Dave Wellman, Sales Manager, Gresham Power Electronics, explains how foundational technologies such as power supplies, which are a vital component in all medical electronics, can be overlooked in a fast moving industry sector.
It is clear that the medical electronics industry has entered an era of dramatic transitions that affect virtually every aspect of the industry from design to marketing. As is often the case with such substantial ‘Big Picture’ changes, it can be easy to overlook how they can affect foundational technologies such as power supplies, which are a vital component in all medical electronics.
Key developments triggered by general technological advances, an ageing population worldwide and even aspects of government healthcare reforms are driving these trends that include the proliferation of wireless technologies and implantable medical devices, the push towards home healthcare options versus in-hospital or outpatient clinic visits and a greater demand for cost containment in medical products and services.
For each of these trends, careful planning of the power design from the beginning can enhance end product feature sets while reducing costs, maximising reliability, and accelerating time-to-market.
Choosing the right power supply design is imperative for an overall successful product. With ongoing goals of increased density, reliability and efficiencies, power ratings need to be reviewed carefully. Other vital considerations include product life, thermal issues, input voltage, load and cooling needs. Other factors especially important for medical electronics vendors include demonstrated reliability and proven quality systems. These encompass control processes for design, purchasing and production as well as product traceability and Corrective and Preventative Actions (CAPAs). Because of increased regulation, medical device designers must now also consider closely their electromagnetic compatibility (EMC) performance. EMC compliance specifications are included in various EN 61000 standards as listed in Table 1.
EN 61000-3-x sets limits and measures methods for low frequency emissions on the AC mains, while EN 61000-4-xx sets limits on susceptibility or immunity of the equipment powered from AC mains. These standards require precise performance so the power supplies or other electronic devices do not affect the safety and effective functionality of other implantable or critical care medical devices.
The stricter standards are also needed to sustain full operation despite noisy and a somewhat less regulated power conditioning environment in the home. For example, an infusion pump malfunctions when a mobile phone is used in close proximity to the patient. Or perhaps, a home ventilator shuts down when the line voltage dips below the minimum operating limit caused by a neighbouring motor or generator use. Or, a remote patient monitoring device is damaged when touched by someone walking around the house and building electrostatic discharge.
Once designers determine the basic power requirements, the final hurdle in power supply product selection involves meeting standards for safety, EMC and regulatory environmental impact compliance.
Today’s medical electronics must meet stringent safety requirements and performance testing adhering to various standards starting with IEC 60601-1 3rd Edition for medical devices. Depending on the end use of the product and its implementation, additional standards such as EN and IEC standards for EMI/EMC, RoHS compliance and CE Mark may also apply.
When selecting an AC or DC power supply (either off-the-shelf or custom) designers must consider the specific performance criteria suitable for their application when defining these requirements. In addition, keep in mind that transition to the IEC 60601-1-2 4th Edition standard on electromagnetic disturbance started in 2014.
Regulatory versus market needs
Generally, manufacturers are only required to have their medical device end products to meet many of the standards, especially those relating to the IEC 60601-1 family of technical standards for medical equipment. However, designers will find that starting out with power supplies designed from the ground up to meet those standards will greatly simplify achieving final compliance while accelerating time to market for their end product, particularly since power and safety issues are the basis for many key facets of the standards.
For example, internal type power supplies are meant to be handled only during the manufacturing process, as parts are installed in end equipment. Therefore, the assumption might be that the power supply should be designed for, and tested based on, that use. However, as internal power supplies are increasingly being designed into portable devices, such as home healthcare equipment, power supplies meeting the more stringent test parameters will provide the end system designer a more robust power supply, potentially allowing easier system compliance to more demanding certifications.
IEC 60601-1 general standard
This key standard is a widely accepted benchmark for medical electrical equipment. Compliance has become a requirement for electrical medical equipment marketed in many regions globally. However, the standard functions on several levels and is not always well understood. While compliance may be a de facto requirement, it is not the same as device approval, only a recognised step in the approval process. The current 3rd Edition of the standard places emphasis on usability engineering, risk assessment and use in home healthcare applications.
In addition to regional variations, the 60601-1 standard also encompasses Collateral Standards (numbered 60601-1-x) that define requirements for certain aspects of safety and performance. Requirements for the general standard may be overridden or bypassed by specific language in the collateral standards that are applicable to specific aspects of product design, such as:
• IEC 60601-1-2 – Electromagnetic Compatibility
• IEC 60601-1-3 – Radiation Protection for diagnostic use of X-rays
• IEC 60601-1-4 – Programmable electrical medical systems
• IEC 60601-1-8 – Alarm systems
• IEC 60601-1-11 – Home healthcare environments.
A further set of particular standards (numbered 60601-2-x) provides supplementary definitions and requirements for specific product types such as defibrillators, MR scanners, electroencephalograms and insulin pumps.
Means of protection
Medical equipment is required to incorporate Means of Patient Protection (MOPP) or Means of Operator Protection (MOOP), with patient protection being the more stringent standard. Primary distinctions between MOPP and MOOP relate to allowable creepage distance, isolations and insulation. Each Means of Protection has a basic level and a 2x level with 2 MOPP referring to the highest level of protection with 4,000V AC isolation, 8mm creepage distance and double insulation.
On the surface, MOOP and MOPP may appear to address distinct markets, however, the increasing use of home healthcare devices is blurring the lines between these applications. For that reason, it can be advantageous to design to the more stringent 2 MOPP levels, as long as it is not cost prohibitive. Increasing volumes resulting from home healthcare demands can, in many cases, mitigate the cost of increased levels of protection.
As mentioned above, final product certification is not dependent upon whether the power supply manufacturer specifies MOOP or MOPP isolation compliance. However, if compliance is not specified, it will be the end product designer’s responsibility to ensure that the final product meets the overall requirements.
EMC compliance requirements
It is important for designers to review and specify their product’s performance acceptance criteria. It is not enough to just state the IEC standard. There are four levels for acceptance criteria, and designers need to be clear on the level that is acceptable for each application.
• Normal performance within the specification limits.
• Degradation or loss of function, which is not recoverable due to damage of equipment or software or loss of data. In all cases, equipment shall not become dangerous or unsafe as a result of the application of the tests. The performance acceptance criteria can be different for the various levels or the test.
Medical device designers must pay particular attention to electrostatic discharge (ESD) issues. The IEC 61000-4-2 standard defines four standard levels of ESD protection, using two different testing methodologies.
Designers should perform an engineering analysis to ensure that there would not be a significant unit cost increase in order for the power supply design to be compliant with the more stringent standard.
From a power supply design perspective, designing a product to comply with the IEC 61000-4-2 ESD requirements can be a challenge at the higher discharge voltages. This becomes even more challenging with a Class II AC input (two-wire, no-earth ground conductor). When the ESD discharge is applied to the output or signal pin, the voltage is developed across various isolating barriers and capacitors. This occurs because the AC mains are virtually grounded at some point, so applied ESD voltage appears between the point where the charge is applied and earth ground.
Without careful consideration of the various discharge paths within the power supply, unexpected arcing and damage to the power supply can occur. Testing to the standard and providing a test report is some assurance that the product will perform well within the confines of the specification.
The evolving market trends and regulatory climate for medical device design can present daunting challenges on many levels, just a few of which have been addressed here. Other important aspects include RFI and wireless considerations, risk management and regional differences, as well as issues relating to various types of devices and where they will be used. In all of these cases, starting with the design of a power supply that addresses known requirements will give any design an edge for speeding development and meeting overall regulatory goals. Choosing a vendor with a depth of medical regulatory experience can further simplify the design process and ensure the smoothest path to market.