Thyristor (SCR)

A Silicon Controlled Rectifier (SCR) is a four-layer PNPN semiconductor device that acts as a bistable switch. It is the most fundamental power device in industrial electronics, used to control large amounts of power with a small gate signal. Understanding its characteristics is essential for GATE and university examinations in power electronics.

Core Concept Explanation

The SCR consists of four alternating semiconductor layers arranged as P-N-P-N, forming three junctions J1, J2, and J3. The three terminals are the Anode (A), Cathode (K), and Gate (G). Under forward bias with no gate signal, J1 and J3 are forward biased but J2 is reverse biased, blocking current flow. This is the forward blocking state.

The two-transistor analogy is the most important model for understanding SCR turn-on. The PNPN structure is split into an NPN transistor (Q1) and a PNP transistor (Q2), where the collector of each drives the base of the other. This forms a positive feedback loop. When gate current triggers Q1, it supplies base current to Q2, which in turn supplies more base current to Q1, until both transistors saturate and the device latches ON.

The condition for latching is that the sum of the current gains must satisfy a1 + a2 >= 1 where a1 and a2 are the common-base current gains of the two equivalent transistors. Once this condition is met, the gate loses control and the device stays ON as long as anode current exceeds the holding current.

Mathematical Expression

The anode current in the SCR can be expressed using the two-transistor model. If IA is the anode current, IG is the gate current, and ICBO is the leakage current of the combined junctions, then the anode current is given by the relation below. The breakover voltage VBO is the forward voltage at which the SCR switches ON without a gate pulse, due to avalanche multiplication at J2.

The latching current (IL) is the minimum anode current required to keep the SCR ON immediately after the gate pulse is removed. The holding current (IH) is the minimum anode current below which the device turns OFF even without gate signal removal. Always IL is greater than IH for a given SCR.

Practical Understanding

In real power circuits, the SCR operates as a controlled rectifier. Once triggered by a gate pulse, it conducts for the remaining part of the positive half cycle in an AC circuit and turns OFF naturally when current falls below holding current at the zero crossing. This process is called natural commutation and is the basis of phase-controlled rectifiers used in motor drives, battery chargers, and HVDC systems.

The SCR can also be turned ON by exceeding the breakover voltage, by excessive rate of rise of anode voltage (dv/dt triggering), by temperature increase, or by light (in LASCRs). Among these, gate triggering is the most controlled and preferred method in practice.

Given:
SCR with holding current IH = 10 mA, latching current IL = 30 mA
Gate current applied IG = 20 mA
Anode supply voltage VA = 200 V, load resistance RL = 50 ohm

Why this formula applies:
At turn-on, minimum anode current must reach IL to latch the device.
IL condition: IA_min = IL = 30 mA

Formula:
IA = VA / RL (once SCR is ON and Vak is approx 1V forward drop)

Substitution:
IA = (200 - 1) / 50 = 199 / 50

Calculation:
IA = 3.98 A

Final Answer:
IA = 3.98 A, which is >> IL = 30 mA, so SCR will latch ON successfully.
If IA drops below IH = 10 mA, SCR turns OFF.

Exam Tip: In GATE problems, remember IL > IH always. If anode current after gate pulse removal is between IH and IL, the SCR turns OFF. Gate current cannot turn OFF an SCR once latched - only reducing anode current below IH achieves turn-off.

Quick Revision

• SCR is a PNPN four-layer device with three terminals: Anode, Cathode, Gate.
• Two-transistor analogy: PNP and NPN pair with positive feedback. Latch condition: a1 + a2 >= 1.
• Latching current IL is required to keep SCR ON after gate pulse removal. Holding current IH is minimum to stay ON during conduction.
• IL > IH always. Gate cannot turn OFF the SCR once latched.
• Turn-ON methods: gate triggering (preferred), dv/dt, breakover voltage, temperature, light.
• Natural commutation occurs at AC current zero crossing when IA falls below IH.
• GATE trap: confusing IL and IH order, or assuming gate can turn OFF the SCR.