Medical electronic equipment is no longer just used in hospitals, which means that trained experts do not always operate it, or 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 utilize 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. This means you cannot tolerate compromises with respect to immunity to interference (EMC/EMI/RFI) and production of conducted or radiated emissions. You cannot compromise safety either - patients have to be fully protected from potentially lethal voltages.

Finally, you need to take account of green legislation including RoHS, CEC/EISA, particularly if equipment is going to be sold around the world. The use of RoHS 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 towards incremental improvements in every aspect of the design, both electrical and mechanical.

The size - power - efficiency trade off

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 does not 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 percent efficient, versus one that is 90 percent efficient, the five percent 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 V or 110 V AC power source being available. It is important to look at how well the efficiency is maintained across the range of input voltages defined in the power supply data sheet, particularly at low line (85-90 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 is active and the efficiency of the power system will vary accordingly.

The frequency - size - efficiency trade off

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 trade-off for efficiency and switching frequency in a typical 200 W power supply produced during the last few years is shown in Figure 1.

(Click on Image to Enlarge)

Figure 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 importantly, the cost for a given power rating.