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MOSFET Junction Temperature Calculator

Estimate MOSFET junction temperature from ambient, dissipation, and thermal resistance (effective RθJA, or case/heatsink-dominant RθJC+RθCA). Optional thermal margin.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance
Input interpretation
Enter values to calculate.
Result
—
Assurance
Engineering
Declared partition coverage
PASS · 3/3 declared partitions (ja, jc-ca, invalid-domain) · Matrix
Known limitations
  • Steady-state thermal; not transient Zth
  • Core CVP does not include live graph, viewport, or pointer interaction.
Model
Steady-state MOSFET junction temperature, temperature rise, and optional margin versus entered Tj,max.
Scope
Steady-state lumped thermal resistance
Verification
Engine tested · Source checked · v1.1.2 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance· View Manifest · CVP overview · Specification
Versions
Calculation 1.1.2 · CVP protocol 1.0.0-proposed
CVP identity
5/5 property · digest 950f87c6bdab
Legacy regression
18/18 tests · Production surface contract 14/14
Reference
O1 model · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session.
Supplemental domain review
Internal · Pass · electrical-engineer · 2026-08-16
Named expert review
Not performed
CVP suite
3/3 golden · 8/8 CVP boundary · 6/6 invalid · 5/5 property · 1/1 cross-interface · 14/14 CVP contract · Manifest
Sources
Sources
Evidence
3 legacy golden · 8 legacy boundary · legacy regression suite · 3/3 oracle-backed golden · 6/6 invalid · Artifact integrity PASS · CalculatorX electrical review (EE-001)
This calculator CURRENT · Public schema 1.1.2 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

JunctionTj = Ta + P · Rθ
Temperature riseΔT = P · Rθ
Thermal marginMargin = Tj,max − Tj
iRθ is effective RθJA, or RθJC+RθCA (exclusive; not equivalent models). RθJA is an application value, not a JEDEC package constant. JC+CA is a case/heatsink-dominant lumped single-path approximation. Steady-state only. Not transient Zth or CFD.

How to use

1

Choose the thermal model

Effective JA uses application RθJA. Case/heatsink uses RθJC plus lumped RθCA when that path dominates. These are not interchangeable algorithms.

2

Enter ambient temperature and dissipation

Ta and MOSFET power dissipation P (device loss converted to heat), not load or output power.

3

Enter thermal resistance for the selected model

For Effective JA, enter an RθJA representative of your PCB and cooling. For case/heatsink, enter both RθJC and RθCA.

4

Read Tj, ΔT, and optional margin

Tj = Ta + ΔT. Margin is Tj,max − Tj when Tj,max is entered. Negative margin means the entered limit is exceeded — not a SAFE/FAIL rating.

Example calculations

Common configurations with formula and result.

ϟ

Effective junction to ambient

Ta=25 °C · P=2 W · RθJA(effective)=40 °C/W · Tj,max=150 °C

Tj=25+2×40
Tj=105 °C · ΔT=80 °C · margin=45 °C
ϟ

Case / heatsink path

Ta=25 °C · P=1 W · RθJC=1.5 °C/W · RθCA=8.5 °C/W (path dominant)

Tj≈25+1×(1.5+8.5)
Tj=35 °C · ΔT=10 °C · path=JC+CA
ϟ

Thermal limit exceeded

Ta=50 °C · P=4 W · RθJA=30 °C/W · Tj,max=150 °C

Tj=50+4×30
Tj=170 °C · margin=−20 °C

Symbols

Common values at a glance.

SymbolMeaningUnit
TjJunction temperature°C
TaAmbient temperature°C
PMOSFET power dissipation (device loss)W
ΔTTemperature rise P·Rθ°C
RθJAEffective / application junction-to-ambient thermal resistance°C/W
RθJCJunction-to-case thermal resistance (case/heatsink path)°C/W
RθCALumped case-to-ambient (interface + heatsink-to-ambient)°C/W
MarginTj,max − Tj when Tj,max is entered°C
i RθJA here is the effective value for the user's board and cooling, not a JEDEC-board package constant. JC+CA ignores parallel PCB/package paths. Measured case/top temperature with ψJT is a separate model (not this page).

MOSFET Junction Temperature calculator specification

Version 1.1.2 · Engine tested · Supplemental domain review · Internal · 2026-08-16

Calculation status

Review policy · Evidence · Reviewed by CalculatorX electrical review (EE-001) (electrical-engineer)

Definition
Tj = Ta + P·Rθ. Use effective/application RθJA, or RθJC+RθCA only when the case/heatsink path dominates. Optional Tj_max yields thermal margin (not a safety certification).
What it calculates
Steady-state MOSFET junction temperature, temperature rise, and optional margin versus entered Tj,max.
Inputs
  • Ta_C
  • P_W ≥ 0
  • Rth_JA or Rth_JC+Rth_CA (not both)
  • optional Tj_max_C
Outputs
  • Tj_C
  • dT_C
  • Rth_C_per_W
  • path
  • margin_C?
Formula
Tj = Ta + P·Rθ; ΔT = P·Rθ; Margin = Tj,max − Tj
Assumptions
  • Steady-state lumped thermal resistance
  • Rθ constant (not Zth(t))
  • RθJA is an effective / application value for the user's thermal environment, not a JEDEC-board package constant
  • JC+CA is a case/heatsink-dominant lumped single-path approximation; it does not model parallel heat-flow through PCB, leads, or package surfaces
Units
  • °C, W, °C/W
Boundary conditions
  • No Rth → MISSING_REQUIRED_INPUT
  • Only one of JC/CA → NEEDS_TWO_INPUTS
  • Rth_JA together with Rth_JC or Rth_CA → CONFLICTING_INPUTS
  • P < 0 → VALUE_MUST_BE_NON_NEGATIVE
  • Rth ≤ 0 → VALUE_MUST_BE_POSITIVE
Example
Ta=25, P=2, RthJA=40 → Tj=105, ΔT=80, margin=45 if max=150
Validation cases

5 published on this page · 18/18 tests · Production surface contract 14/14 · View evidence

  • 25,2,40,Tjmax=150 → Tj=105, dT=80, margin=45, path=JA
  • Ta=25, P=1, RthJC=1.5, RthCA=8.5 → Tj=35, dT=10, path=JC+CA
  • Ta=50, P=4, RthJA=30, Tjmax=150 → Tj=170, margin=−20
  • Rth_JA and Rth_JC+Rth_CA together → error CONFLICTING_INPUTS
  • Ta=25, P=2 (no Rth) → error MISSING_REQUIRED_INPUT
Sources
  • Texas Instruments SPRA953D — Semiconductor and IC Package Thermal Metrics — RθJA, RθJC, and application dependence of junction-to-ambient metrics · accessed 2026-08-15
    Supports: Tj ≈ Ta + P·RθJA as a thermal-metric estimate; RθJA is test-condition specific and is not a universal equivalent of RθJC+RθCA
  • onsemi AND9016 — MOSFET Thermal Resistance — Junction-to-case / case-to-ambient thermal path · accessed 2026-08-15
    Supports: Two-segment RθJC + RθCA for a case/heatsink-dominant path when package-bottom/PCB heat flow is neglected; RθCA lumps interface and heatsink-to-ambient
  • JEDEC JESD51 series — Integrated circuit thermal measurement — Methodology for package thermal metrics
    Supports: Definition of junction-to-ambient and junction-to-case thermal metrics on specified test boards
Last reviewed
2026-08-16
Reviewed by
CalculatorX electrical review (EE-001) (electrical-engineer)
Calculation version
1.1.2

Background

Interpretation and common distinctions.

Estimate MOSFET junction temperature from power dissipation and thermal resistance.

Default: Ta=25 °C · P=2 W · RθJA(effective)=40 °C/W → Tj = 105 °C, ΔT = 80 °C. Optional Tj,max=150 °C → margin = 45 °C.

Supported and not supported

Supported

  • Effective junction → ambient: Tj = Ta + P · RθJA (application RθJA)
  • Case / heatsink path: Tj ≈ Ta + P · (RθJC + RθCA) when that path dominates (lumped single-path)
  • Temperature rise ΔT = P · Rθ and optional margin = Tj,max − Tj
  • API / MCP via electrical.mosfet.tj

Not supported

  • Transient thermal impedance Zθ(t) / pulse heating
  • CFD or multi-heat-source coupling
  • Measured case/top temperature with ψJT (use a dedicated model later)
  • Safety certification from Tj < Tj,max

Agent / API notes

Capability id: electrical.mosfet.tj · tool id: mosfet-tj · pin calculation_version: 1.1.2.

v1.1.0 added dT_C on success and engine CONFLICTING_INPUTS for mixed paths. v1.1.1 tightens the public Input Schema from inclusive anyOf to exclusive oneOf + not. v1.1.2 binds Output Schema path to thermal-resistance fields (oneOf): JA requires Rth_JA with Rth_JC/Rth_CA null; JC+CA requires Rth_JC+Rth_CA with Rth_JA null. Machine path remains JA | JC+CA; those are not interchangeable physical models. Provide Rth_JA xor both Rth_JC and Rth_CA. Clients pinned to 1.0.0, 1.1.0, or 1.1.1 receive VERSION_MISMATCH.

Stable error codes: MISSING_REQUIRED_INPUT, NEEDS_TWO_INPUTS, CONFLICTING_INPUTS, VALUE_MUST_BE_NON_NEGATIVE, VALUE_MUST_BE_POSITIVE, INVALID_NUMBER.

Input schema oneOf + not: Rth_JA with neither Rth_JC nor Rth_CA, or both Rth_JC and Rth_CA with no Rth_JA. Output schema oneOf: path and Rth_* fields must agree. Pin calculation_version: 1.1.2.

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Frequently asked questions

Key distinctions behind the calculation.

Effective JA or case/heatsink?

They are not equivalent algorithms. Effective JA uses Tj = Ta + P·RθJA with an application RθJA. Case/heatsink uses Tj ≈ Ta + P·(RθJC+RθCA) only when heat flow through the modeled case-to-heatsink path is dominant. Mixing the two is schema-invalid and returns CONFLICTING_INPUTS. An incomplete JC/CA pair returns NEEDS_TWO_INPUTS.

Is RθJA a device constant?

No. Enter an effective RθJA representative of your PCB, copper area, airflow, and mounting. Datasheet JEDEC RθJA is primarily a comparison metric on a specified test board; TI notes that 1s vs 2s2p boards can change RθJA by up to about 50%.

Does Tj below Tj,max mean the design is safe?

No. This calculator reports Tj, ΔT, and numeric margin versus the Tj,max you entered. It does not certify safety, reliability, transient peaks, or derating.

What power should I enter?

MOSFET power dissipation — total device loss converted to heat (conduction + switching + other losses), not load or output power. The MOSFET Total Loss calculator can estimate P.