Thermal Resistance Chain

Build a junction→ambient thermal chain, enter power + ambient, and estimate junction temperature with Show Work.

How to Use

  1. Enter power dissipation (W) and ambient temperature (°C).
  2. Build your thermal path as a series chain (each segment adds).
  3. Optionally set a maximum junction temperature to check margin.
  4. Open “Show Work” for formulas and step-by-step results.
Thermal Path View
Series chain: ΔT = P × (Σ Rθ). Estimate Tj from Ta.
P
—
Rθ Total
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ΔT
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Tj
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Status: —
Junction
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Case
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Sink
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Ambient
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Tip: You can model extra segments (board, spreader, enclosure) in the “Custom Segments” list.

Inputs & Chain
All thermal resistances are in °C/W (same as K/W for differences).
Heat generated in the device (electrical loss).
Air temperature around the heatsink / enclosure.
Used to compute thermal margin (TjMax − Tj).
Inputs can be entered/displayed in °C or °F.
From datasheet (package-dependent).
Pad/grease/contact pressure estimate.
Heatsink performance (still air vs airflow matters).
Keeps the readouts clean while preserving precision in Show Work.
Custom Segments (optional)

Add extra series resistances (board, spreader, enclosure, airflow penalty, etc.).

Show Work (step-by-step)
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Work is shown in base units: power in W, thermal resistance in °C/W, temperatures in °C.

Thermal Chain Reference

Core equations:

  • Total thermal resistance (series): Rθ,total = Σ Rθ,i
  • Temperature rise: ΔT = P × Rθ,total
  • Junction temperature: Tj = Ta + ΔT
  • Margin (optional): Margin = Tj,max − Tj
Note: °C/W is numerically equivalent to K/W for temperature differences.

FAQ

What is RθJC vs RθJA?

RθJC is junction-to-case (package). RθJA is junction-to-ambient and depends heavily on the test setup (PCB, airflow, mounting). A chain lets you model your real mounting situation.

Do I add thermal resistances in series?

For a single heat-flow path, yes: each segment adds. Parallel paths (multiple heat exits) need parallel thermal network math (not modeled in this v2.1 tool).

Why does airflow matter so much?

Airflow reduces the sink-to-ambient resistance (RθSA). Datasheet RθSA values usually assume a specific airflow (or still air).

What if my interface is bad?

A thick pad, poor flatness, low pressure, or missing paste can raise RθCS and spike junction temperature. Model it as a larger RθCS (or a custom segment).

Tool Info

Last updated:

Updates may include new presets, improved edge-case handling, and optional parallel-network support.