Industrial Electronics – I (Thyristor)

 

Understanding Thyristors: Layer Diagram, Characteristics, Operating Principle, and Applications:-

In the world of power electronics, thyristors play a crucial role in controlling and converting electrical power. Since their introduction in the late 1950s, they have been widely used in applications such as motor control, power regulation, and rectification. In this blog, we’ll explore the layer diagram of a thyristor, understand its characteristics, learn its operating principle, and finally see some of its real-world applications.

What is a Thyristor?

A thyristor, also known as a silicon controlled rectifier (SCR), is a four-layer, three-junction semiconductor device. It acts as a switching device, allowing current to flow only when triggered. Unlike a transistor that can be continuously controlled, a thyristor is unique—it stays ON once triggered until the current drops below a certain value.

In simple terms, a thyristor behaves like a one-way valve for electricity:

• In the OFF state, it blocks current.
• When triggered, it turns ON and continues conducting until the current naturally falls.

This latching property makes thyristors highly suitable for power control.

Layer Diagram of Thyristor

The structure of a thyristor is based on four layers of semiconductor material (alternating P and N regions).

• Layer sequence: P – N – P – N
• Junctions formed: J1, J2, J3

Here’s a breakdown:

• Anode (A): Connected to the outer P-layer.
• Cathode (K): Connected to the outer N-layer.
• Gate (G): Connected to the inner P-layer near the cathode.

Thus, a thyristor can be represented by the PNPN structure.

When a positive voltage is applied to the anode with respect to the cathode, junctions J1 and J3 become forward-biased, while J2 remains reverse-biased. This reverse-biased junction keeps the thyristor in the OFF state until a triggering signal is applied to the gate.

Characteristics of Thyristor

The V-I (voltage-current) characteristics of a thyristor explain how it behaves under different conditions.

Forward Blocking Region:

• The anode is positive relative to the cathode.
• J1 and J3 are forward biased, but J2 is reverse biased.
• Only a small leakage current flows.

Forward Conduction Region:

• When a gate pulse is applied, J2 breaks down, and the thyristor starts conducting.
• Once ON, it continues to conduct even if the gate signal is removed.
• Current is limited only by the external circuit.

Reverse Blocking Region:

• If the cathode is positive with respect to the anode, J1 and J3 are reverse-biased.
• Only a small leakage current flows until breakdown occurs.

Holding Current (IH):

• Minimum current required to keep the thyristor in the ON state.
• If the current falls below this value, the thyristor turns OFF.

Latching Current (IL):

• Minimum current required to turn ON the thyristor immediately after triggering.

Key points to remember about thyristor characteristics:

• It has three modes: forward blocking, forward conduction, and reverse blocking.
• Once triggered, it cannot be turned OFF by the gate signal.
• To switch it OFF, the current must drop below the holding current.

Operating Principle of Thyristor

The operation of a thyristor can be understood using its two-transistor analogy.

Imagine the four-layer PNPN structure as two transistors connected back-to-back:

• A PNP transistor (T1)
• An NPN transistor (T2)

When a gate current is applied:

• The NPN transistor (T2) starts conducting.
• This provides a base current to the PNP transistor (T1).
• As T1 conducts, it supplies the base current back to T2.
• A positive feedback loop is established, causing both transistors to remain ON.

This regenerative action explains why a thyristor latches ON once triggered.

• Turning ON: Achieved by applying a small gate pulse.
• Turning OFF: Achieved by reducing the current below the holding current or by applying reverse voltage.

Applications of Thyristors

Due to their ability to handle high voltages and currents, thyristors are extensively used in power electronics. Some common applications include.

Controlled Rectifiers:

• Thyristors are used in AC-to-DC converters.
• By controlling the firing angle, the output voltage can be regulated.
• Widely used in power supplies and battery chargers.

Motor Speed Control:

• In DC and AC motors, thyristors control the supply voltage.
• They enable smooth acceleration, deceleration, and speed regulation.

Light Dimmers and Fan Regulators:

• Domestic light dimmers and ceiling fan regulators often use thyristors.
• By adjusting the firing angle, the power delivered to the load is varied.

Overvoltage Protection:

• In circuits prone to voltage spikes, thyristors act as protective crowbar devices.
• They short the supply in case of overvoltage, protecting sensitive components.

Inverters and Converters:

• Thyristors are used in DC–AC inverters and AC–AC converters.
• Essential for applications like induction heating, welding, and UPS systems.

Phase Control:

• By delaying the triggering angle, thyristors control power flow in AC circuits.
• Useful in heater controls and industrial furnaces.

Conclusion

The thyristor is a fundamental building block of modern power electronics. Its PNPN layer structure, unique switching characteristics, and reliable operating principle make it highly suitable for controlling large amounts of power with small gate signals. From household appliances like light dimmers to complex industrial systems like motor drives and power converters, thyristors continue to be indispensable.

--------------------------Industrial Electronics – I (Thyristor)-4.6 Next Page ------------------------------