What is dead time?

In a half-bridge the two MOSFETs must never be on at the same time: if they are, 390 V is shorted straight to ground. So the controller waits a very short time after turning one off before turning the other on.

What does it do? Dead time is the short interval when both the upper and lower MOSFETs are off (usually a few hundred nanoseconds). It prevents the short circuit and also lets the inductor current swing the midpoint by itself, so the MOSFETs turn on with no voltage across them.

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

The two gaps between HO and LO are the dead time. The last slice (FAULT) shows the moment both are on: 390 V is shorted.

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

Phase 1/5: Q1 on

The midpoint is at 390 V; current flows from Q1 into the inductor and load.

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

1. Midpoint

In the dead time the voltage transitions with a gentle slope rather than a sharp edge: the sign of soft switching.

On the scope

Multimeteruse the scope

🔍 How to spot it on a board

•The IC driving two MOSFETs in half- or full-bridge circuits (LLC, inverters, motor drivers)
•On some ICs, a resistor that sets the dead time (DT pin)

🔧 How to measure it

•Look at HO and LO with a two-channel scope (HO floats: isolation transformer or differential probe!).
•You should see a short gap at every transition; they must never overlap.

⚠ Common faults

•Dead time too short or missing: both MOSFETs blow and the fuse goes.
•Dead time too long: the body diodes conduct too much and the MOSFETs heat up.
•A faulty gate resistor can make one MOSFET turn off late and cause a short.

🔒 This content is in the repair pack

The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.

Go further

Where it's used: LLC main supply Concept: bootstrap L6599 pins