Every SMPS has an output rectifier, and every output rectifier is a choice between three technologies that hide different loss envelopes. A fast-recovery diode, a Schottky, and a synchronous-rectifier MOSFET can all be the “right” answer depending on the output voltage, the switching frequency, and the loss split of the specific converter. This article pairs each rectifier to the converter topology that suits it, compares fast-recovery and Schottky parts at 3.3 V, 12 V, and 48 V outputs, works the reverse-recovery loss at 100 kHz, explains when the diode becomes a switch, and closes with a flyback worksheet that sizes the output rectifier from the numbers.
Which Topology, Which Rectifier: The Pairing Map
The output rectifier does not float in space; it lives at the end of a converter topology, and the topology decides which rectifier family fits. The three common topologies — flyback, forward, and LLC — stress the output rectifier differently.
The flyback converter is the low-cost workhorse below a few hundred watts. Its output rectifier sees the switching pulses of the flyback’s discontinuous step, with a high peak-to-average ratio and a demanding recovery event at every turn-off. The forward converter (a full-bridge or half-bridge rectifying stage) spreads the current more evenly. The LLC converter, popular for high-efficiency designs, operates closer to resonance, and its output rectifier sees a smoother, quasi-sinusoidal current with a different recovery profile. Each topology’s waveform shapes the rectifier’s loss split, which is why the pairing map is the first step of the selection. The rectifier selection for SMPS article and the SMPS guide map the topology and the rectifier roles.
The pairing rule follows from the waveform. A topology with harsh, high-dv/dt turn-offs rewards a fast-recovery part; a topology with high average current at low voltage rewards a Schottky’s low drop; a topology where the efficiency budget is tightest rewards synchronous rectification, despite the added control. The map does not pick one family for all — it assigns each topology the family whose loss envelope fits.
FRD vs Schottky at 3.3 V, 12 V, and 48 V Outputs
The voltage-specific comparison is where the selection becomes concrete, because the output voltage changes the ratio between conduction loss and recovery loss, and that ratio flips the family choice.
At a 3.3 V output — the low-voltage rail of a phone charger or a USB-PD stage — the load current is high for the given power, and the Schottky’s low forward drop dominates: the difference between a 0.3 V Schottky drop and a 0.7 V fast-recovery drop is a large share of the output voltage and a large efficiency difference. The Schottky is the clear answer at 3.3 V.
At 12 V the balance shifts. The output current is lower for the same power, so the conduction term shrinks while the reverse-recovery loss grows in relative weight. A fast-recovery part’s higher drop is now a smaller share of the output, and its faster recovery cuts the frequency-dependent loss; the two families end up close, and the choice depends on the switching frequency and the duty. At 48 V the shift completes: the conduction term is a small share of the delivered voltage, the recovery loss dominates at high frequency, and the fast-recovery or SiC part wins where the switching is fast. The 250 V loss worksheet runs this exact comparison numerically on a high-voltage output, and the low-VF Schottky article and the fast recovery diode guide develop the two families.
The pairing map is best read as a table, with the losses made explicit:
| Output | Conduction weight | Recovery weight | Likely best family |
|---|---|---|---|
| 3.3 V, high current | Heavy (low VF wins) | Light | Schottky |
| 12 V, moderate current | Moderate | Moderate | FRD or Schottky by frequency |
| 48 V, low current | Light | Heavy at high frequency | Fast recovery / SiC |
| Low voltage, high duty, tight budget | Heavy | Light | Synchronous rectifier |
The table is the whole selection in one view. The voltages walk the page from low to high while the best family walks from Schottky toward fast recovery, and the synchronous option sits at the corner where current and budget both justify its control overhead. The 250 V loss worksheet and the low-VF Schottky article provide the numeric anchors that make the table a starting point, not a rule of thumb.
Reverse-Recovery Loss at 100 kHz: The Number to Fear
The reverse-recovery loss is the number that separates the engineering from the catalog picking, and it is worth working through because it explains why the frequency, not just the voltage, decides the rectifier.
Every time the output rectifier turns off, the junction must sweep out the stored charge from the forward conduction. That sweep is the reverse-recovery event, and it dissipates an energy per event — call it the recovery energy. Multiply by the switching frequency and the recovery loss appears. At 100 kHz, a modest recovery energy of a few microjoules per event becomes a measurable watt-level loss; at 400 kHz it becomes the dominant term of the rectifier. The fast recovery diode guide and the diode forward voltage article develop the per-event energy and its frequency multiplication.
The fear is justified only at high frequency. At 50/60 Hz the recovery loss is negligible and a standard part suffices; at 100 kHz and above it is the term that decides the family. The honest selection computes the recovery term at the actual switching frequency rather than assuming it, and the frequency column of the sheet is the one most often skipped. The reverse recovery physics article and the SMPS rectifier selection give the full loss-split method this article’s worksheet uses.
Synchronous Rectification: When the Diode Becomes a Switch
Synchronous rectification is the third family, and its name explains its trick: the output “diode” becomes a switch, a controlled MOSFET that conducts during the forward time and blocks during the reverse time, replacing the diode’s forward drop with an on-resistance loss.
The advantage is the loss. A diode’s VF is a semiconductor junction drop — 0.3 to 1.0 V — multiplied by the current. A MOSFET’s on-resistance loss is RDS(on) times the current squared, which at low voltage and high current is far smaller. The price is the controller and the gate logic: the synchronous rectifier must switch at exactly the right instants, adding a driver, timing, and its own small switching loss. The synchronous rectifier MOSFET article and the MOSFET selection guide develop the control and the ratings.
The synchronous option wins where the current is high and the duty predictable — the low-voltage, high-current outputs. It is complex where the duty is erratic or the control cost dominates, which is why it joins the fast-recovery and Schottky choices as a third column of the same selection rather than a universal upgrade. The three families, paired to the topology and the operating point, are the complete output-rectifier menu.
The worksheet is easiest to show as the three-term comparison it really is. For the 12 V, 10 A, 100 kHz flyback example, the loss sheet reads:
| Candidate | Conduction (VF x I) | Recovery (f x E_rec) | Total | Verdict |
|---|---|---|---|---|
| Schottky | 0.3 V x 10 A = 3.0 W | 100k x 2 uJ = 0.2 W | 3.2 W | Wins at 10 A |
| Fast recovery | 0.7 V x 10 A = 7.0 W | 100k x 2 uJ = 0.2 W | 7.2 W | Loses on drop |
| Standard recovery | 0.7 V x 10 A = 7.0 W | 100k x 10 uJ = 1.0 W | 8.0 W | Worst at frequency |
| Synchronous | RDS(on) x I^2 ~ 0.1 W | Control overhead | 1-2 W + control | Wins where budget is tightest |
The table is the method made numeric: the same output, three families, and three different totals that the datasheet catalog would never show. Re-running the table at 48 V and 2 A changes the conduction terms and the verdict with them, which is why the worksheet — not the family name — is the selection tool.
Sizing the Output Rectifier: A Flyback Worksheet
The worksheet closes the selection by turning the comparison into a calculation. The example is a flyback output at 12 V, 10 A average, switching at 100 kHz — the middle of the comparison where the three families are genuinely close.
Step one, compute the conduction term for each candidate: the Schottky’s VF of 0.3 V times 10 A is 3 W; the fast-recovery’s VF of 0.7 V times 10 A is 7 W. Step two, compute the recovery term: at 100 kHz with a fast-recovery energy of 2 microjoules per event, the recovery loss is 0.2 W; with a standard part at 10 microjoules, it is 1 W. Step three, compare the totals at the operating point and add the synchronous option’s RDS(on) and control overhead. The Schottky’s 3 W conduction dominates at 10 A, so the low-VF part wins this worksheet; at 48 V and 2 A, the conduction terms shrink and the fast-recovery part closes the gap. The 250 V loss worksheet is the worked example of this method, and the rectifier selection for SMPS supplies the three-role context.
The output rectifier is not a catalog pick; it is a loss-split decision. The topology sets the waveform, the voltage sets the conduction weight, the frequency sets the recovery weight, and the three families — fast recovery, Schottky, synchronous — each hide a different envelope. Run the worksheet at the operating point, pair the family to the topology, and the output rectifier stops being a guess. The power MOSFET category and the Schottky category supply the parts the worksheet lands on.