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Overview of the application of EMI filters for medical devices

2021/07/19 > Back

In hospital outpatient rooms, operating theatres and wards, electromagnetic interference caused by a medical electrical device with electromagnetic radiation can damage or reduce the performance of other medical electrical equipment, and cause harmful physiological effects through the physical and chemical effects on human, causing harm to human health.

Hospital patients are frequently in a weak condition. Exposure to even small leakage currents can have an adverse effect on their well-being. Depending upon the application, the “allowed leakage current” from the end-product medical equipment (not the power supply alone) can vary from a few µA (micro-amps) to a few hundred µA. The “leakage current” can be defined as the unintended, and potentially harmful, electric current that may pass through the human body. Obviously, medical equipment that has direct physical contact with patients must limit their leakage current to the lowest prescribed levels.

There are three ways of transmitting electromagnetics (medium):

1. Electromagnetics transmitted through space: radiation-type interference, which is very easy to leak out of or into the device from the very small, incomplete seams, connections (joint), vents, poorly shielded control/display panels, inputs/outputs, and cable wiring, etc.

2.Electromagnetics transmitted through conduction: conduction-type interference, this electromagnetic is very easy to leak out of or into the power cord, input/output signal line, ground wire, etc. of the equipment.

3. Electromagnetics transmitted through near field: coupling(Induction)-type interference, this electromagnetic is electromagnetically leaked due to capacitive (electric field effect ) or inductive (magnetic field effect).

The following are medical devices that are more susceptible to electromagnetic interference:

Telemetry
Vital signs monitor like ECG、EMG、EEG etc.
MRI
Electron microscope
Life support machines with microprocessor control or digital lines like infusion pump, hemodialysis machine, ventilator, anaesthesia machine, baby incubator etc.

The followings are electromagnetic interference-prone medical devices:

Electrosurgical generator
Shockwave therapy device (rehabilitation)
X-ray imaging devices
Rapidarc
Medical air compressor
Nursing bed

For interfered devices, the main and effective method we usually use is to install a EMI filter in their power supply. The EMI filter is a part that is often used to comply with EMC requirements for electromagnetic compatibility, and is installed with a power supply (interference source) at one end and a load (device) at the other end, which effectively suppresses interference signals from the grid from entering the device, as well as interference signals generated by the internal circuits of the device from being transmitted to the grid via the power cord. EMI filter is a two-port network, with the characteristics of selecting frequencies, it can allow some frequencies to pass smoothly and block other frequencies, from the point of view, it belongs to the low-pass filter, can transmit the input power of AC/DC to the device while high-frequency interference signal is greatly attenuated. When selecting specifications, the network and parameters of the EMI filter must be selected according to the line and load impedances to be connected at both ends of the EMI filter in order to achieve the maximum possible suppression.

The picture is a summary of our applications in various types of medical equipment, if you are interested please contact us, we will help you speed up the matching to the right EMI filters.

Data source: ntuh.gov.tw; news.eeworld