GATE EE cheat sheet
GATE EE · Keep the key ideas within reach.
Power Electronics
SCR: turns on when anode–cathode voltage exceeds forward breakover and a gate pulse is applied; once latched it stays on until current falls below holding current; latching current is higher than holding current.
Half-wave controlled rectifier with resistive load: average output V_dc = (V_m/2π)(1 + cos α), where α is the firing angle; fully controlled full-wave gives V_dc = (2V_m/π)cos α.
Single-phase full converter with inductive load and continuous current: V_dc = (2V_m/π)cos α; it operates as an inverter when α > 90° and the load can supply power.
Three-phase fully controlled converter: V_dc = (3V_m/π)cos α = 1.35 V_LL cos α for continuous conduction; ripple frequency is six times the supply frequency.
Buck converter: V_o = D V_in in continuous conduction, where D is the duty cycle; boost converter: V_o = V_in/(1 − D); buck–boost: V_o = −V_in·D/(1 − D).
Chopper control: the output voltage is set by the on-time ratio; in continuous conduction the inductor current never reaches zero, and the ripple depends on switching frequency.
Inverter: a single-phase full bridge produces a square wave of amplitude V_dc; PWM controls the fundamental amplitude and shifts harmonics to higher frequencies; the modulation index m = V_ref/V_carrier.
Thyristor protection: a snubber (R–C) limits dv/dt, a series inductor limits di/dt, and a metal-oxide varistor clamps overvoltage; gate drive must be isolated for high-side devices.
Device comparison: the IGBT combines MOS gate drive with low on-state drop, the MOSFET suits high-frequency low-power switching, and the SCR suits high-power line-frequency phase control.