Power Electronics
Power Electronics examines switching devices and the converters built from them: rectifiers, choppers, inverters, AC controllers, and cycloconverters. GATE EE focuses on average and RMS output expressions and on device commutation.
Devices
- An SCR latches on a gate pulse and turns off only when anode current falls below the holding current.
- MOSFET suits high frequency; IGBT suits high voltage and high power.
Controlled rectifiers
- Single-phase full converter with an R load: V_dc = (2V_m/π)cos α.
- A freewheeling diode clamps the output at zero and prevents negative excursion for α > 90°.
Choppers and inverters
- Buck: V_o = D V_s. Boost: V_o = V_s/(1 − D).
- Single-phase square-wave bridge inverter: fundamental RMS = 2√2 V_dc/π; PWM shapes the fundamental and pushes harmonics higher.
Worked example: a single-phase full-wave controlled rectifier is fed from 230 V RMS, so V_m = 325 V, with α = 60° and an R load. V_dc = (2 × 325/π)cos 60° = 206.9 × 0.5 = 103.5 V. The average is reduced by cos α because conduction only starts at the firing angle; the RMS value is not equal to V_dc and must be obtained from the integral whenever heating or device rating is asked.
Traps that cost marks
- Using 311 V for a 220 V RMS supply, or 325 V for 230 V RMS - confirm which is given.
- A purely inductive load needs a freewheeling path, or the single thyristor cannot turn off.
- Ripple frequency is 2f for full-wave and f for half-wave converters.