TO-220 Fast Recovery Diode: Reliable High-Speed Switching for Power Electronics

A TO-220 fast recovery diode is limited by its mounting interface, not its die. What the 0.70 °C/W junction-to-case path means for usable current.

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TO-220 Fast Recovery Diode: Reliable High-Speed Switching for Power Electronics

A TO-220 fast recovery diode is usually bought for its current rating and then limited by something else entirely: the joint between the tab and the heatsink. The die is specified at 0.70 °C/W junction-to-case in the 30 A, 600 V STTH30RQ06, which means a 27 W dissipation raises the junction 19 °C above whatever the case reaches — and the case temperature is decided by the insulator, the mounting torque and the airflow, not by the part number.

Last updated: October 11, 2026

Key Takeaways

TO-220 is the through-hole workhorse outline for power rectifiers: robust mounting, a metal tab for the heatsink interface, and enough body volume for tens of amps.
The binding constraint is usually the case-to-heatsink interface, not the die. Junction-to-case resistance tells you only the first link of the chain.
In ST’s 600 V ultrafast family the 30 A part is rated 0.70 °C/W junction-to-case and the 60 A part 0.38 °C/W — the current rating is the visible output of that thermal difference.
Surge current, leakage and recovery behaviour all worsen with temperature, so the hot column of the datasheet is the design column.
Left unsaid far too often: a TO-220 part in a long-lived platform needs a lifecycle and second-source answer, not just a datasheet.

What the TO-220 Outline Actually Gives You

TO-220 is chosen for mechanical robustness and heat transfer, which is why it still appears on new designs next to much smaller surface-mount parts. The package is a through-hole plastic body on a metal tab; the tab is the thermal and mechanical interface, and everything about mounting a TO-220 flows from it.

The electrical function of a fast recovery diode in this outline is the same as in any other: rectification, freewheeling, clamping, and blocking in one direction while switching off quickly enough not to dominate the loss budget. The STMicroelectronics STTH30RQ06 is a representative part — a 600 V, 30 A ultrafast rectifier with soft recovery behaviour, intended for high-voltage secondary rectification, switching power supplies, and industrial rectification, freewheeling and clamping duties. It is offered in TO-220AC, D²PAK, D²PAK HV, DO-247 and DO-247 LL, which is itself informative: the same silicon moves across outlines as the mechanical and thermal requirements change.

Where the outline earns its place is at the point where currents reach tens of amps and a surface-mount pad can no longer move the heat. A screw-mounted tab with a heatsink behind it is a much larger thermal channel than a pad on a PCB — provided the interface between the two is specified, which is where the design usually becomes vague.

Following the Thermal Chain, Link by Link

Start with the published conduction-loss coefficients and work outward; each link is a number you can obtain or measure.

For the STTH30RQ06, P = 1.05 × I_F(AV) + 0.026 × I_F RMS². Take 15 A average with a 50 % duty cycle, so I_F RMS = 21.2 A: P = 15.75 + 11.7 = 27.4 W, and the junction sits 27.4 × 0.70 = 19.2 °C above the case. Add 2 °C/W for a mica or silicone insulator and thermal compound, plus the heatsink’s own rise above ambient, and the chain is complete: a 30 °C/W-ambient heatsink would put the case 82 °C above ambient, which at a 40 °C ambient exceeds the 175 °C junction limit once the internal rise is added. The model is only as good as its assumptions — insulator resistance and heatsink performance are the two that get taken on trust — but it makes the point that the diode’s own 19 °C is not the problem.

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 (DO-247 LL: 0.90 °C/W)	0.38 °C/W
Surge non-repetitive forward current I_FSM (10 ms sinusoid)	200 A (TO-220AC, DO-247, DO-247 LL)	425 A (DO-247)
Reverse recovery t_rr (1 A, 30 V, −50 A/µs)	40 ns typ, 55 ns max	50 ns typ, 65 ns max
Leakage at V_RRM, 25 °C / 150 °C	40 µA max / 800 µA max	80 µA max / 1600 µA max

Choosing Between 30 A and 60 A in the Same Voltage Class

When two parts share a voltage rating, the larger one is not simply “more headroom” — it is a different thermal design that changes what the surrounding circuit has to do.

The 60 A device halves the junction-to-case resistance and more than doubles the surge rating, which matters if the circuit has to absorb a capacitor-charging inrush or a fault event. It also brings a higher surge current with fewer package options, and a slightly softer, slower recovery at small-signal current (65 ns maximum against 55 ns), which is a consideration if the converter switches at high frequency with a tight ringing budget. The 30 A part covers TO-220AC as well as the D²PAK and DO-247 outlines, which is what you want if the mechanical design is already fixed around a screw-mounted tab.

The decision rule that follows is straightforward. Size the part from the continuous thermal calculation first; then check the surge requirement against the inrush the circuit can actually present; then compare recovery behaviour at your switching conditions; and only then let the mechanical constraints choose between outlines. Doing it in the reverse order — picking the outline first because the board is laid out — is how projects end up needing a heatsink redesign.

The Boundary Conditions That Cancel a Selection

Three datasheet rows do the cancelling, and all three are temperature-related.

Leakage first: 40 µA maximum at 25 °C becomes 800 µA maximum at a 150 °C junction on the 30 A part, and twice that on the 60 A part. In a hot enclosure that is a real current, and in a high-impedance node it is a real design constraint. Surge second: the 200 A and 425 A figures are single 10 ms sinusoidal events at maximum junction temperature, so they size inrush and fault tolerance rather than continuous duty. Recovery third: the 55 ns and 65 ns maximum figures are quoted at 25 °C, and the power-condition rows elsewhere in the same datasheets show how much the numbers move at 125 °C — which is why a part validated on a cold bench has not been validated.

On top of those sits a non-electrical question that belongs in the same review. A TO-220 rectifier in an industrial or automotive platform may stay in production for a decade, and outlines and families do get superseded. Ask about the part’s lifecycle status and its second-source path while the design is still open, and ask for the package and outline drawing, the thermal derating curves and the qualification evidence for the exact variant while you are at it.

FAQ

What is a TO-220 fast recovery diode used for?

Rectification, freewheeling and clamping in switching power supplies, motor drives, inverters and industrial equipment — anywhere a through-hole package with a heatsink tab has to carry tens of amps while switching off fast enough to keep recovery losses in check.

Why does the current rating drop when the same diode is put in a different outline?

Because the current rating is derived from the thermal path. The same silicon in DO-247 LL is rated 0.90 °C/W junction-to-case instead of 0.70 °C/W, and that difference feeds directly into how much average current can be carried at a given case temperature.

How do I calculate the heatsink I need?

Start from the published conduction-loss equation to get the dissipation, multiply by the junction-to-case resistance, then add the insulator and heatsink resistances and check the total against the ambient and the junction limit. The diode’s contribution is usually the smallest part of the chain.

Does a bigger diode always switch better?

No. Larger devices in the same family can be marginally slower at small-signal conditions — 65 ns against 55 ns maximum in the example here — and they carry more recovery charge at high current. Size for the thermal duty, then verify the recovery behaviour at your actual switching conditions.

What should be confirmed before design release?

The exact outline and its junction-to-case thermal resistance, the derating curves, the surge rating with its test condition, the leakage at the hot end of the range, the qualification evidence for the target market, and the part’s lifecycle and second-source status.

Official Resources

References

STMicroelectronics, STTH30RQ06 — 600 V, 30 A ultrafast high voltage rectifier, DS12164 Rev 5 — source of the 30 A rating at a 103 °C case temperature, the 0.70 °C/W and 0.90 °C/W junction-to-case resistances, the 200 A surge rating, the 40 ns / 55 ns recovery figures, the package list 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 rating and the comparative recovery figures. st.com — STTH60RQ06 datasheet (PDF)
Vishay, Ultrafast Recovery — TO-220 FullPAK — product-family overview confirming the commercial range of ultrafast rectifiers offered in TO-220-style outlines. vishay.com — ultrafast recovery TO-220 FullPAK
Eaton, Understanding fast recovery diodes and their uses, Technical Note ELX1366 — source of the recovery mechanism and the dependence of t_rr on current and turn-off slope. eaton.com — ELX1366 application note (PDF)

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