---
title: "Thyristor turn-on and forced commutation"
subject: "Power Electronics"
chapter: 1
key: "semester_7/power_electronics:thyristor-turn-on-forced-commutation"
source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-turn-on-forced-commutation
site: BCT Engineering
---

# Thyristor turn-on and forced commutation

Power Electronics · Chapter 1: Characteristics and specification of power electronic devices

- Asked in: 2074 Asoj, 8 marks
- Video: [Turn OFF Methods of SCR / Commutation Methods of SCR in Power Electronics by Engineering Funda](https://www.youtube.com/watch?v=DuhP_Rhm5fE) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-turn-on-forced-commutation
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1

## Question

Discuss a method of thyristor turn ON mechanism. Also explain about thyristor force commutation techniques.

## Short answer

Thyristor turn-on is normally by a positive gate pulse: gate current raises alpha2, regenerative action drives alpha1+alpha2 to 1, and anode current latches above latching current. Forced commutation in dc circuits uses an external L-C/auxiliary thyristor to force anode current to zero and reverse bias the SCR longer than tq; classes include A-F.

## Answer

A thyristor is turned ON by making its anode current exceed the latching current while it is forward biased. The usual and best method is **gate triggering**; once ON, it can be turned OFF in a d.c. circuit only by **forced commutation**.

#### Methods of turn-on

1. **Forward voltage triggering:** raising $V_{AK}$ above $V_{BO}$ (not used; may damage the device).
2. **Gate triggering:** a positive gate pulse between gate and cathode (normal method).
3. **dv/dt triggering:** a fast rise of anode voltage drives a capacitive current through $J_2$ (unwanted).
4. **Thermal triggering:** high temperature raises leakage (unwanted).
5. **Light triggering:** light falling on the junction creates carriers (LASCR, used in HVDC).

**Two-transistor analogy (gate turn-on mechanism):** the PNPN structure is split into a PNP transistor $Q_1$ and an NPN transistor $Q_2$ connected so that the collector of each drives the base of the other.

```text
          A
          |
        [Q1 pnp] E
          |  \
       B1 |   C1 ----+
          |          |
    +---- C2      B2 +----- G
    |  [Q2 npn]
    |     E
    |     |
    +-----K
```

With common-base gains $\alpha_1$, $\alpha_2$ and leakage currents $I_{CBO1}$, $I_{CBO2}$,
$$
I_A = \frac{\alpha_2 I_g + I_{CBO1} + I_{CBO2}}{1 - (\alpha_1 + \alpha_2)}
$$

When a gate current $I_g$ is injected, the emitter current of $Q_2$ rises, $\alpha_2$ increases, its collector current feeds the base of $Q_1$, whose collector current in turn feeds $Q_2$. This regenerative (positive feedback) action drives $\alpha_1 + \alpha_2 \to 1$, the anode current rises sharply and both transistors saturate: the SCR is latched ON. After that the gate can be removed.

#### Forced commutation techniques

In d.c. circuits the anode current never falls to zero by itself, so an external circuit (L, C and often an auxiliary thyristor) forces it to zero and reverse biases the SCR for longer than its turn-off time $t_q$.

| Class | Name | Principle |
|---|---|---|
| A | Self (load) commutation | Series LC with load; current oscillates to zero |
| B | Resonant pulse | LC across SCR; reverse resonant current cancels load current |
| C | Complementary | Firing a second SCR puts a charged capacitor across the first |
| D | Impulse (auxiliary) | Auxiliary SCR switches a charged capacitor across the main SCR |
| E | External pulse | Pulse from an external source reverse biases the SCR |
| F | Line (natural) | A.C. supply reversal (not forced) |

```text
 Class D (impulse) commutation
        +---- C ------[TA]---+
        |  (-   +)           |
 +Vdc --+--------[T1]--------+--- load ---+
                                          |
 GND -------------------------------------+
```

Firing $T_A$ places the charged capacitor across $T_1$ with reverse polarity; $T_1$'s current is diverted, it turns OFF, and the load current then charges $C$ until $T_A$ also turns OFF.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which turn-on method is the normal and preferred one?
   A. Forward voltage triggering above VBO
   B. Gate triggering with a positive gate pulse
   C. dv/dt triggering
   D. Thermal triggering
   Answer: B. Gate triggering is the normal method; the others are unwanted or may damage the device. (Key point: Normal turn-on method)

2. In the two-transistor analogy, what causes the SCR to latch ON?
   A. Regenerative positive feedback drives alpha1 + alpha2 toward 1, so anode current rises sharply
   B. Alpha1 + alpha2 falls to zero
   C. The gate keeps supplying current forever
   D. The leakage currents cancel
   Answer: A. The collector of each transistor drives the base of the other; alpha1 + alpha2 approaches 1 and both saturate. (Key point: Two-transistor regeneration)

3. In IA = (alpha2*Ig + ICBO1 + ICBO2)/(1 - (alpha1 + alpha2)), what happens as alpha1 + alpha2 approaches 1?
   A. IA goes to zero
   B. IA rises very large, so the SCR turns ON
   C. IA equals Ig
   D. IA becomes negative
   Answer: B. The denominator tends to zero, so the anode current rises sharply and the SCR is latched ON. (Key point: Anode current expression)

4. In Class D (impulse) commutation, how is T1 turned OFF?
   A. Firing TA places the charged capacitor with reverse polarity across T1, diverting its current
   B. The supply is switched off
   C. The gate of T1 is reversed
   D. The load is shorted
   Answer: A. The capacitor reverse biases T1 and diverts its current; the load current then charges C until TA also turns OFF. (Key point: Class D commutation)

## Related in Power Electronics

- [Natural vs forced commutation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Anatural-vs-forced-commutation) — Power Electronics, chapter 1
- [Self and line commutation; minimum gate pulse width](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aself-line-commutation-gate-pulse) — Power Electronics, chapter 1

## Related in other subjects

- [Mechanism AOB driven by gear D, tangential force](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_mechanics_dynamics%3Amechanism-aob-gear-d-tangential-force) — Applied Mechanics (Dynamics), chapter 5
- [Natural and forced convection compared](https://www.bctengineer.com/question.md?key=semester_5%2Fheat_transfer%3Afree-versus-forced-convection) — Heat Transfer, chapter 3
- [Forced damped two-degree-of-freedom system](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Adamped-forced-two-dof-equations) — Theory of Mechanism and Machine II, chapter 7