Full bridge (H-bridge) and SPWM · topology: how it works

The heart of inverters, UPSs and motor drives. Four switches in two diagonal pairs flip the direction of current through the load; the pulse width follows a sine (SPWM), and an LC filter turns the pulses into a clean sine.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

In every carrier period the bridge switches between 'power' (400 V) and 'zero'. The power time ratio follows the sine's present value; the inductor and capacitor average the pulses and a sine remains at the output. The two switches in the same leg never turn on together: there's dead time between them.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/4: + half: Q1 + Q4

Q1 and Q4 on: point A is tied to 400 V and point B to ground. Current flows through the load in the + direction. This pulse is as wide as the sine's present value.

SPWM control (G1–G4): HIGH (≈12 V, switch on) / LOW (0 V, switch off) / LOW (0 V, switch off) / HIGH (≈12 V, switch on)

1. G1 (Q1's gate)

SPWM pulses: they widen towards the sine's peak and narrow towards zero. Active only in the + half.

On the scope

Multimeteruse the scope (floating!)

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

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🔍 How to recognise it on a board

✓Four IGBTs or MOSFETs bolted to a common heatsink (several in parallel per position at high power)
✓A big, heavy filter inductor at the bridge output (often a toroid or E core)
✓A large AC-rated film capacitor
✓A gate driver IC or optical gate driver next to each switch

Where it's used

Hybrid and off-grid solar inverters, UPSs, motor drives, welding machines

Common ICs

EG8010 (SPWM üreteci)IR2110 / IR2113 (gate sürücü)TLP250 / TLP350 (optik gate sürücü)DSP / ARM işlemci

Go further

The stage before: push-pull from battery to DC bus How it works: hybrid inverter Concept: dead time Concept: bootstrap

Other topologies

Flyback Half-bridge (LLC)PFC (power factor correction)Forward Buck (step-down)Boost (step-up)Push-pull MPPT solar charge controller