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An aging population and the rising cost of healthcare around the world are a matter of concern to the general public these days. This is especially true in the U.S.A., where keeping costs under control becomes more and more challenging. In order to reduce the cost of overhead, while offering increased levels of care by way of technology, we are seeing a trend toward medical care at home. Along with this trend is increasing demand for smaller, portable, and efficient medical equipment. While the shorter stay in overcrowded hospitals is saving costs in the health industry, manufacturers of medical devices are being presented with new challenges to minimize the safety risks associated with the use of medical devices. Medical devices used at home are being operated by laypersons with no medical training, instead of trained personnel in a controlled hospital environment. Regulatory requirements have been established to provide safety measures specifically for these users.

As an important addition to the basic standard IEC 60601-1 for medical devices, IEC 60601-1-11 describes requirements about medical equipment used in the home. The developers of such devices or components have to observe additional requirements as it relates to safety in addition to specifications about marking (simple operation) and documentation (for example, understandable operating instructions). The utmost care has to be exercised when it comes to powering the device. The safety requirements according to IEC 60601-1-11 can be summarized as follows:

• Altered ambient conditions (domestic environment)
• Protection Class II
• Minimum IP 21 degree of water and dust protection
• Stricter shock and vibration tests

A key criteria is the responsibility to provide Protection Class II safety, as well as equipment immunity to electromagnetic interference.

Safe Power Supply

Domestic power supply systems, unlike installations in hospitals, are not always reliable or may be insufficiently grounded. A medical device used at home has to compensate for this potential. It is therefore imperative that it be designed with Protection Class II safety.

Protection Class is defined in safety standards that offer protection against dangerous contact voltages. Devices in Protection Class II have double insulation between the mains circuit and the output voltage or metal enclosure. Even if they do have electrically conductive surfaces, they are protected against any contact with other live parts via the double insulation.

The protective insulation has to be reinforced according to the general standard IEC 61140 for protection against electric shocks or be double that of Protection Class I. A second insulation for the power feed is a common solution. Doubling the clearance and creepage distances for all live contacts also has the same effect as a double insulation. This ensures that the two conductors, consisting of the phase and neutral conductor, are sufficiently insulated against all other surfaces that can be touched. If the surfaces that can be touched are electrically conductive, the gaps and insulations also have to be doubled. The clearance and creepage gaps for Protection Class I are 4.0mm between live conductors and metal enclosures or ground conductors. This value is doubled in Protection Class II, corresponding to a gap of 8.0mm between live conductors and a metal enclosure.

Complying with EMC

Since laypersons operate medical devices at home, special attention to the equipment’s ability to effectively suppress interference is needed. This also includes ensuring electromagnetic compatibility (EMC). Most devices today are supplied with power by switched-mode power supplies. These are suitable for the various mains voltages around the world and have a high efficiency, due to the fast switching capability by ICs. These switching ICs, however, cause great interferences that can be measured on the power mains. The EMC standards specify limits for conducted and radiated interfering voltages. This is a precondition for compliance with the EMC standards, which in turn is necessary for the CE conformity marking. The use of a filter is recommended because of the interferences caused by modern electronic devices. A filter can be discreetly mounted onto the PC board or used as a block or in-line filter in combination with the connector. Since the ground connection is usually missing with Protection Class II, the filter has to make do without grounded capacitors. These so-called Y-capacitors are very popular in most filter circuits since they offer good attenuation in the higher frequency range. The two attenuation curves for the same filter — one with (standard) and one without Y-capacitors (Protection Class II) — clearly show the attenuation loss for the asymmetric attenuation in the frequency range above 1.0MHz

The attenuation loss can be compensated by more inductivity or by attaching ferrites to the power cable. But this is not always practical and possible. This is why it is recommended that interferences in this frequency range be dealt with as close as possible to the source of the interference. Connecting capacitors to the internal ground are suitable means in a secondary circuit. Linear chokes may also be suitable, though these have only a limited frequency range.

If interferences occur in a device in a frequency range above 30MHz, shielding should also be provided in addition to the input filter. If the high-frequency source of the interference can be spatially separated from the remainder of the electronic circuits, it is recommended that it be shielded by a metal enclosure. Should this prove impossible, the entire device should be shielded, though this is not always easy since Protection Class II devices do not normally have a metal enclosure. Plastic enclosures with a metal coating on the inside provide effective shielding and are often used as a lower-cost alternative.

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