How to Select Rectifier Diodes for SMPS?

Select a rectifier diode for SMPS by the job it does in the converter, not by its current rating. Match the diode family to the node — Schottky for low-voltage secondary rectification, ultrafast for higher-voltage switching paths, SiC Schottky for high-voltage and high-temperature designs — then size voltage, current, surge, and thermal margins around the worst-case waveform. The diode that keeps total loss inside every limit wins, and that is rarely the part with the biggest headline number.

Last updated: October 11, 2026

The sections below follow the order a design review actually runs in: circuit role, then voltage stress, then current and thermal limits, then recovery behaviour, then the datasheet checks and the EMI consequences. A 5 A output does not automatically mean a 5 A diode is enough; in a compact SMPS a 6 A to 10 A device is often safer once temperature, airflow, and surge current are included.

Which Diode Family Fits Which Node?

Standard rectifier diodes are built for line-frequency rectification, where recovery speed is irrelevant. In an SMPS switching node the same device turns off slowly, so stored charge flows backwards for a moment and shows up as loss, stress, and noise. Fast and ultrafast diodes are the compromise for medium- and high-frequency converters; Schottky diodes give very low conduction loss and are excellent in low-output-voltage secondaries; SiC Schottky diodes suit high-voltage, high-speed, high-temperature operation.

Family	Where it belongs	What it costs you
Standard silicon rectifier	Mains-frequency input bridge, low-frequency rectification	Slow recovery — unusable in a switching node
Fast / ultrafast recovery	Secondary rectifiers and snubber paths in medium- to high-frequency converters	Higher forward drop than Schottky, recovery still finite
Silicon Schottky	Low-voltage, high-current outputs where conduction loss dominates	Leakage rises with temperature; reverse-voltage range is limited
SiC Schottky	High-voltage, high-frequency, high-temperature power stages	Higher forward drop at low current, higher unit cost

Silicon Schottky is not automatically the best choice. It is excellent for low-voltage outputs, but the combination of leakage and reverse-voltage limits pushes higher-voltage designs toward ultrafast or SiC parts. Conversely, paying for SiC in a low-voltage secondary wastes money on a capability the node cannot use. If the output voltage is low, prioritize low forward voltage; if switching frequency and reverse voltage are both high, prioritize recovery behaviour and thermal strength.

How Much Reverse Voltage Margin Does the Diode Need?

Pick a V_RRM rating with margin rather than a minimum match. The margin has to cover input line variation, leakage-inductance spikes, ringing from parasitics, and repetitive stress over the full temperature range — on the primary side and in noisy switching nodes, all four at once. A diode that barely survives the nominal voltage tends to fail in the field, where the nominal is the exception.

Treat the voltage rating as a floor set by the oscilloscope, not by the schematic. Measure the worst-case reverse waveform at maximum input voltage and maximum load, at the lowest expected temperature (where ringing is least damped), and add the manufacturer’s tolerance on top. Push the probe tip against the diode body with a short ground spring: the spike you are checking for does not survive a long ground lead.

How Do You Size the Current Rating?

The average forward current rating has to exceed the real operating current with thermal headroom, and in SMPS that is only the first of four checks. Peak repetitive current governs pulse stress, non-repetitive surge current governs startup and fault events, and the RMS value under the actual duty cycle governs heating. Ratings are quoted at a case or ambient temperature that your board may never hold, so the thermal path converts the rating into a usable number.

Selection factor	What it affects	Why it matters
Average current	Continuous operation	Prevents overheating in steady state
Surge current	Start-up and fault events	Helps survive capacitor charging and spikes
RMS current	Heating under pulsed load	Important in switching converters
Temperature rise	Reliability	Determines the real usable rating

A real SiC part shows how much the current rating depends on case temperature. onsemi specifies the FFSH2065A, a 650 V, 20 A SiC Schottky diode, at 20 A continuous rectified forward current for T_C < 146 °C but 25 A below 135 °C — the same die, two ratings, decided by how well the package is cooled. The same datasheet gives a non-repetitive forward surge of 105 A for an 8.3 ms half-sine pulse and 1100 A for a 10 µs pulse at 25 °C, falling to 1000 A at 150 °C, with a junction-to-case thermal resistance of 0.82 °C/W. A first-cycle inrush event and a repetitive switching stress are three orders of magnitude apart in duration and must be checked against different lines of the table.

What Does Reverse Recovery Really Cost?

Reverse recovery determines how fast the diode stops conducting when the direction of current through it reverses. A slow diode lets stored charge flow backwards briefly, which produces extra loss, raises switching stress on the MOSFET or transformer winding, and radiates noise. Forward voltage matters for the opposite reason: every volt of drop becomes heat, and at low output voltages a small difference in V_F changes efficiency noticeably.

The recovery contribution to loss can be estimated before you build anything. Take the reverse-recovery charge Q_rr from the datasheet, multiply by the reverse voltage the diode blocks and by the switching frequency, and you have a first-order figure for recovery loss:

P_rr ≈ Q_rr × V_R × f_sw

Assume a hard-switched secondary with Q_rr = 30 nC, a blocking voltage of 60 V, and a 100 kHz switching frequency. Then P_rr ≈ 30 × 10⁻⁹ × 60 × 100 × 10³ = 0.18 W of loss that a Schottky or SiC device would not generate, plus the peak reverse current that the switch has to absorb. Double the frequency and the recovery loss doubles with it, while a SiC Schottky diode contributes effectively none — onsemi describes the FFSH2065A as having no reverse recovery and temperature-independent switching. This is a model with stated assumptions, not a bench measurement; substitute the Q_rr and voltage for the part you are actually considering.

Why Does the Diode Run Hot Even at Low Current?

Thermal design is the hidden reason rectifier diodes fail. High forward voltage, slow recovery, poor PCB cooling, or surge pulses can each push the junction beyond what the nominal current suggests. The four things to check are the maximum junction temperature, the thermal resistance from junction to ambient on your actual copper area, the package and mounting method, and the duration and waveform of any surge event.

Package choice is part of the electrical decision, not an afterthought. Package style affects thermal resistance, mounting, and current handling, and a better package can make a borderline diode viable. onsemi publishes 0.82 °C/W junction-to-case for a TO-247 SiC diode and 635 °C/W junction-to-ambient for a small SOD-323 switching diode — the same order of current through the wrong package becomes a completely different thermal problem. Validate at worst-case ambient, highest input voltage, and maximum load, because that combination is where a datasheet rating stops describing your board.

How Do You Match the Diode to the Topology?

Different topologies stress diodes differently, so the circuit position tells you which parameters matter most. Do not choose the diode by current rating alone; the right device for one node can be the wrong one a few millimetres away.

• Flyback secondary rectifier: fast recovery or Schottky, depending on the reflected voltage and output level.
• Forward converter output: ultrafast recovery is usually important because the diode must recover against a driven winding.
• Buck output stage: low forward voltage and low loss dominate, and synchronous rectification may replace the diode entirely.
• Input bridge rectifier: high reverse voltage and surge strength matter most, and recovery speed rarely does.
• Snubber or clamp path: pulse handling and speed matter, and the energy per event has to be checked against the surge rating.

The same principle explains why a diode’s average current rating matters less than where you put it. A part that is comfortably derated in a 200 kHz flyback secondary may be badly overstressed as a freewheeling diode in a hard-switched buck.

What Should You Read on the Datasheet?

Read the electrical table and the test conditions together, not just the front-page headline. A diode that looks stronger on paper may only reach its rating under ideal cooling or a narrow pulse definition, and a leakage figure quoted at 25 °C says nothing about the same part at 100 °C.

• V_RRM, repetitive peak reverse voltage, and the conditions it is quoted at.
• I_F(AV), average forward current, and the case temperature that rating assumes.
• I_FSM, non-repetitive surge current, with the pulse width and waveform.
• V_F at the forward current the circuit actually runs, across temperature.
• t_rr and Q_rr, because recovery loss scales with both.
• I_R, reverse leakage, at the applied reverse voltage and worst-case junction temperature.
• Thermal resistance and the package limits that decide real usable current.

How Do You Reduce EMI with Diode Choice?

EMI rises when the rectifier turns off too slowly or recovers too abruptly, because that transition excites the parasitic inductance around the loop. Choosing a diode whose recovery behaviour matches the switching speed of the converter can reduce ringing, spikes, and noise without adding a snubber. That said, layout can defeat a well-chosen diode, and a snubber should only be added once the rectifier choice is correct — otherwise it hides a problem instead of solving it.

Practical moves: prefer ultrafast or Schottky devices in high-frequency paths, keep general-purpose diodes out of switching nodes, keep the commutation loop small, and check the recovery waveform rather than only its duration. A cleaner diode often simplifies the whole power stage, shrinking the filter and improving compliance margin.

What Should You Ask a Supplier For?

Verify the choice against documents rather than catalogue averages. For any candidate diode, ask for the full datasheet with the recovery and leakage curves at the operating temperature, the qualification report for the exact part number and package, the pulse and surge test conditions behind the I_FSM figure, and the change-notification process that governs the part over the product’s life. Where the part is destined for an automotive programme, ask which AEC-Q101 qualification the device holds and for the PPAP evidence. Documents that are only “available on request” should be requested before the BOM is frozen, not after.

FAQs

Can I use a general-purpose diode such as a 1N4007 in an SMPS?

Usually not in a switching node. General-purpose diodes are too slow for switching positions, which increases loss, EMI, and heat. In a mains-frequency input bridge, where reverse recovery is irrelevant, a general-purpose rectifier is a reasonable and inexpensive choice.

Which diode should I choose for a rectifier position?

Start from the node, not the part number. Identify whether it is an input bridge, a secondary rectifier, a freewheeling path, or a clamp, then match voltage stress, current and surge limits, recovery behaviour, and thermal path. Schottky and SiC diodes are usually preferred where low loss and fast switching matter.

Is Schottky always the best choice?

No. Schottky is excellent for low-voltage outputs, but leakage and reverse-voltage limits can make ultrafast or SiC better in higher-voltage designs. At elevated junction temperature a Schottky diode’s leakage can also become a significant loss term in its own right.

How much voltage margin should I use?

Use enough margin to cover line variation, ringing, and transients, and confirm it against a measured worst-case waveform rather than a calculated typical one. A conservative design is safer than a minimum-rated design, and the extra cost is usually smaller than a field failure.

Why does the diode get hot even at low current?

High forward voltage, slow recovery, poor PCB cooling, or surge pulses can all raise temperature beyond what the nominal current suggests. Check the RMS current under the real duty cycle and the junction-to-ambient path on your own copper area before assuming the current rating was the problem.

Does package choice matter?

Yes. Package style sets thermal resistance, mounting, and practical current handling, and a better package can turn a borderline diode into a viable one. Compare junction-to-case or junction-to-ambient figures between candidates as part of the selection, not after it.

Official Resources

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Related Articles

• What Is a Switching Power Supply?
• What Are SMPS Components and Their Functions?
• Rectifier Selection for SMPS: Three Roles, Three Different Enemies

References

• onsemi — FFSH2065A Silicon Carbide Schottky Diode, 20 A, 650 V datasheet (Rev. 4, May 2024)
• All About Circuits — Rectifier Circuits
• All About Circuits — Fast, Ultrafast, Soft, Standard, Schottky: Selecting the Right Rectifier
• Power Electronics News — How Synchronous Rectification Enhances Power Converter Performance and Efficiency
• JEDEC JC-22 — Discrete diode and thyristor standardization committee
• EDN — Nexperia Shrinks Schottky Footprint with CFP2-HP

Continue through this engineering topic

For the complete selection, design, verification, and troubleshooting path, explore the Rectifier Selection and Ratings.

• Practical guide: How to Choose the Right Ultra Fast Recovery Rectifier Diode?
• Practical guide: 1N4004G Rectifier Diode: Specifications, Polarity and Applications
• Practical guide: 1N4001G Rectifier Diode: Key Specs and Low-Voltage Circuit Checks
• Ask Good-Ark technical support about this application