Chris Collins

Tools · electronics

Thermal design

Junction temperature through θJC, interface, and heatsink; MOSFET, regulator, resistor, and diode losses with temperature feedback; finned heatsink performance in still or moving air; PCB copper as a heatsink; pulsed heating from Foster networks; airflow for enclosures; and chip heat flux with the coolant flow liquid cooling needs.

Added · updated 14 October 2026

How to use it

Temperatures are in °C, thermal resistances in K/W (the same as °C/W).

  • Junction temperature — Tj = Ta + P·(θJC + θCS + θSA). Pick a package preset (typical datasheet figures) and an interface material; θCS comes from the material’s resistance per area and the contact area unless you override it. It also gives the heatsink θSA you need to stay a margin below Tj,max and the most power the part can take without one.

  • Power losses — a MOSFET’s conduction loss rises with temperature (Rds(on) grows about 0.5–0.8 %/K), so loss and temperature are solved together and thermal runaway is reported; switching loss uses ½·V·I·(tr + tf)·f. Linear regulators, resistors (with the usual derating from 70 °C to zero at 155 °C), and diodes are included.

  • Heatsink — a straight-fin extrusion from its dimensions. Natural convection uses the Bar-Cohen–Rohsenow correlation for vertical fin channels, which also gives the optimum fin gap; forced air assumes the flow is ducted through the fins. Radiation from the outer surfaces is added (anodising matters in still air). The plot shows θSA against fin count — too many fins choke natural convection.

  • PCB copper — the copper around a part treated as a circular fin (Bessel-function fin efficiency) cooled from both faces by convection and radiation, with θ against copper area for your weight and double it. Datasheets quote θJA for 1 in² (645 mm²) of 1 oz copper.

  • Pulses & transients — enter the Foster RC network from a datasheet (presets are typical) to get Zth(t), single-pulse heating, and the exact periodic steady-state peak for a pulse train, with the junction waveform.

  • Airflow & enclosures — airflow needed to carry the heat with a given air temperature rise (Q = P/(ρ·cp·ΔT), about 1.76·W/ΔT CFM), the rise with a given fan, and a sealed box’s rise.

  • Heat flux & liquid cooling — heat flux = power ÷ die area in W/cm², against the ~50 W/cm² beyond which forced air stops being adequate (an H100’s 700 W over its 814 mm² die is 86 W/cm²); and coolant flow from Q = ṁ·cp·ΔT for water, PG25 or 50 % propylene glycol — the flow for a temperature rise, or the rise for a flow. Set Devices to multiply up to a rack.

Correlations are for engineering estimates — expect ±25% for heatsinks and ±30% for board copper. Confirm critical designs by measurement (a thermocouple or thermal camera) or against the manufacturer’s curves.

Coolant properties near 30 °C: water from the IAPWS tables (ρ 995.7 kg/m³, cp 4.18 kJ/kg·K); propylene-glycol mixtures from Dow’s DOWFROST engineering data (PG25 ≈ 1,019 kg/m³ and 3.93 kJ/kg·K) — check your fluid’s datasheet. H100 SXM: 700 W (NVIDIA H100 datasheet), 814 mm² die (NVIDIA Hopper architecture whitepaper).

Related: fluid flow (pressure drop through the coolant pipes), AI hardware calculator.

Companion to AI Acceleration: From Transistors to Trillion-Parameter Models: the cooling wall, direct-to-chip liquid cooling and coolant chemistry, flow and ΔT.

Sharing

Share link copies a URL that opens this tool with your current settings — every value it remembers between visits, packed into ?s=. Opening a link replaces your saved settings for this tool.

Linking to an example

Every input can be written by hand under the name it is saved as — jP, jTa, jJC, jSA, hP, aP and so on — for example ?jP=8&jSA=4#junction. Add the tab after #: junction, losses, heatsink, pcb, transient, airflow or liquid. The heat-flux and liquid-cooling tab has plain names:

Parameter Meaning Example

power

Power per device, W

power=700

area

Die area, mm² (blank skips the heat flux)

area=814

count

Devices sharing the coolant loop

count=72

cool

Coolant: water, pg25 or pg50

cool=water

dt

Allowed coolant temperature rise, K

dt=10

flow

A flow in L/min, to get the rise it gives

flow=59

jP, jTa, jJC, jJA, jTjmax, jArea, jSA, jCS, jMargin, tim

Junction tab: power, ambient, θJC, θJA, Tj max, contact area, θSA, θCS override, margin, interface (0–6)

jP=8&jSA=4

aP, adT, aTa, aFan, aArea, aH

Airflow tab: power, rise, inlet temperature, fan CFM, enclosure area, surface coefficient

aP=60&adT=10

example

One of the presets below; parameters written beside it override the preset’s

example=p13-h100-heat-flux

Presets (?example=…; the link opens on the right tab even without #):

example= Shows

p13-h100-heat-flux

An H100: 700 W over an 814 mm² die is 86 W/cm², past the ~50 W/cm² forced air can handle (open)

d2c-coldplate-1kw

A 1,000 W Blackwell-class cold plate on PG25 with a 10 K rise needs about 1.5 L/min (open)

d2c-rack-10k

A 132 kW NVL72 rack at the usual 8–10 K rise (30–32 °C in, 40 °C out): about 200 L/min of PG25 (open)

d2c-rack-59lpm

59 L/min — the GB300 NVL72 figure at 25 °C supply — carrying 132 kW is a ~33 K rise (open)

d2c-rack-177lpm

177 L/min at 45 °C supply: three times the flow for a ~11 K rise (open)

Older links (?s=…) still open as before.

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