Understanding efficiency: looking for the worst-case scenario

June 03, 2014 // By Peter Blyth, XP Power
It’s 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 sold worldwide. Even if a power supply states it has a ‘Universal Input’, that doesn’t guarantee its efficiency matches the headline efficiency for all inputs. The efficiency at the highest input voltage, European mains at 230 VAC, will be different to the efficiency the power supply can achieve at the lowest input voltage, Japanese mains at around 100 VAC or 115 VAC in North America.

The power supply’s efficiency, when it’s working under the most challenging set of operating conditions, may be thought of as the worst-case efficiency. This worst-case efficiency can be calculated by digging deep into the product’s specification, which is essential to ensure the correct product is selected. A product may be selected on headline efficiency alone, perhaps at an attractive cost level, only to find that at the worst-case operating conditions, a cooling fan is needed or a higher power output supply must be used deliver the performance required. Incorrect selection will result in increased cost, which is why it’s vital to work with worst-case efficiencies from day one.

Factors affecting efficiency

Efficiency is calculated as the output power divided by the input power, and is usually expressed as a percentage. The difference between the input power and output power is the power wasted in the power supply as heat.

The input power is the product of the input voltage, current and power factor. If the input voltage (ie. mains voltage) is lower, to supply the same output power the current will have to increase, resulting in greater losses in the power components. The losses in the inductors and transformers are I2R, where R is the resistance of the component. For the same efficiency, halving the input voltage results in twice the input current. In reality the input current is more than double due to the reduction in efficiency caused by increased power losses, resulting in more than quadrupling the power loss of some of the components within the power supply and more than doubling the losses in others.

The same phenomenon exists for the output power, calculated as the product of the output current and output voltage. The optimum output voltage is the highest the PSU can supply; at lower output voltages, currents increase, and some losses increase proportional to the square of the current.

 

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