Thermal Resistance Networks
Model heat flow as an electrical circuit analogy — junction, case, heatsink, and ambient nodes connected by thermal resistances — and calculate exactly how much resistance your heatsink must provide.
Just as electrical current flows through resistors from high voltage to low voltage, heat flows through thermal resistances from high temperature to low temperature.
- Voltage (V) → Temperature (°C)
- Current (A) → Heat Flow (W)
- Resistance (Ω) → Thermal Resistance (°C/W)
- V = I × R → ΔT = Q × Rth
- TJ = Junction Temperature
- TC = Case Temperature
- TS = Heatsink Surface
- TA = Ambient Air
Junction-to-ambient resistance is the sum of three series resistances along the path heat takes leaving the die:
Understanding Each Resistance
Resistance from the semiconductor die to the package surface, fixed by the manufacturer and found on the datasheet. Typical values: 0.2-5 °C/W for power devices, 1-15 °C/W for ICs. Exposed-pad packages (QFN, PowerPAD) can be as low as 0.5 °C/W.
Resistance through the Thermal Interface Material. Depends on TIM type, bond-line thickness, contact pressure, and surface roughness. Typical values: 0.1-2 °C/W. Thermal grease: ~0.1-0.5 °C/W. Thermal pad: ~0.5-2 °C/W.
The heatsink's own thermal resistance — the primary variable a thermal engineer designs for. Depends on fin geometry, surface area, airflow, material, and orientation. Typical values: 0.5-20 °C/W depending on size and airflow.
Enter your device parameters to calculate the required heatsink resistance RθSA step by step.
Temperature at Each Node
How does the required heatsink resistance change with power dissipation, given the fixed RθJC/RθCS and thermal budget from Section 3? The dashed line marks RθSA = 0 — below it, no heatsink can meet the requirement.
Module 3 Quiz
Select the best answer for each question. Instant feedback is provided after each response.