gate voltage is required to maintain it in the ON state. The minimum current necessary to maintain the SCR in the ON state on removal of the gate voltage is called the latching current.

There are two ways to turn it off:

1. Reduce the current through it below a minimum value called the holding current, or
2. With the gate turned off, short-circuit the anode and cathode momentarily with a push-button switch or transistor across the junction.

Thyristor turn-on methods

1. forward-voltage triggering
2. gate triggering
3. dv/dt triggering
4. thermal triggering/Temperature triggering
5. light triggering
6. RC Triggering
7. UJT Triggering

forward-voltage triggering

Forward-voltage triggering occurs when the anode–cathode forward voltage is increased with the gate circuit opened. This is known as avalanche breakdown, during which junction J2 will break down. At sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward current. In this case, J1 and J3 are already forward-biased.

gate triggering

In order for gate triggering to occur, the thyristor should be in the forward blocking state where the applied voltage is less than the breakdown voltage, otherwise forward-voltage triggering may occur. A single small positive voltage pulse can then be applied between the gate and the cathode. This supplies a single gate current pulse that turns the thyristor onto its on state. In practice, this is the most common method used to trigger a thyristor.

dv/dt Triggering

This method is used when high-frequency switching is required. In this method, a rapidly rising voltage is applied across the thyristor, which causes the device to switch to its low-resistance state.

Temperature Triggering/Thermal Triggering

Temperature triggering occurs when the width of depletion region decreases as the temperature is increased. When the SCR is near VPO a very small increase in temperature causes junction J2 to be removed which triggers the device.the thyristor is heated to a specific temperature using a heater element, which causes the device to switch to its low-resistance state.

Light Triggering

 In this method, a light source such as a LED or a laser is used to trigger the thyristor. When the light source is applied to the thyristor's gate terminal, it generates a current that turns on the thyristor.

RC Triggering

In this method, a capacitor is charged through a resistor, and the charged capacitor is then discharged through the thyristor's gate terminal. This causes a pulse of current to flow through the thyristor, turning it on.

UJT Triggering

The Uni-junction transistor (UJT) can be used to trigger a thyristor. When a UJT is connected in series with a resistor and capacitor, and the capacitor is charged to a voltage greater than the UJT's threshold voltage, a pulse of current is applied to the thyristor's gate terminal, turning it on.

Summary

Thyristors are important devices in power electronics and have been widely used in a variety of applications due to their high power handling capabilities, efficient switching characteristics, and low cost.

FAQ

Q1: What is a thyristor?

A thyristor is a semiconductor device that acts as a switch, capable of controlling high voltage and high current in a circuit.

Q2: What are the types of thyristors?

The types of thyristors include silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), and insulated-gate bipolar transistors (IGBTs).

Q3: What is an SCR?

An SCR is a type of thyristor that can control high voltage and high current in a circuit by switching from off to on when a trigger signal is applied to its gate.

Q4: What is a GTO?

A GTO is a type of thyristor that can switch on and off by applying a positive or negative signal to its gate, allowing for greater control over power flow in a circuit.

Q5: What is an IGBT?

An IGBT is a type of thyristor that combines the fast switching capability of a transistor with the high voltage and current handling capabilities of a thyristor.

Q6: What are the applications of thyristors?

Thyristors are used in a wide range of applications, including motor control, lighting control, power supplies, battery chargers, welding, and AC voltage regulation.

Q7: What are the advantages of thyristors?

Thyristors have high power handling capabilities, are durable and reliable, and can be used for a wide range of applications.

Q8: What are the disadvantages of thyristors?

Thyristors have limited control over the amount of power they switch on and off, and can generate high levels of heat during operation.

Q9: How do you test a thyristor?

A thyristor can be tested using a multimeter or an oscilloscope to measure the voltage and current across its terminals and gate. A forward and reverse bias test can also be performed to determine its conductivity.

Q10: What is the difference between a thyristor and a transistor?

A thyristor is a semiconductor device that acts as a switch and can handle high voltage and current, while a transistor is a semiconductor device that amplifies or switches electronic signals.

Q11:Why is a SCR called a Thyristor?

A SCR (Silicon Controlled Rectifier) is called a thyristor because it is a type of semiconductor device that belongs to the thyristor family of devices, which also includes other types such as triacs and diacs. The term "thyristor" comes from the Greek word "thyra," which means door, and "istor," which means "to control." This name reflects the device's ability to control the flow of current in a circuit, acting like a door that can be opened or closed to allow or block the passage of electricity.

Q12:Why do we use a thyristor?

We use a thyristor because it is a semiconductor device that can control the flow of electric current in a circuit, making it useful for applications such as power regulation, switching, and AC voltage control.