SMB Fast Recovery Diode: A Practical Guide For Fast, Reliable Rectification

SMB fast recovery diodes trade footprint for thermal headroom. What a 2 A part really carries, and how package, recovery and surge decide the design.

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SMB Fast Recovery Diode: A Practical Guide For Fast, Reliable Rectification

Two SMT rectifier outlines look interchangeable on a schematic and are not. Take the same 2 A, 600 V ultrafast silicon in ST’s STTH2L06 family: in SMA it is rated 30 °C/W junction-to-lead, in SMB 25 °C/W, and the case or lead temperature it tolerates at full average current differs by 15 °C between them. SMB is the outline you choose when the design is thermally tight, and the price is board area and surge margin.

Last updated: October 16, 2026

Key Takeaways

SMB is a larger SMT rectifier outline than SMA. It buys a shorter, better thermal path to the board, which is why the same die tolerates a higher terminal temperature at the same current.
In the STTH2L06 family the same 2 A, 600 V device is rated 35 °C/W junction-to-lead in DO-41, 30 °C/W in SMA and 25 °C/W in SMB.
At 2 A average with a 50 % duty cycle the allowed lead temperature is 90 °C in DO-41, 100 °C in SMA and 115 °C in SMB.
Surge capability follows the same ordering: 45 A non-repetitive in DO-41, 35 A in the SMT outlines.
A 2 A, 600 V ultrafast part is a secondary-side or low-power-stage device; higher-current bridge duty needs a different class entirely.

Why the Outline Changes the Rating

The die dictates the recovery behaviour; the outline dictates how much current can be pushed through it before the junction gets too hot. SMB is the wider of the two common SMT rectifier outlines, and the extra footprint is spent on thermal contact.

That is not a marketing claim, it is arithmetic in the datasheet. STMicroelectronics offers the STTH2L06 — a 2 A, 600 V ultrafast diode with a 60 ns maximum reverse recovery time and a 175 °C maximum junction temperature — in DO-41, SMA and SMB, and the three variants carry three different thermal resistances with one shared set of electrical ratings. The electrical part of the comparison is fixed; the thermal part is what moves.

Parameter (datasheet values)	DO-41 (STTH2L06)	SMA (STTH2L06A)	SMB (STTH2L06U)
Junction-to-lead R_th(j-l)	35 °C/W (L = 5 mm)	30 °C/W	25 °C/W
Lead or case temperature at 2 A average, δ = 0.5	T_l = 90 °C	T_l = 100 °C	T_l = 115 °C
Surge non-repetitive forward current, 10 ms sinusoid	45 A	35 A	35 A
Forward voltage at 2 A, 25 °C / 150 °C	1.3 V max / 0.85 V typ, 1.05 V max (common)
Leakage at V_RRM, 25 °C / 150 °C	2 µA max / 12 µA typ, 85 µA max (common)

The pattern repeats across the industry because it is physical rather than proprietary. Where a supplier offers the same 1 A silicon in all three outlines — as ST does with the STTH1L06 — the junction-to-lead resistances come out at 45 °C/W for DO-41, 30 °C/W for SMA and 25 °C/W for SMB, and the surge rating drops from 30 A to 20 A when the lead frame is removed. The lesson for a specification is that the package belongs in the thermal calculation, not in the part-number decoration.

A Worked Estimate for an SMB Rectifier

ST publishes the conduction-loss equation, so the first half of the thermal calculation is arithmetic rather than estimation. For the STTH2L06 it is P = 0.89 × I_F(AV) + 0.08 × I_F RMS².

Assume a 2 A average square-wave current with a 50 % duty cycle, so the RMS current is 2 / √0.5 = 2.83 A, and assume the board holds the lead at 110 °C — just inside the 115 °C the datasheet allows for SMB at that current.

P = 0.89 × 2 + 0.08 × 2.83² = 1.78 + 0.64 = 2.42 W.
Junction rise above the lead in SMB: 2.42 × 25 = 60.5 °C, putting the junction near 170 °C against the 175 °C limit.
The identical dissipation in SMA would rise 72.6 °C and in DO-41 84.7 °C — both beyond the limit at that lead temperature.

The conclusion is uncomfortable and useful: at full rated current this device has almost no junction margin left, whichever outline it sits in. It is a model built from published coefficients with stated assumptions, not a measurement, but the shape is right — 2 A is a working figure for a small ultrafast rectifier in a well-controlled layout, not a comfortable one. If the design cannot hold the lead within a few degrees of the datasheet condition, the honest response is to reduce the average current, share it across two devices, or move up a current class rather than to hope the margin appears.

Where an SMB Fast Recovery Diode Belongs

The 2 A, 600 V class is a control-and-secondary part, and the design review should treat it that way.

Its natural duties are freewheeling and clamping in low-power switching supplies, base-drive and gate circuits, polarity protection, and secondary rectification where the output voltage is above the practical Schottky range. ST describes the family as suited to SMPS and base-drive transistor circuits and to high-frequency inverters, freewheeling and polarity protection — which maps onto the same list.

Two boundaries decide whether it survives there. The first is surge: the 35 A non-repetitive rating is a single 10 ms sinusoidal event, so it covers inrush into a capacitor bank but not a repetitive fault current. The second is leakage at temperature, where the specified maximum rises from 2 µA at 25 °C to 85 µA at 150 °C. Where the node is high-impedance or battery-backed, the hot figure is the one to design against. And where the job is a mains input bridge rather than a secondary rectifier, the class changes: a purpose-designed bridge diode such as the 1200 V STBR3012, with a 0.45 °C/W junction-to-case path and a 300 A surge rating, is the appropriate device rather than a bigger SMB part.

FAQ

What is the difference between SMB and SMA diodes?

Mainly size and thermal path. SMB is the larger of the two surface-mount rectifier outlines, which is why the same die is rated 25 °C/W junction-to-lead in SMB against 30 °C/W in SMA — and why it tolerates a higher lead temperature at the same average current.

Does the SMB outline improve the diode’s recovery speed?

No. Recovery is a property of the silicon and its junction design. The outline changes how much heat the diode can move into the board, and therefore how much current you can run before the junction temperature limits you.

How much current can a 2 A SMB fast recovery diode really carry?

Two amps average with a 50 % duty cycle is the datasheet condition, quoted at a 115 °C lead temperature. At that current the junction sits close to its 175 °C limit, so treat 2 A as the ceiling in a well-controlled layout rather than a comfortable operating point.

When should I stop using a small SMT rectifier and use a bridge diode?

When the duty is mains rectification or another high-surge, high-voltage job. A device designed for bridge service raises the voltage rating, the surge current and the thermal path in one step; for example a 1200 V, 30 A part rated at 0.45 °C/W and 300 A surge.

What should be in the specification before release?

The exact package variant, its junction-to-lead or junction-to-case thermal resistance, the derating curves, the recommended pad layout and copper area, the surge rating with its test condition, and the qualification evidence for the target market.

Official Resources

References

STMicroelectronics, STTH2L06 — High efficiency ultrafast diode, Doc ID 10758 Rev 2 — source of the 2 A / 600 V / 60 ns ratings, the 35 / 30 / 25 °C/W junction-to-lead resistances, the 90 / 100 / 115 °C lead temperatures, the 45 A and 35 A surge ratings, leakage and forward voltage at both temperatures, and the conduction-loss equation. st.com — STTH2L06 datasheet (PDF)
STMicroelectronics, STTH1L06 — Turbo 2 ultrafast high voltage rectifier, Doc ID 8321 Rev 4 — source of the 1 A comparison across DO-41, SMA and SMB (45 / 30 / 25 °C/W; 30 A against 20 A surge). st.com — STTH1L06 datasheet (PDF)
STMicroelectronics, STBR3012 — High voltage rectifier for bridge applications, DS11909 Rev 2 — source of the 1200 V, 30 A, 0.45 °C/W and 300 A surge figures used for the bridge-duty comparison. st.com — STBR3012 datasheet (PDF)
Eaton, Understanding fast recovery diodes and their uses, Technical Note ELX1366 — source of the recovery mechanism and the soft-recovery and EMI discussion. eaton.com — ELX1366 application note (PDF)

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