The Hidden Schematic: EMC Threats in Medical Power Supplies

William D. Kimmel and Daryl D. Gerke

Many EMC problems in power supplies—especially for medical electronics—are hidden in parasitic elements. Such factors must be included in early modeling.

Engineers tend to think of power supplies as being primarily associated with power disturbances or conducted emissions out the power cord. These are certainly important issues. But power supplies, or perhaps power conversion issues, go beyond the power cord. Increasingly, power supplies are being used to derive a new voltage source in addition to providing voltage regulation. These devices can be found in portable equipment with no connection to line power. They can even be found sprinkled throughout the electronics modules. As such, the power supply EMC problem covers more than power disturbances and conducted emissions. Radiated emissions must be addressed, as well as electrostatic discharge (ESD) and radiated immunity.

The problem lies with the hidden schematic. These parasitic and nonideal elements aren't indicated on the schematic, but they do exist and should be included in any modeling. These elements include ground impedance, stray capacitance, and inductance in the individual components, in the wires and traces, and between adjacent components and other metallic members.

Although these factors affect the power supply design itself, they are much more of an issue when working with the higher frequencies involved in electromagnetic compatibility (EMC). This article looks at how these hidden factors influence EMC.

The driving EMC requirement in medical electronics is IEC 60601-1-2. The European Union (EU) has taken the lead in adopting these requirements as mandatory for those products being marketed in Europe. The U.S. Food and Drug Administration has not formally adopted these requirements, but it does recommend them.

The EMC Threats in Power Supplies

Basic Interference Issues. Power supplies play a multifaceted role in EMC. They are a source of interference, a recipient of interference, and a conduit of interference.

The primary emission sources originate in the switching devices; the secondary source is from the bridge rectifier, both rectifier noise and diode recovery. Where regulation is done at the bridge using silicon-controlled rectifiers or triacs, the generated interference may rival interference that comes from the switch itself. Power supplies are notorious for generating power-supply harmonics, which are now also limited by the EU.

Power supplies are also the recipients of interference. Although interference originates primarily from the power source, power supplies can also be affected by disturbances from the load side. Radio-frequency interference arriving from either the power source or the load goes after the regulator feedback path, resulting in loss of regulation—usually as a dc offset, but sometimes as a demodulated wave piggybacking on the dc line. Transients of significant magnitude can damage components. Lesser transients can trick the regulator feedback path, causing a momentary sag or surge in supply voltage. Most transients are related to the power source, but occasionally, an ESD problem will surface.

Power supplies also act as conduits between the load and the power source. Internally generated clock noise could go through or around the power supply to the power cord, resulting in radiated emissions. Electric motors, variable- frequency drives, and other electromechanical devices are other sources. Externally generated transients could go through or around the power supply to attack the digital circuits beyond, particularly the reset line.

Power Supply Topologies. Power supply design is a specialized field, especially the design of those intended for use in medical electronics. We don't pretend to be experts in this field—our specialty is EMC—but in our experience, a number of problems can be addressed during the design of a power supply that will minimize electromagnetic interference (EMI) problems without sacrificing safety or efficiency.

It is known that basic convertor design plays a significant role in EMI emissions. Boost or Cuk convertors are generally the quietest, and flyback convertors are the worst. In addition, soft-switch approaches minimize switching transients, reducing emissions still further. So, although selecting the convertor topology is a significant first step in controlling EMI, there are plenty of other factors to consider as well.

Differential- and Common-Mode Interference

The story of EMC in medical power supplies is one of controlling common-mode (CM) currents, both into and out of the power supply. Leakage current limitations make it exceedingly difficult to suppress CM currents. Therefore, any treatment of medical power supplies necessarily includes a discussion of CM currents and how to cope with them.

Differential-mode (DM) currents are the normal mode for both signals and power: currents go out one wire and return along an adjacent path, completing the current loop. CM currents are manifested as currents traveling in the same direction for both signal and return or power and return, with the common return being an unintended path. As such, CM currents serve no useful function. Because CM currents just cause trouble, they can be suppressed with impunity.

Although CM and DM interference exists side by side, DM interference tends to stick to the intended wire path and components, predominantly below 1 MHz. Above 1 MHz, CM interference becomes increasingly important. Higher impedance due to inductance tends to block currents in the intended path, whereas parasitic capacitive and inductive coupling paths become increasingly efficient. CM currents can be difficult to eliminate, especially when leakage currents are a concern. Nevertheless, controlling CM currents starts by controlling DM interference.

The sources of internally generated CM interference are caused by ground impedance and capacitive and inductive coupling. Figure 1 shows an example of CM generation due to ground currents. In radio terminology, this is a dipole antenna terminated by a delta match. The CM generation is proportional to the current, the ground impedance, and the length of the cable connected to the ground (whether connected directly or capacitively). This problem is significant even when a ground plane is present, but becomes a major problem when a two-layer circuit board is used, because ground impedances are much higher. Note that this noise is on ground, and thus it is on all signals referenced to the same ground. Capacitive filtering between signals or on voltage to ground, therefore, would have no effect.

Figure 1. Antenna effects due to ground impedance.

Capacitive and inductive coupling is a more significant factor in power supplies than on digital and analog circuit boards, because of the larger components standing off the board and because of the power-level currents and voltages. Figure 2 shows some of the paths that contribute to CM interference. The primary interference source is the switch, followed by the bridge rectifier.

Figure 2. Parasitic coupling paths in power supply.

Capacitive and inductive coupling paths couple to the various members in the power supply. The inductive coupling paths are primarily magnetic field fringing from inductors and transformers, followed by coupling directly off the switched power-current lines. This can be minimized by adjacent routing of the high current and returns. Controlling these should be done religiously, because even small loop areas can be significant. Capacitive coupling is tougher to suppress because the power supply abounds with coupling paths caused by components that typically stand above the board, losing the protection of the ground plane.

These problems are primarily related to emissions, an issue of major importance in the EU, but less so in the United States. Both the EU and the Food and Drug Administration in the United States are concerned with immunity, which involves the converse: CM-to-DM conversion. If all currents were CM and everything were balanced, there would be no problem.

Unfortunately, circuits are not truly symmetrical. Inevitably, some level of CM interference does get converted to DM interference. If everything were symmetrical, there would be no problem, but any imbalance in the source, the path, or the recipient circuit converts the CM interference to DM interference. The CM-to-DM conversion is not large in terms of percentages. It is important to remember that starting with 1000 V—even where matched circuits exist—a 1% tolerance will cause a significant imbalance. Even if all else were fully balanced, the magnitude of the source could well be sufficient to drive the recipient circuit out of dynamic range.

Although there are significant differences between immunity and emissions, control of both involves the same three principles: keep CM to a minimum, avoid generating CM, and avoid transmitting CM.

Preventing the Generation of Common Mode

CM interference is relatively easy to control in situations that have no leakage current restrictions. A shunt capacitor to the case can effectively divert currents. Where leakage currents are restricted—most notably with patient-connected devices—use of such capacitors is not possible. In such cases, the only recourse is to insert series impedance. For input signal lines, this can often be a resistor in the hundreds or thousands of ohms. For power supplies, this is limited to series inductance—and lots of it. In practice, it is difficult to get enough inductive impedance over the frequency range of interest to be fully effective.

Differential-mode interference is usually much easier to control. Start with a reference level, preferably a plane, which may be isolated (so there are no restrictions in filtering to that reference level). In most cases, the circuits using this reference level can be adequately protected. The problem comes when moving off of that level, and that move is a CM issue.

Minimizing Ground Impedance. This is one of the few cases in which we champion single-point ground concepts. Figure 3 shows a regulator with filtering at the input and output. CM generation is proportional to the indicated ground impedance. There are two ways to reduce this level: reduce the ground impedance to a minimum and reduce the current in the critical path. Reducing ground impedance is not always feasible, although it is definitely minimized by use of a ground plane. It can also be done by reducing the impedance of the common tie point, or point of common connection, as shown in Figure 3. Simply put, the currents from the switching sources should circulate without being in the input-output path.

Figure 3. Regulator filtering.

Minimizing Inductive Coupling. The principal magnetic coupling paths are from inductive elements, followed by those along the high-current switching path. Inductive paths include transformers and inductors, which may couple to adjacent inductive elements, traces, connectors, and even cables. The first line of defense is to select the inductor to minimize leakage inductance. Open cores have magnetic fields that extend well beyond the boundaries of the component itself. Avoid using devices with open flux paths. There is never enough room on board to provide adequate clearances. Pot cores, cup cores, and E cores all provide minimum fringing. Toroids are better than open cores, but these still generate a loop to contend with. Beyond that, spacing between the element and other vulnerable elements (I/O lines, power in/out, and other similar inductors) can also minimize inductive coupling. Figure 4 shows an inductor aligned for maximum coupling to the trace beneath.

Figure 4. Inductor to trace coupling.

Minimizing Capacitive Coupling. The principal capacitive coupling paths are the nodes connected to the high dv/dt elements, notably the switch and the bridge rectifier. These nodes, and the wire directly connected to the nodes, must be isolated from the input-output traces, heat sink, and other metallic structural members. Spacing is important because the field typically falls off with an inverse square law. Where spacing is not possible, Faraday shields can be employed to intercept the capacitive path.

Selecting Filter Components

No treatment of power supplies would be complete without taking a look at the components themselves. We have lost count of the times we have seen cases where the filter failed because component deficiencies were not taken into account. Capacitors suffer from series resonance; inductors suffer from parallel resonance. Once past resonance, the component does not behave as it did when initially purchased. We have often seen that a resonant frequency is much lower than one might guess.

Conclusion

Power supplies are significant sources, recipients, and conduits of electromagnetic interference. Coupling paths abound due to the stand-up nature of power supplies and the high current levels associated with them. These coupling paths can be minimized by careful placement of components on the board and by minimizing loop areas of switching current paths. Keeping ground impedance low is mandatory.

William D. Kimmel, PE, and Daryl D. Gerke, PE, are cofounders of the EMC consulting firm of Kimmel Gerke Associates Ltd., with offices in St. Paul, MN, and Phoenix, AZ. They share more than 60 years in the EMC arena and publish and lecture widely on the subject. They can be reached at 888-EMI-GURU or at http:// www.emiguru.com.