Do you need more than enough? Specifying AC/DC power supplies

One of the most important aspects of designing a power supply into a system is ensuring that it is sized appropriately. Erring on the side of caution by trying to ensure that the supply’s maximum output exceeds that of the load is no longer the right answer in many cases. Customers increasingly need to focus on energy efficiency. The trend is partly driven by the need to cut operating costs and partly by legislation such as the European Union’s EcoDesign Directive.

Under the directive, manufacturers of energy related products need to be able to demonstrate they have taken environmental factors into account. The efficiency of the power delivery sub-system is one of the key factors. It will play a large part in determining how energy will be lost through heat. As a result, choosing a high efficiency PSU (Power Supply Unit) is an important consideration in the design process.

A 200W PSU operating at full load with an efficiency of 85% will lose 30W in waste heat. Not only is that heat wasted, there may be an additional energy cost in forced air cooling to prevent the rest of the system overheating. A PSU that is 90% efficient will cut the power wastage by 10W.

If that PSU is operated below full capacity, it will run at a lower temperature. That allows usage in higher ambient temperatures or with less forced air cooling. However, there can be a trade off between efficiency and headroom. Many PSUs are designed to provide peak efficiency when they are driving a load close to full capacity. But this efficiency can roll off dramatically beneath 70 or 80% of full load. Using a power supply that is oversized for a particular application may result in an undesirable loss in efficiency and excessive heat production.

A potential problem for system designers is that the focus on energy efficiency in electronics has led to the adoption of power saving modes. The resulting load demands can vary widely during operation. Responding to this trend, PSU designers working in the data centre space have embraced initiatives such as 80 PLUS.

Launched in the mid 2000s at a Market Transformation Symposium organised by the American Council for an Energy-Efficient Economy (ACEEE), the 80 PLUS idea was quickly adopted as the basis for PSU efficiency marking by the US Energy Star programme. Supported worldwide, the idea behind 80 PLUS was to make PSUs deliver high efficiency over a larger proportion of the load curve. Recognising that many data centre system PSUs are operated using 1+1 redundancy and current sharing, the maximum efficiency point was centred on 50% capacity.

Ratings range from Bronze to Titanium. At 50% load, Bronze offers an efficiency of 85%. Titanium pushes the peak efficiency to 96%, rolling off towards 94% when operating at 20% load and 91% at full load. Bel and GE provide wide ranges of PSUs that are graded according to the 80 PLUS standards.

An alternative way to approach the issue of variable loads is to use the idea of boost power. This concept is gaining popularity in industrial designs where engineers have to deal with highly capacitive and inductive loads such as motors. As these systems shift between modes, there may be short term peak loads that go some way above normal operation. Motor start up also needs careful handling to deal with high current inrush conditions.

Built for the DIN-rail format commonly used in industrial systems, the Cliq-II and Cliq-M series of DIN-rail supplies from Delta offer an ‘Advanced Power Boost’ of 120% for three seconds or 150% for five seconds, respectively. Alternatively, if a PSU has been derated to operate at a lower output level so that it does not need forced air cooling, it can be ramped up to peak load for short periods of time without necessarily demanding additional cooling. However, this usage of a PSU does call for attention to the thermal conditions to ensure that the short term peaks in heating are dissipated.

To deal with situations such as high current inrush, PSUs such as the Artesyn LCM600 offer constant current modes that limit how much power is delivered to the load during the kinds of demand surges seen during motor startup. As one of a growing number of PSUs that employ digital control, the LCM600’s firmware can be programmed to support a number of different protection strategies so that the integrator can pick the mode best suited to the application.

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