link in the thermal chain if it is not taken into account where the medium itself
discharges theenergy to).
As already described, and set out in detail in chapte
particular attention should
be paid to the thermal contact between the power semiconductor or module and the
surface of the heatsink. This thermal contact depends on the surface qualities, flatness,
contact pressure and insulating and filler substances used.
Currently there is no binding international standard to determine the thermal
performance of a heatsink, andmost suppliers have their own test methods.
Selected heatsink suppliers
4.4.1 Air coolers
When a heatsink cools naturally, its thermal energy is directed into the ambient air that
exists freely in the space around it. Essentially, the heatsink dispels the heat mainly
through convection and partly through thermal radiation. The proportion of energy
dispelled through thermal radiation depends on the shape of the heatsink. In a heatsink
with fins for example, the energy is dissipated by thermal radiation almost exclusively
through the circumferential section of the heatsink, as the clearance between the fins is
often too narrow for the radiation to be directed outwards and instead, the only
exchange of radiation is between the fins that are opposite each other
Therefore, the proportion of radiation does not increase in proportion to the surface area
available for convection.
Heatsinks currently in use are optimised for convection and not for thermal
There are two types of forced cooling: Forced air cooling and liquid cooling. In forced air
cooling, the cool air is moved by a fan and sucked or blown through the fins of the
heatsink, depending on where on the heatsink the ventilator is placed. The thermal
resistance of the heatsink depends on the speed of the cool air blowing past it. The
faster the air speed, the greater the amount of heat that can be dissipated, until a point
at which thermal energy can no longer be dissipated into the air that is flowing past and
the thermal resistance evens out to a value that is more or less independent of the air
speed. The superiority of an air cooler with forced air compared to convection ventilation
can be expressed by a reduction factor f. This reduction factor is dependent on the
stated air speedand is approximately the same as i