IGBT Modules - Technologies, Driver and Application (Second Edition) - page 199

187
Fig. 4.26
Example of the correction factor for a heatsinkR
th
in forced cooling, compared
with convection cooling
The R
th
of the air cooler when changing from convection cooling to forced cooling can
be described as:
convection
,th
forced ,th
Rf
R
⋅ =
Eq. 4.22
With regard to the power P or operating point of the fan in forced cooling, the air speed
and the air pressure or back pressure, the same applies as for the considerations about
a liquid cooler, whichwill be covered inmore detail in thenext section.
4.4.2 Liquid cooling system
In heatsinks with liquid cooling, the thermal energy is dissipated into the fluid that is
flowing through. In most cases, this is water or a mixture of water and glycol, and may
also contain additives. An important criterion in the design of a liquid cooler is the flow
velocity of the cooling medium and the pressure drop in the total cooling circuit.
Pressure dropmeans the difference in pressure that the coolingmedium has to produce
in order to flow in a cooling circuit. The causes of pressure drop are determined by the
frictional and/or gravitational forces that act on the cooling medium, so the specific
pressure drops caused by inlet connecting sockets, bends, etc. in the system have to be
added together to give an overall view. If themanufacturer of the heatsink and the other
components being used has not provided any information in the form of a pressure drop
characteristic as a function of the flow velocity, so as to set the pressure drop for the
entire system, the systemmust bemeasured.
The relationship between the pressure drop and the flow velocity is expressed by an
exponential function. The higher the pressure drop, the greater the resulting flow
velocity
). To achieve the required flow velocity, a pump with the appropriate
pressure rating must be used. If the pressure as a function of the flow velocity of the
pump has been provided
), the operating point can be determined by
superimposing the diagram for the pressure drop as a function of the flow velocity,
which lies at the intersection of the two graphs
).
Several basic points should benoted to reduce the pressuredrop in the system:
The cross-section of the ducts for the cooling medium should be as large as
possible.
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