Heatsink calculator: thermal resistance, the junction-to-ambient chain and sizing a sink
Heat behaves like current: it flows from the hot silicon junction out to the air through a series of thermal resistances, and each one raises the temperature by the watts flowing times its °C/W. The package has one (θjc, junction to case), the grease or pad between case and sink another (θcs), and the heatsink the last (θsa, sink to ambient). So Tj = Ta + P × (θjc + θcs + θsa), and turned round, the heatsink may have at most θsa = (Tj − Ta)/P − θjc − θcs. This calculator does that sum for the common packages and interfaces, says what size of heatsink the answer means, whether the bare package would do, and walks the temperatures up the chain for a heatsink you have.
How to use the heatsink calculator
- Type the power the device dissipates and the ambient at the heatsink (inside the box, not the room).
- Pick the package (its θjc and bare θja are filled in; override nothing unless the data sheet differs) and the interface: grease, dry, a silicone pad, mica.
- Set a junction target if you want something other than 25 °C below the limit. The required θsa appears with a size hint; type the θsa of a heatsink you have to see the sink, case and junction temperatures with it, or pick one from the Common heatsinks tab.
The thermal chain
Tj = Ta + P × (θjc + θcs + θsa) θsa ≤ (Tj − Ta)/P − θjc − θcs
Thermal resistance in °C/W is the analogue of electrical resistance, power the analogue of current and temperature of voltage, and the three resistances are in series. The junction is the silicon itself, the case is the metal tab, the sink is the heatsink's surface, the ambient is the air. Each step is hotter than the next by P × θ, and the whole budget, the allowed junction temperature less the ambient, is shared between them. The package's share is fixed by its design; the interface's by how you mount it; only the heatsink is yours to choose, and it gets whatever is left.
Reading the data sheet
Two figures matter. θjc (RθJC) is junction to case, used when the part is on a heatsink: about 1.5 °C/W for a TO-220, 0.5 for a TO-247, 3–4 for an insulated TO-220F, tens for small plastic packages. θja (RθJA) is junction to ambient with no heatsink, the part in free air or on a minimal pad: 60–65 °C/W for a TO-220, 160 for a TO-92, and it says how little a bare package can shed: a TO-220 at 40 °C ambient can lose only about 1.4 W before its junction passes 125 °C. The maximum junction temperature (150 °C for most silicon, 125 for LEDs and many modules) is the absolute limit; design 25 °C or more below it.
| Package | θjc | θja bare | Tj max | Bare power at 40 °C, Tj 125 °C |
|---|---|---|---|---|
| TO-220 | 1.5 °C/W | 62 °C/W | 150 °C | 1.37 W |
| TO-220F insulated | 3.5 °C/W | 62 °C/W | 150 °C | 1.37 W |
| TO-247 / TO-3P | 0.5 °C/W | 40 °C/W | 150 °C | 2.12 W |
| TO-3 metal can | 1 °C/W | 35 °C/W | 200 °C | 2.43 W |
| D²PAK on 1 in² copper | 1 °C/W | 40 °C/W | 150 °C | 2.12 W |
| DPAK on 1 in² copper | 2.5 °C/W | 60 °C/W | 150 °C | 1.42 W |
| SOT-223 on copper | 15 °C/W | 62 °C/W | 150 °C | 1.37 W |
| TO-92 | 80 °C/W | 160 °C/W | 150 °C | 531 mW |
| High-power LED star | 6 °C/W | 45 °C/W | 125 °C | 1.89 W |
| Power module | 0.25 °C/W | 10 °C/W | 125 °C | 8.5 W |
What θsa means in metal
Still air takes heat from a surface at roughly 10 W per square metre per °C, so a heatsink's thermal resistance is about 1 / (10 × area) with the area in square metres counting both faces: a 100 cm² vertical plate (200 cm² of surface) is around 5 °C/W. Fins multiply the area; a small clip-on for a TO-220 is 15–25 °C/W, a 50 mm extrusion about 5, a 100 mm one 2, and below 1 °C/W you need a big block and usually a fan, which raises the 10 to 40 or more and cuts the resistance to a quarter. The calculator's size hint and plate-area figure come from this; real products are better per volume than a flat plate, so take them as the scale of the thing, not a catalogue number.
| Heatsink | θsa | Carries at 40 °C ambient for a TO-220 with grease, Tj 125 °C |
|---|---|---|
| TO-220 clip-on, 13 mm | 24 °C/W | 3.27 W |
| TO-220 clip-on, 25 mm | 18 °C/W | 4.25 W |
| Flag heatsink, 35 mm | 12 °C/W | 6.07 W |
| Finned, 35 × 35 × 12 mm | 10 °C/W | 7.08 W |
| Extruded, 50 × 50 × 25 mm | 5 °C/W | 12.1 W |
| Extruded, 75 × 75 × 30 mm | 3 °C/W | 17 W |
| Extruded, 100 × 100 × 40 mm | 1.8 °C/W | 22.4 W |
| Extruded 150 mm with a 60 mm fan | 0.6 °C/W | 32.7 W |
| Large forced-air block | 0.25 °C/W | 37.8 W |
Interfaces and mounting
Two flat metal surfaces touch only at their high spots; the air in between insulates. A thin smear of thermal grease fills the gaps and brings a TO-220 interface to about 0.5 °C/W; dry it is 1 °C/W or worse. Where the tab must be insulated from the sink (the tab of a TO-220 is usually the collector or drain), a silicone pad costs 1–2 °C/W, mica with grease about 0.8, and the insulated TO-220F package trades its 3.5 °C/W θjc for no insulator at all. Torque the screw as the data sheet says: too loose and the interface is poor, too tight and the package cracks. Mount the heatsink with its fins vertical and air free to enter below and leave above.
Your device, step by step
- Budget: 125 °C − 40 °C = 85 °C; / 10 W = 8.5 °C/W allowed in all.
- Heatsink: 8.5 °C/W − 1.5 °C/W − 0.5 °C/W = 6.5 °C/W (a medium extrusion, 50–75 mm). Bare, the junction would be 660 °C.
- With 6.5 °C/W: sink 105 °C, case 110 °C, junction 125 °C, 0 °C of margin.
Worked example: 10 W in a TO-220 at 40 °C
A regulator or MOSFET in a TO-220 dissipates 10 W inside a box whose air is 40 °C, and the junction is to stay at 125 °C or less. The budget is 125 − 40 = 85 °C, which over 10 W allows 8.5 °C/W in all. The TO-220's junction-to-case resistance is 1.5 °C/W and a greased interface about 0.5, so the heatsink may have at most 8.5 − 1.5 − 0.5 = 6.5 °C/W: a medium extrusion, 50–75 mm, roughly 76.9 cm² of vertical plate. Without any heatsink the TO-220's 62 °C/W would put the junction at 660 °C, far over; the bare package can manage only 1.37 W. With a 3 °C/W extrusion the sink runs at 70 °C, the case at 75 °C and the junction at 90 °C, 35 °C below the target, and that heatsink could carry 17 W before reaching it.
Questions
Can I use the heatsink's own temperature as a check?
Yes: the sink runs P × θsa above the air, and the junction is P × (θjc + θcs) hotter than that. If you can hold your finger on it (about 50 °C), a TO-220 at 10 W has a junction near 70 °C.
Does a bigger heatsink always help?
Until the package and interface dominate: with θjc + θcs of 2 °C/W, even a perfect sink leaves the junction 20 °C above the air at 10 W. Beyond that, split the power across devices.
What about surface-mount parts?
The board is the heatsink: a D²PAK on a square inch of copper reaches about 40 °C/W, better with thermal vias to an inner plane. Use the DPAK/D²PAK/SOT-223 rows and count the copper as the sink.
Why does my part overheat in the box but not on the bench?
The ambient at the heatsink is the air in the box, 20–30 °C above the room, and still air in a box moves less than on a bench. Use the box temperature, and vent it.