Pushing the limits of ac/dc power design for medical apps

Pushing the limits of ac/dc power design for medical apps

Medical applications are among the most demanding for power supply size, efficiency, performance, and cost

BY PETER BLYTH
XP Power, Sunnyvale, CA
http://www.xppower.com

While it’s clear that reductions in size and improvements in efficiency are constant pressures in the design of ac/dc power supplies, the medical device market has some particular requirements that make the challenge greater still. This article considers these challenges and looks at how recent ac/dc switching power supplies are being designed to meet them.

Medical electronic equipment is no longer just used in hospitals, which means that trained experts do not always operate it nor is it always used in tightly controlled environments. Such equipment is now widely used in doctors’ surgeries, in ambulances, and even in the home.

The requirement for portability in such equipment means that size and 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 might save one-third to one-half of the total volume of a typical unit in this way. The major disadvantage of this approach is fan noise, which disturbs and irritates patients.

Other problems include a significant reduction in reliability the fan will likely be the only moving part in the power supply, and you add a maintenance problem. Due to these issues, system designers are now looking to use convection-cooled power supplies to power their equipment.

Minimizing component-count will help in reducing size and cost, but you will be limited here too. Medical equipment must be reliable in a variety of environments — lives may depend upon it. This means you can’t tolerate compromises with respect to immunity to interference (EMC/EMI/RFI) and production of conducted or radiated emissions. You can’t compromise safety either — patients have to be fully protected from potentially lethal voltages.

Finally, you need to take account of green legislation, including RoHS and CEC/EISA, particularly if equipment is going to be sold around the world. The use of RoHS-compliant components is obligatory, and designing for the highest possible efficiency will not only help in meeting present and future environmental legislation but will also help to ensure 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 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 size-power-efficiency tradeoff

The surface area available to provide cooling will be the limiting factor in how much heat you can dissipate from a convection-cooled power supply – one that 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 great impact here. If you can buy or design a power supply that is 95% efficient, versus one that’s 90% efficient, the 5% difference in efficiency means you need to remove less than half of the heat of the less efficient design. For a 250-W power supply, this means 14.6 W less heat to be dissipated.

Incidentally, because medical power supplies are used in a lot of different environments, you cannot always rely on a 230- or 110-Vac power source being available. It’s important to look at how well the efficiency is maintained across the range of input voltages defined in the power supply datasheet, particularly at low line (85 to 90 Vac). Some units are very much worse than others.

Efficiency will also be affected by load most power supplies operate at maximum efficiency at full rated load. It pays to check out the efficiency you can expect in your individual application. For example, medical equipment operating in standby mode may be consuming far less power than when it’s active and the efficiency of the power system will vary accordingly.

The frequency-size-efficiency tradeoff

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. The tradeoff for efficiency and switching frequency in a typical 200-W power supply produced during the last few years is shown in Fig. 1 .

Fig. 1. Effect on efficiency of reducing component size by increasing switching frequency.

Clearly, you have to reach a compromise between size, efficiency, switching frequency, reliability, lifetime, cooling technique, and, perhaps most important, the cost for a given power rating.

Designing for 90%+ efficiency

The best of today’s 250-W convection-cooled power supplies operate at over 90% efficiency across an input voltage range of 90 to 240 Vac. This level of efficiency is essential in order to keep within an industry-standard 6 x 4-in. footprint while still ensuring adequate heat dissipation without a cooling fan or large external heat sinks.

Over 90% efficiency can only be achieved with near-lossless switching in the active power factor correction circuit, the main converter(s) and the rectifiers. A diagram for a 250-W ac/dc power supply that achieves up to 95% efficiency at 240-Vac input and 92% efficiency at 90-Vac input is shown in Fig. 2 .

Fig. 2. This 250W ac/dc power supply is up to 95% efficient.

From the outset achieving high efficiency was the primary design goal for this power supply. Consequently, for each stage the power loss budget was determined and this drove the choice of circuit topology. Power losses were minimized in each stage, striving to save every milliwatt of unnecessary dissipation. For example, the input filter for the power supply shown above uses very-low-resistance winding wire that virtually eliminates I2 R losses in the chokes.

The EMI filter employed in this design is a two-stage filter with a high-permeability magnetic core. This was 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 being chosen so as not to exceed 300 µA of earth leakage current, as set out in UL60601-1, the most widely referenced medical standard.

A quasi-resonant, lossless power-factor-correction circuit operates in a discontinuous mode. Its operating frequency changes between 30 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, thereby eliminating switching losses.

The main converters are of fixed frequency, resonant, half-bridge design – again with lossless ZCS. Two transformers are employed; the combination has lower I2 R switching loss than if one larger transformer had been used. 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, as illustrated in Fig. 3 . In turn, this halves the value, and size, of the output filter capacitors.

Fig, 3. Combining the outputs of two converters that are 90 out of phase reduces ripple level and doubles ripple frequency.

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 the PFC voltage of approximately 380 to 420 Vdc. This enables the design of the main converters to be optimized around tightly defined voltage parameters, another factor that helps to achieve high efficiency.

The final stage uses synchronous rectification instead of normal diodes as this greatly reduces power loss.

Timing for the boost converter, main converters and synchronous rectifiers needs to be precisely controlled to achieve accurate ZCS. A crystal-controlled clock is used as the timing reference and a divider network is employed to get the desired switching frequency. Using 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, as Fig. 4 demonstrates.

Fig. 4. ZCS switching and the use of resonant converters delivers high efficiency over a wide range of loads and input voltages, not just at full load.

A further benefit of ZCS is the relatively low levels of both conducted and radiated emissions as well as the output ripple and noise. The power supply referred to above exhibits less than 90-mVp-p ripple and noise at 20-MHz bandwidth and is below the level-B limit line for EN55011 for conducted and radiated emissions. As noted earlier, this is another important consideration for medical power supplies as there are more demanding EMC requirements for equipment used outside of hospitals.

Creative mechanical design minimizes size and improves thermal performance. You can greatly improve the thermal performance of a power supply through creative mechanical design. Avoiding hot spots and ensuring the best possible airflow around components that are going to get hot are both important. In the power supply described above, XP Power’s CCM250 (see Fig. 5 ), has input chokes stacked above other components to save board space. Normally this might create hot spots but the low-loss design of the chokes prevents them.

Fig. 5. XP Power’s CCM250 power supply uses creative mechanical design to pack a 250-W (300-W peak) convection-cooled power supply into a 6 x 4 x 1.5-in. format.

All heat-generating components, including the power-factor-correction choke, are bonded directly to the U-channel chassis, which doubles as an effective heat sink, and control circuits are placed on daughter cards mounted at 90 to the main printed circuit board. In other words, full use is made of all the available space by thinking about the mechanical aspects of the unit in 3D at the outset of the design.

Medical applications are now among the most demanding with respect to the size, efficiency, performance and cost of ac/dc power supplies. 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. ■