Medical power supplies: trends, challenges and design approaches

In the last 10 years, a typical convection-cooled, 100-W AC/DC power supply has shrunk from a 4 × 7 inch footprint in 1998 to just 2 × 4 inches today, a reduction over 70%. [Note: power supply sizes are still most commonly defined in Imperial (non-metric) measurements.] This size reduction has had to be managed carefully. Smaller packages mean less area for heat dissipation, which in turn requires higher efficiency. This article explains some of the main design issues and briefly introduces some of the most effective techniques that are now employed to achieve power system design goals in medical applications.

Through both empirical measurement and calculation, estimates of the maximum power loss that a chassis mount or open frame power supply can dissipate as heat for a given footprint are shown in Figure 1. The figures are based on using convection cooling and on maintaining compliance with safety agency requirements. They also take account of providing reasonable operating life and acceptable reliability limits.

Note that forced air cooling can improve the power rating considerably, but at the expense of decreased system reliability, as fans are fundamentally less reliable than the other power system components, and they add to system size and noise. Fan noise is very undesirable in medical applications.

Figure 1: The maximum safe heat dissipation vs. size for power supplies used in medical applications

Figure 2: The minimum efficiency required for a given power supply output to ensure compliance with safety standards

Figure 2 also shows the dramatic effect that a relatively small improvement in efficiency can have on the available power from a power supply for a given heat dissipation. Taking the 20-W power-loss curve, an efficiency gain from 88% to 93% would enable an power supply to deliver over 250 W, instead of less than around 150 W, within a given footprint.

For the power-supply designer, size and efficiency are usually the most important trade-offs. Increasing the switching frequency means that smaller components can be used, notably capacitors and inductors. However, switching losses rise and a power supply that may be 92% efficient at 30 kHz will be only 83% efficient at 200 kHz.

Reliability is always of paramount importance in medical applications, so keeping the power system running well within its maximum ratings is always desirable. Finally, cost is the ever-present final determinant of a power supply's suitability for a given application.

Techniques for managing the design trade-offs
Despite the substantial reductions in power system size over the last decade, no single design leap has made this possible. Rather, a combination of small improvements in both design techniques and components technologies have come together to create the end result. Taking power supply from input to output, these are some of the design approaches that are now adopted.

Two-stage input filters use high permeability cores to minimize size while providing high common-mode and differential noise reduction. Smaller footprints can be realized by stacking components vertically. This can also improve cooling through better airflow.

In many power supplies, it has become economical to use silicon carbide (SiC) diodes in power-factor correction (PFC) circuits. These need no snubber circuits, reducing component count and saving space while giving a typical 1% boost to efficiency.

The main converter topology is critical to efficiency. For power supplies in the 100 to 200 W range, a resonant topology is often chosen. This can virtually eliminate switching losses, enabling smaller heat sinks to be used, thus contributing to the dual goals of smaller size and higher efficiency. In some cases, ceramic heat sinks can replace metal ones.

This results in lower noise because the heat sinks are not subject to capacitive coupling with the drain connections of the switching MOSFETS. Simplified filtering can then be used. An additional advantage of ceramic heat sinks is that smaller creepage distances can be used, compared with those needed for conductive metal heat sinks, so further board space savings are achieved.

The falling price of power MOSFETS has meant that they are now becoming more common than diodes in the main rectifier of switching power supplies. Efficiency improvements of more than 40% in this part of the circuit are possible. For example, a 20 A diode with 0.5 V forward voltage dissipates 10 W, whereas a MOSFET with an 'ON' resistance of, say, 14 mΩ at 100° C dissipates just 5.6 W. Once again, ceramic heat sinks can be used to advantage.

Lastly, control circuits have been greatly simplified in recent years, largely through higher integration of semiconductor functions. Application-specific chips are now available that can provide the main converter voltage and a host of automatic protection features. Comprehensive monitoring and control signals are also more easily implemented thanks to more highly integrated power management devices.

How small can they get?
Figure 3 shows XP Power's ECM140, an example of a compact, efficient power supply that is available today. It has a 3 × 5 inch footprint and is rated at 120 W with convection cooling, or 148 W with forced-air cooling. A 12-V fan supply is included in the design, which has a typical efficiency of 88%. Later this year, AC/DC switching power supplies with medical approvals will take efficiency to levels well in excess of 90%. This will enable 250 W convection cooled units to be compressed into a 6 × 4 inch footprint.

Figure 3: XP Power's ECM140 adopts some of the techniques described to deliver 120 W from a 5 × 2 inch footprint, with 88% typical efficiency enabling convection cooling at this power level

Peter Blyth has been with XP Power (http://www. xppower.com) for over 9 years and has had various roles within the company. He joined in 1997 as an applications engineer, and became the Industry Manager for Medical in 1998. Between 1998 and 2002 he was the Senior Industry Manager - Medical for Europe and in late 2002 moved to the USA to become the Industry Director - Medical for North America.

He is currently based in Anaheim, CA in XP's main R & D center, where his responsibilities include management of XP's medical business in North America, marketing, product development (marketing input) and technical support to customers. Prior to working at XP he worked for the UK Ministry of Defence for 3 years and a small broadcast electronics company in the UK.

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