Convection cooling basics for applications that cannot use a fan
When choosing a power supply for a given application, there are many reasons why you may want to avoid using 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.
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 150W while the force cooled version may have a rating as high at 350W.
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 1).
When specifying a convection cooled power supply for your equipment, you’ll want to choose the most efficient power supply that matches your budget. For example, if your application requires the highest efficiency, XP’s CCB200 boasts an efficiency of 94%; this 200W supply dissipates just 12W from its 3 x 5” footprint (Figure 2). For more cost-sensitive applications, a suitable alternative might be the 150W convection-cooled GCS180, which is 92% efficient.
It’s advisable to check that whatever claims the manufacturer is making about the rating can be backed up by the efficiency, otherwise cooling is likely to be a problem. There are several things to bear in mind that may affect the headline efficiency number quoted on the data sheet. This headline efficiency is usually a best-case-scenario and will therefore not be available over the full spec of the unit.
For example, the input voltage used affects efficiency in real-world applications. Most ac-dc supplies on the market have a universal input for worldwide use. However, in the USA, where mains voltage is around half the European equivalent (230V), double the input current is required to produce the same amount of power. Working with higher currents produces a lot more loss in the various components of the power supply and efficiency suffers. Considering the resistive losses alone, the power lost is I2R, so doubling the input current (from high line voltages to low line) actually multiplies the power lost by a factor of four. Dropping from US voltages (115V) to Japanese mains (90V) increases the input current by a further 28%, creating another 65% more resistive loss. Many data sheets therefore only specify the efficiency at high line voltages.
The next thing to check on the datasheet is the rating curves for the power supply. It may surprise you to learn that there isn’t an industry standard way for manufacturers to gather this rating information – practices vary widely. Products are generally tested in an environmental chamber, but some of these chambers use fans to maintain the temperature, inadvertently creating an air flow around the unit. Obviously, this is to be avoided if you are trying to measure under strict convection conditions since even a small amount of air flow can have a significant effect.
At XP Power, the firm places its product in the chamber inside another box and maintains the ambient temperature based on the temperature inside that box, to try to get an accurate picture of the product’s performance. This creates consistency across design groups and repeatability, which is also a challenge when performing thermal testing. Information like how the product is tested is almost never included on the datasheet but nevertheless has an impact on the final ratings.
In order to meet safety requirements, be it industrial, medical or IT, there are maximum temperatures that certain components (like transformers) can be run at. The thing to remember here is that the safety temperatures are the absolute maximum temperatures allowed, not a recommendation for normal use. If the product was to run consistently at the safety temperature, though it might pass UL approvals, the lifetime of sensitive components would suffer. To deliver a long life, the system should run as cool as possible.
Tips and tricks
Other things to remember when designing for convection cooling include the fact that the power supply is designed to be placed horizontally or vertically. At no time should it be used upside-down. Since heat generated by the power supply will naturally rise, placing the power supply’s PCB above the hot components is not recommended.
Also, don’t forget to consider what other parts of the system are likely to get hot. Are you using a large CPU? A display? What about a motor or pump? These heat-generating components will also require cooling and will add to the waste heat inside the product.
Though convection cooling does not permit as much waste heat as forced air cooling, it is a must for certain applications where a fan is not acceptable. Since the waste heat remains inside the enclosure, it helps to choose a power supply whose efficiency is as high as possible and to check carefully whether the manufacturer’s efficiency claims can actually be met in your application.