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In the design of medical electronic equipment, it is tempting to think that power supplies designed and certified to be safe in industrial applications may be equally suitable. This is not usually the case, because the risks involved are very different. Furthermore, a great deal of the electronic equipment used in hospitals, such as patient monitors, operate with very low-level signals. Medical equipment such as this tends to be more sensitive to electromagnetic interference (EMI), which also makes electromagnetic compatibility (EMC) compliance and performance a concern.

As many hospital patients are in a weak condition, exposure to even small leakage currents can have an adverse effect on their well-being. Depending on the application, the 'allowed leakage current' from the medical equipment can vary from a few µA (microamps) to a few hundred µA. Previously it was the medical equipment that was classified depending upon its application, but now it is the 'applied parts', of which there are three classifications.

Type B, the 'body' applied part, is one that, although it can deliver a current to the patient, is not intended to do so as part of its function (operating room lights). The 'body floating' applied part, or type BF, has a patient connection that is designed to deliver electrical energy or an electrophysiological signal to or from the patient (ECG equipment). Type CF, or the 'cardiac floating' applied part, has a patient connection designed to deliver electrical energy or an electrophysiological signal to or from the patient and is suitable for direct cardiac application (external pacemakers).

The special requirements of medical equipment are reflected in international standards. For most of the world, the safety standards for medical power supplies are contained in the IEC60601-1 standards, with UL, CSA and EN derivatives. The present version of IEC60601-1 is the second edition; the third edition is being reviewed and may commence next year. Another point to consider is that up until now, medical devices and monitoring and control instruments have been exempt from the RoHS and WEEE directives. For many, these are expected to become mandatory sometime between 2010 and 2012.

There are many differences between the two editions, foremost of which is the new requirement for a risk-management process and record/file retention in compliance with the ISO14971 standards. It is therefore expected that future product certifications to IEC60601-1, third edition, may require an audit of the manufacturer's compliance with ISO14971.

The first and foremost requirement of IEC60601-1 standards is for the effective and reliable isolation between the AC input to the power supply, its internal high-voltage stages, and its DC output, as any shortcoming in isolation would result in the risk of electric shock. Several factors contribute to effective isolation, including the spacing between conductors and the electronic components. The IEC60601-1, second edition, sets minimum distances for spacing between these elements and it is important to note that these are greater than the spacing distances prescribed within the relevant standards for ITE (Information Technology Equipment) and industrial power supplies, which are covered by IEC60950-1.

Effective isolation also depends on reliable insulation: most modern medical power supplies use double insulation or reinforced insulation. Supplies, which operate from a 240V AC mains, for example, must withstand a dielectric test at 4kV AC for medical applications, whereas the corresponding figure for ITE/industrial use is only 3kV AC. Power supplies that are approved to less than 4kV AC may be used in medical applications as part of a reinforced barrier, provided there is additional isolation within the medical equipment to achieve the requirements of a reinforced barrier between the AC mains supply and the patient. The third edition of the IEC60601-1 separates the requirements for the patient and operator, whereas the second edition treats them as equal.

The leakage-current needs of the IEC60601-1, second edition, are difficult to achieve while meeting EMC compliance. The maximum permissible earth leakage is 300µA for worldwide approvals, but this figure applies to the end-product, not just the power supply. To allow for additional leakage in other components, it is useful for the power supply to have a lower leakage current. This leads to a trade-off between EMC performance and leakage current.

The most recent advance in medical power-supply design is the implementation of digital control — replacing analogue 'housekeeping' circuits with microcontroller-based solutions. Some manufacturers, including TDK-Lambda with its EFE300M, are introducing new products with full digital control. This, combined with a 4kV AC reinforced input to output isolation and other specifications such as an output-to-ground isolation of 1500V AC, enables these supplies to meet IEC 60601-1, making them suitable for use in B- and BF-type medical applications.