What is bootstrap? How is the upper MOSFET driven?

In half-bridge circuits (LLC, motor drivers, inverters) the upper MOSFET's source rises to hundreds of volts. A small diode and capacitor make the voltage needed to turn it on.

What does it do? Bootstrap makes a 'floating' supply that rides up with the midpoint to drive the upper MOSFET. The capacitor charges while the lower MOSFET is on, and that energy drives the gate while the upper MOSFET is on.

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

Cboot charges while Q2 is on (bottom end at 0 V), then rides up with the midpoint while Q1 is on and powers Q1's gate. That's why bootstrap circuits need the lower MOSFET to switch regularly: the upper MOSFET can't be held on forever.

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: Q2 on: Cboot charges

With the lower MOSFET on, the midpoint is at 0 V. Cboot's bottom end is near ground, so the 12 V supply charges it to ≈12 V through Dboot.

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

1. Midpoint (OUT)

A square wave between 0 and 390 V.

On the scope

Multimeter≈195 V average

🔍 How to spot it on a board

•A small capacitor (100 nF–1 µF) between the driver's VB (or VBOOT) pin and its OUT (VS) pin
•A fast diode from VCC to VB (usually SMD, 600 V+)
•Sometimes a small resistor (a few ohms) in series with the diode

🔧 How to measure it

•Unplugged: measure Dboot both ways on diode mode (≈0.5 V one way, OL the other).
•Remove Cboot and measure it with a capacitance meter.
•Running, VB–OUT should be ≈12 V; this is a floating measurement, use an isolation transformer or a differential probe.

⚠ Common faults

•Dboot open: no HO pulses, only the lower MOSFET runs, no output.
•Cboot lost capacitance: the upper MOSFET doesn't fully turn on, heats up and eventually blows.
•If a MOSFET blew: replacing Dboot and Cboot too is cheap insurance.

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Go further

Where it's used: LLC main supply L6599: the VBOOT and HVG pins Concept: dead time