Power Dissipation Calculator
Calculate junction temperature and thermal management requirements for components.
Total power dissipated by component
Operating environment temp
From component datasheet
From component datasheet (typically 125-150°C)
Thermal Formula
Typical Thermal Interface Materials
| Material | θCS Typical | Notes |
|---|---|---|
| Thermal Paste | 0.2-0.5 °C/W | Best for flat surfaces |
| Thermal Pad | 0.5-2.0 °C/W | Gap filling, easy assembly |
| Phase Change | 0.1-0.3 °C/W | Melts during operation |
| Thermal Tape | 1.0-3.0 °C/W | Adhesive mounting |
| No Interface | 1.0-5.0 °C/W | Air gap, poor contact |
Understanding Thermal Resistance
Heat flows from the component junction (hottest point) to the ambient air through a series of thermal resistances:
Thermal Resistance Chain
- θJC (Junction-to-Case): Internal resistance of the package
- θCS (Case-to-Sink): Thermal interface material
- θSA (Sink-to-Ambient): Heatsink performance
- θJA (Junction-to-Ambient): Total without heatsink
Key Relationships
- Lower θ = better thermal performance
- Resistances add in series
- Temperature rise = Power × Resistance
- Max power limited by junction temp
Thermal Design Tips
PCB as Heatsink
Use thermal vias under components to spread heat to copper planes. Inner layers with 1+ oz copper help significantly.
Airflow
Even 1-2 m/s airflow can reduce θSA by 30-50%. Orient fins parallel to airflow direction.
Derating
Design for 80% of max junction temp for reliability. Every 10°C reduction doubles component lifetime.
Exposed Pads
Solder exposed thermal pads properly. Voids in solder drastically increase thermal resistance.
Spreading
Heat spreads in 45° cone. Larger heatsink base improves performance more than taller fins.
Testing
Verify with thermal camera or thermocouple. Calculations are estimates - always measure.
Frequently Asked Questions
What's the difference between θJA and θJC?
θJA is junction-to-ambient (no heatsink, includes package and PCB). θJC is junction-to-case only. Use θJA for simple estimates, θJC + θCS + θSA for heatsink calculations.
My component has a thermal pad. What θJC should I use?
Components with exposed pads often list θJC for the pad specifically (much lower than for the top). Make sure to use the pad-referenced value and ensure good PCB thermal design.
How do I choose a heatsink?
Calculate required θSA: θSA = (Tj_max - Ta) / P - θJC - θCS. Then find a heatsink with equal or lower thermal resistance. Consider size, mounting, and airflow.