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Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator

Easily calculate and compute heat sink thermal resistance (r_th,sa) & junction temperature (t_j) calculator with accurate equations, live formulas, and expert guidance.

Total heat dissipation from semiconductor device.

Design derated maximum junction temperature (typically 125 °C or 150 °C).

Maximum internal enclosure ambient air temperature.

Datasheet thermal resistance from die to package case tab.

Thermal grease / TIM pad contact resistance.

Calculated Result
1.09 °C/W

Max Heat Sink R_θ,sa

Max Heat Sink R_θ,sa

1.09 °C/W

Required Heat Sink Rth Sa Deg C Per W

1.09

Total Junction To Ambient Rth Deg C Per W

2.29

Operating Case Temperature Deg C

95.3

Operating Sink Temperature Deg C

83

1.09 — °C/W

Calculation Breakdown

  1. Applied FormulaSizes required extruded or forced-air heat sink thermal resistance to keep power transistor junction temperature below maximum safe semiconductor limits.T_j = T_a + P_d (R_{\theta,jc} + R_{\theta,cs} + R_{\theta,sa}), \quad \Delta T = P_d \sum R_\theta

Heat Sink Thermal Design Metric Output

Interactive visualization based on your current inputs

Calculated Value
0.00.30.50.81.1Max Heat Sink R_θ,sa

What Is the Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator?

This tool computes heat sink thermal resistance (r_th,sa) & junction temperature (t_j) calculator with high precision arithmetic.

How Does the Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator Work?

Sizes required extruded or forced-air heat sink thermal resistance to keep power transistor junction temperature below maximum safe semiconductor limits.

Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

Tj=Ta+Pd(Rθ,jc+Rθ,cs+Rθ,sa),ΔT=Pd∑RθT_j = T_a + P_d (R_{\theta,jc} + R_{\theta,cs} + R_{\theta,sa}), \quad \Delta T = P_d \sum R_\theta

Sizes required extruded or forced-air heat sink thermal resistance to keep power transistor junction temperature below maximum safe semiconductor limits.

How to Use the Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator

  1. Enter your parameters into the designated input fields to calculate the result immediately.

Step-by-Step Example Calculation

Heat Sink Thermal Resistance (R_th,sa) & Junction Temperature (T_j) Calculator Example

Input Values:

totalDissipatedPowerWatts:35
maximumAllowedJunctionTempC:125
maximumAmbientTempC:45
junctionToCaseResistanceRthjc:0.85
caseToSinkResistanceRthcs:0.35
Worked Steps: Sample evaluation based on standard reference conditions.

Understanding Your Result

The calculated primary figure indicates the optimal or required °C/W.

Factors That Affect the Result

  • Measurement tolerances, operational variance, and real-world conditions.

When Should You Use This Calculator?

  • Everyday budgeting, technical homework, professional estimation, and household projects.

Assumptions & Limitations

  • Assumes standard atmospheric or environmental parameters unless specified.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Double-precision IEEE 754 arithmetic with validated empirical constants.

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