Single-Stage and Full Thermal-Resistance Chain
Adds a linked single-stage Rja model and a junction–case–heatsink–ambient chain alongside the existing thermal-resistance tool.
Inputs and reverse calculation
Calculated results
| Dissipated power Pd | 2 W |
|---|---|
| Ambient temperature Ta | 35 °C |
| Total thermal resistance Rja | 30 °C/W |
| Junction temperature Tj | 95 °C |
How to interpret the result
Single-stage mode represents the complete cooling arrangement with Rja and explicitly solves Tj, Pd, Rja or Ta; the solved field is read-only. Full mode places Rjc, Rch and Rha in series. Power, ambient or resistance edits calculate every node; a Tj/Tc/Th edit solves the common steady-state power. Negative Celsius temperatures are allowed, down to absolute zero.
Formulas and units
Single stage: Tj=Ta+PdRja, Pd=(Tj−Ta)/Rja, Rja=(Tj−Ta)/Pd and Ta=Tj−PdRja. Full chain: Rja=Rjc+Rch+Rha; Th=Ta+PdRha, Tc=Th+PdRch and Tj=Tc+PdRjc. Temperature inversion: Pd=(Th−Ta)/Rha=(Tc−Ta)/(Rch+Rha)=(Tj−Ta)/Rja.
Worked example
With Pd=2 W, Ta=35 °C and Rja=30 °C/W, Tj=95 °C. With Rjc/Rch/Rha=20/1/15 °C/W, the full model gives Rja=36 °C/W, Th=65 °C, Tc=67 °C and Tj=107 °C. Changing Tj to 125 °C in the same chain gives Pd=2.5 W, Th=72.5 °C and Tc=75 °C.
Scope and limitations
The explicit single-stage solve selector is a site extension that makes the App’s inverse entries clear. Full mode assumes a common steady-state heat flow through three series paths. Do not add datasheet θJA and θJC values measured under different conditions; this is not a multipath thermal network or transient Zth model. Individual resistances may be zero, but total resistance must be positive. A zero-resistance path cannot uniquely solve power, and zero power cannot solve resistance. Dissipated power must be nonnegative; nodes below ambient are rejected for passive-heating inversion. Fahrenheit display changes absolute temperatures only. Thermal resistance stays in °C/W, numerically equal to K/W, without a 32° offset. Device junction limits, derating and design margin are not selected automatically.
Common questions
Why do junction temperature and power change when I edit heatsink temperature?
In full mode the edited temperature becomes the known value. Ambient temperature and the path resistance determine the common power, after which the other nodes are recalculated.
What happens to 30 °C/W when Fahrenheit display is enabled?
This tool keeps 30 °C/W. Thermal resistance is temperature difference divided by power, not absolute temperature, so 30×1.8+32 is invalid. A dedicated °F/W conversion would use only a ×1.8 scale factor.
Can I use a chip datasheet θJA as heatsink resistance?
No. θJA describes the overall path for specified board and ambient test conditions. Heatsink-to-ambient Rha is a different segment. Select parameters consistent with the real heat path and measurement conditions.
Related tools
Formula and data sources
- TI SLVA462: Understanding Thermal Dissipation and Design of a Heatsink
- Nexperia: Understanding Datasheet Thermal Characteristics
Rebuilt from the extracted Electrodoc Pro 5.2 specification. Added, inferred and corrected behavior is identified in the limitations. Updated 2026-11-14.