Analyze heat-sink performance in power-electronics applications, including LED heat sinks, flat-plate heat sinks, aluminum heat sinks, and other thermal solutions. Enter thermal resistance, power dissipation, and maximum ambient temperature to estimate component junction temperature and support reliable thermal design.
Input Parameters
Typical values of thermal resistance for various electronics packages
Package
Junction to Case (°C/Watt)
Junction to Air (°C/Watt)
TO-3
5
60
TO-39
12
140
TO-220
3
62.5
TO-220FB
3
50
TO-223
30.6
53
TO-252
5
92
TO-263
23.5
50
D2PAK
4
35
Thermal Resistance for PCB Copper
Heat Sink
Thermal resistance (°C/Watt)
1 sq inch of 1 ounce PCB copper
43
.5 sq inch of 1 ounce PCB copper
50
.3 sq inch of 1 ounce PCB copper
56
Aavid Thermalloy, SMT heat sink: PN:573400D00010
14
Introduction
Electronics Thermal Heatsink Design Tutorial
To prevent device failure, power components must operate within a specific maximum junction temperature. When designing power electronics, you must consider the device's power dissipation, the ambient environment, and the thermal resistance of the system to ensure the device stays within safe limits.
This tutorial covers the fundamentals of Heat Sink Design, including how to use a calculator, the underlying thermal theory, and how to select the right heat sink type for your application.
1. Heat Sink Calculator Overview
A Heat Sink Calculator is an essential tool for estimating the operating Junction Temperature (TJ) of a power electronic component.
Key Inputs Required:
Maximum Ambient Temperature (TA): The temperature of the air surrounding the device.
Maximum Junction Temperature (TJmax): The limit specified in the component's datasheet.
Power Dissipated (PD): The heat energy generated by the device (in Watts).
Thermal Resistance (Rθ): The resistance to heat flow at various interfaces (Junction-to-Case, Case-to-Sink, etc.).
2. Understanding Thermal Resistance & Temperature
What is Junction Temperature?
The Junction Temperature is the highest operating temperature of the actual semiconductor chip inside the package. It is almost always higher than the temperature of the case or the heat sink.
Goal: The goal of thermal design is to keep the Junction Temperature below the manufacturer's specified maximum (e.g., 125∘C or 150∘C).
How to Calculate Heat Sink Temperature?
Thermal management is often modeled using an electrical circuit analogy (See Ohm's Law for Thermodynamics):
Temperature is analogous to Voltage.
Heat Flow (Power) is analogous to Current.
Thermal Resistance is analogous to Electrical Resistance.
The Thermal Equation
To determine if a heat sink is required (or if a specific heat sink is sufficient), use the following steady-state thermal equation:
TJ=PD×(RθJC+RθCS+RθSA)+TA
Where:
TJ: Junction Temperature (∘C)
TA: Ambient Temperature (∘C)
PD: Power Dissipated by the device (Watts)
RθJC: Thermal Resistance (Junction-to-Case) - From Datasheet
RθSA: Thermal Resistance (Sink-to-Ambient) - Property of the Heat Sink
3. Introduction to Heat Sinks
What is a Heat Sink?
A heat sink is a passive heat exchanger, typically made of metal (aluminum or copper), attached to a device to dissipate heat into the surrounding fluid (usually air).
Conduction: Heat moves from the device to the heat sink.
Convection: Heat moves from the heat sink surface to the air.
How a Heat Sink Works (The 4-Step Process)
Generation: The source (CPU, MOSFET, etc.) generates heat due to electrical resistance or switching losses.
Transfer: Heat travels from the source to the heat sink via conduction. Materials with high thermal conductivity (like Copper and Aluminum) are preferred.
Distribution: Heat spreads throughout the mass of the heat sink, moving from hotter areas near the source to cooler areas at the fins.
Dissipation: Air (or liquid) flows over the heat sink surface. Through convection, the heat is transferred from the fins to the air and carried away.
4. Types of Heat Sinks
Heat sinks are categorized by their manufacturing method and fin structure.
Extrusion
The most common and cost-effective type. Aluminum is pushed through a die to create long shapes with straight fins.
Pros: Cheap, easy to customize.
Cons: Limited fin height-to-gap ratio (typically 6:1), limiting performance in high-density applications.
Bonded/Fabricated Fins
Individual fins are bonded to a grooved base plate using thermally conductive epoxy or brazing.
Pros: Allows for much taller fins (aspect ratios of 20:1 to 40:1) and significantly higher cooling capacity without increasing footprint.
Castings
Made by pouring molten metal into a mold (Die casting or Sand casting).
Pros: Can create complex 3D shapes and high-density pin fins. Good for impingement cooling.
Folded Fins
Corrugated sheet metal (copper or aluminum) is brazed onto a base plate.
Pros: Increases surface area significantly in a small volume; lightweight.
Stampings
Sheets of metal stamped into specific shapes.
Pros: Extremely low cost, suitable for low-power applications where high performance isn't critical.
5. How to Select a Heat Sink
Selecting the right heat sink involves determining the required Volumetric Thermal Resistance. You must also decide between Natural Convection (passive airflow) and Forced Convection (using a fan).
Sizing Guide
The table below estimates the required volume of a heat sink based on the airflow available.
Range of Volumetric Thermal Resistance
Flow Condition
Velocity (m/s)
Volumetric Resistance (cm3⋅∘C/W)
Natural Convection
~0
500 - 800
Low Flow
1.0 (200 lfm)
150 - 250
Moderate Flow
2.5 (500 lfm)
80 - 150
High Flow
5.0 (1000 lfm)
50 - 80
Note: As airflow increases, the efficiency of the heat sink improves, allowing for a smaller heat sink to handle the same thermal load.
FAQ
What does the Heat Sink Temperature Calculator do?
It calculates the junction temperature of power components from inputs such as ambient temperature, thermal resistance values (R1 and R2), and power dissipation. This helps you confirm that your thermal design stays within safe operating limits for components like CPUs, GPUs, and power modules.
What should I enter for Thermal Resistance 2?
Thermal Resistance 2 (R2) is optional and applies to multi-stage thermal paths. If you are unsure, leave it blank or enter 0. The calculator will use R1 (junction-to-case or heatsink resistance) for the primary calculation.
How do I use the typical thermal resistance values table?
The table provides reference values for common package types, such as TO-220 with 3°C/W junction-to-case resistance. Select your component package to automatically fill the Junction-to-Case field and get more accurate results.
What heat sink materials does this calculator support?
It supports aluminum, copper, and PCB-based heat sinks—for example, 1 square inch of PCB copper equals 43°C/W. The tool accounts for material conductivity differences through the thermal resistance inputs, so it works with folded fin, extruded, and cast designs.
How should I interpret junction temperature results?
Compare the result with the maximum limit in your component's datasheet (for example, 150°C). If the value is too high, increase the heatsink size, improve airflow, or reduce power dissipation. The Power field also indicates the safe operating wattage for your device.
Calculate output voltage together with inverting and non-inverting gain for operational-amplifier circuits. Enter V1, V2, Vp, Vn, and resistor values R1 through R4 to obtain accurate results for circuit design, troubleshooting, and op-amp learning.
