Tradeoff for efficiency
ac-dc power supply
Designers face constant pressure to reduce the size and improve the efficiency of their ac-dc power supplies. In addition, the requirement for portability in some equipment means that size as well as weight are prime considerations in the selection of power supplies.
You can always find a smaller power supply or design one by including a fan to provide forced-air cooling. You could save a third to half of the total volume of a typical unit with a fan. Yet the main disadvantage of this approach is fan noise, which, for example, can disturb and irritate patients in the medical market.
There also will be a significant reduction in reliability since the fan likely will be the only moving part in the power supply, and you might be adding a maintenance problem. Due to these issues, system designers are now looking to utilize convection-cooled power supplies to power their equipment.
Minimizing the component count will help reduce size and cost, but you will be limited here too. You can’t tolerate compromises with respect to immunity to interference (electromagnetic capability/electromagnetic interference/radio-frequency interference, or EMC/EMI/RFI) and the production of conducted or radiated emissions. You can’t compromise safety either, as you need to protect users from potentially lethal voltages.
Finally, you need to account for green legislation including the European Union’s Restrictions on Hazardous Substances (RoHS) and California Energy Commission/Energy Independence & Security Act (CEC/EISA), particularly if equipment is going to be sold around the world.
The use of RoHS components is obligatory. Designing for the highest possible efficiency not only will help in meeting present and future environmental legislation, it also will help to ensure the best performance from convection-cooled power supplies.
Breakthrough technologies that have a dramatic impact on power-supply design are rare. Advances in power semiconductor technology have had the most impact, followed by improvements in magnetic materials and capacitors.
Reducing power-supply size without compromising performance means that you have to work toward incremental improvements in every aspect of the design, both electrical and mechanical.
The surface area available to provide cooling is the limiting factor in how much heat you can dissipate from a convection-cooled power supply, which doesn’t need a fan. It follows that the more efficient you make the power supply, the less heat you’ll need to remove and the smaller the unit can be.
What may appear to be small differences can have a great impact here. If you can buy or design a power supply that is 95% efficient versus 90%, the 5% difference means you must remove less than half the heat of the less efficient design. For example, a 250-W supply requires the dissipation of 14.6 W less heat.
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Incidentally, power supplies for portable equipment find themselves in a lot of different environments, which means a 230-V or 110-V ac source is not always available. One needs to look at specified efficiency levels across the range of input voltages in the data sheet, particularly at low line levels, i.e., 85 to 90 V ac. Some units are a lot worse than others.
Load also affects efficiency whereby most power supplies operate at maximum efficiency at their full rated load. It pays to check out the efficiency you can expect in your individual application.
One way to reduce the size of magnetic components and capacitors is to increase the switching frequency of the converter. However, switching losses increase with frequency due to wound component core losses and increased copper/resistive losses caused, in part, by skin effect. Figure 1 shows the tradeoff for efficiency and switching frequency in a typical 200-W supply produced during the last few years.
Designing For Better Than 90%
The best of today’s 250-W convection-cooled power supplies operate at more than 90% efficiency across an input voltage range of 90 to 240 V ac. This level of efficiency is essential to keep within an industry-standard 6- by 4-in. footprint while still ensuring adequate heat dissipation without a cooling fan or large external heatsinks.
Efficiencies over 90% are achievable with near lossless switching only in the active power factor correction (PFC) circuit, the main converter(s), and the rectifiers. Figure 2 shows a diagram for a 250-W ac-dc supply that achieves up to 95% efficiency with a 240-V ac input and 92% efficiency at 90-V ac input.
From the outset, achieving high efficiency was the primary design goal for this power supply. Consequently, the power-loss budget was determined for each stage, and this drove the choice of circuit topology. Power losses are minimal in each stage, saving every milliwatt of unnecessary dissipation. For example, the input filter for the power supply in Figure 2 uses very low-resistance winding wire that virtually eliminates I2R losses in the chokes.
The EMI filter employed in this design is a two-stage filter with a high-permeability magnetic core, carefully selected to attenuate switching noise and to minimize power loss. The other components in the filter are X and Y capacitors with the Y capacitor values not exceeding 300 µA of earth leakage current as set out in UL60601-1, the most widely referenced medical standard.
A quasi-resonant, lossless PFC circuit operates in a discontinuous mode. Its operating frequency changes between 30 kHz and 500 kHz to achieve zero current switching (ZCS) throughout the specified range of loads and input voltages. This is important because it ensures that the voltage switches when the current is truly at zero, eliminating switching losses.
The main converters employ a fixed-frequency, resonant, half-bridge design with lossless ZCS. Two transformers are on board, the combination of which has lower I2R switching loss than if we employ one larger transformer.
The two converters operate at 51.2 kHz, and one of them has its output phase-shifted by 90°. Combining the outputs reduces ripple and doubles the ripple frequency (Fig. 3). In turn, this halves the value and size of the output filter capacitors.
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A feedback loop monitors the power-supply output and varies the boost converter voltage, which in turn varies the voltage at the input to the main converters. The primary purpose of the boost converter is to boost PFC voltage from approximately 380 to 420 V dc. This enables optimal design of the main converters around precise voltage parameters, another factor that helps to achieve high efficiency.
The Last Step
The final stage uses synchronous rectification instead of normal diodes, greatly reducing power loss. Timing for the boost converter, main converters, and synchronous rectifiers requires precise control to achieve accurate ZCS. We use a crystal-controlled clock as the timing reference and employ a divider network to dial in the desired switching frequency.
This approach is crucial for the efficient operation of synchronous rectifiers, especially for higher output voltages. This power-supply architecture results in high efficiency across a wide range of loads and input voltages (Fig. 4).
ZCS also produces relatively low levels of both conducted and radiated emissions as well as output ripple and noise. The power supply described above exhibits less than 90 mV of peak-to-peak ripple and noise at 20-MHz bandwidth and is below the level B limit line for EN55011 for conducted and radiated emissions. Creative mechanical design minimizes size and improves thermal performance.
Parting Shots At Hot Spots
You can greatly improve the thermal performance of a power supply through creative mechanical design. It’s important to avoid hot spots and ensure the best possible air flow around components that are going to get hot.
Combining the best of proven design technologies with creative mechanical design has led recently to the introduction of units that can reach up to 95% efficiency, a figure thought impossible only a few years ago.
Further incremental improvements are going to be harder to achieve. But the decades of experience that many engineers now have in power-supply design, coupled with advances in semiconductor technology, will make them possible.