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Cooling power supply without a fan

Posted: 09 Dec 2013     Print Version  Bookmark and Share

Keywords:power supply  fan  Cool  convection  Conduction cooling 

When selecting a power supply for a particular application, there are many reasons why you may want to go against a fan to cool it.

The audible noise coming from a fan can be a deal-breaker. For equipment destined for laboratory or control room environments, where the operator is in close proximity to the equipment at all times, minimal audible noise is a very desirable characteristic. Medical equipment that is used near to the patient, such as patient monitors and infusion pumps that are near the patient for a long period of time, also need to be as quiet as possible.

Another downside of fans is their reliability. The low lifetimes of these mechanical components can mean they fail in use, leading the power supply to overheat, or require more frequent maintenance or replacement before the end of their life.

For systems which require a high IP-rating, fans are clearly out of the question, since very little ventilation is permitted in order to keep solid and liquid contaminants out. For example, any equipment used in food processing areas will need to have a high IP rating as there will be solid and liquid contaminants present in the environment. Lower IP ratings, perhaps enough to keep out dust from industrial equipment, may condone use of a fan, but often air filters are required. These filters will need scheduled maintenance to clean or replace them periodically, which may be undesirable.

Convection cooling
If your application restricts the use of a fan, you'll need to look at convection or conduction cooling.

Conduction cooling involves bolting the unit to a large heat sink or metal box so the heat is transferred to the outside of the equipment. This is usually reserved for high power applications with larger levels of waste heat, as it can be complex and expensive to achieve. This article will instead focus on convection cooling as a simple approach for lower power equipment.

Convection cooling, put simply, means there is enough free air around the power supply that it can dissipate the amount of heat it needs to without raising the ambient temperature too much. Natural convection currents inside the enclosure cool the unit. As a result, there is a big difference in the power densities offered by power supplies for forced air (fan) cooling and convection cooling, for a given efficiency. A typical 3 x 5" power supply may have a convection cooled rating of 150 W while the force cooled version may have a rating as high at 350 W.

It's important to fully understand the likely constraints that a convection cooled power supply might operate under in order to ensure the lifetime and reliability of your design.

Efficiency is even more important for a convection cooled power supply than for a force cooled one, since all the heat will be staying inside the box; if air can be blown over a unit, dissipating a few more Watts doesn't matter as much. Every efficiency point means less heat dissipated – and technology is improving the efficiencies that can be attained all the time (figure).

Figure: Every percentage point improvement in efficiency has a marked effect on the amount of waste heat generated.

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