One of the challenges of using commercial power supplies in medical instruments is electromagnetic interference (EMI). Commercial power supplies often list “meets Class B” for EMI, but this is often with conditions that are completely unrealistic or impossible for a medical system designer to meet. If designers assume electromagnetic compliance (EMC) in the power supply, they may find themselves chasing EMI problems and making last-minute system design changes, such as adding filtered power entry modules, shielding, and other compensation, before they can launch the product. This article discusses critical details to look for in commercial medical power supplies with respect to EMI. It also explores why it can be advantageous to consider a custom design to meet EMC as well as cost, performance, and lead time requirements.

Medical device designers must limit electromagnetic energy emitted from their products. (Credit: Bear Power Supplies)

Why EMI Matters and How It’s Tested

EMI is a process by which disruptive electromagnetic energy is transmitted from one electronic device to another via radiated or conducted paths, or both. EMI can adversely affect the performance of components, devices, and systems.1 Medical device designers must limit electromagnetic energy emitted from their products. This includes both radiated (through space) and conducted (via cables, power and ground planes, or parasitic capacitance) energy.

Fig. 1 - FCC Part 15 and CISPR 22/32 radiated limits for Class A and Class B at a 3 m antenna distance.2

The limits are set by FCC in the United States and by CISPR (part of the IEC) in the EU. Class A generally applies to industrial settings and includes hospitals. Class B generally applies to residential settings and includes doctors’ offices. Class B requirements are stricter on emissions (see Figure 1), in part because of the noticeable effect of EMI on televisions as higher speed electronics became more common in the home.

In addition to emissions compliance, products need some level of immunity to interference from other sources. Requirements differ, but from a practical view, the product has to work reliably in its intended environment. This article focuses on emissions.

Labs specializing in EMC testing typically charge $800 per half-day in the lab. Therefore companies tend to test for compliance at the end of product development cycle. By understanding and designing for EMC throughout the process, a designer can hope to pass EMC testing first time.

Limits of Standard Medical Power Supplies and EMI

Many system designers presume that a commercial medical power supply will meet Class B radiated and conducted EMI limits. There may even be a note on the data sheet that indicates “meets Class B,” but there will rarely be any information about the test conditions. If a manufacturer signs NDAs, it may be able to obtain the EMC test lab certification report. These reports reveal several ways this presumption can be wrong:

Your specific model was not tested. Bear, for example, recently designed a system using a commercial power supply. The system was failing to meet Class B limits, and Bear could not identify the reason. When Bear obtained the EMC certification report for the commercial power supply, only one model in the line of more than 20 different models (power and output voltage options) had been tested. There should have been no expectation that models with higher power and voltage would also pass, and indeed they did not.

A power supply design and manufacturing company may have basic EMI testing facilities in house, such as this small RF anechoic chamber at Z-axis. A video is available here. (Credit: Bear Power Supplies)

The product was not retested after a design change. Medical power supplies must meet safety requirements outlined in IEC 60606-1, including low leakage current. A common technique to reduce EMI is to add a Y1 capacitor of 1–4.7 nF between the primary and the secondary of the power supply. However, this will result in too much leakage current to meet IEC 60601-1.3

Smaller Y1 capacitors can be used, but additional isolation techniques will be needed. Bear has developed innovative isolation techniques to create power supplies of 600 W with less than 5 μA leakage current that meet EMC requirements.4

Commercial power supply vendors may take one of their standard power supply designs, reduce the Y caps to meet IEC 60601-1 leakage current requirements, and sell it as a medical power supply. But reducing the size of the Y caps also causes higher EMI, and the vendor may not retest this modified design for EMI.

The test did not use a realistic load. About the most forgiving test condition is a purely resistive load very close to the output of the power supply. Yet we have tested commercial power supplies from major suppliers, connected to a simple resistive load on our test bench, and had them fail both conducted and radiated EMI for Class A. And, of course, any real load is more complex than this. Most loads contain some digital logic with their own EMI that must not be passed back onto the input power line.

The tests do not account for cables. Printed circuit boards in a medical system may be inches or even feet apart. The output of a power supply can become quite an RF transmitter under those conditions. For example, 100 MHz radiation is a common problem. This frequency has a wavelength of 9 ft and a quarter wavelength of 2.5 ft — making a 2.5-ft cable a good antenna. A power supply may pass EMC testing with a short wire, but fail with a realistic length cable.

Relying on the internal EMI filter to do the job. Some articles recommend looking for a power supply with an internal EMI filter. In reviewing the EMC test reports of commercial power supplies with internal EMI filters, Bear has found that the test conditions included adding an external commercial EMI filter and encasing the whole system in a conductive box. Clearly the internal EMI filter was not sufficient to do the job.

What This Means for System Designers

Even if a manufacturer chooses a commercial power supply with an internal EMI filter and a claim to meet Class B limits, it is not possible to know whether meeting the limits was achieved with any kind of load, in a metal box, with additional input filters, for the model selected, or with other requirements.

If the final system prototype is sent to an EMC testing lab and fails, the device manufacturer must add shields and external EMI filters and retest. The cost of testing is significant. Even greater is the cost of delayed product launch. Further, added components affect system cost and reliability.4

For these reasons, a custom power supply design can save time and money and improve system performance and reliability. By developing the power supply circuity in parallel with the rest of the system, a designer can optimize the entire system rather than working around and compensating for the limitations of a standard power supply.

It is best to partner with a power supply design specialist. These specialists typically have expertise in EMI reduction techniques. They may have basic EMI testing facilities in house, allowing the device manufacturer to inexpensively check the system design, including the power supply and the load, early in development.

Good communication about how the power supply works with the system will let designers and their partners optimize the system together, rather than separately optimizing the power supply and logic boards. For example, power supply designers can add board-mounted high-frequency filters to the power supply output and reduce emissions from 16 MHz clocks. Power supply developers can design input filters that will be specific to the system requirements, cost less than commercial EMI filters, and achieve better results. They can provide guidance on any number of design choices such as board layout and location of the power supply relative to the logic board to prevent potential EMI problems.

Common Misperceptions about Standard Medical Power Supplies

Using a standard power supply has a number of perceived advantages. These perceptions do not prove true in many cases, including:

  • It’s better to use an existing design than to reinvent one. This may be true if you need a very limited number of voltages and have room to accommodate a standard form factor. Usually there is limited room in a medical device for the power supply. Being able to determine its shape can provide an additional degree of design flexibility.

  • Standard products are reliable, as any issues have been found and fixed. This is typically a reasonable assumption if you derate the power supply by 50 percent. But it is usually better to use a custom power supply that can actually put out its full rated power and that aligns with the actual load.

  • They are already certified to EN 60601-1. This can be helpful, but often redundant. The final product including power supply has to be tested and approved in its entirety. Further, the certified power supply often has restrictions, such as a requirement that it must be fused external to the power supply.

  • They already meet FCC Class B radiated and conducted EMI limits. As described above, this is doubtful. Most off-the-shelf power supplies will pass EMI testing only in very limited conditions.

  • They are produced in volume so they will have short lead time and low cost. As most standard power supplies are made in Asia, the short lead time is certainly not guaranteed. A custom power supply made in the United States can have shorter lead time. And a properly designed custom power supply can save the cost of extras such as EMI filters and thus actually provide a lower system cost.

When considering a standard medical power supply, it is important to anticipate EMI issues even if the power supply has an internal EMI filter and is marketed as meeting Class B requirements. Manufacturers should plan for additional external filters and shielding and allow extra time and money in the budget for additional EMC testing. Alternatively, consider a custom medical power supply design that allows addressing EMI from a systems perspective. Designing the power supply in parallel with the logic provides greater design flexibility, higher system performance, and lower total cost.

References

  1. Fenical, Gary, “The Basic Principals of Shielding,” In Compliance, Annual Guide (03): 56–63, 2014.
  2. Hegarty, Timothy, “An overview of radiated EMI specifications for power supplies,” Texas Instruments white paper SLYY142, June 2018.
  3. Sklepik, Danielle, “Designing a Medical Power Supply,” Power Systems Design, Volume 10 Issue 8 (10):42–46, 2018.
  4. Sklepik, Danielle, “Medical Power Supplies and Capacitor Choices,” Power Systems Design, Vol. 6 Issue 3 (04):27-29, 2014.

This article was written by Michael Allen, President of Z-AXIS, Inc. and Bear Power Supplies, Phelps, NY. He has designed standard and custom power supplies for manufacturers of a variety of medical instruments. For more information, click here .