Design Considerations for Medical Power Supplies

­Medical equipment design is highly regulated, with safety being the main concern. However, the standards must cover a very wide range of equipment types and use environments, from battery-powered monitors to scanners consuming multiple kW and from operating theatres to analysis labs and ‘point of care’ services. These differing conditions are addressed in the standard IEC 60601-1 and its national versions, making it a complex and extensive document.

At the same time, there is inevitable commercial pressure to keep equipment costs low and for the products to include automation, better functionality and, nowadays, AI. This is to enable better patient outcomes and to speed up workflows, particularly in the analysis of bio-samples and clinical data.

1.     Assessing risks

In the later editions of IEC 60601-1, risk assessment and management have become mandatory and form a starting point in medical product designs. The manufacturer must decide whether the product could realistically come into contact with patients and apply the appropriate levels of protection. In the standard, there is a differentiation between a ‘medical device’ (MD) which may contact a patient and an ‘in vitro diagnostic medical device’ (IVD) such as a centrifuge or blood analyzer which would normally not. The risk management procedure must follow the strictures of ISO 14791, ‘Application of Risk Management to Medical Devices’ and the document requirements cover the whole product lifecycle. Considerations are wide-ranging, including risks such as biocompatibility, data and systems security, electricity, moving parts, radiation, and usability. While not all of these might apply to power supplies, either external or built-in, the increasing use of digital control and monitoring in power designs represents a new area where risk of accidental corruption of data, or deliberate tampering with software/firmware must be evaluated and mitigated.

Note that ISO 14971 provides a framework only and does not specify hazards, what risk is acceptable or how to quantify the risk. Figure 1 is a typical risk management workflow.

2.     Safety

Of all the medical equipment requirements, safety is number one and in practice that includes protection against electric shock, fire and mechanical hazards among others. The medical safety standard IEC 60601-1 has adopted the ‘Means Of Protection’ terminology, two ‘MOPs’ being the minimum to prevent shock, following the principle that two failures should occur before equipment could become unsafe. This can be compared with other safety standards that require reinforced isolation, equivalent to two separate protection levels, or one level with enhanced performance such as thick, solid insulation.

The standard defines operator and patient environments and the isolation creepage and clearance distances are different between the two. Two Means Of Patient Protection (2 x MOPPs) for example, requires 8mm creepage and 4kVAC test voltage for up to 250VAC system voltages. When connection to the patient is intended or possible, the medical safety standard requires that consideration be given to the possible failure of other equipment which could leave the patient ‘energized’ with mains voltage. In this case, there should then be no path through normally operating equipment to ground, allowing lethal current to flow. In practice, this means providing at least 1 x MOPP isolation between patient connections and ground in the medical equipment. This requirement is typically not met by power supplies complying with IEC 62368-1 (the standard for audio/video, information and communication technology equipment), where the output may only have functional isolation to ground, if any at all.

The same principle applies to unspecified external signal connections to patient-connected medical equipment and their power supplies. In this case, 2 x MOPP isolation is required from these lines to patient connections as it must be assumed that the signal lines could be energized with AC mains through other faulty equipment. If the connected equipment is specified, then 1 x MOPP to the interface is acceptable. Additional low power DC-DC converters with 1 x or 2 x MOPP isolation and very low coupling capacitance are sometimes used in power lines to meet these requirements (Figure 2).

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Figure 2: An arrangement of PSU and specified connected equipment with MOPP levels

The requirements for isolation for operator protection (Means Of Operator Protection or MOOP) are like those for IEC 62368-1 but there are differences, for example, in fusing arrangements and allowed leakage current.

3.     Leakage Current

Headline safety specifications are for electrical isolation, but laws of physics dictate that there will always be some level of leakage current from the AC mains connections to a power supply chassis or output, through stray coupling capacitance or EMI suppression capacitors. In medical applications, the tolerable level of power supply patient leakage current depends on the use case. Levels specified are for normal and single fault conditions and can be as low as 10µA when the equipment has direct connections to the heart, for example. This is the CF or ‘Cardiac Floating’ case. Table 1 shows the allowed levels.

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Table 1: Permissible levels of ‘auxiliary’ and leakage current according to IEC 60601-1

Medical equipment that only requires MOOP such as is used in labs must still have low overall earth leakage, 0.5mA in normal conditions (0.3mA in the US). This compares with the Energy Source 2 (ES2) limit of up to 5mA at 50/60Hz, according to IEC 62368-1.

4.     EMC Limits

IEC 60601-1-2 lists the medical device EMC requirements referencing CISPR11 and the IEC 61000-4-x series of EMC standards, covering the same emissions and immunity tests need for commercial, industrial and IT equipment. For the latest edition 4.1 of IEC 60601-1-2, a new test was added for immunity to proximity magnetic field in the frequency range of 9kHz to 13.56MHz. This caters for potential sources of interference such as from wireless devices in the varied environments where medical equipment may be used, domestic and care homes, offices, public spaces, emergency vehicles, and hospitals.

In this edition another significant change was to require emissions, AC dips and dropout testing at the extremes of the input AC voltage range, rather than just at nominal, again reflecting the fact that the operating environment can be so varied. The IEC 61000-4-x series includes multiple severity/immunity levels and IEC 60601-1-2 specifies which are to be used, depending on the intended medical application and environment.

It is well known that low leakage current and low conducted EMI tend to be mutually exclusive because ‘Y’ capacitors that attenuate common-mode EMI also couple leakage current to an earthed chassis or to outputs. This has pushed the design of power converters towards topologies that are inherently low noise such as resonant types. Luckily this also normally leads to higher efficiency with its consequent benefits.

5.     Functionality

As medical equipment appears in all care settings, it needs to be practical to use – that is, lightweight, quiet, easily powered or recharged and robust. It must withstand the knocks and falls of continuous use and be resistant to fluids and other foreign material that is inevitably present in the healthcare environment.

What makes care most effective with modern equipment is the ability to collect and analyze data with perhaps wireless connections to aggregate information or to allow remote monitoring and diagnostics. In the home this might be through a Bluetooth^TM connection to an app and, in the laboratory or diagnostics suite, more likely a network connection to local servers or to the cloud. In this setting, speed and efficiency of the workflow is a major performance marker and monitoring of the effectiveness of the equipment is vital to ensure maximum up-time and confidence in collected data. Equipment power supplies that provide digital monitoring of their outputs, with advance warning of AC failure and performance degradation are invaluable in this situation. Additionally, precision remote control of parameters such as output voltage or current facilitate equipment automation such as control of laser diode current in microsurgery or in aesthetic treatments.

Example of a Medical-Rated Power Supply

A power supply product well-suited for medical and industrial applications is the XP Power HPKF3K0 series (Figure 3). Suitable for higher power applications with its 3kW rating, the unit features 2 x MOPP isolation with less than 500µA leakage current, making it suitable for many applications. Medical EMC specifications are met, and the part has sophisticated digital control and monitoring functions including programmable voltage, 0-105% and current, 0-110% through its digital interface, which supports PMBus, CANopen, MODBUS and SCPI protocols for maximum flexibility. The HPF3K0 series holds medical approvals for global markets and is available with 90-264VAC input range and fixed outputs of nominal 24V, 36V, 48V or 60VDC. Efficiency is up to 93% and size is 279.4 x 177.8 x 63.5mm (11 x 7 x 2.5 inches).

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Figure 3: The XP Power HPF3K0 3kW power supply is suitable for medical applications

XP Power

www.xppower.com