Fan cooling power supplies, which airflow direction is better?

Even though the efficiency levels of new power supply designs are now routinely in the 94 to 95% range, the push for higher power densities continues. Fan cooling is one option to reduce the product’s overall size for mid to high power requirements. Should the fan blow air into the power supply or extract air out?  This depends on several factors.

In rack mounted products like the Genesys+™ programmable power supplies the airflow is drawn through the front and out the rear. This is to offer the best possible cooling for these products, to optimize the overall system’s performance and avoid the operator being subjected to hot exhaust air.

For enclosed power supplies, air flow direction can depend upon which direction the system air is being directed.  Having the power supply’s air flowing in the opposite direction to larger system’s fans can cause the airflow to reduce dramatically due to the system’s pressure, and cause overheating.

Where the fan is positioned in a power supply is another consideration.  The lifetime of an electrolytic capacitor is extremely sensitive to heat.  Each 10^oC rise in the capacitor’s temperature will halve its operating life, thus cool air has to be directed accordingly.

Figure 1 shows the top view of a typical product (the cover removed) and the location of the capacitors and fan.  In the case of this power supply, as the input and output connectors are both located on the left side (front) for system wiring access, the fan is situated on the right hand side (rear).

Figure 1: Electrolytic capacitors and fan location

Should the fan blow air out, or in? As with any design, there are advantages and disadvantages.

Figure 2 shows the fan blowing air out (exhaust). 

Figure 2: Fan exhausts the hot air

Advantages

The cool air is drawn in over the output capacitors, this keeps these components cool and their lifetime is improved.

As indicated by the size of the arrows, the speed of the air entering the power supply is lower than the exit speed.  This is due to the cross sectional area of the input being twice that of the output  Lower speed air is less likely to draw in outside contaminants (dust and dirt) which could impact product lifetime.

Disadvantages

The input capacitors receive warmer air, but with an air directing baffle and perforations in the cover this could be mitigated. In general though, the input capacitors are less sensitive than the output filtering capacitors.

Higher speed airflow cannot easily be directed at hot items like magnetics.

Hot air is drawn across the fan bearings, which could affect fan life. If the fan speed is controlled according to ambient temperature, this too would be mitigated. Higher quality or higher temperature fans can also be used.

Figure 3 shows the fan blowing cool air in.

Figure 3: Fan draws in air

Advantages

Cool air is drawn across the fan bearings, increasing fan life.

Fast moving cool air creates backpressure and can be directed at hot areas, like the magnetics, reducing the overall de-rating of the power supply.

Disadvantages

The output capacitors may run hotter.  Larger capacitors can be used, which will have less internal heating and run cooler.

More contamination may be drawn into the power supply.

Many fan cooled power supplies offer “reverse fan” options on their datasheets. Often due to reduced thermal performance within the supply and heated air moving through the fan, additional derating may apply.

Power Guy