Technical Article

The Prescription To Reduce EMI in Medical Equipment

May 14, 2024 by Dermot Flynn

This article addresses electromagnetic interference in power supplies and how to eliminate it.

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

EMI, also known as radio frequency or electromagnetic interference, is nearly everywhere. Sources occur in the natural world in the form of lightning or radiation from the sun. Man-made radiation impacts electrical equipment. Some, like radio, television, and telephone, are intentional. Other sources, such as load distribution, vehicle ignition systems, and power supplies, are unintentional.

In electronics, EMI is unwanted noise caused by an electromagnetic field that may directly affect the performance of equipment, degrading performance, causing malfunctions, or even stopping proper functioning completely. In a medical environment, this can have serious consequences, such as generating erroneous measurements in patient monitors, impacting image quality radiography equipment, and affecting critical surgical systems and their operation. 

 

What Makes Power Supplies Noisy?

Any source of changing voltage or changing current with respect to time will result in ringing (switching noise that occurs due to parasitic components as currents are turned on and off). The switch mode power supply will be full of these events as they happen every switching cycle, and all tracks, points, and components are potential noise sources. Within any given design, there will be loops with rapid current and voltage rises and falls (di/dt and high dv/dt, respectively). Figure 1, for example, identifies the potential areas for high di/ dt in an isolated forward converter, while Figure 2 shows the possible areas for high dv/dt in the same design.

With such high di/dt and dv/dt, generating significant common mode currents and voltage spikes of up to several volts, resulting in ground bounce and EMI, is easy. Another example would be a FET going from a Vds of 200 V in 40 ns, equalling a dv/dt of 5 billion volts per second.

 

Designing To Minimize Emissions

It is practically impossible to eliminate EMI completely but for medical power supplies, this challenge is even greater. For example, conducted EMI is usually reduced by increasing Y-capacitance on the power supply input. However, increased Y-capacitance also increases leakage current, a critical safety specification for IEC 60601-1-2 compliance, as is safeguarding patients and equipment operators.

 

Image used courtesy of Bodo’s Power Systems [PDF]

 

Designing power supplies for good EMI performance is not impossible. With the correct knowledge and experience, limiting EMI can be addressed during the project concept stage. This includes a heavy focus on the design and layout of the PCB, as it plays a significant role in emissions. The first focus is to eliminate, or at least minimize, the emissions from the source of the generators. It is important to consider that receivers also re-transmit, so minimizing the susceptibility of receivers is also necessary. Additionally, it is important to incorporate an EMI filter design, ideally, while keeping filters as physically small as possible, particularly for mobile and portable medical equipment.

PCB layout is often least understood when it comes to designing for EMI. Here are a few things to keep in mind:

  • Minimize loop areas
  • Leave no floating parts, ensuring that all loops are brought back to ground
  • Keep signal and power ground connections separate

This adds complexity to PCB design, particularly when saving layers on multi-layer PCBs while keeping signal and power grounds separate, keeping input and output grounds separate, and minimizing distances from components. Earthing schemes were designed for safety purposes, not EMI, so using ground planes and short connections is vital.

 

Understanding Components for Frequency

The difficulty facing power supply designers is that EMI performance is usually not characterized by the component manufacturer. However, understanding the components is key to knowing how they will behave in a subsystem regarding emissions. Capacitors, wire wound resistors, and even wire leads will vary in behavior as the frequency changes. This holds true for the type of capacitors used, as tantalum capacitors behave differently than ceramic capacitors. Furthermore, parasitic components, ESR (equivalent series resistance), ESL (equivalent series inductance), capacitance, and leakage will all play their part, so knowing and quantifying these are all key to understanding EMI.

Most EMC components are invisible on the circuit diagram as they are either stray or parasitic, so particular attention must be paid to ensure everything is done with sound EMI/RF principles in mind.

 

Designing for EMI Margin

Testing the power supply for compliance at various limits does not guarantee it will pass when installed into a system. The power supply is an active part of the system, which will affect system electronics and can be affected by other noise sources. Selecting a power supply with good emissions and immunity performance will save on a lot of system compliance challenges later in development.

Low-emission power supplies can be produced if they are designed for EMI considerations from the start. For example, the NGB family of medically certified open-frame power supplies from Advanced Energy was specifically designed for exceptional EMI performance with reduced conducted and radiated emissions that simplify system EMI compliance.

 

Figure 3. Advanced Energy’s SL Power NGB425 Series. Image used courtesy of Bodo’s Power Systems [PDF]

 

As Figure 4 shows, the NGB425, 425 W AC/DC power supply, delivers Class B conducted emissions with 6 dB margin and radiated emissions with 3 dB margin.

The NGB425 meets a range of immunity specifications to criteria A (no impact on performance), including:

  • EN61000-4-2 Electrostatic Discharge Immunity, Level 4, 8 kV contact, 15 kV Air
  • EN61000-4-3 Radiated RF EM Fields Susceptibility 10 V/m
  • EN61000-4-4 Electrical Fast Transients / Bursts, Level 4, 4 kV
  • EN61000-4-5 Surge, Level 4, 4 kV Line to Ground
  • EN61000-4-6 Conducted Disturbances Induced by RF Fields, Level 4,
  • EN61000-4-8 Rated Power Frequency Magnetic Fields Test, Level 4
  • EN61000-4-11 Voltage Dips

In addition, the supply is safety certified to the IEC60601-1, third edition, with 2 x MOPP, with both Class I and Class II inputs.

There are several other causes of poor system noise performance, including:

  • Insufficient de-coupling on the PCB or load
  • Poor system earthing
  • Faulty wiring connection or poor cable terminations

Some simple steps can eliminate, reduce, or identify the causes of high-frequency noise:

  • Determine if the noise is conducted or radiated. If changing the position of the power supply or screening improves performance, the noise is likely radiated.
  • Make ground connections (zero volts) to the nearest point on the chassis
  • Twist all pairs of power and sense cables separately

A common assumption is that EMI emissions are highest in full load conditions or over a certain AC line voltage range. In reality, depending on the topology of the power supply, the emissions can vary significantly over line voltage and load range. In some cases, low load performance can be significantly worse than that at full load and low or high line voltage can be worse than the other. Recommendations are made on load and line conditions to perform the verification testing, ensuring proper and thorough evaluation while minimizing the tests required to achieve a high confidence level of compliance.

 

Figure 4. Conducted and radiated EMI plots for NGB425. Image used courtesy of Bodo’s Power Systems [PDF]

 

In the same way that power supply designers need to ensure the sources for radiation are kept to a minimum, system designers need to ensure the overall system has optimum performance when fully integrated. Cabling arrangements and PCB tracking layouts are the greatest contributing factors to system-level EMC performance.

 

Noisy Electronics in Medical Equipment

Power supplies are critical safety components in medical equipment and are, by their nature, “noisy” electronics. However, selecting a power supply manufacturer that has designed EMI margin into their products, coupled with proven performance and medical applications experience, can help system designers meet compliance obligations while not compromising on size and performance.

EMI is a challenge for medical equipment designers. Meeting safety compliance standards is non-negotiable and can directly impact system EMI performance. System EMI compliance can only be verified when the system design is fully completed, and at that time, trying to rectify non-compliance can negatively impact design time and the product launch plan.

Having the means to measure and mitigate EMI in a fully integrated system is a great time-to-market advantage. Testing and modifying labs like Advanced Energy’s Customer Experience Centers can support design and EMI testing needs. Established EMI experts with a worldwide footprint employ a global team of subject matter experts ready to help. From pinpointing layout issues to pre-compliance testing for entire systems, the EMI experts at these global facilities can help meet system needs, deadlines, and budgets.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine and is co-authored by Dermot Flynn and Paul Kingsepp of Advanced Energy.