Power supplies generate waste heat which has to be dissipated. They typically have either convection cooled or forced cooled ratings or, in some cases, both. Forced cooled power supplies may incorporate a cooling fan, or may specify the user cooling required to operate the unit at maximum load and ambient temperature.
Where user cooling is required it is most important that the power supply cooling is adequate for both safe operation and adequate service life. It is very application specific and dependent on the ambient temperature, applied load and physical location with respect to the cooling fan and other system assemblies.
The main difference between convection and force cooled products is in the power density offered. For a given efficiency, convection cooled products offer a lower power density, meaning that they occupy a larger volume. A power supply on a 3” x 5” industry standard footprint may have a convection rating of 150 W while the force cooled version may have a rating as high as 350 W.
Where the power supply has a convection cooled rating, it is intended to be used in an environment where there is free air. The system designer must ensure that there is adequate space around and above the unit for free air convection currents to cool the unit and must also ensure that the ambient temperature local to the power supply is controlled to a level within its maximum ratings.
Force cooled products with integral cooling fans are easy to apply as it is a simple matter of ensuring that the maximum specified ambient temperature is not exceeded for a given load rating and that the intake and exhaust areas are not obstructed.
Typically, power supplies that require the user to provide forced air cooling will specify a minimum required airflow. This is usually for operation at 100% of the power rating at the maximum ambient temperature allowed.
The required airflow is often specified in Cubic Feet per Minute (CFM) which is also the common rating for cooling fans. The effectiveness of cooling fans installed in enclosures must be given consideration, as discussed earlier in this section, and the CFM rating deals in volume of air rather than air speed, which is the important factor. The object is to maintain the components used within the power supply at a safe operating temperature and to ensure adequate service life.
When the required airflow is specified in CFM it assumes that the power supply is installed in an area which is relatively similar to it’s own cross sectional area. This is rarely the case as the power supply is typically used as a sub-assembly within a complete equipment enclosure. It will also assume that the air is directed at the power supply, which may also not be the case, so converting to Linear Feet per Minute (LFM) or meters per second (m/s) provides a more valid criterion as linear air speed measurements specify where the air is flowing and directly relate to heat transfer.
In the case above, the power supply requires forced air of 7 CFM in the direction indicated by the arrow. The cross sectional area is:-
3” x 1.43” = 4.29 inches2 or 0.0297 feet2
Therefore the air velocity required is:-
7/0.0297 = 236 LFM or 1.2 m/s
This air speed can be measured locally to the power supply to ensure that sufficient forced air cooling is being applied.
Evaluation of the Application
The object is to maintain the components used within the power supply at a safe operating temperature and to ensure adequate service life. Given the huge potential for variation between one application and another, the only real test is measurement of the temperature of the critical components within the power supply assembly when installed within the end application under the worst case external ambient conditions. The other option is to model the application exactly using a suitable software simulation.
The criteria for safe operation will be specified for the power supply in question or can be obtained from the manufacturer. For the example above, the specific component temperatures for safe operation are given below; these are typical for a power supply of this type.
While these figures will ensure safe operation they do not give any indication of the service life that can be expected. The lifetime of a power supply is largely determined by the temperature of the electrolytic capacitors, which have a wear out mechanism. As a general rule, capacitor lifetime can be doubled for every 10 °C drop in operating temperature.
The graph below indicates the expected service life of the power supply based on measurement of two key electrolytic capacitors.