Page 78 - Power Electronics Handbook

Liquid cooling 71

d.c.

120Q / lmQ/

11 Tl
Current Ambient temperature
Figure 2.8 Curves used to calculate heatsink thermal resistance

Figure 2.1, whereas the right-hand curves are for a heatsink and give its
thermal resistance. Such a series of curves would be obtained, for example,
in a forced &-cooled heatsink operating under a variety of air speeds. If ll
is the current through the power semiconductor, then PI is the power
dissipated through it, and for an ambient operating temperature of TI the
heatsink must have a thermal resistance of 4"W.

2.5 Liquid cooling

Liquids can be used to cool power semiconductors, and this method is
more effective than air cooling. Several methods exist for liquid cooling:
(i) Interconnecting a series of individual, specially designed heatsinks,
each of which carry a single device. These are all linked together by
pipes through which the cooling liquid flows.
(ii) Mounting the power devices on a common liquid-cooled structure,
such as a hollow bus bar, the components being electrically insulated
from each other.
(iii) Immersing the power devices into the cooling liquid, the components
sometimes having small heatsinks fitted to their bodies.
The liquids used can be water or oil. Water has a high speed of flow but
can cause electrolytic corrosion and can also freeze. De-ionised water is
often used to which has been added a suitable anti-freeze agent. Oil is
more viscous than water and can be inflammable. However, it does not
permit the flow of electrolytic currents and devices can be immersed
directly into it.
Liquid-cooled systems can have a thermal resistance below O.Ol"C/W,
which compares with figures of 0.25'W for natural air-cooled systems
and 0.1"cMr for forced air cooling. The added advantage of liquid cooling