Power Electronics

Power devices are chosen by voltage/current rating and switching speed: SCRs latch and need commutation, MOSFETs switch fastest at low power, and IGBTs combine easy voltage drive with high-power capability.

A phase-controlled rectifier varies its DC output by delaying the SCR firing angle α; for a single-phase full converter V_dc = (2V_m/π)cosα, which even goes negative (inversion) for α > 90°.

A DC chopper (switched-mode converter) produces a variable DC output from a fixed DC input by pulse-width modulation; the duty ratio D directly sets the output — V_o = DV_s for a buck, V_s/(1−D) for a boost.

An inverter converts DC to AC by switching a bridge; a square-wave output has a fundamental of (2√2/π)V_dc, and PWM pushes the harmonics up to the switching frequency where they are easily filtered.

An AC voltage controller varies the rms voltage delivered to a load by phase-controlling anti-parallel SCRs (or a triac); the output rms for a resistive load follows V_s√[(π−α+sin2α/2)/π].