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Addressing patient leakage current issues

11 Feb 2015  | Dante De Guzman

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A more popular solution than a full custom power supply is to modify a standard medical power supply to meet the requirements. Working with customers in real-world applications, XP Power has successfully modified standard medical power supplies to meet BF/CF leakage current requirements, that is, its patient leakage current is less than 10µA (overall compliance will depend on leakage currents from the rest of the system too). Such projects can be based on XP's GCS or ECM60 series, since these meet the isolation requirements of IEC60601-1. Modified versions of the GCS series' LLC topology can meet CF requirements in a 3x5 inch footprint, while modified ECM60 series power supplies have a 2x4 inch footprint and feature universal 90 to 264 Vac input, while complying to EMI class B emissions. Modified power supplies from both of these series have demonstrated patient leakage currents below 10 uA at 264 Vac/60Hz.

A typical way to modify a standard power supply to reduce its input to output capacitance would be to reduce the value of any Y capacitors (bridging capacitors). However, it's not as straightforward as that for every power supply. Aside from the EMI concerns mentioned earlier, even removing the bridging capacitors completely might not be enough to reduce the patient leakage current to below 10µA – the interwinding capacitance is an inherent property of the transformer and cannot be modified, so if that is not low enough to start with, it might not be possible to reduce the output capacitance enough to meet the requirements. Other non-modifiable components like opto-isolators that cross between the primary side and secondary side can add to the output capacitance too. Well-designed power supplies feature low capacitance in these parts as standard.

In summary, medical power supplies must meet stringent patient safety standards, and patient leakage current is often the hardest part of the standard to meet. This leakage current is directly proportional to the input to output capacitance of the power supply, but reducing this capacitance is not always easy because of EMI concerns. Adding a DC-DC converter as an extra layer of isolation is one solution, though it may be costly. Custom power design is a feasible but unpopular option. Alternatively, XP Power works with customers to modify its standard medical power supplies to meet the most stringent patient leakage current requirements, which may be the most cost-effective option.

About the author
Dante De Guzman is with XP Power.

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