TO-247 Fast Recovery Diode: Power Electronics Switching With Confidence

In one 600 V family the 30 A TO-247 diode is rated 0.70 °C/W junction-to-case and the 60 A part 0.38 °C/W. How the thermal path sets usable current.

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TO-247 Fast Recovery Diode: Power Electronics Switching With Confidence

Two ultrafast rectifiers can share a voltage class, a technology and a vendor, and still differ by a factor of two in the current they will carry — because the limit is not the silicon, it is the thermal path out of it. In the same 600 V family, the 30 A part is specified at 0.70 °C/W junction-to-case and the 60 A part at 0.38 °C/W. Choosing a TO-247 diode is therefore a package and mounting decision as much as an electrical one.

Last updated: October 16, 2026

Key Takeaways

A TO-247 fast recovery diode is a through-hole discrete power device that switches off quickly, used for high-power rectification, freewheeling and snubber duties in fast-switching stages.
The package is part of the electrical design: junction-to-case thermal resistance, creepage and clearance, and the mounting interface decide the usable current as much as the die does.
Recovery behaviour is a system property. A hard turn-off raises dI/dt, adds EMI and increases the turn-on loss of the switch beside the diode.
Leakage, surge current and recovery time all worsen with temperature, so a part validated on a cold bench has not been validated.
Lifecycle matters for a TO-247 part in a long-lived platform: ask about end-of-life status and second-source strategy before design freeze, not after.

Same Voltage Class, Different Thermal Path

Read the current rating as the output of a thermal calculation, not as a property of the part number. The table below is drawn from two STMicroelectronics 600 V ultrafast rectifiers whose differences are almost entirely thermal and mechanical.

Parameter (datasheet values)	STTH30RQ06	STTH60RQ06
Average forward current I_F(AV)	30 A at T_C = 103 °C	60 A at T_C = 115 °C
Junction-to-case R_th(j-c)	0.70 °C/W (0.90 °C/W for DO-247 LL)	0.38 °C/W
Surge current I_FSM (10 ms sinusoidal)	200 A (TO-220AC, DO-247, DO-247 LL)	425 A (DO-247)
Recovery I_RM at 125 °C, rated current, 400 V, −200 A/µs	8 A typ, 11 A max	12 A typ, 16 A max
Package variants declared in the datasheet	TO-220AC, D²PAK, D²PAK HV, DO-247, DO-247 LL	DO-247, DO-247 LL

Notice what the second row does to the first. Doubling the current capability while cutting the junction-to-case resistance from 0.70 to 0.38 °C/W is what makes the higher rating usable; without the thermal improvement the larger die would simply run hotter at the same current. It also explains the third row, since a device with a bigger thermal mass and a wider die area can absorb a larger single surge.

The Thermal Chain, Done in Numbers

Junction-to-case is the first link only; the mounting interface and the heatsink are the rest of the chain, and they are usually the ones that were never quantified. ST publishes conduction-loss coefficients for both parts, so the first link can be computed rather than guessed.

Take the 60 A part carrying 30 A average with a 50 % duty cycle, and use the published equation P = 1.05 × I_F(AV) + 0.013 × I_F RMS² with I_F RMS = 30 / √0.5 = 42.4 A.

Conduction loss: P = 31.5 + 23.4 = 54.9 W.
Rise from case to junction: 54.9 × 0.38 = 20.9 °C.
If the mounting interface and heatsink hold the case at 100 °C, the junction sits near 121 °C — inside a 175 °C limit with room for the switching-loss contribution on top.

Run the same arithmetic on the 30 A part at half the current, using its own coefficients (P = 1.05 × I_F(AV) + 0.026 × I_F RMS²): with 15 A average and a 50 % duty cycle, I_F RMS = 21.2 A and P = 15.75 + 11.7 = 27.4 W, giving a 19.2 °C rise. The two designs land in the same place thermally while moving very different amounts of current — and note that this is a model from published coefficients with stated assumptions, not a bench measurement. The measured numbers come from the mounting interface, the insulator, the mounting torque and the airflow, which is why those belong in the design review and not only in the assembler’s hands.

Recovery Behaviour Is a System Property

A fast recovery diode does not switch cleanly by itself; it has to switch cleanly inside your loop. Eaton’s fast recovery application note makes the two halves of this explicit.

The first half is that recovery is not instantaneous and is not constant. The junction must be cleared of stored carriers before it can block, and the time that takes depends on how much forward current was flowing and how fast it is forced to stop. Push the turn-off harder and the recovery current peaks higher, which is exactly the condition that couples into stray inductance.

The second half is the noise consequence. A hard, abrupt recovery produces a fast dI/dt, which through the loop’s inductance produces voltage overshoot and radiates EMI; a soft recovery characteristic spreads the same transition over a gentler edge and mitigates the effect. This is the trade a TO-247 selection actually makes: the largest, fastest device is not automatically the quietest choice, and the ringing budget of the layout belongs in the decision alongside the loss calculation.

The same note draws the technology boundary that puts this package in context. Schottky barrier diodes switch extremely fast but generally block no more than about 100 V, so they sit on the low-voltage secondary side; fast recovery diodes take the high-voltage, high-frequency position; and conventional rectifiers handle the mains-frequency bridge. In a high-power stage, the TO-247 device is usually in the middle of that chain, before the transformer or in a power-factor-correction boost path, where the diode’s recovery current adds to the current the switch must turn on into.

What to Settle Before Design Freeze

Four questions cover most of the risk on a TO-247 fast recovery design, and each has a document behind it.

Thermal: the junction-to-case thermal resistance for the exact package variant, including the low-thermal-resistance option if the board needs it, plus the derating curves rather than the headline rating.
Mechanical: the package and outline drawing, so lead spacing, body height, creepage and clearance, and the heatsink interface are fixed before the layout is frozen.
Qualification: the test evidence that applies to your market — automotive, industrial or renewable — for the exact part and package, not for the family in general.
Lifecycle: the part’s status and its second-source path, because a TO-247 device in a long-lived industrial or automotive platform will outlast several product generations.

Where a figure is not published, the honest answer is that it is available on request rather than assumed, and a supplier who can hand over thermal data, package drawings and qualification evidence for the exact part has answered the question that matters.

FAQ

What does a fast recovery diode do?

It rectifies or freewheels like any diode, but it is built to stop conducting quickly. That short reverse recovery time is what keeps switching losses and stress manageable in circuits that switch at tens to hundreds of kilohertz.

Why do two TO-247 diodes in the same voltage class have different current ratings?

Mostly because of their thermal path. The 30 A part in the example family is specified at 0.70 °C/W junction-to-case and the 60 A part at 0.38 °C/W, so the larger device can move more current at a similar junction temperature.

Is a fast recovery diode the same as a Schottky diode?

No. A Schottky barrier diode is a majority-carrier device with a very low forward drop but a blocking capability that generally stops around 100 V. A fast recovery diode is a p-n device that trades some forward drop for high blocking voltage and controlled recovery.

Why does a hard recovery cause EMI?

Because the abrupt current change multiplied by the loop’s stray inductance produces voltage overshoot and radiated noise. Soft recovery spreads the transition and reduces that effect, which is why softness is evaluated alongside recovery time.

What should be checked before the layout is frozen?

The package and outline drawing against the mechanical and creepage requirements, the junction-to-case thermal resistance and derating curves for the exact variant, the qualification evidence for the target market, and the part’s lifecycle status and second-source path.

Official Resources

References

Eaton, Understanding fast recovery diodes and their uses, Technical Note ELX1366 — source of the recovery mechanism, the relationship between t_rr, forward current and dI/dt, the soft-recovery and EMI discussion, the Schottky blocking-voltage limitation, and the diode-type split across a switch-mode supply. eaton.com — ELX1366 application note (PDF)
STMicroelectronics, STTH30RQ06 — 600 V, 30 A ultrafast high voltage rectifier, DS12164 Rev 5 — source of the 30 A rating, 0.70 °C/W and 0.90 °C/W junction-to-case resistances, 200 A surge current, recovery current and the conduction-loss equation. st.com — STTH30RQ06 datasheet (PDF)
STMicroelectronics, STTH60RQ06 — 600 V, 60 A ultrafast high voltage rectifier, DS12515 Rev 3 — source of the 60 A rating, 0.38 °C/W junction-to-case resistance, 425 A surge current, recovery current, leakage and the conduction-loss equation. st.com — STTH60RQ06 datasheet (PDF)
Vishay, Ultrafast Recovery — TO-247 — product-family overview used to confirm the part numbers and voltage/current spread available in this package. vishay.com — ultrafast recovery TO-247
STMicroelectronics, Ultrafast Rectifier Diodes — product-family page used to confirm the current commercial range of the class. st.com — ultrafast rectifier diodes

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