SMA Fast Recovery Diode: Balancing Efficient Rectification and Compact SMA Packaging

The same 1 A ultrafast die is 45 °C/W junction-to-lead in DO-41, 30 in SMA and 25 in SMB. How SMA package choice, recovery and heat decide the part.

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SMA Fast Recovery Diode: Balancing Efficient Rectification and Compact SMA Packaging

The same 1 A, 600 V ultrafast silicon can be specified at 45, 30 or 25 °C/W junction-to-lead depending only on the package it is put in — DO-41, SMA or SMB. That single row of the datasheet is the real argument for choosing SMA: it is not a smaller version of a through-hole diode, it is a different thermal contract with the board, and the converter’s usable current follows from it.

Last updated: October 11, 2026

Key Takeaways

SMA is the JEDEC DO-214AC surface-mount outline. A fast recovery diode in it is chosen for a compact footprint and a low profile on densely populated boards, not because the package improves recovery.
The package sets the thermal limit. In the STMicroelectronics STTH1L06 datasheet the identical 1 A, 600 V device is rated 45 °C/W junction-to-lead in DO-41, 30 °C/W in SMA and 25 °C/W in SMB.
Recovery behaviour must be matched to the converter, not maximised. A switch that is faster than the loop can tolerate buys ringing and EMI rather than efficiency.
Surge capability falls as the package shrinks: the same device is rated 30 A non-repetitive surge in DO-41 and 20 A in SMA and SMB.
Leakage and forward drop both move with temperature, so the 25 °C column is not the design column.

What the SMA Package Actually Is

SMA is a surface-mount rectifier outline — DO-214AC — sized for automated assembly rather than for heat transfer. Its advantage is board area and profile; its cost is thermal.

The Diodes Incorporated US1A–US1M family is a useful reference point for what fits in the outline: a 1.0 A average rectified output current at a 75 °C terminal temperature, reverse voltage grades from 50 V to 1000 V across the family, a 30 A peak surge overload rating, a moulded plastic case with a UL 94V-0 flammability classification, and a moisture sensitivity level of 1 per J-STD-020 — the last of which matters because it means the part does not require the bake-and-return schedule that a moisture-sensitive package does.

What the package does not change is the recovery mechanism. The diode is still a p-n junction that must be cleared of stored charge before it blocks, and the recovery time still depends on the forward current and the rate of change of current at turn-off. SMA is a mechanical and thermal decision; recovery is a silicon decision; and conflating them is the most common way an SMA selection goes wrong.

One Die, Three Packages, Three Thermal Answers

Where the same device is offered in several outlines, the datasheet becomes a direct comparison of packages. The table below is taken from the STMicroelectronics STTH1L06 datasheet, where the DO-41, SMA and SMB variants share one set of electrical ratings.

Parameter (datasheet values)	DO-41 (STTH1L06)	SMA (STTH1L06A)	SMB (STTH1L06U)
Junction-to-lead thermal resistance, R_th(j-l) (L = 10 mm)	45 °C/W	30 °C/W	25 °C/W
Case or lead temperature at 1 A average, δ = 0.5	T_c = 120 °C	T_c = 135 °C	T_c = 145 °C
Surge non-repetitive forward current, 10 ms sinusoidal	30 A	20 A	20 A
Forward rms current	10 A	7 A	7 A
Forward voltage V_F at 1 A, 25 °C / 150 °C	1.3 V max / 0.85 V typ, 1.05 V max (common to all three)
Reverse recovery t_rr / leakage I_R at 600 V	80 ns max / 1 µA max at 25 °C, 75 µA max at 150 °C (common to all three)

Read the first two rows together and the trade becomes explicit. The SMT packages remove the lead length, so the thermal path shortens and the allowed case temperature rises by 15 °C for SMA and 25 °C for SMB against the axial part — while the surge rating drops by a third. Small package, less thermal path, less surge margin: that is the exchange, and it is visible before any measurement is taken.

A Worked Thermal Estimate for an SMA Rectifier

ST publishes the conduction-loss equation for this device, so the estimate does not have to be guessed. For the STTH1L06 it is P = 0.89 × I_F(AV) + 0.165 × I_F RMS².

Assume a square-wave current of 1 A average with a 50 % duty cycle, which makes the RMS current 1 / √0.5 = 1.41 A, and assume the layout keeps the lead at 100 °C.

P = 0.89 × 1 + 0.165 × 1.41² = 0.89 + 0.33 = 1.22 W.
Junction rise above the lead in SMA: 1.22 × 30 = 36.6 °C, giving a junction near 137 °C against a 175 °C limit.
The same power in SMB (25 °C/W) would rise 30.5 °C, and in DO-41 (45 °C/W) 55 °C.

Two caveats belong with those numbers. This is a model built from published coefficients and stated assumptions rather than a bench measurement, and the junction-to-lead figure assumes the lead is held at the quoted temperature — which is the board’s job, not the diode’s. If the copper under the pad is thinner than the measurement condition used for the datasheet, the effective junction-to-ambient path lengthens and the estimate gets optimistic. That is precisely why the datasheet’s thermal row is the first thing to check and the pad area the second.

Matching Recovery to the Converter, Not to the Catalogue

The fastest available diode is not automatically the most efficient choice for a given converter. What matters is whether the recovery event fits the topology.

A recovery that is too slow for the switching frequency turns into conduction-time loss and heat. A recovery that is faster than the surrounding loop can absorb turns the same energy into a sharper current edge, which through the loop’s stray inductance produces overshoot and radiated noise. Eaton’s fast recovery application note frames that trade in exactly these terms: recovery time is tied to the forward current and the turn-off slope, and soft recovery exists precisely because a hard, abrupt edge is a source of electromagnetic interference.

On top of that sit the two boundary conditions that decide whether an SMA part survives its environment. Leakage is specified at 1 µA maximum at 25 °C and 75 µA maximum at 150 °C on this device — a factor of seventy-five, which matters in any high-impedance node. Forward voltage moves the other way, from 1.3 V maximum at 25 °C to 1.05 V maximum at 150 °C, so a part that looks lossy on a cold bench is not the same part that runs hot. Both effects argue for reading the hot column and the derating curve.

FAQ

What does SMA mean in a diode part number?

It names the outline: SMA is JEDEC DO-214AC, a surface-mount rectifier package. In the STTH1L06 family the SMA variant carries the suffix A, and the SMB variant the suffix U, with identical electrical ratings and different thermal behaviour.

Is a smaller package always worse for a rectifier?

No, but it is a different trade. Surface-mount packages remove lead length and so improve the thermal path to the board, which is why the SMA variant tolerates a higher case temperature than the axial one at the same current. What they give up is surge capability: 20 A instead of 30 A on the same silicon.

Should I choose the fastest recovery diode available?

Only if your converter benefits from it. A recovery edge faster than the loop can absorb produces ringing and EMI rather than efficiency. Choose the recovery behaviour that fits the switching frequency, the snubber and the layout.

How much does the PCB copper area matter?

It sets the second half of the thermal path. The junction-to-lead figure assumes a defined measurement condition; less copper under the pad raises the effective junction-to-ambient resistance and the junction temperature with it. Check the pad area against the condition the datasheet used.

What documentation should accompany an SMA rectifier specification?

The datasheet for the exact package variant with its thermal and derating data, the package and outline drawing including the recommended pad layout, and the qualification evidence that applies to your market — the automotive-qualified equivalent included if the design goes into a vehicle.

Official Resources

References

STMicroelectronics, STTH1L06 — Turbo 2 ultrafast high voltage rectifier, Doc ID 8321 Rev 4 — source of the 1 A / 600 V ratings, the 45 / 30 / 25 °C/W junction-to-lead thermal resistances for DO-41, SMA and SMB, the 30 A and 20 A surge ratings, forward voltage and leakage at 25 °C and 150 °C, and the published conduction-loss equation. st.com — STTH1L06 datasheet (PDF)
Diodes Incorporated, US1A–US1M — 1.0 A surface mount ultra-fast rectifier, DS16008 Rev 11–2 — source of the SMA case description, 1.0 A rated output current, 30 A peak surge overload rating, UL 94V-0 case material and J-STD-020 moisture sensitivity level 1. diodes.com — US1A–US1M datasheet (PDF)
Eaton, Understanding fast recovery diodes and their uses, Technical Note ELX1366 — source of the recovery mechanism, the dependence of t_rr on forward current and dI/dt, and the soft-recovery and EMI discussion. eaton.com — ELX1366 application note (PDF)
STMicroelectronics, STTH60RQ06 — 600 V, 60 A ultrafast high voltage rectifier, DS12515 Rev 3 — used as the high-power reference point for the same recovery family. st.com — STTH60RQ06 datasheet (PDF)

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