EMC Coexistence Challenges for Medical Power Systems

Author : Patrick Le Fèvre, CMO at PRBX

26 March 2024

Figure 1: IEC 60601-1 standard structure with application collateral [Source IEC]
Figure 1: IEC 60601-1 standard structure with application collateral [Source IEC]

The plethora of products transmitting RF signals means that it is very difficult for medical equipment manufacturers to ensure their hardware is secure, when using either published international standards or proprietary protocols (often operating at unlicensed frequencies within the ISM or MICS bands), to properly operate without interfering with or experiencing interference effects from other items of equipment.

Consequently, in ensuring wireless coexistence within medical applications, regulatory bodies around the world have focused on standardising protocols and processes that require power supply manufacturers to include coexistence testing and verification when designing power sources for medical equipment. 

The world would literally stop without access to reliable power. Thankfully, the power industry has a long history in building robust power systems. It 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 the harvesting of energy, making them very sensitive to RF interference (RFI), whilst others might even get their power from radio waves. The coexistence of power supply units (PSUs) with radio signals needs to be considered differently from previous efforts and experiences - especially in relation to medical equipment that is installed outside of professional healthcare environments, such as in the home.

As the number of connected devices and radio transmissions within medical settings has increased, the 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 units that patients’ lives depend upon. Often with reported faults it was very difficult to pinpoint the exact cause, until in-depth investigations revealed that RFI was responsible. In the US, the Food & Drug Administration (FDA) records malfunctions in a central database, which includes a growing number of electro-magnetic compatibility (EMC) problems. Amongst many such cases, 1 example has been selected here that clearly illustrates the complexity of identifying the root cause of electro-magnetic interference (EMI) issues, especially when not in a controlled environment (such as in a home healthcare context).

When welding turns alarms on
A patient with respiratory and heart conditions 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 centre, which received several alarms. After calling the patient, who fortunately was doing very well, all alarms were classified as false, motivating replacement of the monitoring units. Despite replacing these items, warnings were still being received. The respective equipment manufacturers conducted detailed 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 as the alarms came on. Further investigations identified that a nearby industrial site was using high energy welding equipment which, after shoddy maintenance work, had faulty shielding. Radiating radio waves were interacting with sensor control loops, trigging alarms. This example is probably anecdotal, but reflects the complexity of coexistence between vital medical equipment and RFI.

Table 1: Magnetic field testing in 3 spot frequencies added to the IEC 60601-1-2 Edition-4.1 [Source IEC]
Table 1: Magnetic field testing in 3 spot frequencies added to the IEC 60601-1-2 Edition-4.1 [Source IEC]

With the multiplication of such incidents, in homecare and in hospitals too, it is obvious that thorough procedures guaranteeing EMC and immunity to RFI are essential. This has motivated the medical industry and the International Electrotechnical Commission (IEC) to rethink EMI within the medical arena, ensuring that everything works smoothly and safely.

IEC 60601-1/60601-1-2
To guarantee elevated safety levels, the medical industry follows several international standards. For medical electrical equipment (MEE), in 1977 the IEC developed and published 60601-1. This specifies safety and performance requirements for such equipment, and is widely recognised as the benchmark for medical safety. As the breadth and variety of medical-related applications has grown over time, the general standard has been complemented with collateral standards, and particular ones have been through several editions (see Figure 1).

With regard to RFI immunity and EMC, the collateral standard that applies is IEC 60601-1-2. This addresses how medical devices should resist and limit their own electro-magnetic emissions. The 4th edition of this standard was published in 2014, but taking into consideration the rise in connected devices, new operational RF bands and the risk of interference between different items of medical equipment, the IEC SC-62 subcommittee considered the importance of amending the collateral before the next major revision (Edition-5), instead amending Edition-4 with important updates. This was ratified and published in 2020 (referenced as Edition-4.1).

Without going too deep into the latest edition, Edition-4.1 addresses numerous items. Considering the example presented previously, we could list 4 major areas as the core of the amendment: 
  1.  Testing at both minimum and maximum input voltage levels at any given frequency for conducted emissions, voltage dips and short interruptions. 
  2.  Required power frequency magnetic field at either 50Hz or 60Hz, as long as the frequency is the same as what is used to power the medical equipment. 
  3.  Conducted immunity I/O cables <1m are required for all patient cabling.
  4.  A new test specification for enclosure port immunity from close proximity magnetic fields has been added under the medical standard - using IEC 61000-4-39 test techniques requiring magnetic field testing at 3 spot frequencies (30KHz, 134.2KHz and 13.56MHz) (see Table 1).

Figure 2: Simplified representation of MRI equipment and the different fields contributing to creation of the final image
Figure 2: Simplified representation of MRI equipment and the different fields contributing to creation of the final image

Something worth mentioning is that this new table is a compromise on FDA requirements for medical equipment, such as in-vitro diagnostics (IVD) which they used to request met AIM 7351731 RFID immunity testing at 8 different points from 134.2kHz to 2.45GHz. Regarding IVD and implanted equipment, it is important to know that IEC 60601-1-2 does not apply to implants (they have their own standards, e.g. ISO 14117), but it does apply to accessories monitoring or controlling implants from outside the body.

When designing a PSU for medical equipment, because not all tests are applied for all products, it is paramount to consider every aspect of the IEC 60601-1-2 (or regional version) and any applicable EMC tests for each type of medical device/system and their accompanying PSUs. The standard requires some tests for specific products and immunity levels depending on building practices, the type of magnetics, switching frequency, etc. In the test plan and report, the manufacturer must specify and document any areas exposed to external interference, giving consideration to 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 - taking into account new EMC constraints and requirements. More immunity tests might be added from AIM 7351731 to cover sensitive equipment, such as magnetic resonance imaging (MRI) apparatus.

Designing PSUs for high EMC compliance
Taking into consideration that EMC is very important when supplying power to medical applications, PSU manufacturers are looking to reduce EMI by using new switching topologies and advanced shielding. Yet in some extreme applications, like MRI, these technologies are not enough.

Figure 3: Triple outputs, multi-phases, PRBX coreless PSU sustaining B0 field
Figure 3: Triple outputs, multi-phases, PRBX coreless PSU sustaining B0 field

An MRI system will employ an extreme static magnetic field (B0), magnetic field gradients (B1) and the fast evolution of RF pulses (see Figure 2). MRI systems are very sensitive to electro-magnetic noise and the presence of magnetic or conductive materials that can cause image deterioration and might result in artifacts, with the risk of diagnosis errors thereby occurring. To avoid interference, the best practice in powering MRI is to not use alternating current (AC), but rely on direct current (DC) instead, even for lighting. Master PSUs are traditionally positioned outside the shielded operation room, and DC power is distributed to equipment via shielded cables. However, some MRI arrangements require the PSU to be installed within the machine itself, and exposed to very high magnetic fields (up to 5T) without interfering with sensitive equipment.

Because conventional magnetic cores saturate when exposed to the B0 field energy, air-core inductors should be considered (as they have no ferromagnetic core material). A downside of air-core inductors is their low inductance values, which can be compensated for by designing a multi-air-cored power stage operating in parallel. Controlling multiple parallel air-cored PSUs requires implementation of the latest digital control technology, offering high degrees of flexibility in how the different power channels operate. Digital control allows designers to adapt the profile of the PSU to specific conditions. 

Figure 3 shows an example of an advanced air-core PSU, the PRBX GB350. To accommodate the specific MRI, B0, B1 and RF specifications that it has been designed for, the PSU has a 600kHz fundamental switching frequency. With such a switching frequency and its 4 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.

As mentioned earlier, IEC 60601 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 patient and operator safety, but it also provides information on the ‘special environment’ specifications in IEC 60601-1-2 and how that environment is implemented, including how integrity should be maintained during operation. PSUs operating in MRI applications must be tested according to IEC 60601-1-2, but equipment manufacturers may also require in-situ qualification prior to final validation and extra immunity tests specific to their environment.

Conclusion
Society has entered the age of interconnected devices, and the medical industry is rapidly modernising to improve patients’ comfort and wellbeing. A consequence of this is the risk of multiplication of RFI, and that is why the IEC committee is collecting feedback in preparation for its next IEC 60601-1 revision. Until then, power designers are working closely with the medical industry to deliver robust power solutions that can be employed safely in complex environments.


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