Last time, we finished with the point that since all heat flows from hot to cold, a passively cooled device will increase in temperature until it is hot enough to drive the heat into ambient air, (by definition our ultimate heat sink).  This time, we will cover some principles of dissipating heat into ambient air and what that means for cooling the device.  We could have chosen ocean water, such as a power plant would use, or a fuel tank, such as used in some aerospace electronics cooling applications, but most often, the heat is dissipated into the air.  

The short version of this is that a surface must be significantly hotter than air to transfer heat to the air.  Since our focus is on active systems, we will not get into heat sink designs with enhanced surface area (fins).  A quick review of the principals involved will further the discussion and perhaps be useful.  

Although the examples shown in the figures below are specific to electronics the principles are identical whether the device is human skin, a laser diode, or a battery.  In this case, our device is an integrated circuit.  In a typical system, there is an accumulation of temperature rise between the ultimate heat sink and the device to be cooled.  In this case, Ta, the temperature of ambient air, and Tj is the junction temperature at the heat source.  The figure shows the temperature rise from air to the surface (Th) and the temperature rise due to conduction losses (Ti) between the source and the surface.  They are shown as resistors in the schematic. As noted, the device Tj must be hotter than air to overcome the thermal resistance.

Using this same example, values have been applied to the variables to provide a practical example of the temperature rise involved when dissipating heat to air.  For clarity, a temperature rise value of zero has been applied to the internal resistance.   In practice, this is not the case and needs to be known, but for the purposes here it does not matter.  The temperature rise between the surface and ambient air depends on the surface area and whether fans are used.  For a free convection system, using no fans on a flat surface, a reasonable heat transfer coefficient (h) would be 5W/m2K.  For forced convection h would be as high as 50.  Schematic for the expected temperatures at the device for these two conditions.   

From this, the effect of the ambient air temperature on the device temperature is significant.  A 10-degree rise in air temperature will cause a 10-degree rise in device temperature.  The point here is that any system using fans only will not be able to reduce device temperatures below that of the ambient air. 

This is why, with every inquiry, we receive here at Aspen Systems, that we always ask the following three questions. 

  1. What is the cooling load?
  2. What is the required device temperature?
  3. What is the ambient air temperature?

With these three questions, we have a good idea of the type of cooling system that will be required. 

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