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EMC coexistence in medical environments and power supplies – where are we?

From gadget to vital, Connected Devices and Internet of Things (IoT) are everywhere, and we see them gradually filtering through, changing our lives in many ways. The exponential development of small to large products embedding radio-transmitters is driving forwards, albeit not without some concern in terms of interference and equipment disturbance, especially in the case of medical applications where the consequences could be severe!

With the plethora of products transmitting radio signals it becomes very difficult and complex for medical equipment manufacturers to ensure that their equipment is secure when using either published international standards or proprietary protocols of which many are operating at unlicensed frequencies in the ISM (Industrial, Scientific, and Medical) or MICS (Medical Implant Communication Service) bands to properly operate without interfering in, or being interfered with by other equipment. Consequently, in ensuring wireless coexistence in medicals applications, regulatory bodies the world over have focused their efforts to standardize protocols and processes that require power supplies manufacturers to include "Wireless Coexistence" testing and verification when designing power sources for medical equipment.

When the "unpredictable" could happen!
As the world would literally stop without reliable power, thankfully the power industry has a long history in building robust power systems. Industry is perpetually innovating new technologies, improving energy efficiency, reliability and safety throughout. With the rapid development of multiple Connected Devices in medical applications, some of them may be powered by harvesting energy, making them very sensitive to radio interference whilst others might even get their power from radio waves. The subject of power supplies' coexistence with radio signals needs to be considered differently from previous efforts and experiences, and this is especially so with medical equipment that is installed outside professional healthcare-controlled environments, such as in the home.

As the number of Connected Devices and radio transmissions within medical environments has increased, the number of cases of medical equipment reporting false alarms, random failures or malfunctioning, has grown significantly, warning the medical community about the coexistence of multiple radio transmitting equipments that patients' lives might depend upon.

In many cases of reported faults it was very difficult to pinpoint the exact cause, until in-depth investigations revealed radio interferences were the root cause of the problem. In the US, the Food and Drug Administration (FDA) records malfunctions in a central database called "MAUDE", which includes a growing number of EMC problems. Amongst many such cases we have selected one example that clearly illustrates the complexity of identifying the root cause of electro-magnetic interferences (EMI), especially when not in a controlled environment such as in the case of home healthcare.

When welding turns alarm ON
A patient with respiratory and heart problems was connected to a very advanced ventilator at home, coupled to a wireless cardio-monitoring unit. The patient's health was being monitored from a remote healthcare center, which received a series of alarms. After calling the patient, who fortunately was doing very well, all alarms were classified as false, motivating the replacement of the monitoring units. Despite replacing the system, randomly it was still signaling warnings! Equipment manufacturers conducted thorough analysis without finding either hardware or software issues. By coincidence a nurse visiting the patient noticed a strange noise coming from the radio and at the same time the monitoring alarm came on. Further investigations identified that a nearby industrial company was using high energy welding equipment that had, after a maintenance, a default shielding. Radiating radio waves were interacting with the control loop of sensors, trigging alarms. This example is probably anecdotal but reflects the complexity of the coexistence of vital medical equipment and radio interference.

With the multiplication of the incidents, in the case of homecare but also in hospitals it is obvious that thorough procedures guaranteeing Electromagnetic Compatibility and immunity to Radio Interference is a must, motivating the medical industry and the International Electrotechnical Commission (IEC) to rethink Electromagnetic Interference within the medical arena, ensuring that everything works smoothly and safely.

IEC 60601-1 and IEC 60601-1-2
To guarantee the highest level of safety, the medical industry follows several international standards. For Medical Electrical Equipment (MEE), in 1977 the International Electrotechnical Commission (IEC) developed and published 60601-1 that specifies safety and performance requirements for MEE and is widely recognized as the benchmark for medical device safety. As the number and variety of applications in the medical industry has grown throughout the years, the general standard was complemented with collateral standards, and particular ones have been through several editions (Figure 01).

Figure 01 – IEC 60601-1 standard structure with collateral and particular to applications (Source: PRBX)

In the case of Electromagnetic Compatibility and Immunity to Radio Interference, the collateral standard that applies is IEC 60601-1-2, addressing how medical devices should resist and limit their own electromagnetic emissions to ensure device safety and performance. The fourth edition of this standard was published 10 years ago in 2014 but taking into consideration the growing number of Connected Devices, new operational radio frequency bands, and the risk of interference between the different pieces of medical equipment, the IEC subcommittee 62 (SC-62) considered the importance of amending the collateral before the next major revision (Edition-5), instead amending edition 4 with important updates. The amendment was ratified and published in 2020. After a transitional period, the latest edition of IEC 60601-1-2:2014 Amendment 1:2020 (referenced as Edition 4.1), reached the date of application, and regional standards organizations to state the withdrawal (DOW) point e.g., in Europe the DOW date for the EN 60601-1-2:2015/A1:2021 is set for 2024-March-19.

Without going too deep into the latest edition, at a glance in comparison to Edition-4, Edition-4.1 addresses a number of items but considering the example we presented in the introduction, we could list four major areas as the core of the amendment: (1) Testing at both minimum and maximum input voltage levels at any one frequency for conducted emissions, voltage dips, and short interruptions; (2) Required power frequency magnetic field at either 50 or 60 Hz as long as the frequency is the same as what is used to power the medical equipment or medical system ; (3) Conducted Immunity I/O cables less than 1 meter are required for all patient cables and (4) A new test specification for Enclosure Port Immunity to Proximity Magnetic Fields has been added under Table 11 of the medical standard – using IEC 61000-4-39 test and measurement techniques that requires Magnetic field testing in three spot frequencies (30KHz, 134.2KHz, and 13.56MHz) (Figure 02).

Figure 02 - Magnetic field testing in three spot frequencies added to the IEC 60601-1-2 Edition 4.1 (Source PRBX/IEC)

Something interesting and worth mentioning is that this new table is a compromise between the FDA requirement for medical equipments such as In Vitro Diagnostics (IVD) which they used to request those products to meet the AIM 7351731 RFID immunity testing at eight different conditions from 134.2 kHz to 2.45 GHz. Regarding IVD and implanted equipment, it is important to know that the IEC 60601-1-2 does not apply to implants (implants have their own standards, e.g., ISO 14117), but it does apply to accessories that monitor or control an implant from outside the body.

When designing a power supply for medical equipment, because not all tests are applied for all products, it is important to consider all aspects of the IEC 60601-1-2 (or regional version) and all applicable EMC tests for each type of medical device/system and their power supplies. The standard requires some tests for specific products and immunity levels depending on building practices, the type of magnetics, and switching frequency. In the test plan and report the manufacturer must specify and document any areas exposed to external interference and to take into consideration the final application and environmental aspects.

As IEC 60601-1-2 Edition 4.1 becomes the norm, the technical committee is already working on the future and to take into consideration new constraints and requirements for electromagnetic compatibility. More immunity tests might be added from the AIM 7351731 to cover sensitive equipments such as Magnetic Magnetic Resonance Imaging (MRI).

A bit of fun designing power supplies for high EMC compliance
Taking into consideration that EMC is very important when supplying power to medical applications, power supply manufacturers have developed new technologies to reduce EMI by using new switching topologies and advanced shielding. Yet in some extreme applications such as MRI, conventional technologies are not enough.

The Magnetic resonance imaging (MRI) system employs an extreme static magnetic field (B0), magnetic field gradients (B1), and the fast evolution of radio frequency pulses (RF Field) (Figure 03). The MRI system is very sensitive to electromagnetic noise and to the presence of magnetic or conductive materials that can cause image deterioration from which could result artifacts, with the risk of faults in the diagnostic. To avoid interference the best practice in powering MRI is to avoid alternative voltage/current (AC) and to only use continuous voltage/current (DC) even for lighting. Master power supplies are traditionally positioned outside the shielded operation room, and the DC voltage is distributed to the electronic equipment via shielded cables, but some MRI equipments require the power supply to be installed within the machine and exposed to very high magnetic field in a range of 3 to 5 Tesla, without interfering with sensitive equipments.

Figure 03 - Simplified representation of an MRI equipment and the different fields
contributing to create the final image (Source: PRBX/Shutterstock/Pattarawit)

Because conventional magnetic cores saturate when exposed to the B0 field energy, having no ferromagnetic core material, air-core inductors should be considered. One downside of air-cored inductors is their low inductance values, which can be compensated for by designing a multi air-cored power stage operating in parallel. Controlling multi-parallel air-cored power supplies requires implementation of the latest digital control technology that offers a high degree of flexibility in how the different power channels operate. Digital control allows designers to adapt the profile of the power supply to specific conditions. Figure 04 is an example of an advanced air-core power supply, the PRBX GB350. To accommodate the specific MRI, B0, B1 and RF specifications that it has been designed for, the power supply has a fundamental switching frequency of 600kHz. With such a switching frequency and its four phases configured in interleave mode, the unit has a resultant output frequency of 2.4MHz. This allows easier filtering, extremely fast regulation response times, and coherence with the MRI equipment radio compatibility.

Figure 04 - Triple outputs, multi-phases, PRBX coreless
power supply sustaining B0 field (Source PRBX)

As mentioned earlier, the IEC 60601 standard is composed of collaterals and specific standards, and MRI basic safety and performance is covered by IEC 60601-2-33. This document mainly focuses on the patient and operator safety but it also provides information on the "Special Environment" specifications in IEC 60601-1-2 and how the special environment is implemented, including information on how integrity should be maintained during operation. Power supplies operating in MRI environments must be tested according to IEC 60601-1-2, but equipment manufacturers may require in-situ qualification prior to final validation and extra immunity tests specific to their environment.

Technologies presented at recent events; Medica, Embedded World and Mobile World congress confirm that society has entered the age of interconnected devices, and the medical industry is rapidly modernizing to improve patients' comfort and wellbeing. The consequences of this is the risk of multiplication of radio interference, and that is why the IEC committee is collecting feedback in preparation for the next revision of the IEC 60601-1 and collateral standards. Until then power designers are working closely with the medical industry to - even as I write - deliver robust power solutions able to be used safely in complex environments.

About The Author

Chief Marketing and Communications Officer for Powerbox, Patrick Le Fèvre is an experienced, senior marketer and degree-qualified engineer with a 40-year track record of success in power electronics. He has pioneered the marketing of new technologies such as digital power and technical initiatives to reduce energy consumption. Le Fèvre has written and presented numerous white papers and articles at the world's leading international power electronics conferences. These have been published over 450 times in media throughout the world. He is also involved in several environmental forums, sharing his expertise and knowledge of clean energy.


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