Page 76 - Power Electronics Handbook

Heatsinks 69
which is air cooled, whilst Figure 2.6(b) gives an arrangement for the same
device cooled by liquid, as described in Section 2.5.
Heatsinks are usually made from aluminium alloy extrusion, aluminium
being a good conductor of heat, malleable so that it can be readily shaped,
easy to extrude and made with a smooth surface finish. Aluminium gives a
heatsink which is inferior to that made from copper, but it is much cheaper.
Heatsinks are designed with a large surface area, for radiation and
convection of heat, and the weight is minimised. The heatsink may be left
bright, but coloured matt surfaces are more efficient. Black is not
necessarily the best colour since at the temperatures being considered heat
radiation occurs in the infrared region and all enamels, varnishes, anodised
surfaces and oil paints have high emissivities regardless of colour.
Heatsinks are usually designed with fins, the greater the number of fins,
the larger the area for convection cooling, but if the fins are too close
together there is less heat radiation, so a compromise is needed in the
heatsink design. Forced air-cooled heatsinks are three to four times more
efficient than natural cooled systems. Radiation effects are now neghgible,
and since the rate of air flow is less dependent on temperature the thermal
resistance is less variable, so the thermal system can be assumed to be
linear. The air flow is also more independent of heatsink fin spacing and
should be designed to create turbulence over the surface of the fins and
break up any layer of static air.
Electrical isolation is often needed when a device is mounted on a
heatsink, and this can be obtained by using isolating washers. Several
materials are used for these washers; beryllia is the most expensive but has
the highest thermal conductivity and dielectric strength, followed by
hardened anodised aluminium washers which have good thermal
conductivity and dielectric strength. Mica washers were very popular but
they suffer from the fact that they can crack and peel and, because they are
transparent, it is easy for two to become stuck together, which would cause
an increase in the thermal resistance. High-temperature plastics such as
Kapton and Mylar have lower dielectric strength than mica but they are
cheaper, and since they are coloured their shading gives a visual indication
if two are stuck together. An alternative to using an isolating washer is to
spray the heatsink during manufacture with an electrical insulation
material.
The interface between the case of the component being cooled and the
heatsink has a relatively low thermal resistance compared to other parts of
the system. However, the resistance can increase during assembly by a
factor of 10 times unless care is taken to minimise it. This is done by
keeping the mating surfaces clean, by applying adequate mating pressure
and by using a thermal grease between them. This grease, or heatsink
compound, is a silicone material filled with heat-conductive metal oxides.
The grease must not dry out, melt or harden even after operating for long
periods at high temperatures such as 200°C. Figure 2.7 illustrates how the
grease helps to even out the temperatures in the semiconductor package.
Without the grease a slight bump, bow or dust particle causes the
temperature at A to be higher than that at B. The thermal grease replaces
the air and has a much lower thermal resistance so that the temperatures
in the copper tab, and therefore the semiconductor, are more even.