Thermal Resistance Calculator Formula

Understand the math behind the thermal resistance calculator. Each variable explained with a worked example.

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Formulas Used

Junction Temperature

junction_temp = t_ambient + power_w * theta_total

Total Thermal Resistance

total_theta = theta_total

Case Temperature

case_temp = t_ambient + power_w * (theta_cs + theta_sa)

Variables

VariableDescriptionDefault
power_wPower Dissipated(W)5
theta_jcJunction-to-Case (θjc)(°C/W)3
theta_csCase-to-Sink (θcs)(°C/W)1
theta_saSink-to-Ambient (θsa)(°C/W)8
t_ambientAmbient Temperature(°C)25
theta_totalDerived value= theta_jc + theta_cs + theta_sacalculated

How It Works

Thermal Resistance Chain

Formula

T_junction = T_ambient + P x (θjc + θcs + θsa)

Thermal resistances add in series like electrical resistances. Each interface adds temperature rise per watt of dissipation.

  • θjc: junction to case (from datasheet)
  • θcs: case to heat sink (thermal pad/paste)
  • θsa: heat sink to ambient (heat sink rating)

    • Worked Example

    5 W dissipation, theta_jc=3, theta_cs=1, theta_sa=8 C/W, 25C ambient.

    power_w = 5theta_jc = 3theta_cs = 1theta_sa = 8t_ambient = 25
    1. 01Total theta = 3 + 1 + 8 = 12 C/W
    2. 02T_junction = 25 + 5 x 12 = 85C
    3. 03T_case = 25 + 5 x 9 = 70C

    Frequently Asked Questions

    Where do I find theta_jc?

    In the component datasheet under thermal characteristics. It depends on the IC package.

    What is theta_cs?

    Thermal resistance of the interface between IC case and heat sink. Thermal paste: 0.2-1 C/W. Bare contact: 1-3 C/W.

    How accurate is this model?

    It is a simplified steady-state model. Real systems have transient thermal behavior and non-uniform heat distribution.

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