The AC/DC power converter is often the sole interface between a piece of equipment and a building’s electrical installation, which is in turn connected to any number of other off-line systems. It is also the element separating the hazardous high voltage mains from user-facing safety extra low voltage (SELV) networks. Accordingly, AC/DC power converters fall under a great deal of regulatory scrutiny, with mandates ranging from energy efficiency to safety to electromagnetic immunity and emissions. The requirements of each regulatory authority can often spawn contradictions in the resulting engineered solutions.
The design of a power system for medical devices demonstrates the issue, as designers face a prominent trade-off between the attenuation of electromagnetic emissions and the reduction of leakage currents. This relationship is important to understand not only when designing or modifying the power converter, but also when developing the end system — which may have additional mains-connected networks or strenuous touch current constraints for downstream applied parts.
High efficiency power conversion is achieved fundamentally via the deployment of a switch-mode topology. With respect to time, large changes in voltage and current at the circuit’s switching nodes generate an abundance of harmonic content that can easily couple back onto the mains or onto output cabling from which the energy can radiate in space. IEC 60601-1-2:2014 (Ed. 4) imposes strict limits on the escape of this energy. Often, in a switch mode power supply (SMPS) design, the effective solution is to provide a low impedance return path to low potential for the high frequency currents, shunting them away from the device’s input and output (I/O) ports. These paths are often composed simply of Y-capacitors that couple the line and neutral conductors to earth ground and/or Y-capacitors that bridge the primary and secondary returns.
These capacitive channels, however, also provide a potential path for hazardous currents (leakage currents) to flow to earth ground, or secondary during normal operation. The amount of leakage current to earth is tightly limited for medical applications according to IEC 60601-1, and leakage currents to secondary can exceed limits for any applied parts which may be powered from the secondary network. In the event that a suitable path to earth potential is interrupted, and a human body instead becomes the most suitable path, excessive leakage currents could be lethal. Thus, a trade-off is borne whereby the power system designer aims to shunt away as much high-frequency (HF) energy as possible from I/O ports, without allowing excessive hazardous currents to flow in these same paths.
A common implication of this tradeoff is that leakage currents for off-the-shelf AC/DC power supplies are driven to approach the limits dictated by safety standards; this provides maximum emissions margins, leaving room for conducted and radiated energy contributions from the downstream system. The consequence is that it may not be feasible for the system to include other mains-connected networks including any additional external filtering, or even a second off-line converter. If additional mains-connected networks are required, and those networks would act to contribute to the total leakage current in the system, alternative approaches to mitigation of electromagnetic interference (EMI) may be necessary to allow for an increase in the impedance of the shunt paths.
HF attenuation can always be achieved with series inductive reactances rather than with shunt capacitive reactances. In practice, most off-line AC/DC converters incorporate optimized multiorder filters with both series and shunt elements, but if leakage currents are to be reduced, it is always an option to augment the size of the magnetics to offset decreases in shunt capacitance. As a result, the power converter becomes larger, heavier, and more expensive. If system level interactions necessitate the addition of external filtering, the nature of the noise (common code vs. differential mode) should be carefully characterized, and any additional shunts should be deployed judiciously, with careful regard to existing leakage current magnitudes.
A better approach may be to target the HF noise at its source, primarily semi-conductor state transitions. A well-designed SMPS will incorporate effective snubbers around switching elements to reduce voltage transition rates, and as a result reduce the overall harmonic content of the switching waveform. Control of parasitic capacitances between primary and secondary, stemming from optical signal isolators and interwinding capacitances in the isolation transformer, lead to a reduced need for decoupling. If radio-frequency (RF) energy is conducted back from the downstream networks to the SMPS, chokes on interconnecting cables can also help to reduce observed emissions levels.
One might consider deploying a modern resonant topology with zero-voltage (ZVS) or zero-current soft (ZCS) switching. Forcing semiconductor state transitions to occur at natural zero-crossings can drastically reduce or potentially eliminate harmonic-rich voltage and/or current waveform discontinuities. These modern topologies provide many additional benefits in power density and energy efficiency.
Given the natural contradiction between commonplace EMI mitigation design techniques and safety requirements for leakage currents, and how tightly both of these SMPS artifacts are regulated in the medical industry, thorough consideration should be given to the EMI/leakage trade-off when deploying low-leakage systems or systems with multiple mains-connected networks. Good SMPS design practices and modern power conversion topologies can make navigating the trade-off less burdensome, boost design margins, and reduce time to market.