AC-DC power supply design: how to specify capacity
When it comes to an AC-DC power supply, bigger is not always better. One of the most important aspects AC-DC power supply design in 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.
AC-DC power supply design and environmental pressures
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 subsystem 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 power supply unit (PSU) is an important consideration in the design process.
A 200W PSU operating at full load with an efficiency of 85 per cent 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 per cent 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 per cent of full load. Using an AC-DC 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.
80 PLUS Certification requirements
| 80 PLUS certification | 115V internal non-redundant | 115V industrial | ||||||
| % of rated load | 10% | 20% | 50% | 100% | 10% | 25% | 50% | 100% |
| 80 PLUS | --- | 80% | 80% | 80% / PFC 90 | --- | |||
| 80 PLUS Bronze | --- | 82% | 85% / PFC 90 | 82% | --- | |||
| 80 PLUS Silver | --- | 85% | 88% / PFC 90 | 85% | 80% | 85% / PFC 90 | 88% | 85% |
| 80 PLUS Gold | --- | 87% | 90% / PFC 90 | 87% | 82% | 87% / PFC 90 | 90% | 87% |
| 80 PLUS Platinum | --- | 90% | 92% / PFC 95 | 89% | 85% | 90% / PFC 95 | 92% | 90% |
| 80 PLUS Titanium | 90% | 92% / PFC 95 | 94% | 90% | --- | |||
| 80 PLUS certification | 230V EU internal non-redundant | 230V internal redundant | ||||||
| % of rated load | 10% | 20% | 50% | 100% | 10% | 25% | 50% | 100% |
| 80 PLUS | --- | 82% | 85% / PFC 90 | 82% | --- | |||
| 80 PLUS Bronze | --- | 85% | 88% / PFC 90 | 85% | --- | 81% | 85% / PFC 90 | 81% |
| 80 PLUS Silver | --- | 87% | 90% / PFC 90 | 87% | --- | 85% | 89% / PFC 90 | 85% |
| 80 PLUS Gold | --- | 90% | 92% / PFC 90 | 89% | --- | 88% | 92% / PFC 90 | 88% |
| 80 PLUS Platinum | --- | 92% | 94% / PFC 95 | 90% | --- | 90% | 94% / PFC 95 | 91% |
| 80 PLUS Titanium | 90% | 94% / PFC 95 | 96% | 94% | 90% | 94% / PFC 95 | 96% | 91% |
80 PLUS Certification requirements
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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 per cent capacity.
80 PLUS ratings range from Bronze to Titanium. At 50 per cent load, Bronze offers an efficiency of 85 per cent. Titanium pushes the peak efficiency to 96 per cent, rolling off towards 94 per cent when the load is operating at 20 per cent load and 91 per cent at full load. ABB, Artesyn, Bel Power Solutions and Murata provide wide ranges of PSUs that are graded according to the 80 PLUS standards.
Boosting your AC-DC power supply designs
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-III and Cliq-M series of DIN rail supplies from Delta offer an ‘Advanced Power Boost’ of 200% for 5 seconds or 150% for 7 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.
Delta Cliq-M (left)and Cliq-III (right) DIN-rail power supplies
To deal with situations such as high AC current inrush caused by large capacitors in the primary circuit of all switching power supplies, PSUs such as MEAN WELL's ICL series (below) limit how much power is delivered to the load during the kind of demand surges seen during motor start up. The ICL16 and ICL28 are available in both DIN rail and rectangular cases and limit current levels to 16A and 28A respectively, allowing simultaneous switching of power supplies without the risk of tripping circuit breakers.
MEAN WELL'S ICL-16L (left) and ICL-28R (right) series
Applications such as data/server centres often use fan units air conditioning systems to keep power supplies cool. However, unnacceptable noise and short lifetimes can make these systems undesirable. The improvement in efficiency of power supplies over the last decade has allowed manufacturers to design on-board cooling into compact, high power units. One of the latest models available is Bel Power Solutions' ACC600 600W convection cooled power supply for industrial, communications and medical equipment. Alternatively, MEAN WELL's UHP-2500 provides 2500 Watts of power in a conduction cooled, compact 310x140x60mm U frame case.
Bel Power Solutions ACC600 series (left) and MEAN WELL's UHP-2500 series (right)
To cater for the growing diversity of PSU applications in an energy-conscious world, Avnet Abacus can provide an extensive range of off-the-shelf and customised AC-DC power supply solutions made from suppliers such as ABB, Artesyn, Aimtec, Bel, Delta, Excelcys, MEAN WELL and Murata. Avnet Abacus can also help with custom power supply design in conjunction with most of our suppliers. All are leading manufacturers of power supplies, DC conversion modules and LED and lighting specific power solutions. If you have a question, or you would like to discuss your design in more detail, click the Ask an Expert button to get in touch with one of our technical specialists in your local language.
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