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Heat Sink Temperature Calculator

Tj = Ta + P Σθ. 10 W at 25 °C with 1.5+0.5+3 °C/W is 75 °C.

By Jasper · Updated

Calculator

PASS75.00 °CPmax 25 W

Tj = Ta + P (θJC + θCS + θSA). Pmax = (Tjmax − Ta) / Σθ. θSA_req = (Tjmax − Ta)/P − θJC − θCS. Blank Tjmax defaults to 150 °C. Package θ values are not tabulated here — use the datasheet.

Tj
75.00 °C
Pmax
25 W
Σθ
5.000 °C/W
Tjmax − Tj
75 °C
Tj > Tjmax
no
θSA required
10.50 °C/W
Tjmax used
150.0 °C
  • Pass Junction temperature: Tj = 75.0000 °C

Formula

Junction temperature

Tj=Ta+P(θJC+θCS+θSA) Tj = Ta + P (θJC + θCS + θSA)

Power budget

Pmax=Tj,max-TaΣθ Pmax = (Tjmax − Ta) / Σθ

Required heat-sink

θSA_req=Tj,max-TaP-θJC-θCS θSA_req = (Tjmax − Ta)/P − θJC − θCS

Variables

Symbol Meaning Unit Typical range Precision Default
mode Mode — — 1 sig forward
P Dissipation W ≥ 0 µW 4 sig 10 W
Ta Ambient °C — 4 sig 25 °C
θJC Junction-to-case °C/W ≥ 0 °C/W 4 sig 1.5 °C/W
θCS Case-to-sink °C/W ≥ 0 °C/W 4 sig 0.5 °C/W
θSA Sink-to-ambient °C/W ≥ 0 °C/W 4 sig 3 °C/W
Tjmax Tj max (blank → 150 °C) °C — 4 sig 150 °C
Tj Junction temperature °C — 4 sig —
Pmax Power budget W — 4 sig —
sum Σθ °C/W — 4 sig —
m Tjmax − Tj °C — 4 sig —
over Tj > Tjmax — — 1 sig —
θSA_req Required θSA °C/W — 4 sig —

Worked example

10 W, 25 °C, 1.5 / 0.5 / 3 °C/W

Input Value
mode · Mode forward
P · Dissipation 10 W
Ta · Ambient 25 °C
θJC · Junction-to-case 1.5 °C/W
θCS · Case-to-sink 0.5 °C/W
θSA · Sink-to-ambient 3 °C/W
Tjmax · Tj max (blank → 150 °C) 150 °C
Output Value
Tj · Junction temperature 75 °C
Pmax · Power budget 25 W
sum · Σθ 5 °C/W
over · Tj > Tjmax no

Required θSA for Tjmax 125 °C

Input Value
mode · Mode solve_theta_SA
P · Dissipation 10 W
Ta · Ambient 25 °C
θJC · Junction-to-case 1.5 °C/W
θCS · Case-to-sink 0.5 °C/W
Tjmax · Tj max (blank → 150 °C) 125 °C
Output Value
θSA_req · Required θSA 8 °C/W

How it works

Junction temperature is a stack of three thermal resistances, not a single θJA pulled from a blog. Enter dissipation, ambient, and the three θ values from the datasheet and TIM note. The solve mode hides the entered θSA and reports the heat-sink you still need.

Pitfalls

  • Using a website package θ table with no datasheet citation. This page does not invent those numbers.
  • Leaving θCS at 0 when the TIM or insulator is not negligible.
  • Treating Pmax as a continuous rating when the datasheet derates by airflow and altitude.

FAQ

What is Tj for 10 W at 25 °C with 1.5 + 0.5 + 3 °C/W?

75 °C. Pmax at Tjmax 150 °C is 25 W.

What if I leave Tjmax blank?

The page uses 150 °C, a common silicon default. Enter the datasheet limit when you have it.

How do I size the heat sink?

Switch to “solve required θSA”. The entered θSA is ignored in that mode.

Why is there no TO-220 θ table?

Uncited package numbers are worse than a blank. Copy θJC from the datasheet.

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