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Special needs
Safety is one consideration that takes precedence over all others in the design of electronic equipment. It can be tempting, therefore, to think that power supplies that 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 because the risks involved are different. For example, hospital patients are frequently in a weak condition and exposure to even small leakage currents can have an adverse effect on their well-being. The same leakage currents would have no effect on a healthy person and may be acceptable in industrial applications. It is also important to ensure that standard power supply meets the requirements of the IEC 60601 series, the international product safety standards for electrical medical equipment. There follows a list of important points to consider when specifying power supplies for medical applications.
Effective isolation
The most basic requirement is for effective and reliable isolation between the input to the power supply and its output, because 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 these are greater than the spacing distances prescribed by the relevant standards for general-purpose power supplies. This does not automatically disqualify those power supplies from medical use, but some progressive manufacturers have adopted the IEC 60601-1 spacings in many designs.
Effective isolation also depends on reliable insulation. Most modern power supplies use double insulation or reinforced insulation. Its effectiveness is verified by dielectric strength testing, which involves subjecting the insulation to a much higher voltage than the voltage it operates at to ensure that no failure occurs. For example, reinforced or double insulation in supplies that operate from the 240-V mains in the United Kingdom must withstand a dielectric test at 4 kV for medical applications, whereas the corresponding figure for general use is 3 kV. Power supplies that are approved to less than 4 kV may be used in medical applications as part of a reinforced barrier, providing 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.
Leakage and EMC
The leakage current requirements laid down by IEC 60601 are onerous. The maximum permissible earth leakage is 300 μA 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 a lower leakage current. This leads to an interesting conundrum. Electromagnetic compatibility (EMC) performance is another crucial issue for medical power supplies. All modern power supplies are of the switch-mode type, because 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 is necessary to understand 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, hence they are known switch-mode supplies. The outputs from the transformer are converted back to DC and fed to regulators that 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 that is of most interest. The switches are transistors and 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.
Value of zero-voltage switching
Some of the best modern power supply designs, therefore, deliberately slow down the switching operation using zero-voltage switching (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 100 ns compared with 20 ns in a conventional power supply.
By using innovative zero-voltage switching circuitry it is possible to achieve this slow switching without compromising power-supply efficiency. The amount of electromagnetic interference generated is greatly reduced and only a small filter is needed for these supplies to meet the most demanding EMC requirements. With only a modest amount of filtering needed, leakage currents can also be kept low to satisfy another important requirement.
A further benefit is that this circuitry eliminates the need for an interwinding screen in the transformer, another measure that 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 that are compact, lightweight, efficient, cost-effective and reliable. Switch-mode power supplies can meet all of these needs, but not all switch-mode supplies are created equal.
For more information contact Andy Skinner, Advanced Engineering Manager, Lambda UK, Kingsley Avenue, Ifracombe EX34 8ES, UK, tel. +44 1271 856 600, e-mail: powersolutions@lambda-europe.com www.lambda-europe.com.
