Thyristors, TRIACs & Latching Semiconductor Switches
Thyristors use regenerative internal feedback to latch on after a gate trigger; conduction persists until current falls below the holding condition or an external commutation path forces it off.
A thyristor is a latching semiconductor switch. A short gate pulse initiates regenerative carrier action; after anode current exceeds the latching current, the gate no longer controls conduction. The device turns off only when current is forced below its holding current long enough for stored charge to clear.
The regenerative mechanism #
The four-layer p–n–p–n structure can be modeled as coupled PNP and NPN transistors. Each transistor's collector current drives the other's base. When the combined current gains approach unity, positive feedback drives both into conduction.
stateDiagram-v2 [*] --> Blocking Blocking --> Conducting: gate trigger + forward bias Conducting --> Conducting: anode current > holding current Conducting --> Recovery: current forced below holding current Recovery --> Blocking: stored charge removed
Latching current is the minimum anode current needed just after triggering so conduction survives removal of gate drive. Holding current is the lower current below which an established conduction state collapses. Confusing them produces unreliable turn-on near light load.
Device variants #
- An SCR conducts one main-current polarity after a gate trigger.
- A TRIAC is bidirectional and can control both halves of an AC waveform, but triggering sensitivity and commutation differ by quadrant.
- A DIAC is a bidirectional breakover trigger often used with a TRIAC.
- Gate-turn-off and integrated gate-commutated devices add turn-off authority at the cost of substantial gate-current and drive requirements.
Commutation and protection #
Line-frequency AC naturally crosses zero, so a line-commutated SCR or TRIAC may turn off every half-cycle. DC circuits require forced commutation or a turn-off-capable device. Inductive current does not vanish at voltage zero; load phase and stored energy determine whether current has actually fallen below holding current.
Fast can couple displacement current through internal capacitance and cause false turn-on. Excess can concentrate current before conduction spreads across the die. Snubbers, gate impedance, layout, minimum pulse width, and current rise must follow the specific datasheet.
Use thyristors when their latching and high-power strengths match the energy path: controlled rectifiers, AC phase control, crowbars, and very high-power conversion. For freely commanded high-frequency switching, MOSFETs and IGBTs usually offer a more direct control contract.