Safety First

Understanding leakage currents
in medical applications

The 250 W CUS250M power supply from TDK-Lambda optimised for medical applications and is also a good choice for industrial applications where very low values for earth leakage current, touch current and excellent EMC behaviour are important. The power supply complies with IEC 60601-1, 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 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 40 kHz to over 500 kHz. 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).
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 using the AC feed.
Figure 1: The location of the noise reducing
ceramic capacitors in an AC-DC power supply
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