THERMAL DESIGN



THERMAL MANAGEMENT OF ELECTRONICS, HEAT EXCHANGERS GUIDES, FREE SOFTWARE AND CALCULATORS



Let's begin by defining the main terms. The goal of thermal management is to control the temperatures of the electronic devices. But what is

temperature

? Technically speaking, on a microscopic level it is a measure of the average molecular kinetic energy in the matter. The normal flow of kinetic energy is from a higher temperature region (or an object) toward a lower temperature region (or an object). This flow is referred to as heat transfer.








In general, there are three types of heat transfer: conduction, convection, and radiation. Conduction is the collisional transfer of energy between atoms, which occurs in solids. Convection is the motion of molecules in air or fluids. Radiation is the energy flow by electromagnetic waves. In practical electronics primarily only the first two types are noticeable.

Electric currents and alternating electromagnetic fields cause power dissipation in all electronic parts, which results in increase of their temperatures. This in turn affects the reliability and life expectancy of these components. Failure rate and its inverse, mean time between failures (MTBF), are measures of the electronic system reliability. According to Arrhenius model, each 10C rise increases the failure rate by 50%.

At certain point any electronic device can be irreversibly destroyed (typical maximum operating temperature "T" for semiconductors is 125-175 C at their junctions, capacitors 85-125 C, wire insulation- 105-200 C). The thermal management and engineering whose task is to control "T" of the product, is therefore an essential part of electronics design.

When designing a heat sink for a semiconductor cooling, first of all you need to choose the component's maximum operating junction temperature Tjmax (typically, 105-120 oC for commercial parts). Then for convection cooling the required heatsink thermal resistance should be
Rth-hs<(Tjmax-Ta)/P-(Rthj-c+Rthc-hs) oC/W, where Ta- ambient, Rthj-c - thermal resistance between junction and the case from the datasheet (typically 0.5-2.5 oC/W for conventional descrete power packages), Rthc-hs - thermal resistance between the device's case and the heatsink, P- power dissipated by the device in watts.

heatsink design thermal circuit See thermal circuit diagram to the left for an illustration. If you use an insulator, you need to take it into account as well. Once you found the required Rth-hs, you can pick a stamped or extruded heat sink with equal or lower value of thermal resistance. For off-the-shelf parts, Rth-hs is normally specified in the datasheet. Harry Lythall found empirically a "rule of thumb" calculation formula for Rth-hs of home made U-shape folded aluminum sheet. Based on his equation, the required surface area in sq.cm is A=(50/Rth-hs)2.
In general, the main optimization criteria are to maximize the exposed heat exchanger's surface area, and to minimize its weight and the mean distance of the exposed surface from the component to be cooled.

Below you will find free calculators and useful information on the thermal design.

HEAT AND THERMODYNAMICS BASICS
THERMAL MODELING AND ANALYSIS SOFTWARE
THERMAL DESIGN GUIDES, ANALYSIS & APPLICATION NOTES

Heat conduction formulas

A detailed heat transfer textbook for engineering students (conduction, convection, radiation)

The First Law of thermodynamics


PCB TRACE CALCULATOR FOR TEMPERATURE RISE


Heat sink calculation, design and analysis

Thermal conductivity units conversion

Temperature units conversion

Duct flow velocity calculator

Thermal resistance and fin efficiency online calculators

THERMAL DATA OF ELECTRONIC COMPONENTS
Basic thermal management of semiconductor devices

Thermal conductivity in cal/sec and W/m*K for various materials

The design of fan speed control

What you need to know about cooling fans

HEATSINK PROPERTIES AND DESIGN NOTES
How to select an extruded heat sink

Analytical model for simulating electronic systems' thermal behavior

Heatsink design guide





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