--- title: "SCR from complementary BJTs; turning off SCR" subject: "Advanced Electronics" chapter: 7 key: "semester_4/advanced_electronics:complementary-bjt-scr-turn-off" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/complementary-bjt-scr-turn-off site: BCT Engineering --- # SCR from complementary BJTs; turning off SCR Advanced Electronics · Chapter 7: Introduction to Power Electronics - Asked in: 2070 Chaitra, 2+5 marks - Video: [Commutation Circuit Of Scr - Power Semiconductor Device - Power Electronics](https://www.youtube.com/watch?v=cMutyJmjyWU) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/complementary-bjt-scr-turn-off - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7 ## Question Can two complementary BJTs be used to make SCR? Explain about turning OFF of SCR. ## Short answer Two complementary BJTs can form an SCR latch via regenerative feedback but lack power-handling capabilities. SCR turn-off requires reducing anode current below holding current and applying reverse voltage for at least the turn-off time tq; methods include natural (ine) commutation and forced commutation classes A-E. ## Answer #### Can two complementary BJTs make an SCR? Yes, in principle. An SCR behaves like a PNP and an NPN transistor connected in a regenerative loop (two-transistor analogy): the collector of the PNP drives the base of the NPN and the collector of the NPN drives the base of the PNP. ```text A | PNP (Q1) emitter | \ | +--- base Q1 = collector Q2 C1 | +---+--- G (base Q2) | NPN (Q2) | K ``` A small gate current turns Q2 ON, Q2 turns Q1 ON, and Q1 keeps Q2 ON, so the pair latches like an SCR (IA = (α2·IG + ICBO1 + ICBO2)/(1 − α1 − α2)). Such a discrete latch is used in low-power circuits, but it cannot replace a real SCR in power work because discrete BJTs have lower voltage blocking, poorer surge and dv/dt capability, and need careful biasing. #### Turning OFF of SCR (commutation) Once ON, an SCR cannot be turned off by the gate. To turn it OFF, the **anode current must be reduced below the holding current IH**, and then a **reverse voltage must be maintained for at least the turn-off time tq** (about 10–100 μs) so that the junctions recover their blocking ability. Methods: 1. **Natural (line) commutation:** In AC circuits the current falls to zero at the end of each half cycle and the supply reverses, so the SCR turns off automatically. Used in AC controllers and controlled rectifiers. 2. **Forced commutation:** In DC circuits current never becomes zero naturally, so an extra circuit (L, C and an auxiliary SCR) forces it to zero: - Class A: self commutation by resonating load - Class B: self commutation by LC circuit - Class C: complementary commutation (C or L-C switched by another load-carrying SCR) - Class D: impulse commutation by an auxiliary SCR - Class E: external pulse source 3. **Reducing anode current / opening the circuit:** increasing load resistance or interrupting the anode circuit so that IA < IH. 4. **Reverse biasing:** applying a reverse voltage across anode and cathode. ```text iA |‾‾‾‾‾‾\ | \___ IH ___ | \_____/ reverse recovery vAK |____________ | |___ reverse bias ≥ tq ___|‾‾ forward <------- tq -------> ``` The circuit turn-off time must be greater than the device turn-off time tq; otherwise the SCR turns ON again when forward voltage reappears. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Can complementary BJTs form an SCR-like latch? A. Yes, in principle, PNP and NPN in a regenerative loop; but unsuitable for power use B. No, never C. Yes, and it can replace power SCRs fully D. Only two NPNs can Answer: A. Discrete BJTs have poorer blocking, surge and dv/dt capability. (Key point: BJT pair latch) 2. What is required to turn off an SCR? A. Remove the gate current B. IA below IH and reverse voltage for at least tq C. Apply a negative gate pulse D. Raise the anode voltage Answer: B. The gate loses control once ON. (Key point: Turn-off conditions) 3. Why does natural commutation work in AC? A. The gate turns negative B. Resistance increases C. Current falls to zero at the end of each half cycle as the supply reverses D. Capacitors are added Answer: C. Used in AC controllers and controlled rectifiers. (Key point: Natural commutation) 4. What happens if the circuit turn-off time is less than the device tq? A. The SCR stays OFF B. The gate becomes reverse biased C. IH becomes zero D. The SCR turns ON again when forward voltage reappears Answer: D. The junctions have not recovered blocking ability. 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