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Thermal

Junction Temperature Calculator

Calculate semiconductor junction temperature from power dissipation and thermal resistance chain (θJC + θCS + θSA). Essential for transistor, MOSFET, and IC thermal design.

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Formula

TJ=TA+PD⋅(θJC+θCS+θSA)T_J = T_A + P_D \cdot (\theta_{JC} + \theta_{CS} + \theta_{SA})
T_J— Junction temperature (°C)
T_A— Ambient temperature (°C)
P_D— Power dissipation (W)
θ_JC— Junction-to-case thermal resistance (°C/W)
θ_CS— Case-to-heatsink thermal resistance (°C/W)
θ_SA— Heatsink-to-ambient thermal resistance (°C/W)

How It Works

Junction temperature calculator computes semiconductor die temperature from power dissipation and thermal resistance path — essential for reliability analysis, [heatsink selection](/calculators/thermal/heatsink-selection/), and derating calculations. Power electronics engineers, thermal designers, and reliability engineers use this to predict device lifetime and prevent thermal failure. Per JEDEC JESD51-1, junction temperature Tj = Ta + Pd × (θJC + θCS + θSA), where θJC is junction-to-case (0.5-5°C/W for power packages), θCS is case-to-sink (0.1-1°C/W depending on interface), and θSA is sink-to-ambient (1-20°C/W for heatsinks). Exceeding Tj(max) by 10°C halves device lifetime per Arrhenius equation; operating at Tj(max) - 25°C doubles lifetime. TO-220 packages have θJC = 1-2°C/W; D²PAK has θJC = 0.5-1°C/W; QFN packages have θJC = 2-10°C/W depending on exposed pad area.

Worked Example

Calculate junction temperature for IRFZ44N MOSFET switching 10A at 12V with 50% duty cycle in TO-220 package. From datasheet: Rds(on) = 22mΩ at Tj = 25°C, θJC = 1°C/W, Tj(max) = 175°C. Conduction loss: P_cond = I² × Rds(on) × D = 10² × 0.022 × 0.5 = 1.1W. Switching loss at 100kHz: P_sw ≈ 0.5W (estimated from Qg × Vds × f). Total Pd = 1.6W. With TO-220 clip-on heatsink (θSA = 12°C/W) and thermal paste (θCS = 0.5°C/W): Tj = 40°C + 1.6W × (1 + 0.5 + 12) = 40°C + 21.6°C = 61.6°C. This is 113°C below Tj(max), providing excellent reliability margin. Note: Rds(on) increases 1.5× at Tj = 100°C; recalculate iteratively for accurate results.

Practical Tips

  • ✓Use Tj(max) - 25°C as design target for 10-year reliability — per JEDEC JEP122H, this provides 2× lifetime margin vs operating at Tj(max)
  • ✓Thermal interface materials: silicone grease (0.1°C/W), thermal pads (0.3-1°C/W), phase-change materials (0.05°C/W) — select based on assembly requirements
  • ✓For SMD packages without heatsink, θJA on datasheet applies — typical values: SOT-23 = 250°C/W, SOIC-8 = 125°C/W, QFN-16 = 40°C/W with exposed pad soldered

Common Mistakes

  • ✗Using θJA instead of θJC + θCS + θSA — θJA assumes no heatsink and still air; actual thermal path with heatsink is much lower resistance
  • ✗Ignoring θCS (interface resistance) — dry contact is 0.5-1°C/W; thermal paste reduces to 0.1-0.2°C/W; omitting this underestimates Tj by 5-15°C
  • ✗Forgetting temperature dependence of Rds(on) — MOSFETs have positive tempco (1.5-2× at Tj(max) vs 25°C); iterative calculation required for accuracy

Frequently Asked Questions

Immediate effects: increased leakage current (doubles per 10°C), reduced breakdown voltage, potential thermal runaway. Long-term: accelerated electromigration, gate oxide degradation, solder joint fatigue. Per MIL-HDBK-217F, failure rate doubles for every 10-15°C above rated temperature. Exceeding Tj(max) by 50°C can cause immediate destruction.
Heatsink improvements: larger surface area (10× area = 3× lower θSA), add fins, forced air (θSA reduces 3-10× with 1-3 m/s airflow). Interface improvements: thermal grease (θCS = 0.1°C/W) vs dry contact (0.5°C/W). Package selection: exposed-pad packages (QFN, D²PAK) have 5-10× lower θJC than leaded packages (SOIC, TO-92).
θJC (junction-to-case) measures thermal resistance to package surface — fixed by package design (TO-220: 1°C/W, D²PAK: 0.5°C/W). θJA (junction-to-ambient) includes the entire path to air — varies with PCB, airflow, and heatsink. For heatsink calculations, use θJC + θCS + θSA. θJA is only useful for small-signal ICs without heatsinks.
Direct methods: infrared thermography (±2°C accuracy), thermocouple on case (add θJC × Pd to get Tj). Indirect methods: Vbe or Vds(on) sensing (calibrated thermal diode, ±3°C), on-chip temperature sensor (many power ICs include this). Per JEDEC JESD51-14, transient thermal measurement provides accurate θJC characterization.

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