Heat Dissipation Calculator

Junction Temperature

Calculate semiconductor junction temperature: Tj = Ta + (Pd × Rθja)

Power Dissipation (W)

Ambient Temperature (°C)

Thermal Resistance Junction-to-Case θjc (°C/W)

Thermal Resistance Case-to-Sink θcs (°C/W)

Thermal Resistance Sink-to-Ambient θsa (°C/W)

Required Heatsink Thermal Resistance

Calculate maximum heatsink θsa for a given max junction temperature

Power Dissipation (W)

Max Junction Temperature (°C)

Max Ambient Temperature (°C)

θjc Junction-to-Case (°C/W)

θcs Case-to-Sink (°C/W)

Maximum Power Dissipation

Calculate max allowable power for a given heatsink and conditions

Max Junction Temperature (°C)

Ambient Temperature (°C)

θjc (°C/W)

θcs (°C/W)

θsa Heatsink (°C/W)

Formula

Tj = Ta + Pd × (θjc + θcs + θsa) | θsa_max = (Tj_max - Ta)/Pd - θjc - θcs | Pd_max = (Tj_max - Ta) / (θjc + θcs + θsa)

Frequently Asked Questions

What is thermal resistance?

Thermal resistance (θ, in °C/W) describes how much temperature rises per watt of power dissipated across a thermal path. Lower thermal resistance means better heat transfer. The total thermal path from junction to ambient is: θja = θjc + θcs + θsa.

How do I choose the right heatsink?

Calculate the required θsa (sink-to-ambient thermal resistance) using: θsa = (Tj_max - T_ambient)/P - θjc - θcs. Choose a heatsink with equal or lower θsa. Use thermal paste or pads to minimize θcs. Forced air cooling reduces effective θsa by 30-50%.

What is junction temperature and why does it matter?

Junction temperature (Tj) is the temperature at the semiconductor die inside a package. Exceeding the maximum Tj rating (typically 125-175°C) causes permanent damage. For reliability, keep Tj 20-30°C below the maximum. Every 10°C increase roughly halves component lifetime.

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