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Heat Sink Thermal Resistance (θJA) & Junction Temperature Calculator

Calculate power semiconductor junction operating temperature (Tj), required heat sink thermal resistance (θSA in °C/W), thermal paste interface resistance, and safe thermal limits.

40°C = warm enclosure / industrial chassis
Standard silicon max = 150°C (125°C design limit for reliability)
e.g. TO-220 = ~1.0 - 2.0 °C/W, TO-247 = ~0.6 °C/W

Calculation Results

Maximum Allowable Heat Sink Rating (θSA): 1.70 °C/W Requires Medium Extruded Aluminum Heat Sink
Operating Junction Temperature (Tj): 125.0 °C At 125°C Design Ceiling
Total System Thermal Resistance (θJA): 3.40 °C/W
Case Temperature (Tc): 95.0 °C

About This Calculator

Calculate power semiconductor junction operating temperature (Tj), required heat sink thermal resistance (θSA in °C/W), thermal paste interface resistance, and safe thermal limits.

Frequently Asked Questions

What does thermal resistance (θ in °C/W) mean?

Thermal resistance measures a material's resistance to heat flow, expressed as degrees Celsius temperature rise per Watt of dissipated electrical power. Lower thermal resistance indicates better cooling efficiency.

What is the basic thermal Ohm's law equation?

Analogous to V = I * R, the thermal equation is: ΔT = P * θ, or Junction Temperature Tj = Ambient Ta + P * (θJC + θCS + θSA), where P is power in Watts and θ is thermal resistance in °C/W.

Why is Thermal Interface Material (TIM) required between case and heat sink?

Microscopic air gaps between mating metal surfaces trap air, which is a poor thermal conductor (thermal grease conducts heat 100 times better than trapped air).

What is a safe design margin for silicon junction temperature?

While silicon datasheets often rate maximum Tj at 150°C or 175°C, good engineering practice derates operating Tj to 110°C–125°C to significantly increase component lifespan and prevent thermal runaway.