Heat Sink Thermal Calculator
Calculate required heat sink thermal resistance and junction temperature.
Component Values
Results
Rth_ja = Rth_jc + Rth_cs + Rth_sa
Junction Temperature vs Power Dissipated
Junction temperature vs power dissipated
Typical Heat Sink Thermal Resistance
| Heat Sink Type | Rth_sa (°C/W) |
|---|---|
| No heat sink (TO-220) | 50–70 |
| Small aluminum fin | 15–25 |
| Medium heat sink | 5–10 |
| Large heat sink | 1–4 |
| Heat sink + fan | 0.3–1.5 |
Thermal Resistance and Heat Sink Design
Thermal resistance (Rth) measures the opposition to heat flow, in °C/W. The total thermal resistance from junction to ambient is Rth_ja = Rth_jc + Rth_cs + Rth_sa. Rth_jc (junction-to-case) is in the component datasheet. Rth_cs (case-to-sink) depends on the interface material — 0.1–0.3 °C/W with thermal paste, up to 5 °C/W without.
To find the maximum allowed heat sink thermal resistance: Rth_sa_max = (Tjmax − Tambient) / P − Rth_jc − Rth_cs. Choose a heat sink with Rth_sa lower than this value. Always leave 10–20°C of margin below Tjmax.
Thermal paste (thermal interface material, TIM) is essential. Without it, surface roughness creates air pockets that increase Rth_cs dramatically. Good thermal paste like Arctic MX-4 gives Rth_cs ≈ 0.1 °C/W for a TO-220 package.
Thermal resistance chain
Rth_ja = Rth_jc + Rth_cs + Rth_saTj = Ta + P × Rth_jaRth_sa_max = (Tjmax − Ta) / P − Rth_jc − Rth_csKey Points
- Rth_ja = Rth_jc + Rth_cs + Rth_sa — lower is better
- Always use thermal paste to minimize Rth_cs
- Tj must remain below Tjmax at all operating conditions
- TO-220 without heat sink: Rth_ja ≈ 60 °C/W
- Leave 10–20°C margin below Tjmax for reliability
- Forced air cooling can reduce Rth_sa by 3–10×
Applications
- Linear voltage regulator (LM317, 7805) cooling
- MOSFET and IGBT heat sink sizing
- Power transistor biasing and cooling
- Audio amplifier output stage design
- LED driver thermal management
FAQ
How do I choose a heat sink?
Calculate Rth_sa_max = (Tjmax − Ta) / P − Rth_jc − Rth_cs. Select a heat sink with Rth_sa lower than this value. Check both steady-state and transient thermal ratings.
What is thermal resistance?
Thermal resistance Rth (°C/W) is the temperature rise per watt of power. The thermal chain: Rth_ja = Rth_jc + Rth_cs + Rth_sa. Junction temperature: Tj = Ta + P × Rth_ja.
Do I need thermal paste?
Yes. Without thermal paste, air gaps increase Rth_cs from ~0.2 to ~5 °C/W. Always apply a thin, even layer of thermal paste between component and heat sink.
How to calculate junction temperature?
Tj = Ta + P × (Rth_jc + Rth_cs + Rth_sa). Keep Tj at least 10–20°C below Tjmax from the datasheet (usually 125°C or 150°C).
What happens when a transistor overheats?
Exceeding Tjmax causes thermal runaway, parameter drift, reduced reliability, or catastrophic failure. Silicon transistors typically fail permanently above 150–175°C junction temperature.
What is Tjmax?
Tjmax is the maximum operating junction temperature in the datasheet. For silicon BJTs and MOSFETs it is typically 125°C or 150°C. Always derate by at least 10–20°C.
Thermal Resistance Formulas
Junction temperature: T_j = T_a + P_d × θ_jaHeatsink required: θ_sa = (T_jmax − T_a) / P_d − θ_jc − θ_csWhere:
θ_ja = junction-to-ambient (°C/W) — from datasheet
θ_jc = junction-to-case (°C/W) — from datasheet
θ_cs = case-to-heatsink (°C/W) — 0.1–0.5 with thermal paste
θ_sa = heatsink-to-ambient (°C/W) — what we need to find
P_d = power dissipated (W)
Worked Examples
T_jmax = 125°C, θ_jc = 5°C/W, P_d = (Vin−5V)×1A = 5W typical, T_a = 40°C
θ_sa ≤ (125−40)/5 − 5 − 0.5 = 11.5°C/W → use ≤ 12°C/W heatsink
T_jmax = 175°C, θ_jc = 0.4°C/W, P_d = I²×Rds(on) = 25×0.028 = 0.7W, T_a = 50°C
θ_sa ≤ (175−50)/0.7 − 0.4 − 0.3 = 178°C/W → no heatsink needed ✓
Design Tip
Always apply thermal interface material (thermal paste or pad) between component and heatsink. Good paste (like Arctic MX-4): θ_cs ≈ 0.1°C/W. Dry contact: θ_cs ≈ 1–3°C/W. Keep T_j at least 20°C below T_jmax for long-term reliability. Forced air cooling can reduce heatsink thermal resistance by 30–50%.
Did you know? Thermal interface materials (TIM) — thermal paste, pads, or phase-change materials — reduce the contact resistance between a device and its heatsink. Even a thin layer of air between surfaces (thermal resistance ~30–50 °C/W per cm²) can make a heatsink nearly ineffective.