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Medical Supply Safety: The Ultimate AC/DC Challenge?

While there is much discussion around battery-powered devices, there are many situations where a device must be powered by the AC line. This can be due to run-time requirements, excessive size of the requisite battery pack, or even just user convenience. When the AC line is the primary power source, the associated AC/DC supply must, of course, meet multiple safety standards that apply to supplies in general, but the standards and regulations for medical power supplies are in a league of their own.

For example, IEC 62368-1, which takes full effect in December 2020, defines hazards-based safety engineering (HBSE) for audio/video and information/communication technology equipment, including but not limited to their power supplies. Yet that standard doesn’t cover a major application area: patient-connected medical devices. There’s a reason for that.

If you think meeting IEC 62368-1 is a challenge, then look into what designers of medical devices must do to meet IEC 60601-1 (4th edition), Medical electrical equipment – Part 1: General requirements for basic safety and essential performance, which comprises a suite of standards related to the devices having deliberate or likely patient contact during normal use.

This rigorous standard calls out three classifications for the “applied part” which may contact the patient:

  • Type B (body) for applied parts which are generally not conductive and may be connected to earth (as ground), such as x-ray machines, hospital beds, LED operating lighting, and MRI scanners.
  • Type BF (body floating) for applied parts which may be connected to the patient and must be floating and separated from earth, such as blood pressure monitors, ultrasound equipment, and thermometers.
  • Type CF (cardiac floating) for applied parts suitable for direct cardiac connection; these, too, must be floating and separated from earth.

Among the many requirements to meet the IEC 60601 standard are that there be two means of protection (MOP) to prevent applied parts and any other accessible parts from exceeding defined limits on voltage, current, and energy. For example, a qualified earth connection counts as one MOP and basic isolation is also one MOP, while reinforced insulation counts as two MOP.

To add further complications to the situation, the number of required MOP depends on whether this protection is for the device operator (means of operator protection, MOOP) or the patient (means of patient protection, MOPP). Depending on the end use of the system (Type B, BF, or CF), different numbers of MOPP may be needed. For example, power subsystems used for Type BF and CF require two MOPP between primary and secondary sides of the supply.

There’s more to power supply safety than MOP

So far, these requirements on medical supplies are difficult but not extreme. But that’s not all: there are also restrictions on types and amounts of leakage current. Although the specific limits differ depending if it is patient auxiliary current or leakage current, and if it is under normal or fault conditions, the leakage values are on the order of 100 microamps and below. That’s a tough specification to meet in a power supply, as leakage current is a function of transformer design as well as board layout and fabrication. The challenge for power supply designers is to simultaneously provide the required galvanic isolation while also not exceeding the strict leakage limits under both normal and fault conditions.

The mandates on a medical-rated supply don’t end there, as the latest edition of IEC 60601 defines many EMC requirements as well. First, the medical device must demonstrate immunity to RFI/EMI; and while power supplies are generally not as susceptible as the electronics, they still can be adversely affected or “upset” by ESD. Second, the supply must not create EMI/RFI beyond defined limits. The 4th edition of IEC 60601 imposes more-stringent tests on the medical device, including the supply, for ESD; radiated RF immunity; electrical fast transients; and voltage dips, dropouts, and interruptions (Table 1).

Table 1: Key changes to immunity test levels from 3rd edition to 4th edition of IEC 60601-1-2. (Table source: CUI Inc.)

The key takeaway here is that designing, implementing, and getting approval for a medical application line-powered supply which implements the relevant IEC 60601-1 requirements and limits is very difficult. Not only must the basic supply be a “good” one in terms of traditional supply parameters such as voltage and current output, line and load regulation, efficiency, and stability over time and temperature, it must also meet complex and stringent regulatory mandates.

Fortunately, vendors now offer AC/DC supplies which eliminate the need to worry about IEC 60601-1 power supply issues. CUI Inc., for example, offers both open frame units for use within a product as well as encapsulated supplies for external use.

For example, the SWM30-12-NV-P5 is a 12 volt, 3 ampere (A) wall mount adapter that may look just like yet another one of the those basic, cheap “wall warts,” but it certainly is not. This universal input unit (90~264 VAC) is one member of a family of ~30 W fixed output adapters (from 5 V to 48 V DC) which fully meets UL60601 (4th edition) requirements.

Figure 1: The SWM30-12-NV-P5 a 12 volt, 36 watt wall mount adapter is part of a family of single, fixed output AC/DC supplies that meet IEC 60601-1 (4th edition) mandates. (Image source: CUI Inc.)

For designs where the supply is mounted internally, the VMS-100-24 is a 24 volt, 4.2 A supply (Figure 2). It too is a member of a “family” of universal input units which can deliver a single, fixed output (5 to 48 V) at up to 100 W. No-load dissipation is less than 0.5 W and it includes a full range of protections as well as power factor correction, also dictated by regulatory standards.

Figure 2: For internal supply designs, the VMS-100-24 is a 24 volt, 4.2 A unit, part of a 5 to 48 volt family of 100 watt power supplies. (Image source: CUI Inc.)

Conclusion

There’s no way to minimize the reality: designing line-powered medical products is difficult. This is not only because of the basic required medical functionality, but also due to the many regulatory standards for safety which they must meet.

One design strategy for meeting these standards is to begin with components and subsystems which meet the relevant standards as building blocks, and “build up” the design with them. In most cases, it is easier to use such blocks than it is to create a complete system with non-compliant components, and then somehow “upgrade” the design so it meets the requirements. Power supplies such as those from CUI are major elements in implementing this approach.

Related Digi-Key resources:

Medical Power Supplies and the IEC 60601-1 Medical Design Standards

External Medical Power Supplies

High-Density Medical AC-DC Power Supplies

Medical Power Supplies

References (all from CUI)

IEC 60601-1 Medical Design Standards for Power Supplies

IEC 60601-1-2 4th Edition: What You Need to Know

Be Prepared for the 4th Edition of the IEC 60601-1 Medical Standard

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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