FEATURE MEDICAL ELECTRONICS
EMC FILTERS FOR MEDICAL DEVICES Holger Urban, product manager at Schaffner discusses medical grade filter options
E
MC (Electromagnetic compatibility) filters play an important role in
ensuring reliable operation of medical equipment. EMC-compliant design begins with the power entry side of an electronic device. In most equipment a combination of an IEC connector with embedded EMC filter is the most economical solution. With fully isolated power supplies the
need for filters which support class II medical devices feature enhanced creepage distances in the filter and also insulation from the rest of the system. A combination of a plastic housing with an IEC C18 inlet (without earth pin) the required freedom to start an isolated design right away. The EMC Requirements for Diagnostic
Equipment Regarding EMC, IEC/EN 60601 refers to the corresponding limit values of the CISPR standards. Most electrical medical devices are categorised similarly to the generic standards; they are sorted into classes according to use in the home, doctor‘s surgery (B) or in hospitals with separated AC mains (A). Emissions must comply with the limits of CISPR11. For simple electrical components, the reference is CISPR14, lighting facilities refer to CISPR15, and IT systems use CISPR22. In addition to the usual standards Schaffner is following, such as UL60939-3, IEC 60939-3, CAS and CQC, for the FN9274 medical class II compliance is taken into account and in addition the IEC safety certificate shows the compatibility to certain application specific standards including the 2MOPP compliance. Products such as syringe pumps,
medical ventilators, lasers or patient monitoring devices with applied parts in direct physical or electrical contact with patients are defined as medical devices (MD). Operating at 250V AC a 2.5mm clearance distance and 4mm creepage distance are required as basic insulation. The maximum permissible leakage current is 0.1mA in normal operation. The second device group is comprised
of diagnostic units (IVD = in vitro diagnostics), which are used for preparation and analysis of bodily fluids or human tissue. These include centrifuges, haematology analysers, gas or liquid chromatographs for example, as well as various laboratory devices which are classified as medical products but not
28 MARCH 2021 | ELECTRONICS Figure 1: Schaffner‘s “Medical” IEC inlet filter FN9274
for isolated application the commonly used Y-caps (greyed-out components) are not suitable, since there is no connection to protective earth which could be used. Noise, which occurs between the active
conductors, is designated as symmetrical interference (DM = differential mode). All interference on the active
conductors whose current path returns over the earth is so-called asymmetric interference (CM = common mode). The filter X-caps are located between
the active conductors and ensure the differential mode performance of the filter. Due to the lack of Y-caps, which are usually located between the active conductors and the earth, the common mode performance is only based on the inductor and the core material used. According to CISPR 17, the insertion
Figure 2:
Approvals and compliances of IEC inlet filter FN9274, including class II compliance
Figure 3:
Standard filter FN92XX compared to B-type Filters (greyed-out components)
as electrical medical devices. Here, IEC/EN61010 is applied. The limit for leakage current is 0.5mA under normal conditions as well as maximum 3.5mA in case of failure. Power inlet filters, such as the
Schaffner FN 92xx series, are available from 1-15A and should be designed for a maximum permanent operating voltage of 250 VAC with a frequency range of up to 400Hz. All IEC inlet filters with Y-caps (FN92xx) as well as with an integrated earth line choke (FN92xxE) can be used for IVD devices since their leakage current is normally a maximum of 0.16mA to 0.373mA in the worst case. This allows the IEC inlet filters to be used parallel to device-internal power supplies with integrated filtering. Attention must therefore be paid to the total leakage current. Figure 3 shows the typical schematic of Schaffner IEC inlet B-type filters. Here,
Figure 4: FN9274 mounting styles.
Figure 5:
FN9274 Insertion loss comparison 1A to 10A
As a result, the effectiveness of an EMC
filter must be tested inside the equipment. To assist product designers, Schaffner has designed several variants in terms of mechanical mounting styles for the FN9274B. Offering assembly from the front, rear and snap in mount. The output connection type is either wires or fast-on connections, depending on available space and internal interconnects the designer is free to use. The filters are also usable with cables which lock to the IEC inlet according to an industry standard, the additional hatch does not interfere with the use of standard cables, ensuring the inlet is compatible with C13 and C17 couplers. To show (Figure 5) the differences
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regarding the attenuation of the individual filter versions, the 1A and 15A models are compared with each other. Due to the use of nano crystalline core material the attenuation performance is high compared to another standard B- type filter like the FN9222.
/ ELECTRONICS
loss of EMC filters is measured both with 50ohms input and termination impedance. The real insertion loss of an EMC filter depends on the impedance situation in the application. These are not always 50ohms and are mostly of a complex nature.
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