In every switching converter you've studied — buck, boost, flyback, forward — there's a rectifier that conducts when the main switch is off. Traditionally that's a Schottky diode dropping 0.3–0.5 V. At 10 A output, that's 3–5 W burned as heat, capping efficiency around 90%. Synchronous rectification replaces that diode with a MOSFET driven actively in antiphase to the main switch, turning the drop into I·R_DS(on).

Why it matters: A modern 30 V N-channel MOSFET has R_DS(on) of 3 mΩ. At 10 A, that's a 30 mV drop and only 0.3 W dissipation — a 10× improvement. Modern point-of-load converters hit 96%+ efficiency this way, which is why every laptop VRM uses it.

The mechanics in a buck converter: The high-side MOSFET (Q1) conducts during the on-time, dumping current into the inductor. During the off-time, the inductor current must continue flowing. Instead of forward-biasing a freewheeling diode, you turn on a low-side MOSFET (Q2) that shorts the switch node to ground. The MOSFET conducts in its third quadrant (drain-to-source current reversed), but it doesn't care — it's just a resistive channel.

The critical timing problem — shoot-through: If Q1 and Q2 are ever on simultaneously, you've created a dead short from V_IN to ground. The result is hundreds of amps for tens of nanoseconds — instant MOSFET death. The solution is dead time: a deliberate 10–50 ns gap where both gates are low. During this gap, the body diode of Q2 conducts (with its ugly 0.7 V drop), so you want dead time as short as possible without risking overlap.

Rule of thumb for sizing: Choose R_DS(on) such that conduction loss equals switching loss at your nominal load. For a 5 A buck at 500 kHz with a 30 V FET, that typically puts you around 5–10 mΩ. Going lower wastes gate charge (Q_g rises) and gains nothing.

Real-world example: The TI TPS54560 is a non-synchronous buck (uses a Schottky) — fine for prototyping. Its sibling the TPS54302 is synchronous, with both MOSFETs integrated. At 3 A output and 12 V→3.3 V conversion, the synchronous part runs 93% efficient versus 87% for the diode version. That 6% on a 10 W converter saves 700 mW — enough to ditch the heatsink.

One gotcha: Synchronous converters can sink current at light loads (the low-side FET pulls inductor current negative), which wastes energy. Better controllers detect this and drop to diode emulation mode (DCM), turning Q2 off when current crosses zero.