The audible noise of a ventilator can be a problem. For equipment intended for laboratories or control rooms, where the operator is working close to the device, the audible noise should be kept to a minimum. Medical equipment used near the patient, such as vital signs monitors or infusion pumps, for extended periods, also needs to be as quiet as possible.
Another point to consider with fans is their reliability. The short lifespan of these components can mean they fail during use, causing the power supply to overheat, or require more frequent maintenance or replacement before the end of their useful life.
For systems requiring a high IP rating, fans are not suitable, as only minimal ventilation is permitted to keep harmful liquids and solids out of the equipment.
For example, any equipment used in food processing will need a high IP rating because liquids and solids will be present in the environment. With lower IP ratings, it may be sufficient to remove dust and other particles from industrial equipment, even tolerating the use of a fan, but air filters will often be necessary. These filters will require scheduled maintenance for periodic cleaning or replacement, which is not ideal.
Convection Ventilation:
If your application restricts the use of a fan, you will need either convection or conduction ventilation.
Conduction cooling involves bolting and securing the power supply to a large radiator or heat sink, transferring heat away from the device. This is particularly suitable for high-power applications with high heat generation, but it can be complex and expensive to implement.
This article focuses on convection cooling as a simpler approach for lower-power devices.
Convection, or unforced, cooling simply means that there is sufficient free air around the power supply, allowing it to dissipate the necessary amount of heat without significantly raising the ambient temperature. Natural convection currents within the device cool the power supply. As a result, for a given efficiency, there is a significant difference in power densities offered by power supplies that require forced (fan) cooling and those that require convection cooling. A typical 3 x 5” convection-cooled power supply might be 150W, while one requiring forced cooling would be 350W.
It is important to understand the potential limitations under which a convection-cooled power supply will operate in order to ensure the lifespan and reliability of its design.
Efficiency
is even more important for convection cooling than for forced cooling, since all the heat will remain inside the equipment. If some air can be supplied through the unit, dissipating a few more watts, it won't be as significant. Each percentage point increase in efficiency means less heat dissipated, and technology is constantly improving the achievable efficiencies (Figure 1).
When specifying a convection-cooled power supply for your system, you will want to choose the most efficient one that fits your budget. For example, If your application requires the highest efficiency, the XP Power CCB200 achieves 94% efficiency. This 200W power supply dissipates only 12W with its 3 x 5” form factor (Figure 2). For more cost-sensitive applications, an alternative would be the GCS180, which delivers 150W without forced cooling and boasts 92% efficiency.
It is advisable to verify that all manufacturer claims in the documentation can be supported by the actual efficiency; otherwise, cooling is likely the issue.
Several factors can affect the efficiency value provided in the documentation. This efficiency is generally a best-case scenario and therefore will not be the expected performance across the power supply's entire specification range.
For example, the input voltage affects efficiency in real-world applications. Many AC/DC manufacturers specify a universal input voltage. However, in the USA, where the input voltage is approximately half that of Europe (230V), the input current must be doubled to produce the same amount of power. Working with high currents results in greater losses in various power supply components, and efficiency suffers. Considering only resistive losses, the power loss is I²R, so doubling the input current (when switching from a high to a low input voltage) multiplies the power loss by a factor of four. And switching from a US voltage (115V AC) to that of a country like Japan (90V AC) increases the input current by an additional 28%, creating another 65% loss.
Many documentation specifications only provide efficiency figures for high input voltage values.
Convection Values:
The next thing to check in the documentation is the power supply's graph values. You might be surprised to learn that there's no industry-standard way for manufacturers to agree on providing this information—practices vary widely.
Power supplies are tested in climate chambers, but some of these chambers use fans to maintain the temperature, inadvertently creating an airflow around the power supply. Obviously, this needs to be avoided if you want to accurately measure convection conditions, as even a small amount of air can have an effect. At XP, we place our power supplies in the chamber but inside a box, and we maintain the ambient temperature based on the temperature inside that box in order to get an accurate picture of the power supply's performance.
This creates consistency across design groups and repeatability, which is also a challenge when performing thermal tests. Information on how the product is tested is not mentioned in the source documentation, but it does impact the specified values.
When meeting safety requirements, whether for the industrial, medical, or IT markets, there are maximum temperature values for certain components (such as transformers) that must be met. It's important to remember that safety temperatures are the maximum permissible values, not a recommendation for nominal use.
If the power supply has to operate constantly at the safety temperature, even if it passes UL certifications, the lifespan of the components will suffer. To ensure a long lifespan, the system must operate as cool as possible.
Tips and Tricks:
Another thing to remember when designing with convection cooling is that the power supply is designed to be mounted horizontally or vertically. It should never be mounted upside down, as the heat generated by the power supply rises, and it's not advisable to place the PCB above hot components.
Also, don't forget to consider that other components of the system will be hot. Are you using a large CPU? A display? What about pumps or motors? These components, which clearly generate heat, will need cooling and will add heat inside the system.
In summary, while convection cooling doesn't generate as much heat as forced ventilation, in some applications, not having a fan is essential. Since the residual heat remains inside the equipment, it helps to choose a heat source with the highest possible efficiency and carefully check the manufacturer's specifications to ensure it meets your equipment's requirements.
