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Full changelogEngine 1.18.3

Thermal Resistance Network

Thermal

Junction, case and sink temperatures along a series thermal path, and the heat sink you need.

Inputs

Board profile

Fills Ambient (air at the sink) and Junction temperature limit from your board profile.

W
°C
TO-220 ≈ 1–3, TO-247 ≈ 0.5–1, D²PAK ≈ 1–2, SOT-23 ≈ 75
°C/W
Grease ≈ 0.2–0.5, pad ≈ 0.5–1.5, mica + grease ≈ 1–2, dry ≈ 1–3
°C/W
°C/W
°C
°C

Results

Junction TJ
80.0°C
Pass

Checked: TJ ≤ 125 °C, which keeps 25 °C of margin to TJ(max)
Total RθJA

1.50 + 0.50 + 4.00
6.00°C/W
Margin
70.0°C
All results (3)
Case TC
72.5°C
Sink TS
70.0°C
Total rise
+ 30.0°C
Thermal ladder
Junction T_J 80.0 °C
R_θJC
1.50 °C/W
Case T_C 72.5 °C
R_θCS
0.50 °C/W
Sink T_S 70.0 °C
R_θSA
4.00 °C/W
Ambient T_A 50.0 °C

Accuracy

Precision
Resistances in series, exact in steady state only.

Each term is as good as its source: junction to case is the tightest, the interface the loosest, and sink to air depends on the real airflow.

Most of the uncertainty comes from Case → sink (interface). Tighten that first.

Verified against

2 independent cases. See the working.

This is a design aid. The engineer remains responsible for the design and for checking the standard itself.

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Give at least one input a tolerance above zero.

Heat path

The principle

Heat flows from die to air through resistances in series. Power acts as current and temperature difference as voltage, so the drop across each term is proportional to it, and the largest term dominates.

Series thermal path
RθJA=RθJC+RθCS+RθSAR_{θJA} = R_{θJC} + R_{θCS} + R_{θSA}
Node temperatures (working back from ambient)
TS=TA+PtotRθSATC=TS+PDRθCSTJ=TC+PDRθJCT_S = T_A + P_{tot} R_{θSA} \qquad T_C = T_S + P_D R_{θCS} \qquad T_J = T_C + P_D R_{θJC}
Required heat sink
RθSA=TJ(max)−TA−PD(RθJC+RθCS)PtotR_{θSA} = \frac{T_{J(max)} - T_A - P_D (R_{θJC}+R_{θCS})}{P_{tot}}
Ptot is the total power on the sink. PD is the power of the one device you are checking.
Thermal capacitance (transient)
τ=RθCθCθ=m cp\tau = R_{θ} C_{θ} \qquad C_{θ} = m\,c_p
The sink's thermal mass smooths pulses shorter than about τ. Longer pulses see the full resistance.
  • RθJCR_{θJC}junction to case, fixed by the package
  • RθCSR_{θCS}case to sink: interface material and mounting pressure
  • RθSAR_{θSA}sink to ambient: the heat sink
  • PtotP_{tot}total power into the sink from all devices
More detail

Where each term comes from

  • RθJC is set by the package. Nothing outside it changes this term.
  • RθCS is the interface. Real surfaces touch over a few per cent of their area, so dry metal is 1–3 °C/W and grease brings it to 0.2–0.5. An insulating pad or mica washer costs 0.5–2 °C/W. Mounting torque matters as much as the material.
  • RθSA is the sink. In still air it depends on area, fin spacing and orientation. Forced air can improve it 3–10×.

Checking on the bench: ΨJT

Many datasheets give ΨJT, junction to package top, often a few °C/W. Measure the package top with a fine thermocouple; then TJ ≈ Ttop + ΨJT·PD.

Catalogue figures are optimistic. A sink rated at a large ΔT with vertical fins in open air does worse at a small ΔT, with horizontal fins, in a closed box. Allow 20–30 % worse than the catalogue figure, and verify with a thermocouple.
Related: Junction Temperature & Safe Limits for the single-resistance form; Copper Area & Thermal Vias when the PCB is the heat sink.

Engine version ⁨1.18.3⁩