Thermal Considerations for SBC Power Supplies

The single-board computer (SBC) represents an easy-to-integrate solution for many control problems. The popularity of the idea has led to an explosion in the number of SBC offerings on the market that span wide performance and cost demands, ranging from comparatively simple microcontroller-based solutions up to complex but compact mixtures of high-performance processors and field-programmable gate arrays. Often, the need to pack a large amount of compute performance into a small space presents challenges in terms of enclosure and packaging design, with knock-on impacts on the power-supply subsystem.

A rich selection of power-supply solutions has emerged in parallel to the growth of the SBC market. At one level, this makes the design of the final system much easier as it can rely on off-the-shelf solutions; but the demand for small size in many embedded-control projects can affect the choices that need to be made to ensure high reliability and consistent performance.

Higher-performance SBCs can demand high levels of power to support devices that can each consume more than 100 A at 1 W or less. Usually there will be point-of-load (POL) power controllers close to the individual processors or FPGAs; but the core power supply needs to be able to supply a well-regulated voltage at quite high power levels to these POL devices to ensure efficient operation.

Heat has a direct impact on the performance of an electronic system. Electronic circuits, particularly those used for power conversion and delivery, 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.

Cooler running also has a knock-on effect on reliability. Systems will have a lower probability of failing within a given time if they run at lower temperatures. These factors make it important to consider all possibilities when looking at the options for power-supply designs, such as cooling and the load-versus-efficiency curve. There are three main ways in which an electronic unit such as a power supply can lose heat: radiation, convection and conduction. For electronic systems used in most environments, convection and conduction are the most important.

With convection, heat is transferred away from the power supply as energy is transferred from the solid components of the system to air molecules as they move past. The rate of heat loss is proportional to the rate at which the air flows over the system. Therefore, forced-air cooling will provide a greater degree of cooling than the natural movement that results from hot components transferring energy to air molecules.

Forced-air cooling can increase the maximum power available from an off-the-shelf power supply. For example, intended primarily for medical systems, the mains-supplied VMS-160-5 made by CUI can deliver 100 W of power to the load if cooled using forced air delivered at a flow rate of 400 linear feet per minute, and up to 70 W without. The PSU’s single output can provide a voltage down to 5 V, suitable for many off-the-shelf SBC product designs. It has a 12 V auxiliary fan output to help support the forced-air cooling required for higher-power systems.

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. Also, in the context of an SBC-based system, it is important that heat from the power supply is not transferred to the processor complex as this can increase the likelihood of the devices going into thermal shutdown to protect themselves during high-load conditions.

Generally, the high copper content of a PCB as well as the metal within an enclosure help provide good paths for heat flow out of the power supply through conduction. A heatsink mounted on the outside of the enclosure will help divert thermal energy away from the system to where it can be lost through convection. The use of thermal adhesive is recommended to fill any void between the device to be cooled and maximize heat conduction from the device to the heatsink. Bolts or clamps increase contact pressure, which also improves thermal transfer into the heatsink.

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 power supply mounted underneath the SBC will tend to transfer heat to components within the processor complex. If the board is mounted vertically with the PSU to the side, the hot air will have less of an effect. However, more thermally sensitive components may be better placed towards the bottom of the unit.

Where heatsinks are used internally, the fins of the largest of the heatsinks should run parallel to the direction of airflow. Naturally, airflow will be restricted by obstructions, which needs to be taken into account. The way in which air exits the system will help determine how efficient the airflow is. To prevent pressure from building up and to reduce the efficiency of the fan, the cross-sectional area of the exit port for the air should be at least 50 percent greater than that of the entry port.

As well as cooling, power-supply performance is a key contributor to overall thermal behavior. A design of power supply that offers high power-conversion efficiency will need less cooling because it will produce less heat per joule transferred. If a 160 W power supply operates at full load with an efficiency of 85 percent, it will dissipate approximately 24 W in heat when operated at peak load. A power supply just 5 percent more efficient will produce 8 W less waste heat. An example of a high-efficiency AC/DC converter is the NV175 series from TDK-Lambda. It offers conversion efficiency of up to 90 percent, with the highest efficiency available if the PSU is used as a single-output converter and if operated at close to full load.

Figure 1: Efficiency curves for the TDK-Lambda against line voltage, showing the increase in peak efficiency for a configuration with fewer individual voltage outputs.

There will be cases where the environmental constraints mean that ambient temperatures around the equipment will be higher than is desirable for maximum longevity. Industrial applications may demand that the temperature around the system be able to rise to 60°C or higher. In these situations, the power supply specifications need to be consulted to ensure what load levels the system can support. Manufacturers typically provide derating curves that let the system designer trade operating temperature against power output. With derating, the NPS60 produced by Artesyn Embedded Technologies can operate in temperatures as high as 80°C. The PSU has a peak efficiency of 87 percent and can operate at full load in temperatures of up to 50°C. Beyond that point, each output needs to be derated by 2.5 percent per degree of temperature increase.

Figure 2: Typical derating curve for a PSU able to operate at up to 80°C, showing the tradeoff in power output against increasing temperature beyond a limit of 50°C.

Taking these factors into account, through consideration of the thermal parameters that surround the total system design, designers can take full advantage of the availability not only of highly capable SBCs but off-the-shelf power converters.