Key Power Issues for Medical Equipment Designers



Although many electronic design engineers will consider the provision of power for medical applications to be a reasonably well-understood subject, there is one particular area, concerning voltage dips and power interrupts, that demands close scrutiny.

The overall provisions of the IEC 60601-1 safety standard are likely to be familiar territory to any design engineer working in the field of medical equipment. The standard defines the general safety requirements for equipment that has ‘not more than one connection to a particular supply mains and is intended to diagnose, treat or monitor the patient under medical supervision and which makes physical or electrical contact with the patient.’ IEC 60601-1 has been adopted by the US as UL 60601-1, as well as most major industrialized countries, including Canada (C22.2 No. 601.1), the UK and Europe (EN 60601-1), Japan (JIS T0601-1), Australia and New Zealand (AS/ NZ 3200.1).

Safety standard 60601-1 applies to an extremely broad and diverse range of equipment intended for use in medical, dental and laboratory environments. Typical examples extend from small items of equipment such as infusion pump controls and endoscopic cameras, through to much larger systems such as dialysis machines, CTI and MRI scanners and gamma imaging systems.

Build or Buy?

Designing in-house or choosing a commercially available ac-dc power supply for a medical product involves a host of considerations. These include the system’s overall power budget and current and voltage requirements, as well as the power supply’s conversion efficiency, physical size, control and monitoring functions, set-up or programmable features and – not least – its cost. In addition to these factors, it is essential to ensure that the power supply has higher isolation and lower safety ground leakage than a standard non-medical unit, in order to comply with the 60601-1 safety standard.

Since the design of high-efficiency switch-mode power supplies is a specialist task demanding considerable skill and resources, and the medical equipment has to undergo strict compliance testing, most designers will choose to use a standard commercially available medical power supply for their application if one is available, or request a customized unit from a specialist power supply company. By using power supplies that are already pre-approved to the 60601-1 safety standard, medical equipment manufacturers can accelerate compliance testing of their own products and speed time-to-market. Taking this pre-approved route also minimizes the risk of them encountering any unforeseen development problems in an area outside their own field of expertise, which could negatively impact launch timescales.

There are a considerable number of medical power supply manufacturers worldwide, many of which produce technically excellent products. When choosing a particular supplier, it is almost certainly best to look for a company that manufactures a wide range of power supplies, preferably has a proven expertise in both ac-dc and dc-dc conversion technologies, and which has a consistent track record for delivering standard and customized medically approved products to leading medical equipment manufacturers.

Given that nowadays there is a wide availability of power supplies that comply with 60601-1, at first glance it would seem that choosing a suitable unit is merely a case of checking that the product meets all the requirements of the application. However, it is not quite that simple. IEC 60601-1 is a prime example of what is termed a base standard; it covers all the general requirements for electrical medical equipment, but it also has a number of associated standards, known as collateral standards. One of these is IEC 60601-1-2, which defines the rigorous electromagnetic compatibility (EMC) requirements of medical power supplies.

It goes without saying that all 60601-1 compliant power supplies meet the EMC requirements of IEC 60601-1-2, otherwise they would not be approved – in fact, these requirements have been a mandatory condition of sale since 2004. However, meeting the voltage dip requirements of IEC 60601-1-2 – which are themselves the subject of a further complementary pair of IEC standards known as 61000-4-11 and 61000-4-34 – is still a matter involving a degree of controversy.

IEC 61000-4-11 and IEC 61000-4-34 are matched standards that define how equipment must be capable of tolerating voltage dips, voltage variations and short-term power interrupts on the ac mains supply. The standards specify the same depths and durations of voltage dips, and cover both single-phase and three-phase equipment. IEC 61000-4-11 applies to equipment rated at up to 16 amps per phase connected to 50 Hz or 60 Hz ac supply networks, and IEC 61000-4-34 applies to equipment rated at more than 16 amps per phase. Since in many respects the standards are the same, this article limits its discussion to IEC 61000-4-11.

21_a

One of the main problems is that deciding whether or not an item of medical equipment meets the requirements of IEC 61000-4-11 is open to interpretation. In broad terms, the standard stipulates that the equipment should not suffer ‘loss of functionality’ for a 30 percent dip in supply voltage lasting 0.5 s, a 60 percent dip lasting 100 ms, and a 100 percent dip lasting 10 ms. The equipment should also not suffer ‘loss of functionality’ in the event of ac power being removed altogether for a period of 5 seconds. However, the term ‘loss of functionality’ is to some degree subjective, and the compliance test procedure recognizes this fact by defining four distinct classification categories, as shown in Table 1.

Choice of Classification Categories

Provided that the equipment is not intended for critical life-support functions, the choice of which classification category to adopt for compliance testing is left to the equipment designer’s discretion. The designer must also decide what constitutes full functionality – and therefore by definition, what also constitutes ‘loss of functionality.’ This is inevitably something of a gray area. 21_bMost standard low- to medium-power open-frame medical power supplies, which represent by far the largest segment of the market, are too small and inexpensive to satisfy classification level A; achieving lengthy hold-up times at full load with no degradation in output voltage regulation demands the addition of significant holdup capacitance or larger input components for lower voltage operation. A number of power supply manufacturers include this detailed EMC characterization data in their product datasheets or application notes, to help designers decide which classification to use for compliance testing their equipment, and it is worth checking for this information to help narrow the field of choice.

There are a variety of techniques available to medical equipment designers seeking to satisfy the stringent classification level A. They can oversize the power supply for the application, or fit more capacitance to its output – which has maximum limits based on various design criteria. If commercially justified, custom power supplies can also be considered. Another solution is to use a modular power supply, which provides a flexible and cost-effective means of incrementally increasing output capability and adding extra capacitance, to extend the hold-up time in the event of an ac input dip. Some of the better modular medically approved power supplies on the market offer optional power hold-up modules, which can extend the time that the output voltage will be maintained by a significant amount.

Over-specifying a power supply to meet the voltage dip requirements of IEC 60601-1-2 can be an expensive decision. Medical equipment designers would do well to carefully evaluate the system’s overall power budget and required level of functionality, before deciding on a specific power supply.


22_a

Chris Jones is director of product marketing at the Embedded Power business of Emerson Network Power. Jones is responsible for the development of the company’s range of standard embedded power products, from product definition to portfolio management. Jones holds a bachelor’s degree in electrical engineering (BSEE) from West Coast University in Los Angeles, California.

22_b

Conor Quinn is director of technical marketing at Emerson Network Power, with responsibility for embedded power products. Quinn is a regular contributor to the specification and roadmap initiatives of industry groups including PSMA (Power Sources Manufacturers Association), PMBus (Power Management Bus) and PICMG (PCI Industrial Computer Manufacturers Group). Quinn holds a BE in electrical engineering from University College Cork in Ireland and a MSEE and a PhD in engineering from the University of Minnesota.

Share and Enjoy:
  • Digg
  • Sphinn
  • del.icio.us
  • Facebook
  • Mixx
  • Google
  • TwitThis