Safe power for medical devices used at home: Page 3 of 3

February 13, 2015 //By Herbert Blum
Safe power for medical devices used at home
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.
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.

Asymmetric attenuation curves: 1-A filter with Y-capacitors (top line) and without Y-capacitors (bottom line)

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 30 MHz, 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; we recommend 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 metal coating on the inside provide an effective shielding alternative and are often used as a lower-cost alternative.

 

5120: IEC-C18 connector with EMC filter

If a connector with integrated filter is used, it is advisable to rear-mount it to the inside panel. The metal flange of the filter then meets with the shielding inside the plastic enclosure, thus guaranteeing the most complete shielding possible. Make sure that the electrically conductive shield is double insulated against the live conductors. An inlet filter in Protection Class II already satisfies these requirements. The inlet filter together with the enclosure shielding guarantees a very broadband attenuation. This forms the basis for successful proof of the EMC conformity.

Rear side mounting on metallized plastic

Schurter offers various combination connectors that satisfy the requirements of the standard IEC 60601-1-11 which includes Protection Class II variations. These filtered connectors in a Protection Class II design have been high-voltage tested with 4000 V. Every single one of the manufactured components is tested in the final inspection between live and tangible conductive parts.

Examples of conductive parts are the filter shield or the assembly panel. The combination connectors are also protected against solid foreign bodies with a diameter above 12.5 mm, against penetration with a finger, and have an IP 40 degree of protection. Additionally, we recommend a flush-mounted installation in the enclosure so as to achieve the protection against drip, in accordance with IP 21.

Visit Schurter AG at www.schurter.com

About the author:
Herbert Blum is Product Manager EMC at Schurter AG - www.schurter.com

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