Thyristors, Rectifiers & Commutation
SCR characteristics, controlled rectifier outputs, firing angle maths and how a latching device is forced to turn off.
Key formulas
Vdc = (2Vm/π) · cos α
Single-phase full-converter DC output: α = 0° gives the uncontrolled value; α = 90° gives zero; α > 90° with an active load inverts power (inverter mode).
Vdc = (3√2·VLL/π) · cos α
Three-phase bridge (6-pulse) output: Six pulses per cycle → much lower ripple than single-phase; the standard industrial DC drive front-end.
Cheat sheet
- ▸SCR: latches on when gate pulse arrives while forward-biased; turns off only when anode current falls below the holding current — the gate cannot switch it off.
- ▸Firing angle α delays conduction: for a single-phase full converter, Vdc = (2Vm/π)·cos α; half converter (with freewheeling diode) Vdc = (√2·Vm/π)·(1+cos α).
- ▸Freewheeling diode: carries inductive load current when the supply voltage goes negative, preventing negative output voltage and improving power factor.
- ▸Commutation classes: Class A (self/LC resonance), B (resonant LC across SCR), C (complementary SCR), D (auxiliary SCR + capacitor), E (external pulse/transformer).
- ▸DIAC/TRIAC handle AC phase control (fan regulators, dimmers); TRIAC = two anti-parallel SCRs with a single gate, weaker at turn-off.
- ▸dv/dt false triggering: a fast-rising voltage across the SCR charges the internal capacitance and fires it — solved with an RC snubber across the device.
- ▸Latching current > holding current: latching must be exceeded at turn-on; holding is the level below which the device drops out.
Half-controlled vs full-controlled bridge
| Aspect | Half-controlled (semi) | Full-controlled |
|---|---|---|
| Devices | 2 SCRs + 2 diodes | 4 SCRs |
| Firing range | α: 0°–180° (one quadrant) | α: 0°–180° incl. inversion (two quadrants) |
| Cost / complexity | Cheaper, simpler gating | Full regenerative braking possible |
| Ripple & PF | Higher ripple, better PF at high α (freewheel path) | Lower ripple, worse PF at large α |
Top 4 campus interview questions
MCQ mastery quiz
1. An SCR turns off when its anode current falls below the…
2. For a single-phase full converter, α = 60° gives Vdc = (2Vm/π)·cos60° = …
3. The main purpose of an RC snubber across an SCR is to limit…
4. Which commutation class uses a charged capacitor switched by an auxiliary SCR?
5. In a three-phase bridge converter, increasing pulse number from 6 to 12 mainly…