Back to Basics: Thermal management for power supplies
One of the most important considerations in the design and selection of a power supply is its thermal management, says Arun Ananthampalayam of CUI.
Heat-dissipation efficiency has a direct impact on the performance of a power supply. Electronic circuits often perform more efficiently at lower temperatures and will in turn tend to dissipate less energy as wasted heat. The efficiency gains that can be obtained through effective cooling increase significantly as the power output of the overall system increases. Higher temperature operation can also have an effect on reliability. Systems that run cooler will have a lower probability of failing within a given time. These factors make it important to consider all possibilities when looking at the cooling options for power supply designs.
The first law of thermodynamics tells us how much heat needs to be dissipated from a given power-supply design. In brief:
Some of the energy that the supply takes in will be consumed by the internal electronics and converted into heat and this must be accounted for in the power equation. So:
The amount of power dissipated can be derived from the efficiency of the converter, which is calculated as the ratio of Power Out to Power In. The power dissipated as heat is therefore given by:
Power of three
There are three main ways in which an electronic unit such as a power supply can lose heat; radiation, convection and conduction. Radiation through electromagnetic emission provides one source of heat loss but this is rarely the primary means of dissipation.
Convection provides one of the main pathways for heat to be transferred away from the power supply as energy is transferred from the solid components of the system to air as it moves past. The rate of heat loss is proportional to the rate at which the air flows over the system and away into the wider atmosphere. As a result, forced-air cooling — usually driven by fans — will provide a greater degree of cooling than the natural movement that results from hot components transferring energy to air molecules. With natural convection, expansion in air caused by its warming as it passes over hot components provides a degree of movement that allows the heat energy to be distributed through air vents to the outside world. Forced-air cooling provides a steady flow of cooler air to accept heat generated by the power supply’s component but will add acoustic noise to the environment.
Click on image to enlarge.
Conduction through a PCB substrate or system chassis provides a further avenue for removing heat from a power supply although, traditionally, it has been considered as less important than convection. In general, metals provide efficient conduction of heat. When excited by heat, the electrons in a piece of metal can leave their atoms and move within the lattice as free electrons. Kinetic energy from vibrating metals is transferred from hot parts of the metal to cooler parts by the free electrons, which will collide with ions as they move and can be recaptured if they lose enough energy. The high copper content of a PCB as well as the metal within an enclosure helps provide good paths for heat flow out of the power supply through conduction.
A heat sink uses conduction to increase the efficiency of cooling by convection. The heat sink is designed to increase the surface area of a device that is in contact with the surrounding air, helping to increase cooling efficiency.
To maximize heat conduction from the device to the heat sink, the use of thermal adhesive is recommended to fill any void between the device to be cooled, which may be a complete power converter, and the heat-sink surface. Bolts or clamps increase contact pressure, which also improves thermal transfer into the heat sink.
Improving thermal management through design
It is possible to improve heat transfer through suitable choice of materials and structural design. For example, providing efficient cooling via conduction through the baseplate is ideal for systems where active cooling through the use of fans is not desirable; such as professional-audio systems, which often need to be installed in areas where there is minimal noise generated by the electronics.
Click on image to enlarge.
There are two further degrees of freedom to consider in system design for power supplies. One is to choose a power supply that requires a lower degree of cooling through the use of a higher-efficiency design. For example, a 300W power supply that operates at full load with an efficiency of 85 percent will dissipate 45W in heat. A power supply just 5 percent more efficient will need to lose 15W less in waste energy, reducing the requirement for airflow-based cooling.
Another is to choose a power supply that is not run at full load. This makes it run cooler and allows it to be used in a wider range of situations, such as higher ambient temperatures that reduce the rate at which heat can be removed from the vicinity of the power supply, or where the system requirements make it impossible to provide full forced-air cooling.
Airflow
The derating curve for a power supply in the datasheet will show how much the power output needs to be reduced for a given increase in temperature or reduction in airflow.
Forced-air cooling has a significant effect and allows a supply to operate at full power in higher ambient temperatures. In general, forced-air cooling requirements will be expressed in cubic feet per minute in the datasheet. If we look at the derating curve of CUI’s VBM-360, which is both baseplate and forced-air cooled, we see that increasing the airflow to 10 cubic feet per minute (CFM) enables the supply to operate at 100 percent of full load at an ambient temperature of 60°C.
Click on image to enlarge.
Click on image to enlarge.
Figure 3: derating curve VBM-360 under natural convection and a forced (10CFM) airflow
Whether the power supply employs an open-frame or enclosed design has a further effect. If we again use the VBM-360 as an example, its open frame form can run at 100 percent of its rated load from a 220-264V AC supply up to an ambient temperature of 40°C under natural convection. At 60°C, this reduces to 50 percent of its rated load. In the enclosed form, however, it can operate 60 percent of full load at an ambient temperature of 60°C.
To provide the necessary volume of air, the fan should be sized to the cross-section of the power supply to ensure that the air is directed as efficiently as possible over the surface of the surfaces of the components. In general, air should be directed along the long axis of the power supply. However, a further consideration is the layout of any internal heat sinks. The fins of the largest of the heat sinks should run parallel to the direction of airflow.
Airflow will be restricted by obstructions — there are many large, tall components mounted inside power supplies that will impede airflow. To prevent back-pressure from building up and reducing the efficacy of the fan, the exit port for the air should be 1.5 the area of the entry port or more.
It can make sense for the fan to be sized larger than necessary as a larger fan can deliver a greater amount of cooling air but at a lower speed than a smaller fan. By running less quickly, the fan will run more quietly which can be crucial in applications where acoustic noise is a factor but conduction or natural convection cannot be used for cooling.
The orientation of the power supply within the system can also have an effect on cooling performance depending on the layout of the internal components. Because hot air tends to rise, a vertically mounted power supply will tend to transfer heat to other components whereas the hot air in a horizontal design will be pushed more easily towards the exit vents by convection or forced-air cooling.
More efficiency, less noise
As we have seen, the system designer has a number of options available when considering the thermal management of the power supply and the surrounding system. While effective, forced-air cooling is not always the ideal choice for system designers. By trading off power supply capacity against load, by selecting higher-efficiency designs or by moving to advanced products that employ conduction cooling, it is possible to achieve high performance with little to no acoustic noise through the use of non-forced air cooling. Alternatively, fans can be selected that minimize noise emissions and which provide thermal compatibility over a wide ambient temperature range.
Arun Ananthampalayam is product marketing engineer at CUI Inc., and an expert in power supply designs.
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