Medical Electronics
How to select and apply board-mounted DC/DC converters in medical systems
By Rolf Horn, applications engineer, Digi-Key Electronics W
ith competitive pressure on designers of medical system power supplies to increase power density, reduce footprint, and
meet cost constraints—along with meeting extended temperature operation and high reliability requirements—it can be tempting to design their own custom-mounted DC/ DC converter and fully optimize it for the application. Depending upon available resources, in many cases, this may not be the best option as not only is power supply design challenging, medical DC/DC converters are particularly so since they need to be certified to an array of user and operator safety requirements.
These requirements include; IEC/EN/ES 60601-1 3rd edition for 2 x means of patient protection (MOPP) safety, risk management according to ISO 14971, IPC-A-610 Level 3 criteria for electronic assemblies, and electromagnetic compatibility (EMC) compliance according to IEC 60601-1-2 4th edition. In many instances, a 4:1 input range is needed to operate from various battery and vehicle supplies, and it must have high voltage isolation and low leakage current. Instead, designers can select standard board-mounted DC/DC converters in a compact form factor that are certified to all the requirements detailed above, with the added advantage of a mean time between failures (MTBF) of over 1 million hours, as well as numerous control features including; remote on/off control, remote sense, and output voltage trim; plus protection for undervoltage input, short-circuit, overtemperature and overvoltage. This article provides a review of the industry standards to be considered when specifying DC/DC converters for medical applications. It then introduces and discusses the application of an off-the- shelf 60-watt DC/DC converter from TRACO Power that meets all the industry standards for medical systems.
14 December/January 2023
DC/DC converter considerations for medical systems
Industry standards for medical systems design are primarily concerned with safety; safety of patients, safety of equipment operators, and safety of the equipment to protect it from potentially damaging conditions. The concept of “means of protection” (MOP) is the key to understanding and achieving patient and operator safety. Various MOPs are defined as safety insulation, a creepage distance, an airgap, protective impedances, and a protective earth. At a minimum, medical devices must include one MOP to protect patients and operators from the risk of an electric shock should a fault occur. IEC 60601 assigns different MOPs for patients and operators, resulting in specific requirements for MOPP and means of operator protection (MOOP), defined in
terms of an isolation voltage, creepage distance, and insulation level (Table 1). MOPP requirements are more restrictive since patients may have less ability to protect themselves, and depending on the application, one or two MOPPs or MOOPs may be required by IEC 60601.
The level of protection needed is dependent on the specific application. For example, body floating (BF) safety levels are required for applied parts (AP) that are electrically connected to the patient, such as ultrasound equipment and blood pressure monitors, and must be floating and separated from earth. The use of an AC/DC power supply approved for 2 x MOPP safety is one approach to meeting IEC 60601, but it may not be the most cost-effective. Most “medically approved” AC/DC power supplies are not rated for 2 x MOPP and cannot be used in BF
applications. In BF medical applications, part of the system used by the operator needs to meet the less-restrictive 2 x MOOP, while the AP section of the system must be rated for BF safety levels and meet 2 x MOPP. Combining an AC/DC power supply that meets 2 x MOOP with a DC/DC converter that meets the 2 x MOPP is usually the lowest cost solution (Figure 1). This approach can also be useful for medical devices that include battery backup power and need to comply with 2 x MOPP during an AC power failure.
Most off-the-shelf DC/DC converters have isolation ratings of only 500 to 1,600 volts direct current (Vdc) and cannot meet 2 x MOPP. Designers can turn to specialized DC/DC converters with up to 5,000 volts alternating current (Vac) isolation, double insulation, and 8 millimetre (mm) creepage that meet the 2 x MOPP requirements when used with medically approved AC/DC power supplies rated for 2 x MOOP.
Table 1: IEC 60601 requirements for MOPPs are more restrictive than for MOOPs. (Image source: TRACO Power)
Components in Electronics
In addition, standard DC/DC converters have not been subjected to a risk assessment as defined in ISO 14971, which defines the best practices for all life cycle stages of medical devices. This medical device directive also requires manufacturers of DC/ DC converters and other medically approved
www.cieonline.co.uk
Figure 1: An AC/DC power supply with a 2 x MOOP rating can be combined with a 2 x MOPP rated DC/DC converter to arrive at a cost-effective solution for medical device designs. (Image source: TRACO Power)
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