Power Dissipation Calculator
Calculate heat dissipation and junction temperature for semiconductors. Determine heatsink requirements and thermal management.
Calculator
How to Use This Calculator
This power dissipation calculator helps you determine if your component will overheat and whether you need a heatsink. It calculates junction temperature based on power dissipation and thermal resistance.
- Select a Package Preset — Quick start with common package thermal values
- Choose Component Type — Linear regulator, MOSFET, BJT, or resistor
- Enter Voltage and Current — Based on your circuit conditions
- Set Thermal Resistances — From component datasheet (θJC, θJA)
- Enable Heatsink — If using one, enter θCS and heatsink θSA
- Click Calculate — Get junction temperature and thermal status
Thermal Theory
Heat flows from the semiconductor junction to ambient air through a series of thermal resistances. Understanding this thermal path is essential for reliable designs.
The Thermal Model
Think of heat flow like electrical current: temperature difference is like voltage, power is like current, and thermal resistance is like electrical resistance.
Key Formulas
Thermal Resistance Chain
- θJC (Junction-to-Case): Internal to the package, fixed by design
- θCS (Case-to-Sink): Interface material (thermal paste, pad)
- θSA (Sink-to-Ambient): Heatsink performance in still air
- θJA (Junction-to-Ambient): Total without heatsink (from datasheet)
Package Comparison
| Package | θJC (°C/W) | θJA (°C/W) | Max Power* |
|---|---|---|---|
| TO-220 | 1.5 | 62 | 2W (no sink) |
| TO-263 (D2PAK) | 2.0 | 40 | 3W (PCB cooled) |
| TO-252 (DPAK) | 3.0 | 50 | 2.5W (PCB cooled) |
| SOT-223 | 15 | 80 | 1.5W |
| SOT-23 | 50 | 200 | 0.5W |
| QFN (5x5) | 2 | 30 | 4W (with pad) |
*Approximate values at 25°C ambient with typical PCB mounting
Heatsink Selection
When Do You Need a Heatsink?
- Junction temperature exceeds 80% of max rating
- Power dissipation exceeds package capability
- High ambient temperature environment
- Continuous operation at high power
Heatsink Types
| Type | θSA Range | Best For |
|---|---|---|
| Stamp-on (small fins) | 20-40 °C/W | 1-3W, SOT-223 |
| Extruded aluminum | 5-15 °C/W | 5-20W, TO-220 |
| Large finned | 1-5 °C/W | 20-50W |
| Fan-cooled | 0.5-2 °C/W | 50W+ |
Thermal Interface Materials
The interface between component and heatsink adds thermal resistance (θCS):
- Thermal paste: 0.1-0.5 °C/W (best performance)
- Thermal pad: 0.5-2.0 °C/W (convenient, insulating options)
- Mica + paste: 0.5-1.0 °C/W (electrical isolation)
- Dry mounting: 1-3 °C/W (not recommended)
Frequently Asked Questions
What is a safe temperature margin?
Aim for at least 20-30°C below the maximum junction temperature rating. This accounts for ambient temperature variations and component aging. A 50°C margin provides excellent reliability for mission-critical applications.
Why is my linear regulator so hot?
Linear regulators dissipate power proportional to the voltage drop times current. A 12V to 5V regulator at 500mA dissipates (12-5) × 0.5 = 3.5W, which is significant. Consider a switching regulator for large voltage drops or high currents.
How do I find thermal resistance values?
Check the component datasheet. Look for θJC (junction-to-case) and θJA (junction-to-ambient). θJA is useful for designs without heatsinks. For heatsink calculations, you need θJC plus the heatsink's θSA (sink-to-ambient) specification.
Does PCB copper help with cooling?
Yes, significantly. Larger copper areas under and around the component reduce effective thermal resistance. For surface-mount packages with thermal pads (D2PAK, QFN), adequate copper area is essential. Use thermal vias to spread heat to inner layers.
Verify Your Component Selections
After calculating your component values, use Schemalyzer to verify your schematic design. Our AI-powered analysis catches common errors and suggests improvements.
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