Synchronous Rectification: Replacing the Diode with a MOSFET

In a low-voltage, high-current converter, the rectifier can become one of the largest sources of power loss.

A conventional diode is simple: current flows automatically when the device is forward biased. But that simplicity comes with a forward-voltage drop.

Synchronous rectification replaces the diode with a controlled MOSFET so that current flows through a low-resistance channel instead.

The basic attraction is easy to see:

Pdiode ≈ Iavg × VF

versus

Pmosfet ≈ Irms² × RDS(on)

When output voltage is low and current is high, the MOSFET can reduce conduction loss dramatically.

But the MOSFET introduces something the diode did not need: timing.

That is why synchronous rectification is best treated as a system design problem rather than a simple component substitution.

1. Why diode loss becomes painful at low output voltage

Consider a 5 V output carrying 10 A.

If the rectifier diode has an effective forward drop of 0.5 V while conducting, the instantaneous conduction loss can be around:

P ≈ 10 A × 0.5 V = 5 W

Five watts is a substantial penalty in a 50 W output stage.

Now consider a MOSFET with an effective hot RDS(on) of 5 mΩ:

P ≈ 10² × 0.005 = 0.5 W

This simplified example ignores duty cycle, switching transitions, gate-drive loss and parasitics, but it shows why synchronous rectification is attractive.

The lower the output voltage, the larger a fixed diode drop becomes as a percentage of delivered power.

2. The efficiency gain depends on more than RDS(on)

A synchronous MOSFET should not be selected by resistance alone.

The relevant design parameters include:

• VDS margin;
• hot RDS(on) at the actual gate voltage;
• gate charge Qg;
• body-diode forward drop;
• reverse-recovery charge Qrr;
• package resistance and source inductance;
• thermal performance.

At high current, every milliohm in the path matters. PCB copper, transformer winding resistance, vias and connectors can become comparable to the MOSFET channel resistance.

So the useful question is not only “how low is the MOSFET resistance?” but “how low is the resistance of the complete secondary current loop?”

3. The MOSFET has to turn on at the right moment

A diode self-commutates. A MOSFET does not.

The MOSFET should be enhanced when current is flowing in the intended rectification direction and should turn off before current reverses excessively.

Poor timing can produce:

• unnecessary body-diode conduction;
• reverse current from the output back into the transformer or switching node;
• shoot-through in complementary stages;
• extra switching loss;
• ringing and EMI;
• higher device temperature.

The efficiency gain therefore depends on channel conduction time minus the penalties introduced by switching and timing errors.

4. Dead time is necessary — but expensive if excessive

In many synchronous power stages, a short dead time is inserted so that two devices are not simultaneously on.

During that dead time, current may flow through the MOSFET body diode.

Too little dead time can cause cross-conduction or shoot-through.

Too much dead time increases body-diode conduction and can make reverse recovery worse.

A useful way to frame the trade-off is:

Dead time must be long enough for safe commutation, but short enough that the body diode does not become the rectifier again.

Modern controllers may use adaptive timing to reduce this compromise.

5. Reverse recovery still matters

It is easy to assume that once the diode has been “replaced,” diode behaviour is irrelevant.

In reality, the intrinsic body diode may still conduct during startup, dead time and switching transitions.

When current commutates away from the body diode, stored charge can create a reverse-recovery current pulse.

That can increase:

• turn-on loss in the opposing switch;
• current stress;
• voltage overshoot;
• ringing;
• electromagnetic interference.

For higher-frequency designs, reverse-recovery behaviour can materially affect the expected efficiency benefit.

6. Gate-drive strategy depends on topology and cost target

Synchronous rectification can be implemented in several ways.

Common approaches include:

• a dedicated synchronous-rectifier controller;
• controller-integrated secondary-side drive;
• transformer- or winding-derived drive;
• self-driven or semi-self-driven approaches in selected converter topologies.

A dedicated controller can provide better timing control and protection, but it adds cost and circuitry.

A winding-derived or self-driven method can reduce component count, but its timing is more dependent on transformer waveform, load and topology.

The best choice depends on:

• output power;
• switching frequency;
• required efficiency;
• isolation architecture;
• BOM target;
• available gate-drive voltage;
• operating range.

7. Secondary-side synchronous rectification has a strong thermal benefit

The value is not only electrical efficiency.

Reducing rectifier loss can also reduce:

• local temperature rise;
• heatsink requirement;
• PCB thermal stress;
• enclosure temperature;
• required airflow.

That can translate into a smaller or thinner power supply.

In compact chargers, LED power supplies and consumer appliances, a few watts of removed loss can be as important mechanically as it is electrically.

8. Layout becomes part of the switching design

Fast secondary currents can produce high di/dt.

Parasitic inductance in the loop creates voltage according to:

V = L × di/dt

A MOSFET with excellent silicon performance can still ring badly if the current loop is large.

Good layout practice includes:

• minimizing the rectification loop area;
• keeping source and gate-return paths controlled;
• reducing common-source inductance;
• placing the driver close to the MOSFET where practical;
• using wide, low-resistance copper for the output-current path;
• controlling transformer leakage and secondary interconnect length.

Waveform measurement should be performed with appropriate probing because a long oscilloscope ground lead can itself exaggerate ringing.

9. When is synchronous rectification worth it?

The benefit is usually strongest when:

It may be less attractive when output current is very low, when gate-drive overhead dominates, or when the topology makes safe timing difficult.

10. A practical design sequence

A good synchronous-rectifier design flow is:

1. estimate the diode loss in the original design;
2. estimate MOSFET channel loss using hot RDS(on);
3. add gate-drive and switching loss;
4. verify VDS margin including ringing and overshoot;
5. examine body-diode and reverse-recovery behaviour;
6. optimize turn-on, turn-off and dead time;
7. minimize the secondary current loop and gate-drive loop;
8. measure efficiency, waveforms and temperature across the load range.

The final validation should include light load as well as full load because reverse current or poor timing can appear at operating points that are not obvious from the nominal condition.

Key takeaway

Synchronous rectification can turn a lossy diode drop into a much smaller resistive drop, which is why it is so effective in low-voltage/high-current conversion.

But the MOSFET only delivers that benefit when device selection, gate timing, body-diode behaviour, layout and thermal design work together.

Done well, synchronous rectification improves efficiency, reduces temperature and enables higher power density. Done poorly, the timing penalties can quickly erase the expected gain.