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 ($T_J$) of a power electronic component.

Key Inputs Required:

• Maximum Ambient Temperature ($T_A$): The temperature of the air surrounding the device.
• Maximum Junction Temperature ($T_{Jmax}$): The limit specified in the component's datasheet.
• Power Dissipated ($P_D$): The heat energy generated by the device (in Watts).
• Thermal Resistance ($R_{\theta}$): The resistance to heat flow at various interfaces (Junction-to-Case, Case-to-Sink, etc.).

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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., $125^\circ C$ or $150^\circ C$).

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:

$T_J = P_D \times (R_{\theta JC} + R_{\theta CS} + R_{\theta SA}) + T_A$

Where:

• $T_J$: Junction Temperature ($^\circ C$)
• $T_A$: Ambient Temperature ($^\circ C$)
• $P_D$: Power Dissipated by the device (Watts)
• $R_{\theta JC}$: Thermal Resistance (Junction-to-Case) - From Datasheet
• $R_{\theta CS}$: Thermal Resistance (Case-to-Sink) - Interface material/Grease
• $R_{\theta SA}$: Thermal Resistance (Sink-to-Ambient) - Property of the Heat Sink

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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)

1. Generation: The source (CPU, MOSFET, etc.) generates heat due to electrical resistance or switching losses.
2. Transfer: Heat travels from the source to the heat sink via conduction. Materials with high thermal conductivity (like Copper and Aluminum) are preferred.
3. Distribution: Heat spreads throughout the mass of the heat sink, moving from hotter areas near the source to cooler areas at the fins.
4. 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.

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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.

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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 ($cm^3 \cdot ^\circ C/W$)
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.

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