How it works
A single-phase full-bridge (H-bridge) inverter uses four switches: Q1Q4 ON for positive half-cycle, Q2Q3 ON for negative. Simple square-wave output has fundamental V1 = (4/π)Vdc ≈ 1.27 Vdc — too much harmonic distortion for sensitive loads. A three-phase VSI (voltage source inverter) with 6 IGBTs uses 120° or 180° conduction to produce a stepped output; the phase voltage has harmonics at 5th, 7th, 11th, 13th... (6k ± 1 series). Total harmonic distortion THD = √(V3² + V5² + V7² + ...)/V1; for a square-wave THD ≈ 48%. Sinusoidal PWM compares a sinusoidal reference (50 Hz) with a triangular carrier (typically 2–15 kHz) and switches the IGBT when reference crosses carrier — harmonics shift to sidebands around the carrier frequency, easily filtered.
Key points to remember
Modulation index Ma = Vpeak_reference / Vpeak_carrier; for Ma = 1 (linear modulation), fundamental output = Vdc/2 for a half-bridge. Over-modulation (Ma > 1) increases fundamental but reintroduces low-order harmonics. Dead time (typically 1–4 µs) is inserted between complementary IGBT gate signals to prevent shoot-through (upper and lower devices ON simultaneously, shorting the DC bus). Current-controlled PWM uses hysteresis bands — when actual current exceeds reference + Δi, upper switch turns off; when it drops below reference − Δi, upper switch turns on. Harmonic elimination (selected harmonic elimination, SHE) computes specific notch angles to cancel chosen harmonics.
Exam tip
Every Anna University power electronics paper asks you to derive the Fourier coefficients for a single-phase square-wave inverter output and show that even harmonics are absent due to half-wave symmetry — practise this derivation so you can complete it in under five minutes.