POWER ELECTRONICS – PAGE 7
Silicon Controlled Rectifier (SCR)
1️⃣ Introduction
SCR (Thyristor) is a controlled rectifying device. Unlike a diode, it can be turned ON at a desired time using a gate signal.
2️⃣ Terminals
- Anode (A)
- Cathode (K)
- Gate (G)
3️⃣ Construction
SCR is a four-layer device:
PNPN Structure
It consists of three junctions: J1, J2, J3
4️⃣ Working Principle
Forward Blocking Mode
- Anode positive w.r.t cathode
- Device OFF
- Small leakage current flows
Gate Triggering
- Gate pulse applied
- SCR turns ON
- Large anode current flows
Reverse Blocking Mode
- Anode negative
- SCR behaves like reverse biased diode
5️⃣ Important Parameters
- Latching Current (IL) → Minimum current to turn ON SCR
- Holding Current (IH) → Minimum current to keep SCR ON
- Breakover Voltage
- Forward Voltage Drop
6️⃣ V-I Characteristics
- Forward blocking region
- Breakover point
- Forward conduction region
- Reverse blocking region
7️⃣ Advantages
- High power handling
- Controlled operation
- High efficiency
- Used in converters, inverters, choppers
GATE / PSU Focus: V-I characteristics graph, latching vs holding current difference, triggering concept, PNPN structure.
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