Electromagnetic Compatibility in Medical Equipment

A Guide for Designers and Installers

William D. Kimmel, Daryl Gerke

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351453370
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Microwaves
• Index: Technology & Engineering

Co-published with the IEEE Press, this book is a practical, hands-on guide to EMC issues for medical device designers and installers. It addresses electromagnetic interference and covers the basics of EMC design, physics, and installation, minimizing theory and concentrating upon the correct way to ground and shield. Covering EMC from the inside out, the book provides the basics of electronics, discusses and evaluates problems and common causes, and explores effective remedial techniques at three levels: circuit, box, and interconnect. It contains appendices that provide important reference material such as constants and conversion factors.

1

INTRODUCTION

Modern medical devices are packed with electronics, ranging from sensitive analog amplifiers to sophisticated microprocessors. Unfortunately, these same devices can be adversely affected by EMI (Electromagnetic Interference) problems. Furthermore, these EMI problems can be compounded due to critical missions (human life may depend on proper operation) and harsh EMI environments (ranging from the operating room to emergency vehicles to patient homes). Designers and users of modern medical devices need to know how to identify, prevent, and fix these EMI problems.

Common EMI threats can cause upsets and even damage to sensitive medical devices. In the following chapters we will discuss the physics of EMI, then we will start from the inside of electronics and work to the outside. We will discuss the problems and give you some common cases and effective remedial techniques at three levels–circuit level, box level, and interconnect levels.

The leakage current limit is the single most important EMI–related concern in medical device design because it is almost impossible to adequately filter signal and power lines given the current limit. This major design challenge essentially mandates that EMI issues be addressed from the very start of a design project. Designers need also be aware that medical devices will soon be subject to new EMI regulations in Europe, and that similar requirements may be implemented in the United States. As currently proposed, these requirements reflect the real operating environment, providing helpful guidance for designers. As is true of all design issues, however, the sooner all of these EMI issues are addressed, the less expensive and more successful the final design will be.

WHAT ARE THE THREATS?

EMI has two faces–emissions and immunity (also known as susceptibility). Emissions will come from your equipment, possibly to interfere with the operation of nearby electronic equipment. On the other hand, external electrical energy may adversely affect your equipment. Thus, your equipment may be the source of interference or the recipient. These threats may be propagated in two ways: radiated and conducted, or a combination of the two.

If you are only concerned about design regulatory issues (see Chapter 2), the requirements are easy to identify–just read the specification. These requirements are not arbitrary, but represent hundreds of “engineer years” of research, analysis, and committee work to arrive at these levels. As a result, the recommended levels represent real world threats.

But suppose you have already met your applicable standards, and you are still getting EMI reports from the field. Remember, these standards are only good guides and cannot possibly cope with every condition in the field. Maybe your equipment is placed very close to an ESU (electrosurgical unit), or maybe you have some high level transmitters nearby. You will need to identify the actual threat before you take corrective action.

Let us take a look at the principal threats to your equipment: The sources may be transient in nature or may be nearly continuous. The three common EMI threats are RFI (radio frequency interference), ESD (electrostatic discharge), and power disturbances. Each are common sources of problems to electronic equipment, although they have widely different signatures and typically impact different circuits.

Emissions

Emissions are a measure of electrical energy emitted from your equipment. Some medical devices generate high electrical energy as a part of its function, and this, of course, is a concern. But most electronic equipment generates so little energy that it is a potential threat only to nearby radio equipment. Accordingly, emissions control is primarily a regulatory issue: The emissions may not adversely affect nearby medical equipment, but the general regulatory limits will still apply.

Radio Frequency Interference

As the name implies, RFI deals with threats in the RF, or radio frequency, range. Traditionally, this begins at about 10 kHz on the low end, and usually extends to 500 or 1000 MHz for commercial applications, and to 40 GHz (radar frequencies) for military or aircraft applications. RFI is usually continuous (long relative to circuit response time) rather than transient.

Electrostatic Discharge

Electrostatic discharge follows a gradual charge buildup. Actual charge buildup will occur over a period of time, generally seconds or more, and usually poses no threat to electronics. When discharge occurs, it takes only a few nanoseconds, and this is what causes the ESD problem. Actual discharge can be from a human body to or near the equipment in question, or it may be accumulated by the equipment (as in the case where the equipment is portable or on rollers) and discharged from there.

Power Quality

Power quality, or lack thereof, is simply a deviation from the 50 or 60 Hz ideal sine wave power we have come to hope for (or demand). Power disturbances can have many sources and take many forms, both continuous and transient. Power disturbances are often generated locally by nearby noisy equipment, and, thus, cannot be controlled by the power company. Power disturbances are tough to eliminate completely, so it is usually necessary to cope with them. These disturbances can have a variety of signatures, depending on the nature of the source, but only a few types of disturbances are of significance in electronics.

Self-Compatibility

Self-compatibility deals with threats internal to your medical electronics equipment. Simply, your equipment is prevented from satisfactory operation by an electrical interference generator in your equipment. Two common cases are when you are dealing with very sensitive equipment (sensitive to interference even from ordinary digital electronics) and when you have a high energy generator that is powerful enough to interfere with nearby equipment, including the electronics within. This situation often surfaces when you are integrating purchased modules into your system.

SOURCES, PATHS, AND RECEPTORS

When dealing with any EMI problem, it is important to divide the problem into three categories–a source, a receptor, and a path coupling the source to the receptor. All three must exist for there to be a problem and, therefore, eliminating any one of these will eliminate the problem. But in any specific case, some solutions are more practical than others. For example, if your equipment is upset by a nearby radio transmitter, you usually cannot eliminate the source–but you can use shields or filters to block the path, or you can harden the receptor at the circuit level. So, you attack the problem where you have some control over the outcome. Equipment designers, of course, will concentrate on hardening the circuits at the receptor. Let us look at the common sources, paths, and receptors for each of the threats in turn.

Sources

RFI Sources

What are some typical RF sources? In today’s modern society we are literally awash in RF energy from a wide range of sources. These include natural and man-made sources, and intentional and unintentional radiators. Table 1-1 shows some typical high level RF sources we might find in the medical environment.

The electric field strength of an RF source drops off inversely with distance from the source. Thus, a low power walkie-talkie located a meter away poses a much greater threat than a high powered commercial broadcaster a kilometer away. Even worse–this handheld source is mobile, making it difficult to trace.

For example, a maintenance person keys a handheld radio in the hallway and upsets a sensitive piece of equipment, and then walks away unaware of the problem he or she just caused. Do not overlook this threat–it may explain some mysterious field failures. With the proliferation of handheld radios and cellular phones, we always put these sources on our suspect list.

Mobile radio transmitters are also a threat, particularly near emergency vehicles. These transmitters have higher power than the handheld types and they often pose a threat at distances of 10 meters or more. Any equipment in or near the emergency wing of a hospital will be exposed to these threats on a regular basis. Any medical devices mounted in or carried into an emergency vehicle are exposed to even higher levels of RF energy and must be protected.

Many common medical devices use RF energy and can pose a threat to nearby electronic equipment. These include diathermy units, MRI systems, lasers, and ESUs. The ESU used in an operating room is particularly nasty, as it “sprays” RF all over the place, upsetting even robust electronics, not to mention sensitive monitoring equipment.

Table 1-1. High Level RF Sources

Category	Name	Frequency Range
Medical devices(unintentional radiators)	Diathermy ESU MRI Lasers	27 MHz-500 MHz 30 kHz-100 MHz 60 MHz 27 MHz (varies]
Radio transmitters (intentional radiators)	Television Radar AM radio FM radio Land mobile*	54 MHz-800 MHz 1 CHz-40 GHz 550 kHz-1.6 MHz 88 MHz-1 08 MHz 30-50 MHz 150-170 MHz 450-500 MHz 900 MHz
Other	Arc welders RF heaters	2-20 MHz 13.5, 27, or 40 MHz typical

* Land mobile includes police, fire, ambulance, pagers, and walkie-talkies.

Other potential RF threats include theft detectors, RF welders, and RF heat-sealing equipment. While not commonly found in the medical environment (although arc welders may be involved in hospital construction projects), they can cause problems to patients equipped with portable electronics, and you should be alert for these unexpected threats.

The measure for RF problems is the “electric field intensity,” given in volts/meter. This can be measured with a field strength meter, and, in simple cases, can also be predicted. For today’s electronics failures typically occur in the 1–10 volts/meter range, although we have seen failures on some sensitive systems in the 0.1 volt/meter range. Unfortunately, nearby radio transmitters can cause levels in the 1–100 volts/meter range, depending on the power level and the distance from the transmitter. This is why most medical RF regulations now specify test levels in the 3-10 volts/meter range. But even this is not enough for equipment used in emergency vehicles carrying radio transmitters, which may experience field strengths as high as 200 volts/meter.

ESD Sources

ESD requires charge buildup before discharge can occur. Actual charge buildup originates by rubbing two materials together (at least one of which is a dielectric), resulting in an accumulation of positive charge on one material and a negative charge on the other. A useful table, called the Triboelectric Series (Appendix B), places common materials on a relative scale, from the more electropositive to more electrone...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• PREFACE
• ACKNOWLEDGMENTS
• 1. INTRODUCTION
• 2. EMI STANDARDS FOR MEDICAL DEVICES
• 3. THE PHYSICS OF EMI
• 4. COMPONENTS AND CIRCUITS
• 5. PRINTED CIRCUIT BOARD DESIGN
• 6. GROUNDING FOR EMI CONTROL
• 7. POWER AND POWER FILTERING
• 8. INTERCONNECT AND MOUNTING
• 9. SHIELDING AND SHIELDING MATERIALS
• 10. CABLES AND CONNECTORS
• 11. SPECIAL EMI PROBLEMS IN MEDICAL ELECTRONICS
• 12. SYSTEMS CONSIDERATIONS
• GLOSSARY
• APPENDICES
• INDEX