Medical Electronics
Figure 3: Earth leakage path
Figure 6: CUS250M series of medical power supplies
Table 1: Maximum earth leakage current Figure 4: Touch current path
Touch current flows from the medical device’s enclosure or parts thereof, excluding patient connections, and is accessible to patients or operators in normal use through an external path (not PE) to the earth (Figure 4). The touch current is also measured on the output terminals, which may be accessible to the patient depending upon the final system. Patient leakage current flows from patient connections of the electrical medical equipment (Applied Part) via the patient to earth, this is considered a Normal Condition (NC). An Applied Part is a part of the equipment that under normal operating conditions contacts the patient. There are three main categories:
B rated (Body): Table 2: Maximum touch current
May be connected to earth Normally not conductive and can be immediately released from the patient Examples: Medical lasers, MRI body scanners, phototherapy equipment, beds and cables
BF rated (Body Floating):
Has conductive contact with the patient Examples: Blood pressure monitors, incubators, ultrasound equipment CF rated (Cardiac Floating): May come in direct contact with the
heart Example: Dialysis machines Figure 5: Patient leakage path
Patient leakage current flows from patient connections of the electrical medical equipment – the Applied Part - via the patient to earth (Figure 5).
Power supply design Table 3: Maximum patient leakage current
www.cieonline.co.uk.
Ensuring that the internal electrical noise generated by the switching converter
is minimised allows the value of the capacitors to be reduced. This, in turn, reduces the power supply’s leakage current and still retains compliance to the EMI standards.
The design specification for TDK- Lambda’s 250W rated CUS250M power supply had aggressive targets for leakage currents and EMI performance. The engineering team knew that this could not be an afterthought. A patent pending output terminal / inductor combination was developed to reduce common mode noise, a custom ferrite core was designed for the power factor correction (PFC) circuit to reduce EMI and, importantly, the engineering team took significant care in the layout of the printed circuit board (PCB).
The result was a leakage current of less than 150µA (at 264Vac 63Hz - the worst case voltage and frequency), a touch current of <10 µA (Class I) and <70µA (Class II, no ground connection). Despite low leakage currents, the EMI performance for both Class B conducted and radiated emissions had significant margins.
Summary
There are large numbers of companies offering medical power supplies. It is recommended to select one that has extensive experience in assisting customers. Every medical development has an overall system leakage current budget. Knowledgeable field application support can interface between engineering teams, offering advice and even providing a minor product change that can save the customer both time and money.
https://www.emea.lambda.tdk.com/uk Components in Electronics May 2023 15
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