TDK-Lambda Nieuws

The specification of power supplies for medical applications is a task which needs to be approached with care. In particular, as Andy Skinner of Lambda UK explains, it is never safe to make assumptions about the suitability of power supplies which were primarily designed for industrial use.

In the design of electronic equipment, there is one consideration that takes precedence over all others. This overriding concern is safety and it is just as important in industrial or domestic equipment as it is in equipment for medical applications. It might be tempting to think, therefore, that power supplies which have been designed and proved to be safe in industrial applications would be equally suitable for use in medical equipment.

Unfortunately this is not always the case as the risks involved are different. For example, hospital patients are frequently in a weak condition. Exposure to even small leakage currents can, therefore, have an adverse effect on their well-being. The same leakage currents would have no effect on a healthy person and might be acceptable in industrial applications.

Further much of the electronic equipment used in hospitals, such as patient monitors, works with very low-level signals. This equipment is much more sensitive to electromagnetic interference than most of the equipment used in industry, which also makes EMC performance a key concern in medical applications.

The special requirements of medical applications are, as might be expected, reflected in international standards. For most of the world, including Europe and North America, these standards are the IEC 60601 series. In this short article it is not possible to explore these wide-ranging standards in detail, but it is useful to consider how they affect the design and specification of power supplies.

The first and most basic requirement is for effective and reliable isolation between the input to the power supply and its output, as any shortcoming in isolation would result in a higher risk of electric shock.

Several factors contribute to effective isolation, including spacing between conductive parts. The IEC 60601 standards lay down minimum distances for spacing, and it is important to note that these are greater than the spacing distances prescribed by the relevant standards for industrial and general-purpose power supplies.

This does not automatically disqualify those power supplies from medical use as some progressive manufacturers, including Lambda, have adopted the IEC 60601-1 spacings in many designs. This is, however, an important point to check when specifying power supplies.

In addition to adequate spacings between conductors, effective isolation also depends on reliable insulation. Most modern power supplies use double insulation or reinforced insulation, the effectiveness of which is verified by dielectric strength testing. This involves subjecting the insulation to a much higher voltage than that at which it operates, and ensuring that no failure occurs.

Once again, medical requirements differ from those for standard power supplies. Reinforced or double insulation in supplies which operate from the UK 240V mains, for example, must withstand a dielectric test at 4kV for medical applications, whereas the corresponding figure for industrial use is just 3kV. As with the spacings, this difference must be taken into account when choosing a power supply. Power supplies that are approved to less than 4kV may be used in medical applications as part of a reinforced barrier provided that the insulation provided by the power supply is regarded as a lesser ‘basic’ or ‘supplementary’ barrier. In this case additional isolation is required within the end equipment to achieve the requirements of a reinforced barrier between the mains supply and the user.

The leakage current requirements laid down by IEC 60601 are also onerous. The maximum permissible earth leakage is 300?A to allow for worldwide approvals, but this figure applies to the equipment as a whole, not just the power supply. To allow for additional leakage in other components it is, therefore, highly desirable for the power supply to have an even lower leakage current.

This leads to an interesting conundrum: as we have noted, EMC performance is another crucial issue for medical power supplies. All modern power supplies are of the switch-mode type, as these are small, efficient and cost effective. Switch-mode supplies, however, generate electromagnetic interference and require the incorporation of filters to limit the effect.

The capacitors in these filters allow a small amount of leakage current to flow and the more effective the filter at suppressing the interference, the more leakage it is likely to produce. It seems, therefore, that there is a trade-off between EMC performance and leakage current.

For conventionally designed switch-mode supplies this is indeed true, but EMC performance can be improved by methods other than simply providing more filtering. A better approach is to minimise the amount of interference that the power supply generates in the first place. To explain how this can be achieved it’s necessary to understand a little about how switch-mode power supplies work.

Essentially, they first convert AC power from the mains into DC. This DC is converted back to AC, but at a much higher frequency than the mains supply, so that it can be applied to a lightweight compact transformer to produce the required output voltages. The DC to AC conversion is carried out by a switching circuit, which is why these products are called switch-mode supplies.

The outputs from the transformer are converted back to DC and fed to regulators which ensure that the output voltages remain stable when the current drawn from the supply varies. Current limiters, to protect against overloads, are also usually incorporated. From the EMC point of view, however, it is the switching circuit which is of most interest.

The switches are transistors and are usually arranged to switch as quickly as possible because this helps to minimise losses in the power supply. Unfortunately the faster the transistors switch, the more interference the switching circuit generates.

Some of the best modern power supply designs, therefore, such as those in Lambda’s NV range, deliberately slow down the switching operation using special ‘zero-voltage switching’ or ‘ZVS’ circuits. These still allow relatively fast switching of the transistors whilst achieving voltage transitions (rise and fall times) that are much slower. These transitions in a ZVS circuit may be of the order of 100ns compared to 20ns in a conventional power supply.

By using innovative zero-voltage switching circuitry, Lambda’s designers have managed to achieve this slow switching without compromising power supply efficiency. The amount of electromagnetic interference generated is, however, greatly reduced and only a small filter is needed to for these supplies to meet the EMC requirements of even the most demanding medical applications. With only a modest amount of filtering needed, leakage currents can also be kept low satisfying another important requirement.

A further benefit is that the new circuitry eliminates the need for an interwinding screen in the transformer, another measure which was traditionally employed to improve EMC performance. Eliminating the screen not only allows a physically smaller transformer to be used, thereby reducing the overall size of the power supply, but also further increases efficiency.

Modern medical equipment needs power supplies which are compact, lightweight, efficient, cost-effective and reliable. Switch-mode power supplies can meet all of these needs but, as we have seen, not all switch-mode supplies are created equal.

Specifiers and designers should, therefore, take care to choose products from a reputable supplier, preferably with proven experience in the medical field and with a good understanding of the special demands of this market segment. They should also take care, especially when tempted to use a standard power supply, to ensure that their choice fully satisfies the provisions of IEC 60601-1.

Andy Skinner is Advanced Engineering Manager of Lambda UK