Basic Thermal Equation and Heat Transfer

Thermal Equation Parameters

Many parameters contribute to a design's thermal circuit, including the

device's maximum power consumption for the design, the maximum

environment temperature, package characteristics, and airflow at the

device.

Maximum Power Consumption (P)

Use the power calculator values from design simulations in the Altera

Quartus® II software (or the device's power calculator at

http://www.altera.com) to estimate the maximum power consumption

of the device. Once a prototype design is available, measure the actual

power consumption and use this value for thermal calculations.

Maximum Temperature (TJ & TA)

The maximum ambient and junction temperatures are found in the data

sheet for the device under Device Absolute Maximum Rating and the

operating junction temperature is found under Device Recommended

Operating Conditions. The temperature must be kept within the

maximum conditions or damage could occur. The junction temperature

should be kept within the recommended operating conditions to ensure

the device achieves the performance reported by the Quartus II software.

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HEAT TRANSFER Basic Theory

The rate at which heat is conducted

through a material is proportional

to the area normal to the heat flow

and to the temperature gradient

along the heat flow path. For a one

dimensional, steady state heat flow

the rate is expressed by Fourier’s

equation:

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Basic HEAT TRANSFER FUNDAMENTALS and Heatsink

HEATSINK BASICS

All semiconductor devices have some electrical resistance, just

like resistors and coils, etc. This means that when power

diodes, power transistors and power MOSFETs are switching

or otherwise controlling reasonable currents, they dissipate

power — as heat energy. If the device is not to be damaged by

this, the heat must be removed from inside the device

(usually the collector-base junction for a bipolar transistor, or

the drain-source channel in a MOSFET) at a fast enough rate

to prevent excessive temperature rise. The most common way

to do this is by using a heatsink.

To understand how heatsinks work, think of heat energy itself

as behaving very much like an electrical current, and

temperature rise as the thermal equivalent of voltage drop. We

also have to introduce a property of materials and objects

known as thermal resistance, which behaves in a very similar

way to electrical resistance: the more heat energy ‘flowing’

through it, the higher the temperature rise across it. As you

might imagine metals like copper and aluminium have very low

thermal resistance, while air tends to have a relatively high

resistance. So do many plastics and ceramic materials.

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HEAT TRANSFER FUNDAMENTALS

Introduction

The objective of thermal management programs in electronic packaging is the efficient removal of heat from the semiconductor junction to the ambient environment. This process can be separated into three major phases:

1) heat transfer within the semiconductor component package;

2) heat transfer from the package to a heat dissipater (the initial heat sink);

3) heat transfer from the heat dissipater to the ambient environment (the ultimate heat sink)

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Heatsink