Understanding AC/DC power supply efficiency

The drive for smaller power supplies goes hand in hand with a need for greater efficiency. There's a direct relationship here because the smaller the power unit, the smaller the area available for dissipating heat through conduction and convection. Through both measurement and calculation, XP Power has developed a guide to the maximum power loss that can be tolerated for a given size of power supply, based on using convection cooling.

This is the loss that keeps all components in the unit operating within their temperature ratings. Of course, forced-air cooling can have a big impact, however, in many applications, forced-air cooling is to be avoided, not least because adding a fan to a power supply inevitably decreases its reliability, adds cost, creates noise, and introduces a maintenance headache.

The most important thing to remember when investing in a power supply is that efficiency is affected by the way in which the product is operated and the environment in which it finds itself.

As a result, power supply efficiency is usually specified based on the operating conditions that are most favourable to the figure concerned, for example, at maximum rated load. Whilst economic necessity will mean that you are unlikely to specify a power supply rated at much more power than you need, it could be that the maximum rating is only needed for a relatively small proportion of the unit's operating time in your application. For the rest of the time, it will be operating below full load, and efficiency is likely to be much lower than the headline figure.

Another important factor is the AC input voltage to the power supply.

At the lowest acceptable AC input, often referred to as ‘low-line', efficiency is likely to be appreciably lower than when the power supply is operating at its maximum rated input, or ‘high-line'. It is common for ‘universal AC input' to mean a range of input voltages from 90 VAC to 230 VAC. The less impact that low input voltage has on overall efficiency, the easier it will be for you to calculate the likely efficiency and heat losses that need to be taken into account in a variety of operating environments. An example of a power supply that achieves almost a flat efficiency curve over its input range is XP Power's CCM250. This 250 W AC/DC power supply unit is rated up to 95% efficiency at 240 VAC input and 92% efficiency at 90 VAC input.

The final point to be aware of is that for a given input voltage, the efficiency falls in line with output voltage. A short form data sheet will often refer to a family of products with various DC output options. The headline efficiency figure of perhaps ‘88% typical' may refer to a unit with 48 V DC output. If you are looking for the 5 V output model within the range, the efficiency is likely to be somewhat lower. Once again, digging deeper into the detailed data is essential to understand efficiency that's likely to be encountered in your application.

Find out more about how XP Power can supply the most efficient power supply for your application by visiting www.xppower.com.