Thermal management has a significant impact on maintaining the lifespan and reliability of power supplies and system subassemblies.
It's common knowledge that if heat is dissipated within an enclosed space or case, the temperature in that space will rise. As the ambient air temperature increases, both the power supply and the load will heat up even more, possibly exceeding their maximum operating temperatures.
It's important to avoid this situation, as heat is the primary cause of unreliability and/or reduced lifespan of power supplies within an electronic system. This is because the durability of electrolytic capacitors is closely linked to a lower operating temperature of the case. This is where cooled power supplies come into play.
How Cooled Power Supplies Work:
Some power supplies are designed to be forcibly cooled by a system fan, which removes excess heat from the case. In these cases, the airflow required for proper cooling will be specified in the power supply's datasheet. It's important to note that this is the airflow required at the power supply's location; a fan placed even a short distance away will not be sufficient.
Since air always follows the path of least resistance, only a portion of the air driven by the fan will reach the power supply where it's needed. Internal panels (baffles) that direct airflow help channel the air along the required path to reach and cool the target components.
How to calculate airflow
Whether the equipment needs to operate at a lower temperature or the power supply requires cooling by means of an external fan, the airflow should be calculated following these steps:
Setting the Maximum Temperature:
First, the maximum operating temperature at which the power supply or electronic components can safely operate must be established. For the power supply itself, this is typically 50°C and is usually related to safety certifications or the need for a lower value to increase component lifespan. A general rule is that a 10°C reduction in the case temperature of an electrolytic capacitor doubles the power supply's lifespan.
Next, the highest air temperature in the environment of the cabinet housing the power supply must be considered. The difference between these two temperatures will be the maximum permissible temperature rise. For example, suppose the power supply can operate at an ambient temperature of 50°C. If the power supply case is designed to operate in an unconditioned environment—where the maximum temperature could reach 40°C—the total permissible temperature rise would be 10°C.
Determining Power Dissipation:
The next step is to establish the amount of power that will be dissipated. The total power dissipated within the enclosure is the sum of the power used by the load, plus the power lost by the power supply as waste heat. For example, if the load consumed by the electronics is typically 260 W, and assuming the power supply has an efficiency of 80%, then the total heat dissipated is 260 W / 0.8, or 325 W. From this, the required airflow volume can be calculated.
There is a simple, universal formula for calculating the airflow needed to maintain a specific temperature increase for a given amount of heat, which uses a constant of 2.6.
The formula is: airflow (in m³/h) = 2.6 * total power dissipated (in W) / allowable temperature increase (in °C). In our example, the required airflow would be 2.6 * 325 W / 10 °C = 84.5 m³/h
How to Control Airflow
Unfortunately, finding a solution isn't as simple as calculating the required airflow. Using the calculations above, you should be able to choose a suitable fan power for the airflow, but in reality, a housing will have a natural resistance to airflow, also known as pressure drop.
Pressure Drop:
Pressure drop reduces the fan's free airflow, which is the volume of air released through the device over a period of time. Pressure drops will also vary between applications, depending on the size and location of the PCB, the size of the intake and exhaust grilles, and the cross-sectional area through which the air flows.
Things get a bit more complicated because pressure drop also depends on the air velocity as it passes through the enclosure. Furthermore, pressure drop is in turn affected by air velocity. Higher air velocity will result in greater pressure drop, but this drop reduces the air velocity. Without careful fan selection, the fan could be ineffective. For example, in an application where the resulting pressure drop and air velocity reach an equilibrium point below the level required to remove heat from the enclosure.
Determining the actual pressure drop for each application would be too complex, as this requires a detailed understanding of fluid dynamics equations. However, we can approximate it using the device characteristic curve shown below in Figure 1. This will give us an initial starting point that can be used for a more detailed evaluation.
Figure 1 - Device characteristic curve
Considering the airflow calculated earlier, the curve indicates a pressure drop of 11 Pa. Therefore, we know we need a fan capable of delivering an airflow of 84.5 m³/h with a pressure drop of 11 Pa.
Each fan manufacturer publishes a chart for each fan showing the airflow at different pressure drops. Figure 2 below shows the curves for five fans. The light-colored cone indicates the optimal operating range for each fan. In our example, fan number five would be needed to guarantee the required airflow of 84.5 m³/h with a pressure drop of 11 Pa.
Figure 2 - Fan flow rates at different air pressures
How to Manage Power Supply Lifespan:
The power supply designer considers all these factors before manufacturers apply design power reduction rules to ensure adequate product lifespan. These rules do not take into account the mission profile, environment, mounting orientation, positioning, surrounding space, applied load, and cooling/ventilation systems installed in the end equipment. Capacitor lifespan, especially in convection or natural cooling environments, should be reassessed based on the installation.
How can users manage the lifespan of power supplies?
Case Temperatures:
It is clear that measuring the applied ripple currents (the AC portion of the current signal) is not practical. However, since both the overall equipment design and the power supply design determine the effective operating temperature of the component, a good indication of the life of each capacitor can be obtained by measuring its case temperature and applying the Arrhenius equation and mission profile to the base life specified by the component manufacturer.
Lifespan:
Many power supply datasheets, such as those for the XP GCS series, identify the key components that determine the product's lifespan. This is especially true for those requiring external cooling from the end device and those designed for convection-cooled applications.
Considering this helps the system designer determine the power supply's lifespan in the end application. The mechanical diagram below identifies the components, and the curves indicate the power supply's expected lifespan based on the temperature of two capacitors (C6 and C23).
Once the required pressure drop and airflow have been established, there are some other considerations to keep in mind. As mentioned earlier, for general equipment cooling, the fan can be located anywhere as long as air flows through the heat-generating components.
Forced Cooling:
In the case of a power supply designed for forced cooling, the amount of air flowing over it is critical for proper and reliable operation. If the fan cannot be placed directly next to the power supply, or if it is not possible to direct the entire airflow over it, the chosen fan must have a much higher capacity rating.
Some fans are specified with an air velocity in linear feet per minute (LFM), while others have a volumetric capacity in cubic feet per minute (CFM) or cubic meters per hour (m³/h). To convert between these units, you need to know the cross-sectional area of the fan's ventilation system; this is the only reliable way to do so. For a forced-cooled power supply, the required airflow can be indicated either as a velocity rating, such as LFM, or as a volumetric rating, such as CFM.
Dust Filters:
Equipment with fans typically has filters to prevent unwanted dust from entering. These filters increase airflow resistance, contributing to pressure loss, so their presence must be taken into account. More importantly, as the filter becomes clogged with dirt, pressure loss can increase significantly, and a fan that initially performs adequately could become inefficient after extended use. For this reason, dust filters should be cleaned or replaced regularly.
How to minimize noise:
Adding a fan to a piece of equipment makes it audibly noisy. Some applications cannot tolerate any noise, such as hospitals or recording studios. Even for use in noisy environments, minimizing interference like this is usually desirable. This can be achieved through various methods.
Fan Bearings:
One way to minimize noise is to use a fan with a higher-quality bearing. Fans with ball bearings, which use a ring of balls around the shaft to solve problems of uneven wear and rotor wobble, are generally quieter than fans with sleeve bearings (enclosed in their sleeve-shaped structure) and have the advantage of a longer lifespan. Of course, there are fans that use oil impregnated inside the sleeve bearings, which can counteract this.
Blade Speed
: For a given air volume, a larger fan is usually quieter than a smaller one because it requires a lower blade speed. The noise generated by the fan blades passing near a fixed part of the fan, such as a strut or guard grille, must also be considered. If the guard grille can be moved even slightly away from the fan blades, the noise can be reduced.
Another way to minimize this type of disturbance is to reduce the fan's supply voltage. Fans are specified with an operating voltage range, and those with DC input typically rotate at a speed that depends on the actual DC voltage supplied. The slower the fan rotates, the less audible it will be.
How to Use Data to Manage Temperature:
Thermal management of modern power supplies is becoming increasingly important due to higher power densities and smaller heatsink volumes. Because of this, datasheets now contain essential information for system designers. This ensures that power supplies do not operate at excessively high temperatures by establishing specific maximum temperatures for selected components.
Once the fan has been chosen using this method, a final check should be performed by measuring the temperatures of these components in the final configuration. If it appears that the component temperature will exceed the value indicated in the datasheet, the airflow and its direction should be reassessed.
Please feel free to contact us for more information on proper power supply cooling.
About the author: Andrew Bryars
Andrew Bryars graduated with honors in Electrical and Electronic Engineering and has been working in the power supply industry since 1990. He has been with XP for over 25 years and has held various engineering, sales, and management positions, culminating in his current role as Senior Product Manager.
