MOSFET Thermal Runaway Calculator
Self-consistent junction temperature with Rds(on) temperature feedback — converges to the real Tj or flags thermal runaway.
The naive estimate using 25 °C Rds(on) underestimates loss by 40–70 % in real designs — always use the self-consistent (hot) value. The positive tempco is also why paralleled Si MOSFETs share current naturally (hotter part takes less). This covers conduction loss; in hard-switched converters add switching loss separately before sizing the sink.
MOSFET Thermal Calculator computes the self-consistent MOSFET junction temperature with Rds(on) feedback — free, instant and private in your browser. Power designers whose 'cool' 25 °C calculations keep producing hot boards use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About MOSFET Thermal Runaway Calculator
MOSFET Thermal Calculator computes the self-consistent MOSFET junction temperature with Rds(on) feedback using the standard engineering relation: iterate Tj = Ta + I²·Rds(25)·(1 + 0.004·(Tj−25))·θja until it converges — or diverges (runaway). Worked live: 8 A through a 12 mΩ FET at θja 62 °C/W and 50 °C ambient settles near 124 °C with Rds grown ~40 %. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use MOSFET Thermal Runaway Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use MOSFET Thermal Runaway Calculator?
- ✓Implements the real formula — iterate Tj = Ta + I²·Rds(25)·(1 + 0.004·(Tj−25))·θja until it converges — or diverges (runaway) — with the substitution shown, not a black box
- ✓Built for power designers whose 'cool' 25 °C calculations keep producing hot boards
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate mosfet thermal?+
The self-consistent MOSFET junction temperature with Rds(on) feedback follows iterate Tj = Ta + I²·Rds(25)·(1 + 0.004·(Tj−25))·θja until it converges — or diverges (runaway). For example, 8 A through a 12 mΩ FET at θja 62 °C/W and 50 °C ambient settles near 124 °C with Rds grown ~40 %. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Why is my MOSFET hotter than the datasheet math predicted?+
You used the 25 °C Rds(on). Silicon Rds grows ~0.4 %/°C, so a junction at 125 °C carries 1.4–1.8× the resistance — and the loss — of the cold number. The self-consistent (iterated) answer is the only honest one for conduction-loss sizing.
Is positive tempco good for anything?+
Yes — current sharing. Parallel Si MOSFETs self-balance: the hotter one's resistance rises and sheds current to its partners. (Bipolars do the opposite and need emitter ballast.) The same tempco is what makes single-device thermal runaway possible with poor cooling, though.
Is the MOSFET Thermal Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
Related Electronics tools
PCB Trace Width Calculator (IPC-2221)
Minimum trace width for your current and temperature rise — IPC-2221 formula with copper weight, layer choice and a step-by-step worked example.
● LiveMicrostrip Impedance Calculator
Z₀ of a surface microstrip from width, height and εr (IPC-2141A), plus effective dielectric constant and propagation delay.
● LiveStripline Impedance Calculator
Characteristic impedance of an embedded stripline trace from geometry and εr — the inner-layer companion to the microstrip tool.
● Live