Sunday, March 14, 2010


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.

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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