Medical Device Link.

Originally Published MEM Fall 2005

EMI

Paying attention to fundamental problem sources during the product design stage can prevent interference issues from affecting equipment performance.

William D. Kimmel and Daryl D. Gerke

Device and system designers encounter a wide variety of emissions and immunity problems. Some electromagnetic interference (EMI) problems are common to a range of technology sectors, while others are specific to a single field of business. Medical device electronics, for example, involve some unique problems. These concerns are partly because of the need for patient safety and partly because of the sensitive nature of physiological signal levels.

The medical electrical equipment standard, IEC 60601, the second edition of which is soon to be succeeded by a third, pretty much drives the international medical device market today. Its emphasis is on immunity, as might be expected, but emissions also are considered. Some EMI problems are specific to medical equipment, while others are common to numerous industries.

1. Ground Impedance

Ground impedance comes first because it occurs so often. The overwhelming majority of high-frequency problems, whether relating to emissions, self-compatibility, or immunity, have high ground impedance at the root. These are not low-frequency ground loop issues, nor earth grounds. These are problems caused by local ground impedances such as those found on circuit boards or in cables. High-impedance ground paths are the principal contributor to cable shielding failure and common-mode currents.

Simply put, wires and traces are always high impedance. This is why a ground plane should be used at high frequencies: to keep the ground impedance as low as possible. How high is a high frequency? That depends on the application. But, with a wire or trace, the inductive impedance is already higher than the resistance of the path at audio frequencies, and it is significant at 1 MHz. At that point, designers should avoid using wires or pigtails for grounds. A good rule of thumb is that the inductance of a wire is about 20 nH per inch of length. A 1-in. wire or trace has an impedance of 2πfL, or 12 Ω at 100 MHz—hardly a short circuit.

At radio frequencies, any length of wire should be viewed with suspicion. A useful rule is to keep the path width at least one-fifth that of the length. Thus, a 5-in.-long ground strap should be at least 1 in. wide, and preferably wider.

2. Poor Cable Shielding

When an emission or radio-frequency (RF) immunity problem is encountered, it almost always involves the cable. Ground impedance, as just discussed, plays a key role in cable termination performance.

Figure 1. The cable shield is not properly terminated. (click to enlarge)

The only fully satisfactory treatment for high-frequency RF interference (RFI) and radiated emissions is a circumferential cable termination at both ends. The shield fails if the connector does not mate with the shield (see Figure 1). Single-point grounding is good for audio frequencies but unacceptable for radio frequencies.

Unfortunately, patient-connected cables are essentially impossible to shield effectively. The cable cannot be terminated at the patient end, so the shield cannot be grounded at both ends. Further, the shield cannot be effectively grounded at the equipment end, either, as isolation must be maintained. This puts the onus of patient connection on filtering rather than shielding.

For cases in which the patient is not connected, or where isolation need not be maintained, good cable shielding practices can be implemented effectively. A visual inspection gives a pretty good indication of cable shielding effectiveness. However, a problem may still be encountered in the field or on the test floor. There are several possible causes.

The cable shield may be single-point grounded. Cable shields must be grounded at both ends any time the cable length is 1/20 of a wavelength or longer. A quarter-wavelength is a worst-case condition. A cable purchased from the local computer store is almost assuredly grounded at one end only—and more than a few such cables are not grounded at either end. If the cable is grounded at both ends, it will be so labeled on the box. That is worth checking to be sure. Unfortunately, it is often difficult to tell whether cable shields are terminated circumferentially. Many purchased cables use pigtails, which, as noted in the previous section, are bad news for electromagnetic compatibility (EMC).

The cable shield may be damaged. Many cable shields are Mylar foil, which is not very robust. Even with careful handling, the shield could rupture somewhere along the cable, degrading the shielding effectiveness. The trouble with this problem source is that the rupture is not visible, nor will it be detectable with a digital voltmeter.

Finally, the cable shield could be grounded through the drain wire. This causes a problem if the drain wire is attached to a pin in the connector, making it almost completely ineffective, or to a screw post inside the connector, in which case shielding effectiveness is reduced. It is important to note that this creates a pigtail.

3. Emissions from Switching Power Supplies

Figure 2. Interference sources and coupling paths in a switch-mode power supply. (click to enlarge)

The problem of emissions from switching power supplies or dc-dc convertors goes back to the early days of emission requirements. The switching node—usually the collector or drain—of a power supply couples noise into the heat sink, creating a conducted emissions problem, as shown in Figure 2.

This problem with power supplies is universal, but it is especially acute in medical electronics, where leakage current limitations apply, because the key filtering technique—common-mode or Y capacitors—is unavailable. It is vital to reduce the generation of common-mode currents, so any methods available may be used. The key is to provide differential-mode filtering as close to the offending elements as possible, and to maintain as much physical isolation of the noisy node as can be managed. Heat sinks should be well bonded to the appropriate reference plane, usually power common.

4. Power Supply Filters

As just mentioned, whatever emissions cannot be contained within the switcher itself must be contained at the power line filter. The power line filter is also an important factor in RF immunity, as RF often enters the equipment through the power cord.

Common-mode currents often are handled by shunting the currents to the case, with small capacitors (Y capacitors) generally placed in a conventional power line filter. These capacitors are not used in medical-grade power line filters, as they result in excessive leakage currents. Thus, a key suppression method is unavailable. The only remaining method is providing lots of series impedance by means of inductors, which are bulky and generally undesirable.

Building a power line filter is not for amateurs; designs using cookbook filter theory are doomed to failure. Someone who is going to model a filter should be sure to account for all the parasitic elements within the filter components, in the wiring, and between adjacent components. Power line filter companies have a good handle on the problem, of course, and their filters have been proven to work.

5. LCD Emissions

Figure 3. Emissions from an LCD. (click to enlarge)

The increasing use of liquid-crystal displays (LCDs) has created a problem with emissions. Inevitably, the problem surfaces with the data cable, usually a flex cable from the circuit board driver to the LCD panel. The currents going up to the LCD do not all return on the cable; a small fraction remains as common-mode current, exciting the LCD. As discussed in problem 1, ground impedance is again the issue here (see Figure 3).

A number of steps can be taken in this case. First, the currents need to be returned to the source—the driving circuit board—with as small a loop area as possible. The return-path impedance might be reduced by running a ground strap under the flex cable. But better would be to provide direct shunts from the LCD panel back to the circuit board. Effectively, this means grounding the panel to the circuit board, at all four corners if possible. Most LCDs have a metal plate at the back of the panel as part of the frame. If a particular display does not, the designer will need to add one. A ground plane is necessary to keep emissions from coming directly off the front of the panel, which may happen even if the perimeter is grounded. If a device has a metallic enclosure, then the designer should ground the LCD to the enclosure around the entire perimeter.

6. Stray Internal Coupling Paths

At higher frequencies, internal coupling paths, including but not limited to crosstalk, become very important. Although these paths can be anywhere in the equipment, their most common location is at sensitive patient-input cables, where component and trace placement will make the difference between success and failure.

There are a couple of possibilities for dealing with this problem source. One is capacitive coupling from the inductor or ferrite. These components are usually inserted at inputs and outputs to block RF interference from entering or exiting. In such a role, the side of the inductor facing the noise source carries considerable high-frequency voltage, which will capacitively couple to any nearby metallic member, including ground planes, circuit board traces, and heat sinks. The solution is careful placement to avoid coupling to sensitive recipients.

Figure 4. Stray capacitance between the chip and connector can bypass onboard filters. (click to enlarge)

Capacitive coupling from board elements to connector pins is an option, too (see Figure 4). But this path effectively bypasses any filter elements that may have been placed on the board. A ferrite placed outside on the cable often works better than a ferrite placed inside on the circuit board. The solution is to intercept the coupling path with a Faraday shield, which can be located at the offending chip or at the connector. In either case, the shield connects to circuit ground.

While coupling to the connector is a common cause, other capacitive coupling paths also cause problems, including signal and power traces, heat sinks, and any other metallic elements in the immediate vicinity of the circuit board.

7. Component Parasitics

In addition to these stray paths, deficiencies in the components themselves can limit performance.

Parasitics are the stray reactive elements found in every component, whether a passive element or active device. Ideal components such as may be characterized in the classroom exist nowhere outside of the textbook.

All capacitors have series inductance, creating a series resonant circuit. All wound inductors have interwinding capacitance, creating a parallel resonant circuit. These circuits resonate at much lower frequencies than might be thought, and are, of course, ineffective above resonance. Most capacitors resonate at well below 100 MHz, and most wound inductors resonate below 20 MHz. Even small transformers resonate below 5 MHz. So designers should make sure their selected component is functional at the frequency range of interest. It is virtually certain that filter and decoupling elements are being operated above resonance.

Turning from passive to active components, the same problem presents itself. This includes stray capacitance and inductance in the lead frame and bond wires, as well as in the die itself, which results in internal pin-to-pin crosstalk. This crosstalk problem is getting worse, what with decreasing pin pitches and higher speeds. These parasitic elements are also interacting with external circuit components, creating a dazzling array of resonances.

8. Inadequate Signal Returns

A common situation with cables, especially intrasystem cables, is that there are not enough return lines. The design thinking has been that anything more than one wire for ground return is wasted.

This is another case where high ground impedance is the source of a problem. The signal goes out on one line, and most of it comes back on the return as intended. But unless the return-path impedance is zero, a small portion of the current returns on a stray path. That portion may be as small as one part in a thousand, but even that is often enough to create a common-mode cable emissions problem.

In the extreme case, the voltage drop on the return is enough to cause data errors, creating a signal integrity problem.

How many returns are needed depends on data rates and run length. For high speeds, those above 100 MHz, designers should figure on one return for each signal line. For lower speeds of less than 10 MHz, one return for every five signal lines is recommended.

9. Discontinuous Return Paths

Many printed circuit board (PCB) problems can be traced to a discontinuous return path. Grounding plays a key role in all aspects of EMC, including in PCBs. The EMC problem in PCBs is becoming more acute as speeds rise.

Figure 5. Gaps in the ground or power plane destroy the return-current path and create a slot antenna. (click to enlarge)

The problem revolves around the signal current loop. Ideally, the signal goes out a trace and returns immediately under the trace. The laws of physics dictate that the current follows the minimum-energy path, which usually means the smallest loop area. But all too often the return path is broken by a discontinuity; that is, the trace crosses a cut in the plane or passes through a via and changes reference planes (see Figure 5). When something like that happens, it creates a discontinuity, energizing the slot and causing signal reflections.

A good design rule is to spend as much time considering the return path as is spent on creating the signal path. Designers should avoid crossing slots, should drop a ground via if ground planes must be switched, and should put in a capacitor near the via if they are using Vcc as a reference plane. Clearly, this cannot be done for all signals. The best thing is to concentrate on the critical signals, clocks, and data buses in the case of emissions, and on strobe lines for immunity.

10. ESD in Metallized Enclosures

Figure 6. The ESD path in a plastic enclosure (a) is circuitous, by contrast with the discharge route in a metallized enclosure (b). (click to enlarge)

The problem of electrostatic discharge (ESD) often occurs as the result of an afterthought. Designers who use plastic enclosures add metallization only when forced to, in order to minimize emissions or RF immunity problems. To be effective, the metallization needs to be brought right up to the seams to provide conductive continuity between mating surfaces (see Figure 6). Unfortunately, this creates a whole new set of ESD contact points. The designer may create a new problem while solving another one.

This can be a tough nut to crack. The designer has essentially three choices: to redesign the equipment to eliminate the need for metallization, to recess the metallization so that the arc cannot get to it, or to take extra care to close the gaps in the shield. Basically, the problem arises not from the presence of the metallization but from the gaps in the shield. It is hard to get good closure with a metallized box. But once the gaps are closed, the shield should perform well with respect to ESD.

Far and away the best defense against ESD is to prevent discharge in the first place, leaving only indirect discharge as an effect needing to be handled. That means that the designer has to recess metal members beyond the reach of static discharge, a design approach that is not always feasible.

If discharge cannot be avoided, then it will take a very good shield to cope with ESD.

Conclusion

Medical equipment electronics see many of the same EMI problems that designers of other kinds of electronic products do, but certain problems are unique to medical devices. Surprisingly often, these EMI problems occur because the fundamentals have been ignored; some of the problems described in this article have been around for decades. Others have appeared recently and are growing increasingly common.

The problems analyzed here are central to equipment design, and all are manageable. Designers who work to avoid them will avoid a lot of headaches.

William D. Kimmel, PE, and Daryl D. Gerke, PE, are partners in Kimmel Gerke Associates Ltd., an EMC consulting firm with offices in St. Paul, MN, and Mesa, AZ, that specializes in EMC design, troubleshooting, and seminars. The NARTE-certified EMC and ESD engineers may be reached by phone at 888-364-4878 or via their Web site at www.emiguru.com.

Copyright ©2005 Medical Electronics Manufacturing