Core equation
| Heat flow | ΔT = P · R_θ (analogous to V = IR) |
|---|---|
| Junction temp | T_J = T_A + P · (R_θJC + R_θCS + R_θSA) |
| Derating | Target T_J ≤ 0.8 × T_J,max |
Approximate heat transfer coefficients (W/m²·K)
| Mechanism | h |
|---|---|
| Natural convection (air) | 5 – 25 |
| Forced convection (air) | 10 – 500 |
| Natural convection (water) | 50 – 1 000 |
| Forced convection (water) | 300 – 12 000 |
| Boiling water | 2 500 – 25 000 |
| Radiation (painted matte) | 5 – 10 (at 300 K) |
Heatsink rules of thumb
- Orient fins vertically for natural convection.
- Leave 25 mm clearance above/below fins.
- Black-anodize fins for +10–15% radiative boost over bare aluminum.
- Forced air (small fan) typically halves R_θSA.
- Heat pipe inside bigger sinks spreads heat from the die footprint across the full fin area.
- TIM: use just enough to wet the surface — excess increases resistance.
Ballpark TO-220 dissipation
| Setup | R_θJA | Safe P @ 25 °C ambient |
|---|---|---|
| No sink | ~60 °C/W | ~1.5 W |
| Small clip sink | ~20 °C/W | ~4 W |
| Medium extruded | ~8 °C/W | ~11 W |
| Large + fan | ~3 °C/W | ~30 W |
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