Heat Sink Thermal Resistance Calculator
Calculate junction temperature, thermal resistance path, and heatsink requirements. Predict component cooling for power supply, audio, and RF designs.
Thermal Resistance Path
Total Thermal Resistance: θja = θjc + θcs + θsa
Junction Temperature: Tj = Ta + (θja × P)
Required Heatsink: θsa = (Tj_max - Ta) / P - θjc - θcs
Typical Values:
- θjc (IC package): 0.5–5 °C/W
- θcs (thermal paste): 0.1–0.5 °C/W
- θsa (small heatsink, natural): 5–20 °C/W
- θsa (large heatsink, forced): 0.5–5 °C/W
Common Power Devices & Tj_max
| Device Type | Tj_max (°C) | Typical Package |
|---|---|---|
| Op-Amp (general) | 150–200 | DIP, SO-8 |
| Linear Regulator (78xx) | 150 | TO-220 |
| BJT Power Transistor | 150–200 | TO-220, TO-3 |
| MOSFET (logic-level) | 150 | SO-8, DFN |
| Power IGBT | 150–175 | TO-247, TO-220 |
Frequently Asked Questions
What is thermal resistance?
Thermal resistance (θ, in °C/W) measures how effectively heat flows from a component to ambient air. It's analogous to electrical resistance: ΔT = θ × P, where P is power dissipation in watts. Lower thermal resistance means better heat dissipation and cooler component operation.
What is junction temperature?
Junction temperature (Tj) is the temperature inside the semiconductor die where heat is generated. It must stay below the maximum rating (typically 150°C for silicon) to ensure reliability and prevent thermal runaway. Formula: Tj = Ta + (θja × P).
How do I calculate thermal resistance?
Total thermal resistance from junction to ambient: θja = θjc + θcs + θsa, where θjc is junction-to-case (from datasheet), θcs is case-to-heatsink (thermal interface), and θsa is heatsink-to-ambient (depends on airflow).
What is thermal interface material (TIM)?
TIM (thermal paste, pads, or adhesives) fills microscopic air gaps between component case and heatsink. Typical θcs for good-quality thermal paste is 0.1–0.5 °C/W. Poor or missing TIM dramatically increases θcs and junction temperature.
How does airflow affect heatsink performance?
Natural convection (still air) vs. forced convection (fan) significantly impacts θsa. Forced convection can reduce θsa by 50% or more. Always verify heatsink performance curves include your airflow conditions.
What heatsink size do I need?
Rearrange: θsa = (Tj_max - Ta) / P - θjc - θcs. If your calculated θsa is negative, you need a larger heatsink or forced cooling. Online heatsink databases let you search by required θsa.
Why do IGBTs and MOSFETs overheat?
Gate drive losses, conduction losses (I²R), and switching losses all dissipate as heat. At high currents, even small die resistances create significant junction heating. Proper heatsinking and thermal management are critical in power electronics.
Can I use a heatsink without thermal paste?
No. Microscopic surface roughness creates air gaps with extremely high thermal resistance. Always apply a thin layer of quality thermal paste. Typical improvement: 5–10°C reduction in junction temperature.
What is transient vs. steady-state thermal response?
Steady-state is the final equilibrium temperature after power has been applied for sufficient time (typically seconds to minutes). Transient is the temperature rise during the first few milliseconds. Short pulses may not reach steady-state.