Feature: Medical
250VAC, a 2.5mm clearance distance and 4mm creepage distance are required as basic insulation, with maximum permissible leakage current of 0.1mA in normal operation. Another device group comprises diagnostic units (IVD = in
vitro diagnostics), used for preparation and analysis of human tissue and bodily fluids. Among them are centrifuges, hematology analysers, gas or liquid chromatographs, and various laboratory devices classified as medical products but not as electrical medical devices. Here, IEC/EN61010 applies; the limit for leakage current is 0.5mA under normal conditions, with a maximum of 3.5mA in case of failure. Power inlet filters, such as the Schaffner FN92xx series, are
available from 1-15A and should be designed for a maximum operating voltage of 250VAC with a frequency to 400Hz. All IEC inlet filters with Y-caps (FN92xx), as well as integrated earth line chokes (FN92xxE), can be used for IVDs, since their leakage current is usually a maximum of 0.16mA, or 0.373mA in the worst case. Tis allows the IEC inlet filters to be used with device-internal power supplies with integrated filtering. However, attention must be paid to the total leakage current. Te schematic above right is that of a Schaffner IEC inlet B-type
filter. Here we need to mention that for isolated application, the commonly used Y-caps are not suitable since they afford no connection to protective earth. Noise, which occurs between active conductors, is considered
symmetrical interference, or DM = differential mode. All interference on the active conductors whose current path returns over the earth is so-called “asymmetric interference”, or CM = common mode. Te filter X-caps are located between the active conductors, and
ensure the filter’s differential mode performance. Due to the lack of Y-caps, usually located between the active conductors and earth, the common-mode performance is based only on the inductor and the core material used. According to CISPR 17, the insertion loss of EMC filters is
measured both with 50Ω input and the termination impedance. Te frequency response curve is displayed in all Schaffner data sheets. Te real insertion loss of an EMC filter depends on the impedance of the application; they are not always 50Ω, and are mostly complex, having to test the EMC filter’s effectiveness inside the equipment.
Panel-mounted IEC socket with EMC filter To assist product designers, Schaffner offers several mounting
styles for the FN9274B. Offering assembly from the front, rear and snap-in mount, the output connections are either wires or fast-on, depending on the available space and internal interconnects. Te filters are also usable with cables that 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. Te graphs below show the differences in attenuation of
individual filter versions, comparing the 1A (leſt) and 15A (right) models. Due to the use of nano crystalline core material, the attenuation performance is high compared to another standard B-type filter like Schaffner’s FN9222.
Standard filter FN92XX compared to B-type filters (the greyed-out components)
Figure 5: FN9274 insertion loss comparison 1A to 10A
www.electronicsworld.co.uk March 2021 27
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