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Online estimation of the heat load
that can put on a PCB. The procedure implements the
cooling efficiency method.
A more elaborated implementation is found in the
Bload
code. It can in addition handle
serial sub rack cooling, natural convection,
sub rack impacts and
flow channels with fans.
Exact values for the cooling efficiency can be calculated with
the Btemp code.
Related links
Thermal design for electronics
Thermal online tools
Unit conversion
Comments
General
The
cooling efficiency method is a simple and
powerful front end design method. The basic idea is to
replace the lay out impact with an experience value, called the
cooling efficiency.
Experiance values
A good default value for signal processing multi-layer PCBs is
70%. If the PCB is a power application it is lower.
If the PCB is larger than 200x200 mm, reduce the
value by 10% for each 100 mm. If the sub rack inlet conditions
creates considerable disturbances the value can be 30%
higher. The 100% limit can be exceeded if there
are heat sinks.
To read more about the cooling efficiency concept
download
chapter 3 of the theory document. (pdf-file).
Inputs
Height
Is the length of the PCB in the air flow direction.
Width
Is the width of the PCB.
Air inlet temperature
The air inlet temperature is the temperature of the air that
enters the PCB gap. In most cases it is the ambient enclosure
temperature but some times the air is pre-heated because it
has passed over other heat sources or because there is a
re-circulation of exhaust air into the inlet air.
Most thermal design is targeted to deal with the worst
temperature case. The ambient enclosure temperature is then
given by the environmental specification. For telecom
equipment it is 45 or 50 C, for consumer products it is
usually 40 C.
Maximum PCB temperature
The maximum PCB temperature that can be tolerated depends on
the quality of the components. Commercial grade components are
guaranteed to function safely at 70 C ambient temperature.
Industrial grade components are classified for 85 C.
Most passive components will essentially take the same
temperature as the PCB. The critical temperature for the active
components is the chip temperature but solder fatigue problems may
also limit the PCB temperature.
The maximum PCB temperature is always found below one of the
active components. A reasonable default assumption is that
the temperature of the surrounding passive components is
5 C lower.
The maximum allowed PCB temperature for commercial grade
components can therefore be estimated to 70+5=75 C.
If the passive components have industrial grade but not the
active components, it is probable that the chip temperature of
one of the active components sets the limit. A default
assumption of 85 C is reasonable for such a case.
If all components are of industrial grade one could use 85+5=90 C
as the maximum PCB temperature.
Note that an estimation never is complete until the most
critical chip temperature has been checked.
Gap
Is the wall-to-wall distance to the side PCBs.
Cooling efficiency
Is the cooling efficiency value.
Start and end velocity
The air velocity is defined as the average air velocity in
the gap if there were no components. The calculation is
made for 30 points between the start and end velocities.
Outputs
Air efficiency
The air efficiency is a measure on how well the air is used for
cooling. It is defined as the ratio between the temperature
increase in the air and the maximum temperature difference
between the PCB and the incoming air. The maximum value is 100%.
The Air Efficiency Concept article treats this subject more
in depth.
Editing
Save last/Delete last
Saves or deletes curves in the diagram. Up to 5 curves
can be saved. Saved curves are colored.
Note
Clicking in the diagram opens a panel with tabled results
that can be copied and pasted into a spread sheet.
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