Leland Teschler, Executive Editor, Design World
The effects of electromagnetic interference on medical gear made headlines in the 1990s when the FDA’s Center for Devices & Radiological Health found that the presence of low-level EMI kept some sleep-apnea monitors from noticing that babies had stopped breathing. EMI was also the culprit in other problems back then, ranging from power wheelchairs that started rolling when EMI caused their brakes to release, to EMI from electrosurgery units that made anesthetic gas monitors stop working.
Fast-forward to today: Design practices to prevent EMI have grown more stringent. But the potential for interference problems have risen with the pervasive use of wireless technology in all manner of medical gear. In particular, the switching power supplies now nearly ubiquitous in medical equipment are getting close scrutiny for RFI problems because of their potential for radiating harmful interference.
Most modern switching supplies internally operate in the range of hundreds of kilohertz to a few megahertz. That means the strongest upper harmonic frequencies they generate tend to be not much higher than a few tens of megahertz – typically not an interference problem for modern-day wireless gear. But the next generation of new designs for switch-mode power supplies push operating frequencies higher as a way of cramming circuitry into smaller and smaller volumes.
An indication of the trend can be seen in the recently developed Dart power supply from Finsix, a start-up with roots in the Mass. Institute of Technology. The Dart fits in the palm of your hand but puts out 65 W, as much as a conventional brick power supply for laptops. The key to its small size is its switching power supply operating at VHF switching frequencies – around 100 MHz, per company literature. That puts its strongest upper harmonics squarely in the UHF frequency bands occupied by communication gear such as the Global Positioning System and narrowband radio modems used for controlling power distribution networks.
The concern over interference problems is such that the newest version of EN60601-1-2, the International Electrotechnical Commission (IEC) 60601-1-2 safety standard for medical equipment, calls for tougher levels of EMI and RFI (radio frequency interference) performance.
Problem is, the EMI/RFI provisions in this standard have been handled in a way that has caused confusion among users and makers of power supplies. “The third edition [of EN60601-1] is still in force for safety requirements. The fourth edition [which contains the new EMI/RFI requirements] will come into force for some parts of the world in 2017. By end of 2018, it will be required everywhere,” says Kevin Parmenter, VP applications at power supply maker Excelsys Technologies and chair of the Power Sources Manufacturers Assn.’s safety and compliance committee.
“To be compliant, you have to meet both the third-edition and fourth-edition specs now. Big manufacturers have already done fourth-edition compliance because it can take three to five years to get a new design into production. And meeting the EMI levels spelled out in the fourth edition is challenging because of all the noise a switching supply generates. But power supply makers that use well-defined design methodologies should be in good shape.”
Power supply makers are also having to make design changes to meet requirements spelled out in the third edition of EN 60601-1-2. For one thing, the third edition calls for more isolation of circuit components. Creepage and clearance allowances have grown. Creepage is the shortest path between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured along the surface of the insulation. Creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of electric discharges near an insulation surface.
Clearance is the shortest distance between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured through air. Clearance distance helps prevent dielectric breakdown caused by the ionization of air. The dielectric breakdown level is also influenced by relative humidity, temperature and degree of pollution in the environment.
To make matters a bit more confusing, the creepage and clearance distances spelled out in the third edition of EN 60601-1-2 must be further increased to handle another requirement: Operation at altitudes above 5 km. High-altitude safety is now mandated in China via the Chinese standard GB 4943.1-2011 and is becoming important in Chile, India, Peru and other countries with mountainous terrain. The creepage and clearance distances that are adequate for safety at sea level aren’t acceptable at high altitudes. The reason: Air’s capacity to act as an insulator diminishes with altitude, forcing designers to position conductors farther apart in the interest of safety.
Designers typically plan for high altitudes by employing a printed circuit board layout standard called IPC-2221B, then applying a multiplier to the distances it specifies – usually 1.48 X.
But not all power supply makers currently meet third-edition requirements. “Some suppliers who aren’t good at it are still in the second edition, even though the third edition is the current law of the land for safety,” says Parmenter. “There are probably over 500 suppliers worldwide that market internal power supplies. Probably less than 10 make products you’d want to put in a medical system. Compliance really thins the herd.”
The third edition of IEC 60601-1 also spells out that equipment exhibit a high degree of isolation between the AC input to the power supply, the internal high-voltage stages, and the DC output. For example, medical supplies that operate from a 240-Vac mains must withstand a dielectric test at 4 kVac for medical applications (compared to 3 kVac for industrial applications).
More stringent as well is the treatment of “touch current.” Touch currents refer to those that may follow leakage paths from an enclosure to a patient or operator. Touch current levels allowed within the third edition of IEC 60601 are no more than 100 μA for normal operation and 500 μA for a single fault condition.
Touch current differs from the total patient leakage current. The total patient leakage current test measures the leakage current when all parts of the medical device that touch the patient are, in fact, in contact with the patient. The test is designed to ensure the patient is protected when there are multiple connections and leakage paths.
In leakage-current measurements, the concept of an “applied part” is important. The term “applied part” refers to the portion of the medical device that may physically touch the patient during normal operation. The 60601-1 standard divides applied parts into three classifications going from least- to most-stringent shock protection levels: B (body), BF (body floating), and CF (cardiac floating).
Type B applied parts may be connected to earth ground. Type BF and CF are separated from earth and are considered floating. Power supply isolation voltages vary depending on the type rating.
Type B classification goes on applied parts that are normally not conductive and can be immediately released from the patient. Examples include MRI body scanners and hospital beds. Type BF applied parts have conductive contact with the patient or touch patients for a long duration. Examples include blood pressure monitors and ultrasound equipment. Type CF applied parts are those that may come into direct contact with the heart, such as dialysis machines.
Building in clearance and creepage
The requirement for lower leakage currents and higher isolation levels can be met mainly by spacing circuit components farther apart. But that doesn’t mean that medical power supplies are bigger than industrial supplies with the same power ratings.
“You might think the more severe creepage and clearance ratings would force the supply to be bigger. But you are talking an increase spacing of only few millimeters,” says TDK-Lambda Americas Inc. Director of Marketing Tom Tillman. “So medical supplies can be the same overall size as general-purpose models. In fact, many power supply makers don’t design separate units for medical and offer both ITE 60950 and IEC 60950-1 in a single model. Usually the only visible difference is the medical safety approval on the label and dual fuses for both the line and neutral connections, where general purpose units typically just have one.”
It ‘s also worth noting that the IEC60601-1 third edition requires a risk management process and record retention in compliance with a standard for the application of risk management to medical devices, ISO14971.
“The risk management requirement is coming from hospitals that must show a risk management process to get accreditation. These requirements trickle down to power supply makers,” says Excelsys Technologies’ Parmenter.
Meeting the IEC60601-1 risk management requirements can be a big undertaking for power supply makers because it involves aspects of the manufacturing process. It may entail meeting the requirements of another standard, ISO 13485, that spells out a quality management system for the design and manufacture of medical devices. “When medical equipment makers are evaluating a supplier of power supplies, they’ll first ask about ISO 9000, then about whether the manufacturing facility complies with ISO 13485. Manufacturers want to know this because if you are compliant, they are,” says Parmenter. “And the ISO 9000 quality standard is a prerequisite of ISO 13485. All in all, it can take a couple years to get certified to 13485.”
Equipment makers might wonder whether they can get a qualified medical power supply from a supplier lacking ISO 13485 status. Parmenter says that’s possible, but ISO 13485 compliance makes it easier for the equipment maker to get certification themselves. “Eventually, it will be the law of the land that all medical power supplies will have to be made in 13485 facilities.”
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