Medical Electronics


Power supply leakage currents in medical devices


By David Buck, product manager, TDK-Lambda EMEA I


EC 60601-1 is a series of international standards for basic safety and essential performance of medical electrical equipment. It is widely used in many countries and is now deemed a requirement for most commercially available products. One key aspect of the standard is avoiding an electric shock, which could cause injury or even death, particularly due to excessive leakage currents.


Leakage current


Leakage current is the current measured flowing through the protective conductor to ground. If there is no ground connection, it is the current that could flow to ground if a conductive path is present, which includes the human body. In modern residential and office applications, if that current exceeds dangerous limits, the Residual Current Device (RCD) would trip, isolating the equipment or device from the AC source. Many people will have experienced this if they have cut the AC cable while using a garden hedge trimmer.


For healthcare applications, the maximum limit for leakage currents is set much lower, depending on the type of medical device being used. Medical leakage currents are not intended to be applied to a patient.


How does an AC-DC power supply generate leakage currents? A certified medical power supply must comply with many other restrictions besides the leakage current limitations. These include EMC (Electro Magnetic Compatibility) for conducted and radiated emissions, along with immunity against voltage dips, electrostatic discharges (ESD), input transients, RF (radio frequency) and even magnetic fields. Meeting these EMC standards is a legal obligation in many countries. It ensures that electrical and electronic equipment functions correctly


14 May 2023 Components in Electronics www.cieonline.co.uk.uk Figure 1: The location of the noise reducing ceramic capacitors in an AC-DC power supply


in its environment and does not generate unacceptable disturbances that could affect other equipment.


Power supplies use high frequency, high voltage switching circuits, typically operating between 40kHz to over 500kHz. These circuits generate high frequency electrical noise. To pass the conducted and radiated emissions standards, filtering circuitry is required. Ceramic filter capacitors are used to reduce electrical noise (Figure 1).


Figure 2: Y capacitors on the TDK- Lambda CUS30M


Capacitors CY1 and CY2 are used in conjunction with filter inductors (not shown). High frequency noise is diverted away from the AC source, through the capacitors into the earth ground (Protective Earth) connection. This technique avoids interference with other equipment also utilising the AC feed.


Capacitor C3, connected between the primary and secondary sides of the transformer, reduces high frequency noise and also represents the interwinding


capacitance of the transformer. C4 reduces output noise and has little effect on the leakage current, the dominant components being CY1 and CY2 between line and earth and neutral and earth. Lower capacitance values of CY1 and CY2 will result in a lower leakage current. But it may then cause the power supply to fail the conducted and radiated EMI standard requirements.


Types of medical leakage currents IEC 60601-1 refers to the following three main types of medical leakage current. They are measured under two conditions - Normal Conditions (NC) and Single Fault Conditions (SFC). Under a Single Fault Condition, for example, where the earth connection on a pluggable device is disconnected, a higher leakage current limit is allowed according to IEC 60601-1. Earth leakage current flows from the primary circuitry through or across insulation into the Protective Earth (PE) conductor. No current flows from the enclosure via the Patient or Operator when touching the medical device (Figure 3).


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