Thyristor Characteristics: A Deep Dive

 

In our previous post, we explored the world of thyristors, introducing them as powerful switches for high-voltage applications. Now, we delve deeper into their fascinating characteristics, understanding how these devices operate and what sets them apart.

Demystifying the Thyristor Structure: Layers and Junctions

At the heart of a thyristor lies its unique structure. It's a four-layer (PNPN) semiconductor device comprised of three pn junctions. Each junction acts like a gatekeeper, controlling the flow of current. The device boasts three terminals:

• Anode: This is the positive terminal, where current enters the device.
• Cathode: The negative terminal, where current exits the device.
• Gate: This terminal plays a crucial role in controlling the thyristor's on and off states.

Fig 1

the figure depicts the symbolic representation of a thyristor alongside a sectional view of its three pn junctions.

Understanding the Cross-Section:

Imagine splitting the thyristor's cross-section  into two distinct parts: an NPN section and a PNP section  This visualization aids in comprehending how current flows within the device.

Fig. 2

Unlocking the Thyristor's Operational States

Now, let's delve into the different states a thyristor can be in:

• Forward Blocking (Off-State): When a positive voltage is applied to the anode relative to the cathode, junctions J1 and J3 become forward biased, allowing current to flow through them. However, junction J2 remains reverse biased, restricting substantial current flow. In this state, only a minimal leakage current, known as the off-state current (ID), passes through the device.

• Forward Conduction (On-State): If the anode-to-cathode voltage (VAK) is significantly increased, the reverse-biased junction J2 undergoes avalanche breakdown. This breakdown occurs at a specific voltage called the forward break-down voltage (VBO). Since J1 and J3 are already forward biased, a large number of carriers flow freely across all three junctions, resulting in a significant forward anode current.

The thyristor transitions to the on-state, where the voltage drop is minimal (typically around 1V) due to the ohmic resistance within the four layers. An external impedance or resistance (RL) limits the anode current in the on-state .

• Latching Current (IL): To maintain the required carrier flow across the junctions and keep the thyristor conducting, the anode current must exceed a value known as the latching current (IL). If the current falls below this level while reducing the anode-to-cathode voltage, the device reverts to the blocking state.

Latching current (IL) is essentially the minimum anode current needed to sustain the thyristor's on-state after being triggered by a gate signal and subsequently removing the signal.

• Holding Current (IH): Once a thyristor conducts, it behaves like a regular diode, and control over the device is lost. However, if the forward anode current is reduced below a specific level called the holding current (IH), a depletion region forms around junction J2 due to the reduced carrier flow. This reduction in current triggers the thyristor to return to the blocking state.

The holding current (IH) is typically in the milliampere range and is lower than the latching current (IL). In simpler terms, IL is always greater than IH (IL > IH). Holding current (IH) represents the minimum anode current required to maintain the thyristor's on-state.

• Reverse Blocking State: When the cathode voltage is positive relative to the anode, junction J2 becomes forward biased. However, junctions J1 and J3 are reverse biased, resembling two series-connected diodes with reverse voltage applied. In this state, the thyristor is in reverse blocking, allowing only a small reverse leakage current (IR) to flow through the device.

Additional Notes:

• While a thyristor can be turned on by exceeding the forward voltage (VAK) beyond the breakdown voltage (VBO), this method can be destructive. In practical applications, the forward voltage is kept below VBO, and a positive voltage is applied between the gate and cathode to trigger the on-state. 

source:
[1]Power Electronics: Circuits Devices and Applications Mohammad H Rashid, Pearson 4th Edition, 2014