Model
| ΔT = P · R_θ | Like Ohm's law — temperature rise for given heat |
|---|---|
| R_θJA | Junction to ambient — no heatsink |
| R_θJC | Junction to case — datasheet value |
| R_θCS | Case to sink (TIM + contact) |
| R_θSA | Sink to ambient — heatsink rating |
| Series | R_θJA = R_θJC + R_θCS + R_θSA |
Typical values
| Element | R_θ (°C/W) |
|---|---|
| TO-220 no heatsink | 60–80 |
| TO-220 small heatsink | 10–20 |
| TO-220 with fan | 1–5 |
| TO-247 big extrusion | 0.5–2 |
| Thermal pad (TIM, 1 mm) | 0.5–2 (1.5 °C·cm²/W) |
| Thermal grease, good | < 0.1 °C·cm²/W |
| BGA package | 5–15 (to PCB) |
Heatsink selection
- Target T_J < 0.8 × T_J,max for reliability (e.g. < 100 °C if rated 125 °C).
- Sizing: R_θSA_max = (T_J,max − T_A − P · R_θJC − P · R_θCS) / P.
- Natural convection limits — above ~10 W per TO-220, add forced airflow or larger sink.
- Mount with TIM — a dry joint is typically 5–10× worse than paste.
Notes
- R_θJA depends heavily on PCB copper area — datasheet values assume specific standards (JEDEC 51).
- Electrical analogy: temperatures are voltages, power is current, R_θ is resistance, thermal capacitance smooths transients.
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