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Power Electronics8–10 min read

Synchronous Buck Converters

Replace the freewheel diode with a controlled MOSFET to reduce loss while managing dead time and reverse current.

Mental model

A synchronous buck replaces a one-way check valve with a second controlled valve. It wastes less pressure, but both valves must be timed carefully.

+VDCQHQLSWLCoutLOADVout
A controlled low-side MOSFET replaces the freewheel diode to reduce conduction loss.
UNDERSTAND THE BEHAVIOR
01

A conventional buck uses a diode to carry inductor current when the high-side switch is off. A synchronous buck replaces that diode with a low-side MOSFET. When timed correctly, current flows through the MOSFET's low channel resistance instead of a relatively large diode forward drop.

02

The two MOSFET commands are complementary with dead time. During dead time the body diode often conducts briefly. Too little dead time risks shoot-through; too much increases diode loss. Controllers may adapt dead time because the best value changes with devices, current, temperature and layout.

03

At light load, the inductor current may reach zero and try to reverse. Some controllers allow reverse current for continuous conduction; others enter diode-emulation or pulse-skipping modes to improve efficiency. The operating mode affects ripple, noise and whether energy can flow back toward the input.

04

Higher efficiency does not remove switching loss, gate-drive loss, inductor loss or layout requirements. Compare total loss across the expected input, output and load range instead of comparing only diode drop with RDS(on).

Why it matters

Replace the freewheel diode with a controlled MOSFET to reduce loss while managing dead time and reverse current.

Core ideas

  • The low-side MOSFET performs synchronous rectification.
  • Dead time trades shoot-through risk against diode conduction loss.
  • Light-load control determines whether negative inductor current is allowed.
Key equationsDiode loss estimate: P ≈ IavgVFMOSFET conduction estimate: P ≈ Irms²RDS(on)Ideal buck relation: Vout ≈ D × Vin
EXAMPLE

Work through a practical example

1Compare a 5 A freewheel path with VF = 0.5 V against a 15 mΩ MOSFET.

2The diode estimate is 5 × 0.5 = 2.5 W while the MOSFET estimate is 5² × 0.015 = 0.375 W during equal conduction time.

3The real comparison must scale each loss by its conduction interval and add switching, body-diode and gate-drive losses.

4Verify controller behavior at light load and during startup before assuming current always remains positive.

Common mistake

Assuming the low-side MOSFET can simply stay on whenever the high side is off, including conditions where reverse current or startup states are unsafe.

RememberThe low-side MOSFET performs synchronous rectification.
TEST YOUR UNDERSTANDING

Three quick checks

Choose one answer per question, then check your work. Each answer includes an explanation.

1. What replaces the freewheel diode in a synchronous buck?
2. Which loss comparison is a useful first estimate?
3. What may a controller do at light load to avoid negative inductor current?

Practice and check your understanding

Try each question before opening the worked answer.

1. Why can a synchronous buck be more efficient at low output voltage?

Show answer and reasoning

A low-RDS(on) MOSFET can have much less conduction loss than a diode with a fixed forward drop.

2. What carries current during dead time?

Show answer and reasoning

Usually a body diode or another commutation path, so excessive dead time can add loss even though it helps avoid shoot-through.

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Technical references and further reading

References are for further study, not endorsements. Report an error or suggest an improvement.

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