EMC & Thermal Management EMC filters for medical devices By Holger Urban, product manager at Schaffner E
MC filters play an important role in ensuring reliable operation of medical equipment. In addition to the interference suppression of the components used in switching power supplies or controller systems, their central use in the power supply allows a flexible device design. 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. The metal housing of the IEC inlet filter, with a flat connection to the device mounting wall, creates a clear separation between the non-suppressed interior of the device and the external environment. With these components, there is no crosstalk even within the smallest space inside the device.
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 household, 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.
IEC/EN 60601/10 for Medical Electrical Equipment
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 250 V AC a 2.5 mm clearance distance and 4 mm creepage distance are required as basic insulation. The maximum permissible leakage current is 0.1 mA 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
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include centrifuges, haematology analysers, gas or liquid chromatographs for example, as well as various laboratory devices which are classified as medical products but not as electrical medical devices. Here, IEC/EN61010 is applied. The limit for leakage current is 0.5 mA under normal conditions as well as maximum 3.5 mA in case of failure. The EMC requirements for so-called IVDs are governed by the harmonised EN 61326 standard. 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 0.16 mA and max. 0.373 mA at worst. 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.
In a typical schematic of Schaffner IEC inlet filters, X-caps as well as Y-caps are used. The X-caps are located between the active conductors; the Y-caps are located between the active conductors and the earth. They must comply with increased requirements for electric strength, self-healing, insulation, lifetime, etc., as described in IEC/EN 60384-14 due to their direct and constant connection to the AC mains.
The B-types contain no Y-caps to fulfil the leakage requirements. Integrated earth line chokes (e.g. Schaffner FN92xxB and FN92xxEB) can be used for MD equipment. Conducted interferences are basically distinguished in two types. The first is noise, which occurs between the active conductors, and is designated as symmetrical interference (DM = differential mode).
Secondly. all interference on the active conductors whose current path returns over the earth is so-called asymmetric interference (CM = common mode). So, the Y-caps suppress mainly the CM noise while the X-caps suppress the DM noise. A further core element of the EMC filters is thus the common mode choke. It attenuates asymmetric interference running from the device to the AC mains, as well as
Figure 1: Schaffner‘s “High performance” IEC inlet filter FN9280/90
DM noise with a low-pass filter out of its leakage inductance, and the mains-sided X-cap. The CM choke also suppresses mains disturbances and so contributes to improved device immunity.
The B types of the Schaffner IEC inlet filters are designed without Y-caps due to the above- mentioned leakage-current requirements of MDs. Filter families should also be available with an earth line choke (E-design) to suppress ground loops in devices which are linked up to an additional connecting cable, such as interface or bus. The resistance R ensures that the caps are discharged, and no shock-hazard voltage remains on the poles, when the equipment is unplugged. Thus, the selection of Option R (bleed resistor) is always recommended if no additional measures were taken inside the device.
According to CISPR 17, the insertion loss of EMC filters is measured both with 50 ohms input and termination impedance. The frequency response curve is displayed in all Schaffner data sheets. These diagrams are helpful for an initial orientation. The real insertion loss of an EMC filter depends on the impedance situation in the application. These are not always 50 ohms and are mostly of a complex nature.
In practice, an inductive part on the mains side and a capacitive part on the load side are often added. As a result, the effectiveness of an EMC filter must be tested inside the equipment. Schaffner has a worldwide network of engineering organizations and
can offer support. Besides the standard program, customer-specific solutions can also be implemented on request. Versions with smaller Y-caps with leakage current < 0.1 mA are also feasible.
To assist product designers Schaffner has ensured that the external dimensions of their dual-stage version FN 9290 are identical to the single-stage version with earth line choke. For the new dual-stage family, a filter has been developed which has the same leakage currents as the FN 9280 and which nevertheless delivers an improved attenuation of up to 80 dBuV regarding the common- mode suppression (curve B).
The common-mode suppression (curve B) according to CISPR 17 of the dual-stage filter FN 9290 has a higher attenuation of up to 20 dB in the frequency range between 1 MHz and 30 MHz which is important for the common-mode noise suppression. For the differential-mode suppression (curve A), the attenuation of the dual stage filter is higher by approx. 20 dB at 150 kHz, which represents the starting point in the generic standards for conducted interferences. Schaffner FN92xx Series ultra-compact EMC filters are instrumental in the quick and simple qualification of electronic devices according to international requirements. They effectively control electromagnetic interference allowing the design of modern electronic devices to be made easy and reliable.
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Components in Electronics June 2020 29
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