Convection cooled power supplies are ideal for applications where acoustical noise cannot be tolerated or when there is a possibility that dirt, dust and other materials can be drawn in by a cooling fan. With power supply efficiencies now commonly in excess of 90%, more fan-less products are entering the market, particularly in the 100-300W power range.
At first glance it seems that they are easier to install. No need to direct forced air at the power supply or calculate how much air is needed – the product will operate correctly without any concern. There are, however, some considerations that the user should be aware of.
The definition of convection cooling is the transfer of heat by the movement of fluids or gas. Hot air rises, being is less dense than cold air, causing the circulation of air through or around a heat dissipating product.
In the case of an open frame power supply, it is often mounted horizontally on standoffs to provide clearance for solder joints and under board components. Figure 1 shows the direction of the air currents, with the cooler air being drawn in at the bottom.
The speed of this air is quite low, in the order of 0.3m/s, but it is enough to reduce the power supply’s component temperatures and avoid “hot spots”. The safety bodies (BSI, TUV, UL, etc) perform thermal testing in this orientation and state on the conditions of acceptability that thermal measurements should be repeated in the end equipment.
To avoid restricting the natural convection air currents, sufficient clearance should be provided around the power supply. A 50mm space is considered adequate.
In some applications a vertical orientation is required (figure 2) and care should be taken to examine if heatsinks and other bulky parts will restrict the air currents. To increase field life it is good practice to ensure that electrolytic capacitors are situated at the bottom where they will operate cooler.
Often manufacturers will state a recommended mounting orientation and suggest a derating factor if that is not possible. Mounting a convection cooled power supply upside down (with the circuit board at the top – Figure 3) can dramatically reduce field life. This is usually forbidden.
If you have concerns, consult the manufacturer’s installation manual, or contact their technical support. With high density power supplies, there is usually a list of maximum component temperatures for the critical parts. To avoid potential delays during safety certification, it is best to check those before submitting your product.