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Understand thermal management for power supplies

Posted: 23 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? thermal management? heat transfer?

One of the most important considerations in the design and selection of a power supply is its thermal management. Here we examine the pathways for heat transfer and how power supply designs have evolved to enable effective heat dissipation and deliver greater performance.

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:

Power In = Power Out.

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:

Power In = Power Out + Power Dissipated as Heat.

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 Out * (1 C Efficiency)/Efficiency.

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 coolingusually driven by fanswill 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.

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 maximise 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.

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 per cent will dissipate 45W in heat. A power supply just 5 per cent more efficient will need to lose 15W less in waste energy, reducing the requirement for airflow-based cooling.

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