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Application Note AN-1057
Heatsink Characteristics

Table of Contents Page
Introduction ......................................................................................................... 1 Maximization of Thermal Management ............................................................. 1 Heat Transfer Basics.......................................................................................... 1 Terms and Definitions ........................................................................................ 2 Modes of Heat Transfer ...................................................................................... 2 Conduction .......................................................................................................... 2 Convection.......................................................................................................... 5 Radiation ............................................................................................................. 9 Removing Heat from a Semiconductor........................................................... 11 Selecting the Correct Heatsink ........................................................................11

In many electronic applications, temperature becomes an important factor when designing a system. Switching and conduction losses can heat up the silicon of the device above its maximum Junction Temperature (Tjmax) and cause performance failure, breakdown and worst case, fire. Therefore the temperature of the device must be calculated not to exceed the Tjmax. To design a good ThermalManagement solution, the Tj should always be kept at the lowest operating temperature.

Application Note

AN-1057

Heatsink Characteristics
By Llew Edmunds, International Rectifier. References to Aavid Thermalloy,

Topics Covered
• Introduction • Maximization of Thermal Management • Heat Transfer Basics • Modes of Heat Transfer: • Conduction • Convection • Radiation • Removing Heat from aSemiconductor • Selecting the Correct Heatsink • Extrusion Data • Temperature and Length Correction Factors • Thermal Modeling Capabilities

Introduction
In many electronic applications, temperature becomes an important factor when designing a system. Switching and conduction losses can heat up the silicon of the device above it’s maximum Junction Temperature (Tjmax) and cause performance failure,breakdown and worst case, fire. Therefore the temperature of the device must be calculated not to exceed the Tjmax. To design a good Thermal Management solution, the Tj should always be kept at the lowest operating temperature.

Maximization of Thermal Management
Thermal management should be determined at the board layout design stage, not later. It is feasible and less costly to determine thethermal load at your design process of the pcb board. The ability to design in optimal solutions, more flexibility, more choices, and also to save possible device failures after the design has been finalized. Most of the problems occurring at the end of the design cycle are due to thermal management considerations.

Heat Transfer Basics
Heat transfer occurs when two surfaces have differenttemperatures, thus causing heat energy to transfer from the hotter surface to the colder surface. For example, voltage is the driving force that causes current to flow. By analogy, temperature is the force that causes heat to flow. If the temperature difference is increased, the amount of heat flow will be increased: Heat transfer α temperature difference

Terms and Definitions
Heat Load (W)Ambient Temperature (Tamb) Maximum Junction Temperature (Tjmax) Amount of heat energy produced. Usually defined as: (voltage drop across device) x (current flowing through device) Temperature of air immediately around device to be cooled Maximum allowable temperature that the device will see at it’s silicon junction The resistance that the heat energy meets in its flow from hot to cold. This is the...