Half-bridge (LLC) · topology: how it works

Two MOSFETs take turns to swing the midpoint between 0 and 390 V. Thanks to the resonant capacitor (Cr) and inductor (Lr), the MOSFETs switch at zero voltage, so they barely warm up.

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

The IC drives the two gates in turn, with a short dead time so both MOSFETs never conduct together and short the 390 V rail. It regulates by changing frequency: more load, lower frequency.

Measuring points: tap the numbers on the diagram

Phase 1/2: Q1 on

The upper MOSFET is on: the midpoint goes to 390 V and current flows through Cr–Lr–primary. On the secondary, D1 conducts.

1. Midpoint (Q1 source / Q2 drain)

A square wave between 0 and 390 V at about 50% duty. In an LLC the output is set by frequency, not duty cycle.

On the scope

Multimeter≈195 V average

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Real circuit: circuit tour and fault scenarios

The same topology with every part you'd find on a real board. Follow how it works step by step with the "Circuit tour", or pick a fault scenario below to see what to measure, in order, on the schematic.

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

The IC drives HO and LO HIGH in turn, with a short dead time between so both MOSFETs never conduct together and short the 390 V rail. It lowers the frequency as the load rises and raises it as the load falls.

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

Phase 1/2: Q1 on (HO)

The IC turns Q1 on from HO. The midpoint rises to 390 V; current flows through Cr and Lr into the primary. On the secondary, D1 conducts.

L6599 HO / LO outputs: HIGH (≈12 V, switch on) / LOW (0 V, switch off)

1. Midpoint (Q1 source / Q2 drain)

A 50% duty square wave between 0 and 390 V. In the dead times the voltage swings softly (zero-voltage switching).

On the scope

Multimeter≈195 V (average)

🛠 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

✓Two stacked MOSFETs on the same heatsink (a half-bridge)
✓A large, usually film-type resonant capacitor in the primary path
✓A separate resonant inductor, or leakage inductance built into the transformer
✓A centre-tapped secondary with two diodes (or a dual Schottky)

Where it's used

LED TV power boards (main supply), 150–500 W supplies, server power supplies

Common ICs

L6599 →NCP1396FAN7621TL431 →

Go further

How it works: TV power board Repair Workshop: TV power board

Other topologies

Flyback PFC (power factor correction)Forward Buck (step-down)Boost (step-up)Full bridge (H-bridge) and SPWM Push-pull MPPT solar charge controller