Power Electronics and Control Devices
This domain tests semiconductor devices that switch or convert power, and the phase-control calculations that go with them.
Core concepts
- The SCR is a four-layer latching device: a gate pulse turns it on, and it stays on until anode current falls below the holding current. Latching current is higher than holding current.
- The DIAC is a bidirectional trigger device; the TRIAC conducts in both directions and suits AC phase control (lamp dimmers, fan regulators); the UJT makes a simple relaxation-oscillator trigger; the IGBT and power MOSFET handle high-frequency switching.
- Controlled rectifiers use firing angle α: half-wave V_dc = (V_m/2π)(1 + cos α); full-wave V_dc = (V_m/π)(1 + cos α).
- Converters by function: chopper (DC→DC), inverter (DC→AC), cycloconverter (AC→AC), rectifier (AC→DC).
- A snubber (RC network) limits dv/dt; a freewheeling diode clamps inductive spikes.
Worked example
A single-phase half-wave controlled rectifier has V_m = 100 V and α = 60°. V_dc = (100/2π)(1 + cos 60°) = (15.92)(1.5) = 23.9 V. Note that with α = 0 the same formula gives V_m/π = 31.8 V, the maximum, and output falls as α increases.
Traps
Confusing latching and holding current; expecting an SCR to turn off when the gate signal is removed; measuring α from the wrong zero crossing; using the full-wave formula for a half-wave circuit; forgetting that an SCR is unidirectional while a TRIAC is not.