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EETE MAR 2015

POWER MANAGEMENT Safe power for medical devices used at home By Herbert Blum 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 USA, 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 towards medical care at home. Along with this trend is increasing demand for smaller, more 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). 5008: C8 appliance inlet with filter C8 C18 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 most certainly the responsibility to provide Protection Class II safety, as well as equipment immunity to electromagnetic interference. Safe power supply for medical equipment used at home 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 C18 PEM connector 5707 with V-Lock locking system and IP 65 protection 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 through the double insulation. The icon for Protection Class II symbolizes two insulation layers and is affixed to all Protection Class II devices. 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 mm between live conductors and metal enclosures or ground conductors. This value is doubled in Protection Class II, corresponding to a gap of 8 mm between live conductors and a metal enclosure. A C8 style IEC connector with a maximum rated current of 2.5 A is suitable for all devices with outputs up to approx 500 W. Matching connecting cables are available with 2-pin Euro and National Electrical Manufacturing Association (NEMA) plugs. The C18 connector, with a rated current up to 10A VDE and 15 A UL is available for medical devices with higher outputs. Complying with EMC for domestic medical equipment 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 on 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 MHz. Icon for Protection Class II Herbert Blum is Product Manager at Schurter AG – www.schurter.com 32 Electronic Engineering Times Europe March 2015 www.electronics-eetimes.com


EETE MAR 2015
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