1Thermal budget

Device power loss (W)

Junction temperature limit (°C)

Ambient temperature (°C)

Junction-to-case, Rjc (K/W)From the device datasheet — the die-to-case thermal resistance.

Thermal interface material (case-to-sink)

TIM thickness (mm)

TIM conductivity (W/m·K)

Contact area (mm²)

Rcs (computed) = 0.8333 K/W

2Heatsink

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Sink-to-ambient, Rsa (K/W)From a heatsink datasheet, or 0 to only see the required budget.

Results

Required Rsa (budget)

4.167K/W

max sink Rth to hold Tj ≤ 150°C

Rsa used

1.5K/W

direct entry

Resulting junction temperature

96.7°C

Margin

53.3°C

Reference & assumptions

New to Heatsink Thermal Design? Read the guide: Heatsink Sizing: Junction-to-Ambient Rth and Natural-Convection Fin Arrays — a plain-language explainer of the standard behind this calculator.

The thermal budget is plain series thermal-resistance algebra: Tj = Ta + P·(Rjc + Rcs + Rsa). Fin-array sizing treats each inter-fin gap as an isolated vertical flat plate (Churchill-Chu correlation, the vertical-plate analogue of the horizontal-cylinder correlation used elsewhere in this project) — a disclosed simplification that ignores inter-fin channeling (the Elenbaas/Bar-Cohen effect that reduces the convection coefficient at very tight spacing). It is accurate at the commonly-cited near-optimal spacing range (roughly 6mm and above); a warning appears below that. Fin efficiency uses the standard adiabatic-tip-corrected rectangular-fin formula (η = tanh(mLc)/mLc). Radiation is applied over the same effective area as convection, a disclosed simplification rather than a full per-fin view-factor treatment. Natural convection only — does not model forced-air/fan cooling.

Validated: the Rth-budget algebra and TIM t/(k·A) resistance were checked against hand-worked cases (exact match). The vertical-plate Churchill-Chu correlation's Rayleigh/Nusselt/h output was independently re-derived from the documented formula and matched exactly, its constants (0.825, 0.492) confirmed distinct from the horizontal-cylinder correlation's (0.6, 0.559), and the resulting h checked against the well-known 2-25 W/m²K range for natural convection in air. The rectangular-fin efficiency formula was checked against its exact tanh(x)/x identity and its η→1 limit as h→0. The full fin-array solve was checked for energy-balance self-consistency (computed convection + radiation heat flow matches the target loss at the solved base temperature) and for a physically-expected monotonic drop in Rsa as fin count increases.

Calculation steps

1. Case-to-sink (TIM) resistance

Rcs = t / (k · A)

t = 1 mm, k = 3 W/m·K, A = 400 mm²

Rcs = 0.8333 K/W

2. Junction-to-ambient thermal budget

Rth(total, allowed) = (Tj_max − Ta) / P; Rsa(required) = Rth(total) − Rjc − Rcs

Tj_max = 150°C, Ta = 40°C, P = 20 W, Rjc = 0.5 K/W, Rcs = 0.8333 K/W

Rsa(required) ≤ 4.1667 K/W (total budget 5.5 K/W)

3. Resulting junction temperature

Tj = Ta + P·(Rjc + Rcs + Rsa)

Rsa used = 1.5 K/W

Tj = 96.7°C vs Tj_max 150°C — margin 53.3°C — pass

Heatsink / Junction-to-Ambient Thermal Calculator