Figure 2 So we can now use this knowledge to calculate just how much more
efficient each subsequent design must be in order to maintain the same reliability.
Pdiss_inch3 = (Pout ((1- / ))/ (L x W x H) Eqn (3)
Where Pdiss_inch3 = Power to be dissipated per cubic inch L = Length (inches) W = Width (inches) H = Height (inches)
There is a direct correlation between Power dissipated per cubic inch, and thermal rise of internal components. In turn this will have a direct impact on the lifetime expectation of that component. If we look at this through the eyes of Arrhenius’ equation, we can see that many common chemical reactions at room temperature will double for every 10 degree Celsius rise in temperature.
If you cannot make your designs sufficiently high enough, then you will need to consider options in dealing with getting heat away from key components.
Thermal operating limits There can be up to three methods by which heat is transferred from one location to another. Conduction is the transfer of heat from one material to another. Convection is where heat is transferred from a heat generating material to a moving surrounding fluid. Thermal radiation is the electromagnetic radiation emitted by a body due this its temperature. On a power supply we can see this in the form of 1. Heat transfer of kinetic energy from one molecule to another through a solid medium where a temperature difference exists between the two bodies. This is referred to as Conduction cooling. 2. Convection is generally air, but in some cases we are starting to see fluid being introduced, and is governed by Newton's law of cooling. 3. Thermal radiation is the electromagnetic radiation emitted by a body due this its temperature. A good everyday example of this would be the sun radiating heat onto the earth’s atmosphere. Unlike the other methods mentioned above this is not a linear event.
Heat sinks v fans
If the above methods of passive cooling are not enough, then you may need additional cooling in form of a heat sink or forced air-cooling. You can either use a fan in your design or specify a level of forced air-cooling for the customer. Remember, forced air-cooling functions by the volume of air that it can force across a component. You will also note that the volume of airflow will be fixed for any given fan, and velocity will vary with the cross section of the fan.
The optimum solution would be a combination of the two methods above. Heatsinks can be incorporated to take heat away from heat generating components, and with careful design, you can use fans to effectively remove the heat from these heatsinks. As you reduce the size of your power supply, the key to maintaining
reliability is a combination of reducing losses and effectively dealing with heat flow away from key components. As size decreases, losses need to be reduced accordingly. This approach allows you to maintain reliability. The most important factor is good, careful design based on sound experience, resulting in known safety margins.
Excelsys Technologies |
www.excelsys.com
Shane Callanan is Director of Applications Engineering Manager at Excelsys Technologies
May 2013 CIE Power Supplement 13
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