Understanding efficiency: looking for the worst-case scenario

June 09, 2014 // By Peter Blyth
Understanding efficiency: looking for the worst-case scenario
Peter Blyth of XP Power advises that looking at worst-case scenarios can help to understand power supply efficiency issues.

It is standard practice to put the best-case scenario on data sheets, but how does that differ from the efficiency that can actually be achieved in your application?

Efficiency is one of the key parameters to consider when selecting the correct switch mode power supply. Pressure on equipment designer to deliver more functionality in a small size can result in more power being required which has a direct effect on the form factor of the power supply. The consequence of this is power supplies now have to deliver more output power in a smaller form factor. This, coupled with the need to meet more demanding environmental legislation and to minimise or eliminate fan cooling, is forcing equipment designers to look for more efficient power supplies.

An efficient power supply means less power is wasted as heat, which is the biggest factor in reliability of electronic components. Efficiency therefore has a big effect on the reliability and lifetime of the end equipment. Selecting an efficient power supply may also mean the equipment can be designed for operation without a cooling fan, reducing the audible noise, which is very desirable in many applications.

When deciding on a particular power supply for a piece of equipment, the minimum efficiency required for the equipment to run without a cooling fan, or with a certain lifetime guaranteed, may be calculated. The designer then turns to power supply data sheets to decide whether a particular supply meets those minimum efficiency criteria.

Equipment designers should be aware that the efficiency figure quoted on the manufacturer’s website or data sheets is almost certainly a best-case scenario. The headline efficiency found on marketing material is true only when the power supply is run under favourable, or indeed, optimum, conditions. The actual conditions the power supply will experience be used in could be very different.

For example, it’s common for the same model of industrial and medical equipment to be

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