Synchronous Rectification vs. Diode Rectification: When Is the MOSFET Worth It?

Design decision · DC-DC converters

A MOSFET can cut rectifier conduction loss, but it adds gate-drive power and timing constraints. A worked estimate helps show when that trade is likely to pay off.

Replacing a rectifier diode with a MOSFET looks like an easy efficiency win: a diode may drop several tenths of a volt, while an enhanced MOSFET presents a small resistance. In a real converter, however, the MOSFET needs a driver and careful timing. At light load, the added overhead can use up the gain.

The useful question is not whether synchronous rectification is more efficient in theory. It is whether it reduces losses across the load range that matters in your design—and whether that improvement justifies the extra control and validation work.

What changes when the diode becomes a MOSFET?

In a buck converter, the inductor current continues during the interval when the main switch is off. A diode provides a passive freewheel path. In a synchronous buck, a controller drives a low-side MOSFET so its channel carries most of that current instead.

The channel’s conduction loss is approximately proportional to current squared and hot on-resistance. A diode’s loss is roughly its forward drop multiplied by current. The MOSFET can reduce the conduction component, especially in a high-current, low-voltage output stage. It also introduces gate-drive loss, body-diode conduction during dead time, and reverse-recovery effects.

Diode and MOSFET freewheel paths in a buck converterTwo simplified buck stages compare a passive freewheel diode with a controlled low-side MOSFET. Both connect the switch node to ground while the inductor delivers current to the output. Diode rectification Synchronous rectification High-side FET SW FreewheeldiodeGND Inductor → output High-side FET SW Low-sideMOSFETGND Passive path; fewer control requirementsLower channel drop; driver and timing required
Figure 1. Simplified comparison of the freewheel path. The MOSFET is actively driven during most of its conduction interval; its body diode may conduct briefly during dead time.

A worked comparison: 24 V to 12 V, 10 A

Consider an idealized buck converter operating in continuous conduction. At 24 V input and 12 V output, the duty cycle is about 50%, so the freewheel interval is also about half a switching period. Assume a 10 A load, a 0.7 V diode drop, and a synchronous MOSFET with 5 mΩ hot on-resistance. These values illustrate the method; they are not a recommendation for a specific device.

For the diode, the first-order conduction estimate is 0.7 V × 10 A × 0.5 = 3.5 W. For the MOSFET channel, it is 10² A² × 0.005 Ω × 0.5 = 0.25 W. The rough difference is 3.25 W of conduction loss that the synchronous design may avoid before its added losses are counted.

Illustrative conduction-loss comparisonFor the stated 24 to 12 volt, 10 amp example, the estimated diode conduction loss is 3.5 watts and MOSFET channel conduction loss is 0.25 watts, excluding added synchronous rectification losses. Conduction loss only · illustrative assumptions Diode3.5 W MOSFET0.25 W 01.753.5 W The gap is not the converter’s net saving; add gate-drive, dead-time and switching losses.
Figure 2. The example leaves a 3.25 W conduction-loss budget for the extra synchronous-rectifier losses before the MOSFET option stops winning on this one operating point.

That 3.25 W is not the predicted efficiency improvement. Add the MOSFET’s gate-drive demand, switching-related losses, body-diode conduction during dead time, reverse recovery and any circulating current. Use the hot RDS(on) at the gate voltage the controller actually provides, and include inductor ripple when estimating RMS current.

When a diode may still be the better choice

A passive diode is often the sensible option when output current is low, the converter runs intermittently, or simplicity and low component count matter more than peak efficiency. It also avoids complementary gate timing and the risk of reverse current caused by an unsuitable control strategy.

At light load, fixed switching and gate-drive losses can take a larger share of total output power. A controller with diode-emulation or pulse-skipping modes may reduce reverse current, but the behavior depends on the controller and operating mode. Compare efficiency across the actual load profile rather than at one full-load point.

If the MOSFET option wins, check these four items

  1. Hot RDS(on) and rectifier RMS current. Use the datasheet value at the intended gate drive and operating temperature, not a room-temperature headline.
  2. Dead-time loss and shoot-through margin. Too much dead time increases body-diode conduction; too little can turn both switches on together. Include propagation delays and tolerances.
  3. Gate charge and reverse recovery. Check driver capability, switching frequency, body-diode Qrr, output-capacitance energy and the resulting switch-node waveform.
  4. Real thermal and voltage conditions. Include measured overshoot, board layout, package, cooling and the required safe-operating margin.

A synchronous rectifier IC or controller provides timing and gate-drive functions; the MOSFET remains the device that carries the power current. They are separate parts, and their voltage, timing and drive specifications must work together.

Use this article with the product pages

This comparison is intended to answer one design question: is the efficiency benefit worth the additional switch and control? For broader device fundamentals, see the two MOSFET guides. For product matching and topology-specific support, continue to the relevant application pages.

MOSFET Solutions for Synchronous RectificationApplication overview and device-matching information MOSFET Solutions for DC-DC ConvertersPower-stage applications and product paths MOSFETs: A Comprehensive OverviewRatings, loss estimates and thermal checks MOSFET Types and ApplicationsChannel polarity, device technologies and common uses

When the MOSFET path is justified, browse the dtPower Semi low-voltage MOSFET range and verify each candidate against its exact datasheet and converter conditions.